Enter the ‘Synestia’

by Paul Gilster on May 24, 2017

What happens when giant objects collide? We know the result will be catastrophic, as when we consider the possibility that the Moon was formed by a collision between the Earth and a Mars-sized object in the early days of the Solar System. But Sarah Stewart (UC-Davis) and Simon Lock (a graduate student at Harvard University) have produced a different possible outcome. Perhaps an impact between two infant planets would produce a single, disk-shaped object like a squashed doughnut, made up of vaporized rock and having no solid surface.

Call it a ‘synestia,’ a coinage invoking the Greek goddess Hestia (goddess of the hearth, family, and domestic life, although the authors evidently drew on Hestia’s mythological connections to architecture). Stewart and Lock got interested in the possibility of such structures by asking about the effects of angular momentum, which would be conserved in any collision. Thus two giant bodies smashing into each other should result in the angular momentum of each being added together. Given enough energy (and there should be plenty), the hypothesized structure should form, an indented disk much larger than either planet.



Image: The structure of a planet, a planet with a disk and a synestia, all of the same mass. Credit: Simon Lock and Sarah Stewart.

Moreover, this process should be widespread (if generally short-lived) in young, evolving planetary systems. As planet formation ends, planetary collisions should produce rapidly rotating, partially vaporized rocky objects. The researchers developed a computer code called HERCULES that allows them to calculate the physical structures of bodies in varying temperatures and rotational states. There are combinations of rotational rate and thermal energy that make it impossible for a planet to rotate like a solid body. Instead, we get an inner region with its own rotation connecting to a disk-like outer region moving at orbital velocities.

The paper on this work notes that “…the structure of post-impact bodies influences the physical processes that control accretion, core formation and internal evolution. Synestias also lead to new mechanisms for satellite formation.” Moreover, Stewart and Lock believe that rocky planets are vaporized multiple times during their formation. Thus synestias should be a common outcome in young systems. From the paper:

…there is a corotation limit for the structure of terrestrial bodies that depends on mass, compositional layering, thermal state, and AM [angular momentum]. We have named super-CoRoL [corotation limit] structures synestias. Synestias typically consist of an inner corotating region connected to an outer disk-like region. By analyzing the results of N-body simulations of planet formation, we found that high-entropy, highly vaporized post-impact states are common during terrestrial planet accretion. Given the estimated range of planetary AM during the giant impact stage, we find that many post-impact structures are likely to be synestias.

Remember that a sufficiently large impact will have produced molten or gaseous material in vast quantities, expanding in volume and responding to all that angular momentum. One outcome, given the size of the impacting objects and the energy involved, could be a disk of material surrounding the impacted planet. But the researchers believe a synestia is likely at some point, perhaps lasting as little as a hundred years. For the same amount of mass, a synestia would be much larger than a solid planet with a disk of material around it.

Could we ever hope to observe such an object given how short its lifetime is expected to be? Perhaps, and not just by a stroke of good luck. For Stewart and Lock argue that a synestia formed from larger objects like gas giants or even stars could potentially last much longer. That would make a synestia a possible observable in young extrasolar systems. With that in mind, it will be interesting to see whether the HERCULES code produced in this work will find its way into new studies of planet formation and evolution.

The paper is Lock & Stewart, “The structure of terrestrial bodies: Impact heating, corotation limits, and synestias,” Journal of Geophysical Research: Planets 122 (2017). Abstract / preprint.



TRAPPIST-1h: Filling in the Picture

by Paul Gilster on May 23, 2017

One of the worst things we can do is to get so wedded to a concept that we fail to see conflicting information. That’s true whether the people involved are scientists, or stock brokers, or writers. It’s all too easy to distort the surrounding facts because we want to get a particular result, a process that is often subtle enough that we don’t notice it. Thus I was interested in what Rodrigo Luger said about his recent work on the outermost planet of TRAPPIST-1:

“It had me worried for a while that we were seeing what we wanted to see. Things are almost never exactly as you expect in this field — there are usually surprises around every corner, but theory and observation matched perfectly in this case.”

And that’s just it — in exoplanet research, we’ve come to expect the unexpected. So when Luger (a doctoral student at the University of Washington) went to work on this intriguing star some 40 light years from Earth, and its seven now famous planets, he was understandably edgy. Would TRAPPIST-1h turn out to orbit just where his team had projected?


Image: A size comparison of the planets of the TRAPPIST-1 system, lined up in order of increasing distance from their host star. The planetary surfaces are portrayed with an artist’s impression of their potential surface features, including water, ice, and atmospheres. Credit: NASA/R. Hurt/T. Pyle.

You’ll recall that it was the original survey led by Michaël Gillon (University of Liège, Belgium) that identified three planets around TRAPPIST-1 in 2016, a number that jumped to seven in a 2017 paper that drew worldwide attention, especially because several of the planets appeared to be within the star’s habitable zone. TRAPPIST-1h was a problem because Gillon and team were only able to observe a single transit. What Luger proceeded to do, working with Gillon as one of his co-authors, was to follow up on that work with an international effort that studied 79 days of K2 data, snaring four transits of TRAPPIST-1h in the process.

The bit about seeing what you want to see comes from the fact that the planets of TRAPPIST-1 are locked into an orbital resonance, making their orbital periods mathematically related. Luger and colleagues wanted to use the orbital velocities of the better understood TRAPPIST-1 planets to make a prediction about the orbital velocity and period of TRAPPIST-1h. Six possible resonant periods for the planet came out of these calculations, but subsequent data ruled out all but one. Would resonance unlock the problem?

The answer was yes. The K2 data confirmed the prediction, showing us that TRAPPIST-1h is in a frigid 18.77 day orbit around the ultracool dwarf star. At this distance from the host, the planet receives about as much energy per unit area from its star as Ceres receives from the Sun.

Image: The animation shows a simulation of the planets of TRAPPIST-1 orbiting for 90 Earth-days. After 15 Earth-days, the animation focuses only on the outer three planets: TRAPPIST-1f, TRAPPIST-1g, TRAPPIST-1h. The motion freezes each time two adjacent planets pass each other; an arrow appears pointing to the location of the third planet. This complex but predictable pattern, called an orbital resonance, occurs when planets exert a regular, periodic gravitational tug on each other as they orbit their star. The three-body resonance of the outer three planets causes the planets to repeat the same relative positions, and expecting such a resonance was used to predict the orbital period of TRAPPIST-1h. Credit: Daniel Fabrycky/University of Chicago.

So now we have a seven-planet chain of resonances. We’ve seen other multi-planet resonances — four-planet resonances exist at Kepler-80 and Kepler-223 — but TRAPPIST-1, as in so many things, ups the ante considerably. The resonance picture here also gives us ideas about the history of this system, which is thought to be somewhere between 3 billion and 8 billion years old (and if that range of possibilities isn’t a reminder of how much we have to learn about dating cool stars like this, I don’t know what is).

From the paper:

The resonant structure of the system suggests that orbital migration may have played a role in its formation. Embedded in gaseous planet-forming disks, planets growing above ∼ 1 MMars create density perturbations that torque the planets’ orbits and trigger radial migration. One model for the origin of low-mass planets found very close to their stars proposes that Mars- to Earth-sized planetary embryos form far from their stars and migrate inward. The inner edge of the disk provides a migration barrier such that planets pile up into chains of mean motion resonances.

Thus we’re looking at possible migration inward of a set of planets that migrated in what Luger calls ‘lock-step.’ Suddenly TRAPPIST-1 becomes an excellent test case not only for planet formation but planet migration theories. The paper continues:

This model matches the observed period ratio distribution of adjacent super-Earths if the vast majority (∼ 90%) of resonant chains become unstable and undergo a phase of giant impacts. Some resonant chains do survive, and a handful of multiple-resonant super-Earth systems have indeed been characterized. The TRAPPIST-1 system may thus represent a pristine surviving chain of mean motion resonances.

This would have been, the authors believe, a slow migration given the low mass of the TRAPPIST-1 planet-forming disk, and the fact that the planets themselves are low in mass. Perhaps this explains why the TRAPPIST-1 resonant chain is less compact than in systems with more massive planets, and why its unique stability has survived.

And what more can I say about K2? Despite everything going against the TRAPPIST-1h work, including not just the drift and jitter of the spacecraft in its less than optimum state but the faintness of the occasionally flaring target, K2 nonetheless delivered the goods. It’s a real testimony to those working the mission that we’re still pulling useful data out of the K2 observations, and testimony as well to the quality of the team working TRAPPIST-1h.

The paper is Luger et al.,”A seven-planet resonant chain in TRAPPIST-1,” Nature Astronomy 1 (2017), 0129 (abstract / preprint).



Best Images Yet of Fomalhaut Debris Disk

by Paul Gilster on May 22, 2017

The ongoing dimming of Boyajian’s Star will result in a flood of new data from a wide variety of instruments worldwide, excellent news for those trying to piece together what is happening here. I hope you saw Tabetha Boyajian’s interview with David Kipping over the weekend, but if not, you can see it archived here. I tracked the story on Twitter all weekend, and as I did so, I was reminded of the recent news about Fomalhaut, where massive comets may explain what we are seeing in the star’s debris disk. You’ll recall that early in the work on Boyajian’s Star, comets were one explanation for its anomalous light curves, and it will be interesting to see whether the cometary hypothesis can stand up to the influx of new information.

Interesting as well to look at the new data in terms of Kepler’s, asking whether this is a periodic dimming, and hence not the result of intervening material between us and the star.

We’ll see! And as I continue to keep one eye on my Twitter feed, let me segue into Fomalhaut.

A Stunningly Imaged Debris Disk

The ALMA radio telescope (Atacama Large Millimeter/submillimeter Array) has given us the first complete view of the star’s prominent circumstellar disk in millimeter-wavelength light. One of the brightest stars in our sky, Fomalhaut (α Piscis Austrini) is about 25 light years from the Sun, an A-class star whose excess emission in the infrared has flagged the presence of a circumstellar disk. We actually have a K-class star here as well (TW Piscis Austrini) and an M-class dwarf (LP 876-10), making up a widely spaced triple system. And around it we have a band of icy dust some 2 billion kilometers wide, circling the star 20 billion kilometers out.

What is striking about the composite image (incorporating Hubble’s optical data in blue as well as the ALMA results) is how well-defined the disk of debris and gas appears. We had earlier ALMA imagery taken when the array was still under construction in 2012 that showed about half of the debris disk, but we now get the complete view, one whose data reveal similarities between cometary material in our own Solar System and the debris around Fomalhaut.

“ALMA has given us this staggeringly clear image of a fully formed debris disk,” said Meredith MacGregor, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and lead author on one of two papers accepted for publication in the Astrophysical Journal describing these observations. “We can finally see the well-defined shape of the disk, which may tell us a great deal about the underlying planetary system responsible for its highly distinctive appearance.”


Image: Composite image of the Fomalhaut star system. The ALMA data, shown in orange, reveal the distant and eccentric debris disk in never-before-seen detail. The central dot is the unresolved emission from the star, which is about twice the mass of our sun. Optical data from the Hubble Space Telescope is in blue; the dark region is a coronagraphic mask, which filtered out the otherwise overwhelming light of the central star. Credit: ALMA (ESO/NAOJ/NRAO), M. MacGregor; NASA/ESA Hubble, P. Kalas; B. Saxton (NRAO/AUI/NSF).

You’ll also note in both images in today’s post that the disk seems brighter in two places, at about the 1 and 7 o’clock positions. This is intriguing: It tracks a prediction made last year by MIT’s Margaret Pan, who contributed to both the papers that have just appeared on the ALMA work. Pan pointed out that the dusty material in the Fomalhaut disk travels more slowly at apoapsis, when the disk is at its furthest from the star. The slowdown allows denser concentrations of dust to form, which show up as bright millimeter-wavelength emission.

This is a relatively young system at 440 million years old, making it about a tenth the age of our Solar system, and we may be seeing a system going through its own version of our Late Heavy Bombardment. Some 4 billion years ago, the Earth was frequently impacted by asteroids and comets, material left over from the formation of the system.

The Fomalhaut system is also familiar in that carbon monoxide shows up strongly in the ALMA findings. The data peg the relative abundance of carbon monoxide and carbon dioxide around Fomalhaut as roughly similar to what we see in comets found in our Solar System. Comet collisions may be releasing the gas in these quantities, perhaps even the impacts of huge comets of the kind that some have suggested around the enigmatic Boyajian’s Star.


Image: ALMA image of the debris disk in the Fomalhaut star system. The ring is approximately 20 billion kilometers from the central star and about 2 billion kilometers wide. The central dot is the unresolved emission from the star, which is about twice the mass of our sun. Credit: ALMA (ESO/NAOJ/NRAO); M. MacGregor.

We already have an intriguing object here in the form of Fomalhaut b, which has been directly imaged, in a 1700 year highly elliptical orbit with apastron at about 300 AU. It is currently 110 AU from its host. Are there planets within the ring? Evidence points in that direction. The researchers were able to develop computer models that, along with the ALMA data, helped them calculate the width and geometry of the Fomalhaut disk. The disk has similarities to our own Kuiper Belt, as the paper notes, and its shape is telling:

The Fomalhaut debris disk is similarly narrow to the main classical Kuiper Belt in our own Solar System, which is radially confined between the 3:2 and 2:1 orbital resonances with Neptune implying a fractional width of ∼ 0.18 (Hahn & Malhotra 2005). In contrast, both the HD 107146 (Ricci et al. 2015) and η Corvi (Marino et al. 2017) debris disks appear much broader with fractional widths of > 0.3.

That very narrowness has been seen as an indicator of planets, and not just by the authors of the current papers. But compelling as the evidence is, it is not yet conclusive:

Boley et al. (2012) propose that the narrow ring observed in Fomalhaut may also result from interactions with planets, namely two shepherding planets on the inner and outer edges of the belt. If the structure of the belt is indeed due to truncation by interior and exterior planets, we would expect to see sharp edges. However, given the resolution of our observations (∼ 10 AU) compared with the width of the belt (∼ 14 AU), we are unable to place any strong constraints on the sharpness of the disk edges.

Seeing the Fomalhaut disk at his level of detail gives us insights into early planet formation as we try to tease out the underlying planetary system that seems to be emerging. And the chemical similarities within this band and comets in our own system tell us that the processes occurring here may well parallel conditions in our own outer system’s infancy.

The papers are Macgregor et al., “A Complete ALMA Map of the Fomalhaut Debris Disk” (preprint) and Matrà et al., “Detection of exocometary CO within the 440 Myr-old Fomalhaut belt: a similar CO+CO2 ice abundance in exocomets and Solar System comets,” accepted at the Astrophysical Journal (preprint).



New Dip for Boyajian’s Star

by Paul Gilster on May 19, 2017

Twitter action has been fast and furious with this morning’s news of the first clear dip in light from Boyajian’s Star (KIC 8462852) since the Kepler data.

I’m on the road most of today and so couldn’t get off a full post, but I did want to pass along Tabetha Boyajian’s newsletter, short but sweet.

Hello all,

We have detected a dip in progress!

Not much time to share details – we are working hard coordinating followup observations.

Here is a snapshot of LCO data for the Month of May. Stay tuned!

~Tabby et al.


And here is Jason Wright’s video chat on this event during his visit to UC Berkeley.



Detecting Photosynthesis on Exoplanets

by Paul Gilster on May 18, 2017

Although many of the nearby stars we will study for signs of life are older than the Sun, we do not know how long it takes life to emerge or, for that matter, how likely it is to emerge at all. As we saw yesterday, that means plugging values into Drake-like equations to estimate the possibility of detecting an alien civilization. We can’t rule out the possibility that we are surrounded by planets teeming with non-sentient life, fecund worlds that have no heat-producing technologies to observe. Fortunately, we are developing the tools for detecting life of the simplest kinds, so that while a telescope of Colossus class can be used to detect technology-based heat signatures, it can also be put to work looking for simpler biomarkers.

Svetlana Berdyugina (Kiepenheuer Institut für Sonnenphysik and the University of Freiburg), now a visiting scientist at the University of Hawaii, has been leading a team on such detections and spoke about surface imaging of Earth-like planets at the recent Breakthrough Discuss conference. The emphasis was on Proxima b, but these techniques can be applied to many other systems within the 60 light year radius that Colossus should be capable of probing.

Studying exoplanets in different orbital phases allows us to acquire a surprising amount of information, using techniques that have already been deployed for the study of the surfaces of stars. We have to take into account factors like clouds, seasonal variation in albedo, and the variability of the host star as we consider these matters, but given the signal-to-noise ratios that the specs for a Colossus-like telescope imply, we should be able to discern not only variations between land masses and oceans but the photosynthetic biosignature of local plant coverage.

Berdyugina’s team includes Jeff Kuhn (University of Hawaii), and university colleagues David Harrington and John Messersmith, along with Tina Šantl-Temkiv (Aarhus University, Denmark). The idea they have explored in a 2016 paper in the International Journal of Astrobiology (citation below) is to use the properties of light to detect photosynthesis.

What can we say about the detectability of biomolecules that capture photons and store their energy in chemical bonds? In green plants, the resulting chemical energy converts water, carbon dioxide, and minerals into oxygen and energy-rich organic compounds. Photosynthetic pigments can absorb solar light in the visible range and create the chemical bonds with which it can be stored for use. Chlorophyll pigments, for example, absorb blue to red light while reflecting some part of the green at visible wavelengths, which accounts for our perception of green plants. The useful fact is that all incident infrared light is reflected, giving us a marker if we can find a way to observe it. This sharp rise in reflectivity has been called ‘the red edge.’

We use these facts already in studying our own planet through Earth-observing satellites like Landsat, which can map changes to Amazon forest cover by imaging in multiple bands falling on either side of the ‘red edge.’ I also note work from Giovanna Tinetti (University College London), which estimates that 20 percent of a planet’s surface must be covered by plants and free from clouds in order for the imprint of vegetation to show up in a global spectrum. It will be interesting to see whether, as it continues to develop, Berdyugina’s work agrees with this figure.

But back to the paper. The key to the work is polarization, the oscillation of light in certain directions as opposed to light that oscillates in all directions at once. We learn that the infrared light reflected by a leaf is polarized nowhere near as markedly as the visible light reflected off it. This means that polarizing filters can be used with sufficient contrast to detect biopigments, each of which, like chlorophyll, has its own signature in polarized light. Moreover, the current work shows that polarized light can distinguish clearly between the biosignatures of photosynthesis and light from oceans, minerals and atmospheres.


Image: A green leaf absorbs almost all red, green and blue light (RGB), but it reflects and transmits infrared light (shown in grey). The reflected infrared light is only weakly polarized due to the reflection of a healthy leaf, but the reflected RGB light is strongly polarized due to biopigments. Measuring the amount of polarized light at different colors reveals the signature of the leaf biopigments. Green sand reflects and polarizes sunlight almost equally in all wavelengths, which distinguishes it from a leaf that is a similar color. Similarly, yellow plants are different from yellow sand, etc. Credit: S. Berdyugina.

In their 2016 paper, the researchers discuss their development of a detection mechanism based on polarimetry, working with a laboratory experiment measuring the optical polarized spectra of samples both biological and non-biological. The paper investigates a range of biomolecules that capture stellar photons and store their energy in chemical bonds, examining plants with various pigments and likewise measuring non-biological materials like rock and sand.

The results, drawn on modeling of the polarized spectra of Earth-like planets in a variety of configurations — degree of surface coverage by photosynthetic organisms, empty land areas and ocean — show how useful polarized spectra can be at detecting photosynthetic pigments. Bear in mind when considering future observations that even with upcoming giant telescopes, we will not be able to image a planetary surface directly. Instead, we will use changes in the rotational signature of the planet as it moves about its star to learn about surface properties.

This rotational signature should prove extremely helpful, as the paper notes, although it requires large telescopes. Moreover, the contrast achievable in detected light depends on the kind of star we are dealing with and the wavelengths we are working at. From the paper:

It is feasible that the contrast down to 10–8 can be achieved with the current technology. However, to collect the necessary amount of photons in order to achieve such a high contrast for small planets requires extremely large telescopes. It appears that 25–40 m telescopes will be able to see only a few such planets. Large telescopes, such as the 75 m Colossus telescope are needed to investigate hundreds of Earth-like planets in stellar habitable zones (Kuhn & Berdyugina, 2015), but even such large telescopes will be able to detect their light with a sufficient SNR at very low spectral resolution or in broad bands. The fact that absorption and polarization features of biopigments are extremely broad allows for filters designed to provide enough detail on their possible photosynthetic origin.

Such filters give us, in other words, a tool that can distinguish a living from a non-living world. And not just in terms of vegetation. The paper goes on to note that the same biosignature is produced by bacteria and archaea, both of which use biopigments to harvest stellar light, as well as to protect themselves from dangerous UV radiation. Applying the techniques in this paper to the study of microorganism signatures through polarized light is an ongoing project.

This work may remind you of Nancy Kiang’s work on the spectral signatures of photosynthesis at NASA Goddard. Finding the right absorption band in the spectrum of an exoplanet can be tricky, as Kiang has shown by studying the colors of pigments and how they might change depending on the spectral class of the host star (see Beyond the Red Edge). On Earth, the colors of our land plants depend upon pigments that absorb in the visible blue and red, giving us green and yellow plants, but M-class dwarfs may have their peak absorption in the blue and near-infrared part of the spectrum, as Berdyugina and colleagues note. Thus we have to bear in mind how photosynthetic pigments might adapt depending on incident starlight. From Berdyugina et al:

It is worth also to note that a lack of blue photons in cool M stars will probably require a more complex mechanism for splitting water molecules into hydrogen and oxygen involving three or four photons instead of two as it occurs in terrestrial organisms. Therefore, understanding properties of radiation reflected from various photosynthetic organisms may help to identify such life forms on distant planets. This is the primary goal of our study.

Thus the paper identifies the signatures of biological pigments that can be used for biomarker detection, developing models of Earth-like planets with different coverage conditions of land and ocean, vegetation and clouds. The investigation of polarization winds up showing that linear polarization becomes the most potent method for detection of such biomarkers, citing “…very sensitive and rather unambiguous detection of photosynthetic pigments of various kinds.”

The paper is Berdyugina et al., “Remote sensing of life: polarimetric signatures of photosynthetic pigments as sensitive biomarkers,” International Journal of Astrobiology 15 (1): 45-56 (2016). Full text.



A ‘Census’ for Civilizations

by Paul Gilster on May 17, 2017

We’ve been talking about the Colossus project, and the possibility that this huge (though remarkably lightweight) instrument could detect the waste heat of extraterrestrial civilizations. But what are the chances of this, if we work out the numbers based on the calculations the Colossus team is working with? After all, Frank Drake put together his famous equation as a way of making back-of-the-envelope estimates of SETI’s chances for success, working the numbers even though most of them at that time had to be no more than guesses.

Bear in mind as we talk about this that we’d like to arrive at a figure for the survival of a civilization, a useful calculation because we have no idea whether technology-driven cultures survive or destroy themselves. Civilizations may live forever, or they may die out relatively quickly, perhaps on a scale of thousands of years. Here Colossus can give us useful information.

The intention, as discussed in a paper by Jeff Kuhn and Svetlana Berdyugina that we looked at yesterday (citation below), is to look out about 60 light years, a sphere within which we have numerous bright stars that a large instrument like Colossus can investigate for such detections. We’re making the assumption, by looking for waste heat, that civilizations living around such stars could be detected whether or not they intend to communicate.

Screenshot from 2017-05-17 08-41-14

Image: Figure 1 from Kuhn & Berdyugina, “Global Warming as a Detectable Thermodynamic Marker of Earth-like Extrasolar Civilizations: The case for a Telescope like Colossus.” Caption: Man-made visible light on the Earth in 2011. From DMPS/NASA. The brightest pixels in this 0.5 × 0.5 degree resolution map have a radiance of about 0.05 × 10−6 W/cm2/sr/micron. Credit: Jeff Kuhn/Svetlana Berdyugina.

Let’s take the fraction of stars with planets as 0.5, and the fraction of those with planets in the habitable zone as 0.5, numbers that have the benefit of Kepler data as some justification, unlike Drake’s pre-exoplanet era calculations. Kuhn and Berdyugina have to make some Drake-like guesses as they run their own exercise, so let’s get really imaginative: Let’s put the fraction of those planets that develop civilizations at the same 0.5, and the fraction of those that are more advanced than our own likewise at 0.5. These numbers operate under the assumption that our own civilization is not inherently special but just one of many.

Work all this out and we can come up with a figure for the fraction of civilizations that might be out there. But how many of them have survived their technological infancy?

Let me cut straight to the paper on the outcome of the kind of survey contemplated for Colossus, which is designed to include “a quantifiably complete neighborhood cosmic survey for [Kardashev] Type I civilizations” within about 20 light years of the Sun, but one that extends out to 60 light years. In the section below, Ω stands for the ratio of power production by an extraterrestrial civilization to the amount of stellar power it receives (more on this in a moment).

From the paper:

…current planet statistics suggest that out of 650 stars within 20 pc at least one quarter would have HZEs [Habitable Zone Earths]. Assuming that one quarter of those will develop Ω ≥ 0.01 civilizations, we arrive at the number of detectable civilizations in the Solar neighbourhood ND = 40fs, where fs is the fraction of survived civilizations (i.e., civilizations that form and survive). Hence, even if only one in 20 advanced civilizations survive (including us at the time of survey), we should get a detection. Taking into account the thermodynamic nature of our biomarker, this detection is largely independent of the sociology of detectable ETCs.

Independent because we are not relying on any intent to communicate with us, and are looking for civilizations that may in fact be advanced not far beyond our own level, as well as their more advanced counterparts, should they exist.

Suppose we detect not a single extraterrestrial civilization. Within the parameters of the original assumptions, we could conclude that if a civilization does reach a certain level of technology, its probability of survival is low. That would be a null result of some consequence, because it would place the survival of our own civilization in context. We would, in other words, face old questions anew: What can we do to prevent catastrophe as a result of technology? We might also consider that our assumptions may have been too optimistic — perhaps the fraction of habitable zone planets developing civilizations is well below 0.5.

But back to that interesting figure Ω. The discussion depends upon the idea that the marker of civilization using energy is infrared heat radiation. Take Earth’s current global power production to be some 15 terawatts. It turns out that this figure is some 0.04 percent of the total solar power Earth receives. In this Astronomy article from 2013, Kuhn and Berdyugina, along with Colossus backers David Halliday and Caisey Harlingten, point out that in Roman times, the figure for Ω was about 1/1000th of what it is today. Again, Ω stands for the ratio of power production by a civilization to the amount of solar power it receives.

The authors see global planetary warming as setting a limit on the power a civilization can consume, because both sunlight from the parent star as well as a civilization’s own power production determine the global temperature. To produce maximum energy, a civilization would surely want to absorb the power of all the sunlight available, increasing Ω toward 1. Now we have a culture that is producing more and more waste heat radiation on its own world. And we could use an instrument like Colossus to locate civilizations that are on this course.

In fact, we can do better than that, because within the 60 light year parameters being discussed, we can study the heat from such civilizations as the home planet rotates in and out of view of the Earth. Kuhn and Berdyugina liken the method to studying changes of brightness on a star. In this case, we are looking at time-varying brightness signals that can identify sources of heat on the planet, perhaps clustered into the extraterrestrial analog of cities. A large enough infrared telescope could observe civilizations that use as little as 1 percent of the total solar power they intercept by combining visible and infrared observations. A low value of Ω indeed.

Screenshot from 2017-05-17 08-41-55

Image: Figure 3 from the Kuhn/Berdyugina paper “Global Warming as a Detectable Thermodynamic Marker of Earth-like Extrasolar Civilizations: The case for a Telescope like Colossus.” Caption: Fig. 3. Expanded view of a representative North American region illustrating temperature perturbation due to cities (left, heated cities are seen in red) and corresponding surface albedo (right). From NEO/NASA.

You can see what a challenge this kind of observation presents. It demands, if the telescope is on the ground, adaptive optics that can cancel out atmospheric distortion. It also demands coronagraph technology that can distinguish the glow of a working civilization from a star that could be many millions of times brighter. And because we are after the highest possible resolution, we need the largest possible collecting area. The contrast sensitivity at visible and infrared wavelengths of the instrument are likewise crucial factors.

I’ll refer you to “New strategies for an extremely large telescope dedicated to extremely high contrast: The Colossus Project” (citation below) for the ways in which the Colossus team hopes to address all these issues. But I want to back out to the larger view: As a civilization, we are now capable of building technologies that can identify extraterrestrial cultures at work, and indeed, instruments like Colossus could be working for us within a decade if we fund them.

We can add such capabilities to the detection of non-technological life as well, through the search for biomarkers that such large instruments can enable. More on that tomorrow, when I’ll wrap up this set on Colossus with a look at photosynthesis signatures on exoplanets. Because for all we know, life itself may be common to habitable zone planets, while technological civilization could be a rarity in the galaxy. Learning about our place in the universe is all about finding the answers to questions like these, answers now beginning to come into range.

The Colossus description paper is Kuhn et al., “Looking Beyond 30m-class Telescopes: The Colossus Project,” SPIE Astronomical Telescopes and Instrumentation (2014). Full text. The paper on Colossus and waste heat is Kuhn & Berdyugina, “Global warming as a detectable thermodynamic marker of Earth-like extrasolar civilizations: the case for a telescope like Colossus,” International Journal of Astrobiology 14 (3): 401-410 (2015). Full text.



Colossus and SETI: Searching for Heat Signatures

by Paul Gilster on May 16, 2017

Yesterday we looked at the PLANETS telescope, now under construction on the Haleakala volcano on the island of Maui. What will become the world’s largest off-axis telescope is considered a pathfinder, part of the progression of instruments that will take us through the array of sixteen 5-meter mirrors that will be called ExoLife Finder, itself to be followed by Colossus, an instrument comprised of 58 independent off-axis telescopes. Colossus will use ultra-thin mirror technologies and interferometric methods to achieve an effective resolution of 74 meters. And it will be optimized for detecting extrasolar life and extraterrestrial civilizations.


Image: Artist’s rendering of the Colossus telescope. Credit: Colossus/Dynamic Structures Ltd.

How to build something on such a scale? The design work is being handled by a consortium led by Jeff Kuhn (University of Hawaii), Svetlana V. Berdyugina (University of Hawaii/Kiepenheuer Institut für Sonnenphysik), David Halliday (Dynamic Structures) and businessman Caisey Harlingten, backed by an international team of astronomers associated with the PLANETS Foundation, as we saw yesterday. Building an instrument of this scale calls for innovation across the board, especially in terms of reducing weight and heightening resolution.

Thus Colossus relies upon extremely lightweight mirrors that deploy electromechanical force actuators that control the mirror’s shape and provide its stiffness. These mirrors are not separated from their electromechanical backing structure after manufacturing, depending on a network control system to fix their shape. In this overview of the Colossus design, they are described as ‘live mirrors,’ unlike normal telescope optics because they have much less mass and can be created without conventional grinding.

Civilization and Heat

An instrument like this has sufficient aperture and scattered light suppression to detect exoplanet biomarkers and, if they exist, the markers of extraterrestrial civilizations. It’s on this latter issue that I want to focus today. Over the past few years, we’ve delved into what is being called ‘Dysonian SETI,’ the search for other civilizations not through dedicated beacons but astronomical evidence of their activities. The reference to Freeman Dyson goes back to his description of spherical structures for gathering the total luminosity of a host star, the so-called Dyson sphere, or as it is also imagined, the Dyson ‘swarm’ of energy-gathering technology.

Richard Carrigan, a scientist emeritus in the Accelerator Division at the Fermi National Accel­era­tor Laboratory, has run searches for such objects using data from the Infrared Astronomical Satellite (IRAS) mission (1983), which he believes sensitive enough to find Dyson spheres out to about 300 parsecs. But he is hardly the only one to mount such searches. The Russian radio astronomer Vyacheslav Ivanovich Slysh likewise surveyed infrared data for Dyson signatures, as did M. Y. Timofeev, collaborating with Nikolai Kardashev, in an attempt to scan the same IRAS data.

Carl Sagan, working with Russell Walker, was analyzing “The Infrared Detectability of Dyson Civilizations” (a paper in The Astrophysical Journal) back in the 1960s, noting the problems of distinguishing a Dyson sphere signature from natural phenomena. I won’t go deeper in this direction, though if you’re interested, the archives here cover the various search attempts as well as the ongoing work of the Glimpsing Heat from Alien Technologies group at Ohio State (see Archaeology on an Interstellar Scale and G-HAT: Searching for Kardashev Type III for more references on recent work). The point is that we have yet to find something that can be identified as a Dyson sphere or swarm despite repeated attempts.

The building of Colossus would allow us to move beyond the enormous Dyson constructs (spherical structures with planetary-like radii) to examine much weaker, but surely more likely, heat signatures from an active extraterrestrial civilization. Running a civilization takes power, and we know that by virtue of the laws of thermodynamics, power produces heat. Notice that in both Dysonian searches and these attempts to find heat as a byproduct of a civilization’s ongoing activities, we are not assuming any intent to communicate on the part of the extraterrestrial culture. We are simply trying to observe the unavoidable consequence of being a tool-using civilization that has reached a certain level of development.

In a paper looking at Colossus and its application to this search, Jeff Kuhn and Svetlana Berdyugina explain the point this way:

Waste heat is a nearly unavoidable indicator of biological activity, just as the energy that civilization consumes is eventually reintroduced into the planetary environment as heat. On planetary scales, biologically produced heat tends to be spatially clustered, just as an ET civilizations’ technological heat is difficult to distribute uniformly. Planetary surface topography and the efficient tendency for population to cluster in agrarian and urban domains leads to heat ‘islands’ (cf. Rizwan et al. 2008).The temporal and spatial distribution of this heat can be an observable ‘fingerprint’ for remote sensing of civilizations. Here we argue that we may soon be in a position to detect this thermodynamic signal from Type I, nearly Earth-like civilizations.


Image: The Earth at night seen from space (NASA). Colossus will be able to detect similar patterns of advanced civilization heat islands. Credit: Colossus consortium.

A search for Dyson spheres assumes a Kardashev Type II civilization, one capable of using the total energy output of its system’s star, according to the scale Nikolai Kardashev devised in 1964. But Kuhn and Berdyugina argue that an instrument like Colossus is capable of looking for Kardashev Type I, those civilizations capable of using all the energy available to their planet from its star. The argument here is that Type I civilizations (we are sometimes said to be at about Kardashev level .07) will inevitably evolve toward greater power consumption.

The correlation between power consumption and accumulated information content is a strong one in our society. In fact, we humans collect information with a doubling time on the order of two to three years, while our power consumption increases at a pace that outstrips population growth (global power consumption grows by about 2.5 percent per year, while the world’s population grows at something less than half this rate). The assumption, then, is that even a very efficient advanced civilization will still have high power requirements because of the cost to build and use its base of information. As cultures mature, information content grows.

But as we’ll see tomorrow, there are limits on the power a civilization can consume at the planetary level. And waste heat radiation can become a powerful signature for detection with the right equipment. A tool like Colossus, operating not with a wide field of view like most of the giant telescopes in the pipeline but observing only a few arcseconds of the sky at a time, would be capable of studying nearby planets in the habitable zone of their stars to detect such waste heat. A survey of stars within roughly 60 light years of the Sun could thus help us identify an extraterrestrial civilization or, just as important, demonstrate the lack of same.

More on Colossus tomorrow as we look at its methods, and address the question of whether technological civilizations survive. We can’t know the answer to this yet, but beginning a statistical survey of nearby stars is one way to get a glimpse of our own possible destiny. We also need to think about giant telescopes and their capabilities at detecting photosynthetic organisms in extrasolar systems. We may not find civilizations, but we can still find life.

The Colossus description paper is Kuhn et al., “Looking Beyond 30m-class Telescopes: The Colossus Project,” SPIE Astronomical Telescopes and Instrumentation (2014). Full text. The paper on Colossus and waste heat is Kuhn & Berdyugina, “Global warming as a detectable thermodynamic marker of Earth-like extrasolar civilizations: the case for a telescope like Colossus,” International Journal of Astrobiology 14 (3): 401-410 (2015). Full text. For the overview on Colossus, see the project’s home page.



PLANETS Telescope: Building Toward Colossus

by Paul Gilster on May 15, 2017

Let me call your attention to the PLANETS telescope, now seeking a funding boost through an ongoing Kickstarter campaign. Currently about halfway built, the PLANETS (Polarized Light from Atmospheres of Nearby ExtraTerrestrial Systems) instrument is located on the 10,000 foot Haleakala volcano on the island of Maui. When completed, it will be the world’s largest off-axis telescope (at 1.85 meters) for night-time planetary and exoplanetary science. And it’s part of a much larger, scalable effort to find life around nearby stars in as little as a decade.

An off-axis design removes obstructions to the light path like the secondary mirror supports that can cause diffraction effects and lower image quality in axially symmetric reflective telescopes. Here light from the primary mirror is deflected slightly out of the incoming lightpath, limiting diffraction and scattered light. The PLANETS Foundation, the international collaboration of scientists and engineers behind the new telescope, sees it as a test of “low scattered light off-axis optics” as well as cutting edge “thin mirror technology.” The lightweight PLANETS mirror is 90 percent polished — using a tool called HyDRa (developed at the National Autonomous University of Mexico) that has demonstrated 1/100th of a wavelength polish — and key mechanical components of the off-axis design are waiting to be built.

The PLANETS instrument will be optimized for studying the exo-atmospheres of the rocky planets in our own Solar System, but will also delve into the atmospheres and surfaces of bright nearby exoplanets and examine circumstellar disks in young stellar systems. It also sets the stage for biosignature detection as we begin to upgrade its scalable technologies.

As a pathfinder, the PLANETS instrument is the beginning of a 10-year roadmap that aims to make telescopes that are lighter and less costly than the large instruments we currently use to probe the universe. The goals here are impressive: To create a census for life on several hundred of the nearest habitable zone exoplanets. The next step would be an instrument called ExoLife Finder, a circular array of sixteen 5-meter mirrors, using the ‘printed mirror’ technology and lessons learned from the PLANETS telescope to create a hybrid interferometer. With a total diameter of some 40 meters, ELF would be the first telescope to create surface maps of nearby exoplanets, including the one on our doorstep, Proxima b.

But beyond ELF we have Colossus, consisting of 58 independent off-axis telescopes that combine their data using interferometric methods to produce a 74-meter diameter effective resolution. Colossus and its capabilities will be the subject of tomorrow’s post, but for today I’ll note that the $600 million instrument could itself be built in a scant 96 months, according to the PLANETS Foundation site, once funding has been secured. An array based on scalable Colossus concepts could even become an optical system for beamed sailcraft of the kind envisaged by Breakthrough Starshot. But before we do all this, we have to build PLANETS.

The PLANETS telescope is backed by a number of academic sources including Japan’s Tohoku University, Germany’s Kiepenheuer Institute and the Institute for Astronomy in Hawaii, along with technology organizations like HNu Photonics and Dynamic Structures. Completion is expected in 2019, with a total cost of $4 million and approximately $500,000 left to raise. Thus the Kickstarter campaign is a cog in a larger effort. The initial Kickstarter goal of $20,000 goes toward finishing the polishing of the secondary mirror for the instrument, with a stretch goal of $45,000 that would be applied to building the primary telescope support system.

This instrument will demonstrate the ultra thin mirror concepts and hybrid interferometry needed to create an ELF within five years, and a Colossus within a decade. If you can help, please join this effort, and note the ExoCube, a 3D laser engraved glass map of potentially habitable worlds, that is available to supporters in a variety of styles featuring a range of mineral sphere ‘planetary’ add-ons. The Kickstarter site’s videos give you the overview.


Tomorrow we’ll delve deeper into Colossus and talk about the markers it could identify not only in terms of biosignatures but signs of possible technological civilizations.



Synchrony in Outer Space

by Paul Gilster on May 12, 2017

As we watch commercial companies launching (and landing) rockets even as NASA contemplates a Space Launch System that could get us to Mars, it’s worth considering just which future we’re going to see happen. In this essay, Nick Nielsen thinks about making the transition between an early spacefaring civilization to a truly system-wide space culture, and one capable of moving still further out. No technologies arise in isolation, and the financial and social contexts of the things we do interact in ways that make predicting the long haul a dicey business. There is, as Nielsen reminds us, no unilateral history, but just how contingency and serendipity will shape what we achieve in space is no easy matter to untangle. Herewith some thoughts on history, context and attempts to put a brake on rapid change.

By J. N. Nielsen

me small

Diachronic and synchronic historiography

In historiography a distinction is made between the diachronic and the synchronic, which is usually explained by saying that the diachronic is through time and the synchronic is across time. I don’t find this explanation helpful, so I say instead that the diachronic is succession in time and the synchronic is interaction in time. Of course, all interaction in time involves succession in time, so that a synchronic perspective involves some “width” of the present, thus in approaching history from a synchronic perspective we need to agree (even if only tacitly and implicitly) on the width of the present. [1]

The width of the present in synchronic historiography ideally could be set by any temporal parameters we choose. If we extrapolated from the punctiform present (an instantaneous present with no width) to ever-greater parameters for the present, mathematical induction would eventually lead us to a “present” that encompasses the whole of history. It could be argued that Big History is exactly this synchronic perspective that encompasses the whole of time, but I will not take this up at present, as I want to engage in a more conventional exercise in synchronic historical thinking that initially will take a period of a few decades—say, more than a decade but less than a century—as my temporal parameters, and then will be extended to a longer duration. However, I do want to add one highly unconventional element to synchronic historiography by applying this mode of historical thought not to the past, but to the future, though I will also reach a little bit into the past as well.


[“A perfectly scaled diagram showing the orbital altitudes of several significant satellites of earth. all planets and orbital distances are drawn to scale and the altitude data was collected from many Wikipedia articles and various other sites.” Image by Rrakanishu]

Origins of the Space Age

The Space Age began during the Cold War, and arguably as a technological spinoff of ballistic missile research. The first true ballistic missile was the German V-2 produced at the end of the Second World War, which was, for its time, a highly sophisticated rocket. The US and the USSR competed in scooping up the best German scientists after the war in order that these scientific minds should be put to work on the arms of the new superpowers that emerged from the war. Thus the Space Age was conceived in war and brought to its first successes in war. However, many within this war industry were visionaries who viewed their war work as a necessary evil in order to move humanity to the point at which military spinoff technologies could independently contribute to human progress (sort of like the idea of “Atoms for Peace”). Perhaps the best examples of this attitude are to be found in Wernher von Braun and Sergei Korolev.

After the US won the “Space Race” to get to the moon first, budgets for space exploration dwindled and less heroic space missions became the norm. One of the last Apollo capsules was employed in the Apollo-Soyuz mission, which linked up the US and the USSR spacecraft in orbit, and which initiated an era of international cooperation in space that has endured to the present day. Though this cooperative effort has been rocky at times, the funding of the effort has been kept sufficiently low that little has been at stake in terms of geopolitical-level goals. This sort of low-temperature, back-burner space effort could continue indefinitely with little to no impact upon the wider world.

The greatest geopolitical impact of the Space Age to date has been the ability to monitor Earth from orbit. The satellite industry is now an industry in every sense of the term. Many nation-states build and deploy satellites, and many different companies, both government SOEs and private industry, compete for the launch business to place satellites in orbit. The extent to which we today inhabit a planetary civilization—with all that implies in terms of both our global reach and our global limitations—is testified to by this satellite industry, which affects every other industry on the planet, as most of these satellites are built for the purposes of monitoring Earth and facilitating human activity on Earth. A planetary civilization in an equilibrium state thus extends a little beyond its homeworld, to include the orbital trajectories around its planet, in order to more effectively control, administer, and monitor that planetary civilization.

The greatest impact of the Space Age from a scientific point of view, but almost irrelevant from a geopolitical standpoint, has been the exponential improvement in our scientific knowledge of our solar system and the universe beyond by means of automated probes that have traveled throughout our solar system, and a little beyond, collecting data and transmitting it back to Earth. For a relatively small investment in contemporary terms, these instruments placed beyond the atmosphere of Earth, and in some cases on the surfaces of other planets in our solar system, have revolutionized cosmology and our understanding of the universe. While it is commonplace to belittle human achievement in the name of Copernicanism, it really is remarkable that we have managed to take the measurement of the universe entire while our species is still confined to a single planet.


[Alejo Fernández’s “Virgin of the Navigators” (1531-1536) vividly shows
the intersection of medieval Spanish piety and the new Age of Discovery.]

Extraterrestial buildout: the next developmental stage in spacefaring civilization

A planetary civilization more-or-less in an equilibrium state could continue in this manner, with a rudimentary presence in space, without moving appreciably beyond this level of spacefaring development. I take this rudimentary level of spacefaring to be consistent with a planetary civilization in its mature stage of development, when it is capable of satellite technology. However, if a planetary civilization begins to make the transition to a more robust spacefaring posture, it begins to exceed planetary equilibrium and begins the process of becoming a system-wide civilization within the planetary system of its homeworld. This will be the next developmental stage of our civilization if we choose to make that transition. I once called this process extraterrestrialization (which is, admittedly, a long and awkward word with many syllables), though now I prefer to call it “buildout.”

In the present context I will employ “buildout” as a general term meaning the building of any infrastructure that allows a civilization to make the transition to another stage of development. Thus the buildout of a transoceanic seafaring capacity was a necessary (but not a sufficient) condition to the Age of Discovery and the voyages of Columbus and Magellan. This buildout can also be thought of as the mature expression of the commercial trading networks of late medieval European civilization, at which point this regional civilization may have stagnated, though this stagnation did not in fact occur. The buildout of a rudimentary spacefaring capacity in the form of satellite technology can be thought of as the mature expression of a planetary civilization, but it also suggests the buildout of a more robust spacefaring capacity that is the necessary but not sufficient condition of a durable, self-sufficient human presence beyond Earth.

Buildout always shades over imperceptibly into that which comes next, but there is no inevitability as to what follows. [2] The intimations of future possibilities for civilization within the scope of our present institutions may remain undeveloped if a civilization stagnates at any one developmental stage. The spacefaring capacity we currently possess, enabling us to be active in Earth orbit, could hint at further spacefaring to come, or it could simply be the mature expression of a planetary civilization at equilibrium, needing this orbital capacity to monitor and manage the only world that it has. As Freud once said, sometimes a cigar is just a cigar.


[We already have a blooming, buzzing confusion in Earth orbit as the result of all our satellites and space junk. This will only become more confusing over time.]

The blooming, buzzing confusion beyond Earth [3]

What I want to do here is to consider the buildout to the next stage of spacefaring civilization as the parameters of a “wide” present to be analyzed synchronically. This stage of spacefaring buildout will span from our present spacefaring capacity to a civilization on the threshold of an interstellar spacefaring capacity, but not yet having attained that goal. [4] The central lesson of a synchronic perspective on history is that social events do not occur in a social vacuum; technological developments do not occur in a technological vacuum; political events do not occur in a political vacuum, and so on. Moreover, these developments contextualize each other, so that social events also do not occur in a technological vacuum, political vacuum, etc.

Present spacefaring capacity includes satellite construction and launch, the construction and launch of scientific instruments destined for points throughout the Solar System, and a continual human presence in Earth orbit in the ISS since 02 November 2000. This human presence in LEO is a research station and not a settlement. The ISS is entirely dependent upon supply from Earth. There is no industrial infrastructure off the surface of Earth, whether an agricultural industry, or a manufacturing industry, or any other kind of industry, by which the ISS could even potentially support itself, either existentially or financially. [5]

The immediate future of spacefaring comprises both government and private sector initiatives that build on present spacefaring capacity, as well as plans for human missions to Mars. However, the immediate future is likely to be distinguished from the immediate past by the rapidly increasing private sector involvement in space. While private sector involvement in space is primarily driven by wealthy individuals who are engaged in aerospace for personal reasons rather than financial gain, the potential for financial gain, while largely unrealized, is present. The satellite launch industry is, as noted above, a true industry with competition among launch services, and no longer an exclusively government-sponsored undertaking. In so far as the reusable rockets of SpaceX and Blue Origin can bring down launch costs, these are potentially viable, profit-making businesses. Moreover, since the owners of these businesses engage in aerospace initiatives for personal fulfillment, it is likely that profit and growth in this sector will be re-invested in order to pursue more ambitious goals.

While I have great admiration for the work that Blue Origin and SpaceX have done in producing reusable rockets, and it seems likely that reusable rocket technology will reduce the cost of access to space as it matures, the engineering solutions brought to the problem by Blue Origin and SpaceX are not the only engineering solutions possible. There are not merely one or two different technologies that might be employed for cheaper access into space, there are quite literally dozens of different technologies with real promise in this field. I have previously addressed this in my Centauri Dreams post How We Get There Matters: the particular technologies we use will influence our spacefaring activities, because each technology has its individual possibilities and limitations. The actual technologies that come into routine use will be the result of a synchronic process that is ultimately as large and as complex as our industrial infrastructure that produces and uses the technology.

What this means is that technologies that enter into large-scale industrial production will be partially selected by what technology works best, and we don’t know what technology works best until we actually test these technologies under real-world conditions. But this isn’t the only influence on the process of bringing technologies to market. There will also be a question of financing, and which enterprise gets financed will partially be a function of the social and political network of the individuals involved in bringing technologies to market. If venture capitalists find you to be likeable and believable, you will probably get more funding than some other company whose founders are less likeable or less persuasive, even if the latter has a better product than the former. [6]

If a technology is not just an improvement of some degree, but an order of magnitude better than previous technologies [7], this technology will likely come into use at some point in time, but the industrialized economy has a remarkable ability to maintain non-optimal products in use—partly from social inertia, partly from risk aversion, partly from investor’s fear of financing stranded assets—as is witnessed by the QWERTY keyboard.


Synchronic interaction in markets

Energy markets are a perfect example of maintaining non-optimal technologies for social and economic reasons. We have the technology at present to supply the entire world with clean, non-polluting energy, but utilities continue to build fossil fueled power stations. The electrical generation industry changes very slowly, with the planning, financing, construction, and operation of generation facilities taking decades, and their decommissioning also spread over decades.

Energy markets are also a perfect illustration of synchrony in action; though they change slowly, they do eventually change in response to market forces. In my post Synchrony in Energy Markets I attempted to show the interaction of energy markets as technology changes and market forces interact with changes in human society. The failure to understand energy markets and how they function dynamically leads to fixations like the idea of “peak oil,” as though oil were the only fuel used, and the only fuel possible, for an industrialized economy, and as though technology never changes, never becomes more efficient, and industry never substitutes inputs or produces a different product. It would be possible for a totalitarian government to enforce the monopoly of a single fuel, and, to the extent that it was successful, it would weaken itself by making its economy vulnerable to competitors free to experiment with alternatives, which alternatives may prove to be more efficient or more effective.

Fuels are not only inputs of energy into an industrialized economy, fuels are also commodities, and commodities are traded on commodity markets. The same observation holds for the fixation on the dangers of fiat currencies, with the alternative being a currency based on some commodity, usually a durable metal, and this durable metal usually being gold. But gold (or silver) is a commodity like any other commodity, and, in an economic system of any degree of openness, will be traded. [8] Markets are not perfect, and are subject to manipulation and artificial limitation, but they do, on the whole and on the large scale, eventually bring supply and demand into equilibrium, and markets do this by efficiently allocating capital according to the signals conveyed by prices.

If you choose gold as the basis of your currency, then the discovery of a large gold mine in another nation-state, or the discovery of a new and more efficient way of refining gold (e.g., refining gold from seawater) will impact your economy in ways that you cannot control. If you have a gold-based currency and someone steers an asteroid into Earth orbit with more gold on it that is available to mine near the entire surface of Earth, as that gold is funneled into the terrestrial economy it will radically alter the commodity market for gold, and your gold-based currency will be impacted. You could try to limit this impact; you might even go to war with the extraterrestrial gold importer in order to try to control the flow of gold, but ultimately this is likely to be pointless. Market forces on a planetary scale are far stronger than any one nation-state. You might slow the transition to gold being plentiful, but you won’t stop it. If one gold importer is stopped, another, having learned hard lessons from the failure of the first, will do the same thing, but they will be smarter about it and harder to stop.

My point here is not to write an essay in economics, and I am not making any predictions, but rather I want to show the technologies of a spacefaring civilization in the context of the industrialized infrastructure that would produce them, and this industrialized infrastructure is dynamic, flexible, open to change, and, above all, multifarious. When a door closes, the saying goes, a window opens, and this is precisely what happens in an industrialized economy of planetary scale. Even legal limitations to commerce, backed by force of arms and penalties, will be circumvented by smugglers and black markets. As we know from sad experience, the very fact of a black market, by raising the penalty for failure, increases the price of banned commodities and so draws in competitors.

The examples of synchronic interaction in energy markets and currencies should give the reader a sense of the complexity of synchronic interactions that can also shape technologies, which, like energy and currencies, are commodities, but which are also tools that we employ in order to achieve some end, such as the buildout of spacefaring civilization. That these technologies are tools means that they will be employed to their best advantage in order to achieve some end; that these technologies are also commodities means that their employment will be measured against the employment of other technologies in competition with them.


[The tail fins of the 1959 Cadillac: glory days for Detroit.]

The problem of diachronic extrapolation

The simplest and most straight-forward way to construct a scenario for the future is a linear extrapolation of trends in the present. This is the fundamental conceit of induction: the future will be like the past. [9] Only, it won’t. Or, rather, the future won’t be exactly like the past. The extent to which we can interpret the future as being like the past defines the limits of inductive inference. Given the proper framework of scientific abstraction, this inductive extrapolation is unproblematic, but we aren’t always able to find the right framework, i.e., the best simplified model that captures the essential features while avoiding unnecessary complexity.

Diachronic extrapolation takes as its point of departure a single factor and extends into the future the apparent developmental trajectory of that single factor up to the present. The tendency of futurism to engage in single-minded diachronic extrapolation (something that I wrote about in The Problem with Diachronic Extrapolation and Diachronic Extrapolation and Uniformitarianism) magnifies an already human, all-too-human tendency to focus on a single factor, but the spacefaring future, if it comes about, will not be about a single technology, or a single industry, or about a single artificial habitat in Earth orbit, or a single settlement on Mars, or a single effort to reach the stars. All of these will be pursued simultaneously, and the technologies and engineering solutions that are developed will be found to interact and to be useful to each other in unexpected ways. Contingency and serendipity play a definitive role in the unfolding of history, and diachronic extrapolation by definition does not suggest contingent and serendipitous factors.

When someone (or some group of persons) is deeply invested in a single project, this personal investment becomes an impediment to understanding events divorced from this project, i.e., events that do not appear in the diachronic extrapolation of this project. The very fact that some individual has devoted their entire life to a single project makes them an expert judge on their area of specialization, but a poor judge of the big picture. Someone who invests their life in a project inevitably sees the world through the lens of that lifelong effort, and, while this is a valuable perspective, it involves a bias. Here a quote from James Madison is relevant: “No man is allowed to be a judge in his own cause, because his interest would certainly bias his judgment, and, not improbably, corrupt his integrity.” [10] We make an effort not to place individuals in the position of being a judge in their own cause because it is a moral hazard to do so, and we should exercise the same restraint in seeking an overview of the development of spacefaring civilization.

I do not wish to criticize or to belittle those who have invested their entire careers in a single area of expertise, largely limited to the production of single product. On the contrary, the plurality on which a large economy thrives only comes about as a result of dedicated groups of individuals who focus on their project to the exclusion of all other projects. We need to have dedicated industries around reusable rockets (like SpaceX and Blue Origin), hybrid engines (like Reaction Engines Limited), reusable space planes (like Sierra Nevada Corporation), Plasma engines (like Ad Astra Rocket Company), and more and better beside these efforts, but the future does not belong to any one of them alone. [11]

Processes that take place on a civilizational scale, and especially on the scale of planetary civilization, are intrinsically distinct from local efforts that focus on single industries or single technologies. I have observed elsewhere (cf. Detroit and Babylon) that Detroit in its heyday was a city based on a single product employing a single technology according to a single business model. This was one of the most successful industries in the middle of the twentieth century, but the product, the technology, and the business model all changed, and Detroit fell from a city of two million to a city of less than a million. If Detroit manages to build itself up again, this will not come about because of a renaissance in the American automobile industry, but because of some other product or service that arises from the social milieu of the city.

A diachronic extrapolation of Detroit’s economic success in the middle of the twentieth century, plotting the graph of its growth and tracing the same path into the future, would have given us Detroit as the largest and most successful city in the US, and a future of glorious chrome monstrosities, masterpieces in their own right, but not responsive to the social context in which they appear. When Detroit fell from its preeminence in post-war industrial production, that was not the end of civilization, but the end of the preeminence of a single city. A civilization consists of many cities, and different cities with different cultures produce different products by way of different business models.

What is true of a planetary-scale civilization will be true at a greater order of magnitude for a spacefaring civilization, and also for the buildout of a spacefaring capacity that will make a spacefaring civilization possible. If we take one technology or one industry and extrapolate its development into the future, the more we extrapolate, the more we will get the future wrong.

The more capacious and diverse futures imagined in Star Trek and Star Wars are closer to the reality of the synchrony of spacefaring, with many different space ships and many different robots, etc. These visions of a mature spacefaring future are often ridiculed as being a kind of naïve reflection of our present planetary civilization, but this kind of criticism fails to distinguish between the perennial and the ephemeral in history. We would expect that the future will take a form in which perennial verities of civilization are re-expressed in ephemeral terms, which latter will eventually be antiquated in their turn and replaced by further innovations that will continue to reflect the underlying intrinsic properties of civilization. If we could get into a time machine and visit a far future of a mature spacefaring civilization, some aspects of it would be strangely familiar us precisely because it grows out of familiar (and perennial) forces that manifest themselves in any large-scale social organization.


[A famous passage from Dryden’s Amphitryon; there are no knock-down arguments in history.]

There is no unilateral history

In addition to those who see the future through the lens of a single technology or a single industry, and so only see the future conditioned on that single factor, there are those who deny the possibility of large swathes of the future based upon a single factor taken to be a knock-down argument. When I have written posts about individual projects that might be pursued by, or which might develop from, spacefaring civilization, I have noticed that the comments often take these individual instances out of their (future) historical context and place them in splendid isolation, and then the project taken in isolation is subject to a critique conceived in isolation. Granted, it is necessary to place projects in isolation in order to subject them to a rigorous analysis, but if any of these projects come to fruition, they will come to fruition in a context of others projects in development and eventual implementation.

It is in the spirit of splendid isolation that it is said that human beings won’t settle Mars because conditions on Mars are more difficult than Antarctica or the Gobi Desert, and we haven’t settled these latter places (as though no legal impediments existed to settling the Gobi Desert), and that if we cannot even settle another terrestrial planet in our solar system, human beings have no future away from Earth (as if Mars were the only destination in space). It is said that human beings won’t live off Earth because there is no gravity in space and gravity on Mars (or the moon) is too weak and human beings cannot be healthy under these conditions (as though there were nothing that could be done about this), that it takes too long to get to Mars (as though transportation technology never changes), and, of course, the old, familiar line that we shouldn’t go into space when there is so much that remains to be done on Earth (as though going into space is going to make things worse on Earth, rather than better, contrary to evidence of the entire space program to date).

There is no single knock-down argument against a spacefaring future, just as there is no single technology or single industry that will determine the entire future course of history. There may well be single factors that are disproportionately difficult to resolve, and so disproportionately absorb energy and resources in pursuit of a solution, and there will almost certainly be single industries that dominate the effort for a time, but when considering something as complex as a civilization, and more especially the developmental trajectory of a civilization, no single factor will ever explain everything, and if a single factor explains a great many things at present, it will not continue to explain as many things in the future. As Fernand Braudel stated, “…we can no longer believe in the explanation of history in terms of this or that dominant factor. There is no unilateral history. No one thing is exclusively dominant…” [12]


[Walter Benjamin]

Walter Benjamin’s bombshell

When we understand synchrony in markets and social institutions, and that no single factor is exclusively dominant in the development of history, we understand why it is so disastrous for governments or other powers to put themselves in the position of picking winners and losers: they almost always make the wrong choices. We can, however, think of these mechanisms of top-down control, whether by governments or dominant industries, and their repeated record of making bad choices, as a social mechanism for slowing down economic growth and the disorienting social change that follows from rapid economic growth. There is a limit to the magnitude of social change that any society can absorb without experiencing catastrophic failure.

Walter Benjamin let slip a fascinating remark to this effect in a posthumously published fragment on one of his most famous late essays, “On the Concept of History” (a work that has significantly influenced my own thought). The collected papers of Benjamin include “Paralipomena to ‘On the Concept of History’,” and among these supplementary texts we find this bombshell:

“Marx says that revolutions are the locomotive of world history. But perhaps it is quite otherwise. Perhaps revolutions are an attempt by the passengers on this train—namely, the human race—to activate the emergency brake.” [13]

The Soviet Union embodied the ideology of which Benjamin was an apologist, and the Soviet Union presented itself to the world as a revolutionary new approach to industrialization that would transform the world in its image. While the western world was struggling with the Great Depression, the Soviet Union under Stalin was building entire cities from scratch, [14] and the modern graphics used to convey this message of socialist industrialization also presented the Soviet Union as being in the vanguard of history, an image driven home by the Soviet’s early successes in the Space Race. Under the glitz and the glamour, however, the Soviet project was more about exercising control over the direction of history and not allowing it to careen headlong into a chaotic future. This control came at a cost, and that cost was the Soviet economy eventually grinding to a halt. The emergency brake had been applied too suddenly and with too much pressure, and the result was a trainwreck.

Revolutionary and reactionary social and political movements alike can serve as an emergency brake on history, but this brake was applied more gently on the other side of the Iron Curtain. I have argued elsewhere (cf. Late-Adopter Spacefaring Civilization: the Preemption that Didn’t Happen), had funding for space exploration continued at the rate experienced during the Space Race that human civilization might have become spacefaring civilization in the second half of the twentieth century. It is often said that these Space Race levels of expenditure were ruinous, but I am not aware of any studies that show that even extravagant expenditures on space exploration have negatively impacted terrestrial events, while numerous studies have shown the spinoff benefits of the space program for terrestrial civilization. However, one can easily imagine that the rapid social change that would have followed from the early emergence of spacefaring civilization, following so closely on the heels of the industrial revolution, might have been more than western civilization could have borne without cracking. And so the emergency brake was applied, and instead of careening into an unknown future, we retreated to the certainties of life on Earth. Spacefaring civilization had to wait.


[Fernand Braudel made the Annales school of historiography prominent and emphasized the study of the longue durée.]

The longue durée of early spacefaring civilization

While it could have started earlier, but is only now on the horizon, the buildout of a robust spacefaring infrastructure will in any case require centuries to come to maturity, and several centuries of unified development like this is an appropriate period to study in terms of the longue durée. The idea of the longue durée is essentially that of treating an entire macrohistorical period [15] in terms of synchrony (though this is not how Braudel, who brought the idea to prominence, formulated the longue durée). It is often said that journalists write the first draft of history. If this is true, then we can think of the longue durée as the final draft of history, purged of the breathless onrush of events, having risen above mere contingency and party faction alike, with, “…all the heavy thickness of social reality, resistant to all inclemency, to crises and sudden shocks…” [16]

As the buildout of early spacefaring civilization unfolds, the developments that constitute this buildout will occur in the context of other developments occurring simultaneously, and these developments will shape each other as they interact. Individual human beings and human institutions will, in turn, react to these developments as they occur, and these actions and reactions will constitute the background for further developments. In contradistinction to the diachronic perspective, which takes a single factor and extrapolates beyond the present from this point, the synchronic perspective places the same single factor in the context of other such factors. The interaction that synchrony studies is the sum total of every possible diachronic extrapolation overlapping and intersecting with every other extrapolation. The furthest extent to which we can push this synchronic interaction is the longue durée.

Needless to say, the development of spacefaring civilization will also take place against the background of continuing developments of terrestrial civilization, with all that this entails for continuity: both the timeless verities of the human condition, in its geographical and biological context, and the unprecedented social developments of the fulfillment of planetary industrialized civilization, which latter can also be studied in terms of the longue durée. The synchrony of terrestrial developments will overlap with the synchrony of extraterrestrial developments, as the longue durée of planetary civilization will overlap with the longue durée of early spacefaring civilization.

We can think of synchronic historiographies of planetary civilization and early spacefaring civilization as a kind of temporal or historical form of Copernicanism. As the Copernican principle reminds us that the Earth is a planet among other planets, the sun a star among other stars, and the Milky Way a galaxy among other galaxies, the synchronic perspective on history reminds us that any given event is an event among other events, that any given technology is a technology among other technologies, and so on. We might, then, call the present effort an attempt at Copernican historiography, and Copernican historiography teaches us the importance of displacing any one perspective from the center of our concern, including any perspective entailed by seeing development through the exclusive lens of a single technology. The perspective of the longue durée facilitates our ability to understand history in this way, because we are not focusing on individual events, persons, or technologies, but on a complex structure in time that constitutes a macrohistorical period.


Beyond synchrony: the axes of historiography

In my paper A Manifesto for the Scientific Study of Civilization I wrote:

“One form that the transcendence of an exclusively historical study of civilization can take is that of extrapolating historical modes of thought so that these modes of thought apply to the future as well as to the past (and this could be called history in an extended sense). To recognize the role of the future in the concept of civilization is not intended to call into question the value of history and tradition, but to supplement it, and to supplement the concept of civilization with the idea of its future is to pass beyond history sensu stricto.”

The present essay on synchrony in early spacefaring civilization is an attempt to put this principle into practice; it has been my purpose to employ traditionally historical modes of thought in order to explicate the future. I have here focused on synchrony because I wanted to make a particular point about focusing on particular technologies of spacefaring in isolation, but a comprehensive study of early spacefaring civilization would not be limited to synchrony.

The use I have made in the present essay of concepts from historiography can be extrapolated more generally to other concepts from historiography, specifically, what I have elsewhere called the axes of historiography. If we take two distinctions common in historiography—synchronic and diachronic on the one hand, and on the other hand nomothetic and ideographic—and make these the axes of a chart, we have four permutations of historiography: nomothetic synchrony, ideographic synchrony, nomothetic diachrony, and ideographic diachrony. All of these concepts have potential for elucidating the study of the future by way of the methods of history.

The macrohistorical period that I have here identified as the longue durée of early spacefaring civilization can be given an account in terms of lawlike interactions (nomothetic synchrony), contingent interactions (ideographic synchrony), lawlike succession (nomothetic diachrony), and contingent succession (ideographic diachrony). In order to do justice to these ideas it would be necessary to given an account of Heinrich Rickert’s exposition of the nomothetic and the ideographic, but this must be a task for another time.



[1] We could posit a law of history such that the greater the separation of two or more objects or events in diachronic time, the less their interaction in synchronic time. This implies, conversely, that the greater the interaction between two or more objects or events in synchronic time, the less is their separation in diachronic time. In other words, interaction is proportional to proximity. The most dramatic interactions will be seen between objects or events in close temporal proximity. I will not develop this thought further at present.

[2] It has become a commonplace to cite the buildout of the Chinese imperial navy and its voyages during the early Ming dynasty (from 1405 to 1433) under Zheng He, taking this as an instance of a seafaring capacity for expansion that was not exploited and was subsequently dismantled.

[3] The reference, “blooming, buzzing confusion” is to a passage in William James: “The baby, assailed by eyes, ears, nose, skin, and entrails at once, feels it all as one great blooming, buzzing confusion; and to the very end of life, our location of all things in one space is due to the fact that the original extents or bignesses of all the sensations which came to our notice at once, coalesced together into one and the same space.” (Principles of Psychology, Chapter XIII) I have elsewhere used this phrase to convey the sense of synchronic historiography.

[4] Of course this assertion admits of exceptions. If, for example, the Breakthrough StarShot initiative manages to successfully launch a “starchip” to a nearby star, and this occurs within the temporal window of the buildout here considered, this will be like our early automated probes of the solar system, which (over time) transformed our scientific knowledge, but which had little impact on large-scale economic and political developments as they occurred. Indeed, this kind of “exception” underscores two points already made above: A) the importance of a synchronic perspective on buildout, which takes account of minor developments in marginal fields as part of the whole ensemble of the present, and B) how the buildout of a civilization always shades over a little into the next developmental stage of a civilization (whether or not that development comes to fruition).

[5] I can imagine a reader at this point asking, “What about 3D printing?” So-called 3D printing is not the same as an industrial infrastructure. (I say “so-called” only because 3D printing is not really printing, it is automated manufacturing.) 3D printing requires the resources of an industrialized economy for its inputs, and for the construction and maintenance of the printer itself. For a complete industrialized infrastructure, there needs to be a supply chain from raw materials to finished products; 3D printing can fulfill some of the roles in this supply chain, but it cannot substitute for them all.

[6] Venture capital firms, too, are subject to winnowing over the selection pressure of what products and services prove to be robust in the market. If they are too subject to manipulation and bias, they throw their money away on debacles like Theranos and Juicero, and if this happens often enough they will exhaust their capital and be forced to cease operations.

[7] Peter Thiel, in his book Zero to One: Notes on Startups, or How to Build the Future, emphasizes the importance of magnitude of order improvements as being especially desirable from the standpoint of venture capitalists, and therefore, also, for the successful entrepreneur, but I will point out that what Thiel says about order of magnitude improvements may be a necessary condition of a technology that will attract funding, but it is not a sufficient condition.

[8] Currency markets are a world unto themselves, because currencies are those commodities that are both bought and sold in and of themselves, as well as being the measure by which other commodities are bought and sold.

[9] There is a long-running controversy in the philosophy of science over the nature of induction. Hume is usually interpreted as having shown the ultimate shortcomings of induction, but since this is a philosophical embarrassment, it is usually only spoken of in hushed tones. Popper claimed that induction is superfluous and that science actually proceeds by deduction. Carnap defended induction. Suffice it to say that this controversy has taken on a life of its own and, if the reader is interested to follow up on this, I recommend the bibliography to the article “The Problem of Induction” in the online Stanford Encyclopedia of Philosophy.

[10] The Federalist Papers, no. 10, James Madison.

[11] The use of the phrase “killer app” plays into a kind of lazy diachronic extrapolation, and in so far as anyone is waiting for the “killer app” of space travel they will be disappointed. Killer apps are products with a definite lifespan, backed by investors who want to, “Get in. Get rich. Get out.” This is a good way to make money in the short term, but the economy on the whole is the sum total of overlapping products like this, and not any one individual product. Moreover, the economy also includes all the failed products that don’t make anyone rich, and the fortunes that these failed products take with them when they fail.

[12] “The Situation of History in 1950,” Fernand Braudel, in On History, Chicago and London: University of Chicago Press, 1980, p. 10. This was Braudel’s inaugural lecture at the Collège de France.

[13] Benjamin, Walter, “Paralipomena to ‘On the Concept of History’,” in Selected writings: Walter Benjamin, Volume 4, 1938-1940, edited by Howard Eiland and Michael W. Jennings, Harvard University Press, 2003, p. 402. Benjamin’s often cryptic “On the Concept of History” is greatly enhanced by these supplementary texts, which exhibit some of Benjamin’s thought processes while writing. Also, the Harvard edition has a lot of helpful notes. Benjamin committed suicide a few months after this was written, and I cannot help but wonder if he would have further developed this insight if he had not chosen to end his life.

[14] To understand how the Russian revolution’s vision of socialist industrialization was celebrated across all sectors of society it is instructive to read Yevgeny Yevtushenko’s “Bratsk Station,” which is an epic poem chronicling the building of a hydroelectric power station in Siberia. The human cost of socialist industrialization was high, as the absence of democratic mechanisms and press freedoms meant that abuses went unchecked, ultimately undermining the legitimacy of a state that claimed to represent the working class.

[15] I specify “macrohistorical period” because many traditionally recognized periods in the development of western civilization—the renaissance, the Reformation, the Enlightenment, romanticism, etc.—are episodic in comparison to the longue durée. Braudel might well have called these familiar periods conjunctures. Indeed, all of these named periods might be taken together as “early modern European history,” and the centuries of early modern history could well be studied in terms of the longue durée. Within the longue durée of spacefaring buildout we might similarly define periods of conjuncture, just as I have, in this essay, distinguished the earliest phase of spacefaring from 1957 to the present day.

[16] Fernand Braudel, “History and the Social Sciences: The Longue Durée,” in On History, Chicago and London: University of Chicago Press, 1980, p. 130. Machiavelli, by contrast, might be taken as a master of the history of the event, or episodic history (histoire événementielle), with his focus on fortune (Fortuna).



The Sounds of Europa

by Paul Gilster on May 11, 2017

Although there are no plans at present to send a lander to Europa, we continue to work on the prospects, asking what kind of operations would be possible there. NASA is, for example, now funding a miniature seismometer no more than 10 centimeters to the side, working with the University of Arizona on a project called Seismometers for Exploring the Subsurface of Europa (SESE). Is it possible our first task on Europa’s surface will just be to listen?

The prospect is exciting because what we’d like to do is find a way to penetrate the surface ice to reach the deep saltwater ocean beneath or, barring that, any lakes that may occur within the upper regions of the ice shell. The ASU seismometer would give us considerable insights by using the movements of the ice crust to tell us how thick it is, and whether and where ocean water that rises to the surface can be sampled by future landers.


Image: Close-up views of the ice shell taken by the Galileo spacecraft show uncountable numbers of fractures cutting across each other. Reddish colors (enhanced in this view) come from minerals in ocean water leaking through the shell and being bombarded by Jupiter’s radiation. The ASU-designed seismometer would land on the shell and detect its movements. Credit: NASA/JPL-Caltech.

Europa’s story is all about tides. The moon (a bit smaller than our own Moon) is constantly being tugged by the large Galilean moons Io and Ganymede, preventing its orbit from circularizing completely. In turn, that small orbital eccentricity allows Jupiter to stress the ice shell. Alyssa Rhoden is an ASU geophysicist working on the SESE project. She points out in this ASU news release that seismometers can tell us how active the ice shell is.

Acknowledging that we’re dealing with a geologically young surface — probably between 50 to 100 million years old, based on crater counts and resurfacing — Rhoden adds: “It may have undergone an epoch of activity early in that period and then shut down.” Equally plausible is the idea that even today the shell is undergoing uplifts and fracturing from below, with the opportunity for ocean water to reach the surface. Recent observations of plumes on Europa, based on Hubble data from 2012 and 2016, support the idea.

Seismometers would help us detect ongoing activity in the shell. ASU envisions a seismometer mounted on each leg of a lander — four to six seismometers in all, depending on lander design. These would be driven deep into the ground, avoiding the kind of loose surface materials that would isolate the instruments from seismic waves passing through the shell. And that calls for the kind of rugged instrument ASU is building. Able to operate at any angle, the prototype can survive landings hard enough to ensure deep penetration for each seismometer.

Edward Garnero is an ASU seismologist who points out that the instrument package will need to sample a wide range of potential vibrations, combining observations from each seismometer to pinpoint the source of seismic activity:

“We can also sort out high frequency signals from longer wavelength ones. For example, small meteorites hitting the surface not too far away would produce high frequency waves, and tides of gravitational tugs from Jupiter and Europa’s neighbor moons would make long, slow waves.”

The sound of Europa? Garnero adds:

“I think we’ll hear things that we won’t know what they are. Ice being deformed on a local scale would be high in frequency — we’d hear sharp pops and cracks. From ice shell movements on a more planetary scale, I would expect creaks and groans.”


Image: Four sensors arranged in a box measuring about 10 centimeters on a side make up the test module for the SESE project seismometer. The various sensor orientations allow the instrument to work no matter how it lands on the surface. Credit: Hongyu Yu/ASU.

The Seismometers for Exploring the Subsurface of Europa project avoids the mass-and-spring sensor concept used in conventional instruments because that design is delicate enough that it needs to be put in place without any serious jolts and must be installed in an upright position. The SESE seismometer avoids those problems and uses a micro-electromechanical system with a liquid electrolyte as its sensor, offering high sensitivity to a wide range of vibrations.

Finding pockets of water within the upper ice would offer further areas of astrobiological interest and add to the likelihood of nutrients being transported from the ocean to the surface. Thus the findings of a seismometer like this could be crucial for future lander missions. Galileo imagery has shown us long linear cracks and ridges broken by areas of disrupted terrain where surface ice has refrozen. If Europa remains active today, we can use what SESE hears on the surface to predict the best areas for future lander operations.

“We want to hear what Europa has to tell us,” adds Hongyu Yu (ASU School of Earth and Space Exploration), who heads up the project. “And that means putting a sensitive ‘ear’ on Europa’s surface.”