Centauri Dreams
Imagining and Planning Interstellar Exploration
Strategies for Life on Titan
Back in September of 1961, Isaac Asimov penned an essay in Fantasy & Science Fiction under the title “Not As We Know It,” from which this startling passage:
…when we go out into space there may be more to meet us than we expect. I would look forward not only to our extra-terrestrial brothers who share life-as-we-know-it. I would hope also for an occasional cousin among the life-not-as-we-know-it possibilities.
In fact, I think we ought to prefer our cousins. Competition may be keen, even overkeen, with our brothers, for we may well grasp at one another’s planets; but there need only be friendship with our hot-world and cold-world cousins, for we dovetail neatly. Each stellar system might pleasantly support all the varieties, each on its own planet, and each planet useless to and undesired by any other variety.
Asimov’s idea, prompted by a monster movie excursion with his children, was to look at realistic ways that life much different from our own could emerge. Here he anticipated our discussions of habitable zones and just what they imply, for we usually speak of a world being habitable if liquid water can exist on its surface. Asimov would have none of that because he wanted to know what kind of life might emerge in the hottest and coldest places in the Solar System. Reprints of the essay inspired James Stevenson, a graduate student at Cornell University, whose recent work on the astrobiological possibilities on Titan has energized wide discussion.
Image: Are there ways life could emerge on Titan? A panorama of the shoreline where Huygens touched down, stitched from DISR Side-Looking and Medium-Resolution Imager Raw Data. Image credit: ESA / NASA / JPL / University of Arizona / Rene Pascal (panorama).
Collaborating at Cornell with astronomer Jonathan Lunine and chemical engineer Paulette Clancy, Stevenson went to work on a cell membrane that could function in a cold and methane-rich environment. Clancy specializes in chemical molecular dynamics, while Lunine’s background includes working on the Cassini mission. With non-aqueous life on the table (Lunine had received a grant from the Templeton Foundation to study the possibilities), Clancy’s expertise seemed made to order. She comments on the work in this Cornell news release:
“We’re not biologists, and we’re not astronomers, but we had the right tools. Perhaps it helped, because we didn’t come in with any preconceptions about what should be in a membrane and what shouldn’t. We just worked with the compounds that we knew were there and asked, ‘If this was your palette, what can you make out of that?'”
It’s an interesting palette in a very interesting place. Liquid methane is the only liquid other than water that forms seas on the surface of a planetary body in the Solar System. The paper also notes the intriguing fact that there is an unknown process at work on Titan’s surface that consumes hydrogen, acetylene and ethane — these reach the surface out of the atmosphere but do not accumulate. Finding a cell membrane mechanism for Titan’s methane seas becomes an exercise in astrobiology that we can hope one day to weigh against data from the surface.
Using molecular simulation strategies given the challenges of cryogenic experimentation, the researchers screened for the best candidates for self assembly into membrane-like structures. The result: A cell membrane the researchers call an azotosome, made out of nitrogen, carbon and hydrogen molecules already known to exist in Titan’s frigid seas. If Earth life is built around the phospholipid bilayer membrane — water-based vesicles made from this are known as liposomes — then a methane-based membrane like the azotosome could be the Titanian analog, a flexible and stable cell membrane able to function at temperatures of -180 °C. From the paper:
In a cold world without oxygen, we suggest that the vesicles needed for compartmentalization, a key requirement for life, would be very different to those found on Earth. Rather than long-chain nonpolar molecules that form the prototypical terrestrial membrane in aqueous solution, we find membranes that form in liquid methane at cryogenic temperatures do so from the attraction between polar heads of short-chain molecules that are rich in nitrogen. We have termed such a membrane an azotosome. We find that the flexibility of such membranes is roughly the same as those of membranes formed in aqueous solutions. Despite the huge difference in temperatures between cryogenic azotosomes and room temperature terrestrial liposomes, which would make almost any molecular structure rigid, they exhibit surprisingly and excitingly similar responses to mechanical stress.
Image: A representation of a 9-nanometer azotosome, about the size of a virus, with a piece of the membrane cut away to show the hollow interior. Credit: James Stevenson.
Could such membranes form on Saturn’s largest moon? We already know that a liquid organic compound called acrylonitrile can be found in the atmosphere there, and the researchers believe that an acrylonitrile azotosome compound would offer indigenous life the same kind of stability and flexibility that phospholipid membranes bring to life on Earth. Studying the metabolism and reproduction of the hypothesized cells is the next order of business, but Lunine talks of one day going well beyond theory to float a probe on Titan’s seas to sample its organics directly.
None of this demonstrates that life is present on Titan, but focusing on the availability of molecules that can form cell membranes helps us understand the kind of chemistries we need to look for under cryogenic conditions. In their conclusion, the authors talk about the ‘liquid methane habitable zone,’ a wonderful reminder of how our views on astrobiology are expanding.
The paper is Stevenson, Lunine & Clancy, “Membrane alternatives in worlds without oxygen: Creation of an azotosome,” Science Advances Vol. 1, No. 1 (27 February 2015), e1400067 (full text).
Were There Planets Inside Mercury’s Orbit?
With the Mercury Messenger mission now coming to its end, it seems an appropriate time to speculate on why our inner Solar System looks the way it does. After all, as we continue finding new solar systems, we’re discovering many multi-planet systems with planets — often more than one — closer to their star than Mercury is to ours. We have Kepler to thank for these discoveries, its data analyzed in a number of recent papers including one arguing that about 5 percent of all Kepler stars have systems with tightly packed inner planets. The awkward acronym for such systems is STIP.
Well, maybe it’s not all that awkward, and Kathryn Volk and Brett Gladman (University of British Columbia) have good cause to deploy it in their new paper, which focuses on this topic. They’re wondering why our Solar System lacks planets inside Mercury’s orbit, and they point to the paper I mentioned above (Lissauer et al, 2014) as well as another by Francois Fressin and colleagues that concludes that half of all Kepler stars have at least one planet in the mass range from 0.8 to 2 Earth masses with orbits inside Mercury’s distance from our Sun, which is 0.39 AU, or 58.5 million kilometers.
Image: The Caloris basin and adjacent regions on Mercury. Recent exoplanet discoveries raise the question of why our Solar System lacks planets inside Mercury’s orbit. Can instabilities in the early Solar System help us find the answer, while at the same time explaining some of the planet’s peculiarities? Image credit: JHU/APL.
Taking as an hypothesis that nearly all F, G and K-class stars originally form with planets well within Mercury’s orbital distance, Volk and Gladman ask whether the reason we find systems without such planets today is that instabilities have destroyed these worlds through generations of catastrophic collisions and gradual re-formation, leaving (in our case) Mercury as the surviving relic. It is true that STIPs can be dynamically stable over long time-frames (hence we see the Kepler examples), but the absence of tightly packed inner worlds around many stars is here taken as the result of a ‘metastable planetary arrangement’ that leaves one or no short period planets. The Kepler STIPs we see, then, are those that have survived this process.
The authors use the Kepler data to generate systems similar to those we have uncovered, allowing them to ‘evolve’ computationally to study system dynamics, taking some simulations well beyond the first collision to see how the instabilities multiply. An initial collision often produces second collisions at higher speeds. While low-speed impacts can occur in some systems, producing far smaller amounts of debris and subsequent accretion, a fraction of STIPs experience heavy perturbation that can lead to the destruction of their inner worlds. From the paper:
Our experiments show that instability timescales in these systems are distributed such that equal fractions of the systems go unstable (reach a first planetary collision) in each decade in time (Fig. 2). This logarithmic decay is not unknown in dynamical systems (eg., Holman & Wisdom (1993)) and is presumably related to chaotic diffusion and resonance sticking near the stability boundary. After a brief, relatively stable initial period, the systems hit instability at a rate of ?20% per time decade, with half of the systems still intact at ?100 Myr. The exact decay rate may be influenced by our usage of the current Kepler STIPs sample (perhaps the most stable); however if this decay rate held, at ?5 Gyr 5–10% of STIPs would not yet have reached an instability, in rough agreement with the observed STIPs frequency.
Turning the results on our own Solar System, they find that the orbits of the three outer terrestrial planets (Venus, Earth, Mars) remain unaffected on 500 million year timescales by the presence of additional planets totaling several Earth masses, all of the latter inside a distance of 0.5 AU from the Sun. Dynamical instabilities would have initiated a sequence of collisions among these worlds that left Mercury as the sole survivor. The authors argue that it is possible for the orbits of the outer terrestrial planets to remain unperturbed as the inner planets fall victim to these events.
Various issues are explained by this scenario. A series of collisions concentrates iron into the surviving remnants, which accounts for Mercury’s high density. The authors also ask whether instabilities in the inner system approximately 4 billion years ago could account for the Late Heavy Bombardment (sometimes called the ‘lunar cataclysm’), when Mercury, Venus, Earth and Mars experienced a high number of impacts. From the paper:
Gladman & Coffey (2009) estimated that 10–20% of large (m to 100 km) debris originating near current Mercury would strike Venus, with 1–4% impacting Earth (?0.1% strikes the Moon). The Earth’s impact rate would peak ?1–10 Myr after the event and decay on ?30 Myr timescales as Mercury and Venus absorb most of the debris; this is a plausible match for the cataclysm’s final stages (Cuk et al. 2010). A bottom-heavy size distribution for the 1–100 km debris could explain the recent finding (Minton et al. 2015) that a main-belt asteroid source would produce too many impact basins during the cataclysm.
Such an event might be studied by future sampling missions, for:
STIP debris would likely be mostly silicate-rich mantle material similar but not identical to main-belt asteroid compositions, consistent with cataclysm impactor compositions inferred via cosmochemical means (Joy et al. 2012). The smallest dust (being blown out hyperbolically) could impact the Earth-Moon system. We estimate that 10?11 of the departing dust would strike the Moon, at vimp ?30 km/s. If any dust or meteoroid projectiles were retained, fragments might be found in regolith breccias compacted during the cataclysm epoch.
This is an interesting model, and the authors point out that it gets us out of the difficulty of assuming an inner protoplanetary disk edge that has to be adjusted to account for Mercury and the lack of worlds interior to its orbit. We have a model that would leave the orbits of the existing terrestrial-class worlds unaffected by the series of collisions and disintegrations that Mercury emerged from, although the recipients of catastrophic amounts of early debris. The model also accounts for the apparent instability of Mercury’s orbit on a 5 to 10 billion year time-frame.
The paper is Volk and Gladman, “Consolidating and Crushing Exoplanets: Did it happen here?,” submitted to Astrophysical Journal Letters (preprint). Thanks to Andrew Tribick for the pointer to this paper.
Seeing Ceres: Then and Now
I’m interested in how we depict astronomical objects, a fascination dating back to a set of Mount Palomar photographs I bought at Adler Planetarium in Chicago when I was a boy. The prints were large and handsome, several of them finding a place on the walls of my room. I recall an image of Saturn that seemed glorious in those days before we actually had an orbiter around the place. The contrast between what we could see then and what we would soon see up close was exciting. I was convinced we were about to go to these worlds and learn their secrets. Then came Pioneer, and Voyager, and Cassini.
And, of course, Dawn. As we discover more and more about Ceres, the process repeats itself, as it will again when New Horizons reaches Pluto/Charon. Below is a page from a book called Picture Atlas of Our Universe, published in 1980 by the National Geographic. Larry Klaes forwarded several early images last week as a reminder of previous depictions of the main belt’s largest asteroid, or dwarf planet, or whatever we want to call it. Here the artwork isn’t all that far off the mark for Ceres, though Vesta would turn out to be a good deal less spherical than predicted. No mention of a possible Ceres ocean in the depictions of this time; all that would come later.
Image: Ceres and other asteroids as seen through the eyes of artist Davis Meltzer in 1980, with the Moon as a background.
The recent Dawn imagery has us buzzing about the two bright spots on Ceres that, of course, were unknown to our artist in 1980. From 46,000 kilometers, all we can do is admit how little we know, which is more or less what Andreas Nathues, lead investigator for the framing camera team at the Max Planck Institute for Solar System Research (Gottingen) does:
“The brightest spot continues to be too small to resolve with our camera, but despite its size it is brighter than anything else on Ceres. This is truly unexpected and still a mystery to us.”
Image: This image was taken by NASA’s Dawn spacecraft of dwarf planet Ceres on Feb. 19 from a distance of nearly 46,000 kilometers. It shows that the brightest spot on Ceres has a dimmer companion, which apparently lies in the same basin. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
Chris Russell, principal investigator for Dawn, speaks of a possible “volcano-like origin” of the two bright spots, but adds that we have to wait for better resolution to make any serious geological interpretations. The wait won’t be all that long (for better resolution, at least) given that we’re just days away from entering orbit on March 6. Could there be a better approach to this small world than this one, already focusing on something no one had expected to see?
In 1961, in an illustration from The Universe (New York: Morrow), we find Ceres again displayed with companion objects like Vesta and Pallas (I’m afraid I don’t know the name of the artist). Here the round, cratered Ceres is reasonably accurate, and you’ll note the size comparison, with Ceres tucked up inside Texas. At the bottom of the image is Eros, shown here as an object the size of Manhattan and described in the caption as “Flying end over end through space like an island torn from its moorings…”
And here is Eros as it appeared in the Astronomy Picture of the Day in 2001.
Image: Orbiting the Sun between Mars and Earth, asteroid 433 Eros was visited by the robot spacecraft NEAR-Shoemaker in February of 2000. High-resolution surface measurements made by NEAR’s Laser Rangefinder (NLR) have been combined into the above visualization based on the derived 3D model of the tumbling space rock. NEAR allowed scientists to discover that Eros is a single solid body, that its composition is nearly uniform, and that it formed during the early years of our Solar System. Credit: NEAR Project, NLR, JHUAPL, Goddard SVS, NASA.
When we get cameras in the vicinity of objects that for so long were just smudges in even the best telescopes, we sometimes find ourselves surprised and delighted, as witness the volcanoes of Io or, for that matter, the cryovolcanism and ‘canteloupe terrain’ on Neptune’s moon Triton. Sharpening the view takes us out of the realm of the artist’s imagination and into the world of concrete measurement. Giving up earlier visions can be poignant, as we learned with Mariner 4’s 1965 flyby of Mars, when a vegetative and even fertile Mars suddenly became a fantasy forever lost. But as Ceres is proving right now, the discovery of the unexpected is a much greater reward.
Astrobiology: A Cautionary Tale
We’re discovering planets around other stars at such a clip that moving to the next step — studying their atmospheres for markers of life — has become a priority. But what techniques will we use and, more to the point, how certain can we be of their results? Centauri Dreams columnist Andrew LePage has been mulling these matters over in the context of how we’ve approached life on a much closer world. Before the Viking landers ever touched down on Mars, a case was being made for life there that seemed compelling. LePage’s account of that period offers a cautionary tale about astrobiology, and a ringing endorsement of the scientific method. A senior project scientist at Visidyne, Inc., Drew is also the voice behind Drew ex Machina.
by Andrew LePage
Every time I read an article in the popular astronomy press about how some new proposed instrument will allow signs of life to be detected on a distant extrasolar planet, I cannot help but be just a little skeptical. For those of us with long memories, we have already been down this road of using remote sensing techniques to “prove” life existed on some distant, unreachable world, only to be disappointed when new observations became available. But instead of a distant extrasolar planet, over half a century ago that planet was our next door neighbor, Mars.
Back when I was in high school in the late 1970s, I enjoyed spending time during study hall going through science books and magazines, old as well as new, in the school library. Among the interesting tidbits I read about were spectral features known as “Sinton bands” and how in the early 1960s these were considered the latest evidence of life on Mars. Of course by the time I was reading this, I knew from the then-recent results from the Viking missions that the explanation for these and other observations was simply incorrect. So what ever happened to these Sinton bands and the interpretation they were evidence of life on Mars?
In the years leading up to the beginning of the Space Age, the general consensus of the scientific community was that Mars was a smaller and colder version of the Earth that supported primitive plant life akin to lichen. This view was based on a large body of observational evidence gathered over the first half of the 20th century. A firmly established wave of darkening was observed spreading over the spring hemisphere of Mars each Martian year which was widely seen as being the result of plants coming out of their winter slumber much as happens on Earth each spring. This interpretation was bolstered by visual observations that the dark regions of Mars appeared to have a distinct green hue just as one would expect from widespread plant life.
Other observations of Mars during this period lent further support to the view that the Red Planet could support simple life forms. The general consensus of the astronomical community at this time based on analyses of decades of photometric and polarimetric measurements of Mars indicated that the surface pressure of the Martian atmosphere was about 85 millibars or about 8.4% of Earth’s surface pressure. Carbon dioxide and water vapor were detected and nitrogen was widely expected to be the major atmospheric constituent just as it was on Earth. No large bodies of water were visible on the surface and the climate was certainly colder than on Earth as a whole owing to Mars’ greater distance from the Sun, but the surface temperatures at the equator easily exceeded the freezing point of water during the summer so that liquid water was expected to be available. While not an ideal environment by terrestrial standards, it seemed that Mars had conditions that would be expected to support life much like the high arctic here on Earth.
Image: This was the best photograph of Mars available before the Space Age taken at the Mt. Wilson observatory in 1956 – the same year Sinton bands were discovered. Credit: Mt. Wilson Observatory.
To further test this view, American astronomer William Sinton (1925-2004) decided to use the latest technological advancements in infrared (IR) spectroscopy to obtain observations of Mars during its especially favorable 1956 opposition. On seven nights during the fall of 1956, Dr. Sinton used the 1.55-meter Wyeth Reflector at the Harvard College Observatory to make IR spectral measurements using a lead sulfide detector cooled using liquid nitrogen to vastly improve its sensitivity. He made repeated measurements between the wavelengths of 3.3 to 3.6 μm in order to sample the spectral region where resonances from the C-H bonds of various organic molecules would create distinctive absorption features. His analysis found a dip in the IR spectrum of Mars near 3.46 μm which resembled his IR spectrum of lichen. This finding and his conclusions were published in highly respected, peer-reviewed astronomical publication The Astrophysical Journal.
Encouraged by these initial results, Dr. Sinton repeated his measurements using an improved IR detector on the 5-meter Hale Telescope at the Mt. Palomar Observatory (then, the largest telescope in the world) during the following opposition of Mars in October 1958. His new observations had ten times the sensitivity of his original measurements and now covered wavelengths from as short as 2.7 μm out to 3.8 μm. In addition to absorption features attributable to methane and water vapor in Earth’s atmosphere, Dr. Sinton identified absorption features centered at 3.43, 3.56 and 3.67 μm that appeared to be weaker or absent in the brighter areas of Mars. Dr. Sinton concluded that inorganic compounds like carbonates could not produce the observed features. Instead they must be produced by organic compounds selectively concentrated in the dark areas of Mars that were already known to be greener. While the features he observed were not a perfect match for any known plant life on Earth, he concluded that they were due to organic compounds such as carbohydrates produced by plants on the surface of Mars. These findings and conclusions were again published in a well-regarded, peer-reviewed scientific journal, Science.
While there was naturally some healthy skepticism about the findings, they were seen by many as supporting the generally held view that Mars was the home of simple, lichen-like plant life. In order to better observe what became known as “Sinton bands”, the Soviet Union even planned to include IR instrumentation to measure these spectral features from close range on the first pair of spacecraft they launched towards Mars in October 1960. Unfortunately, both Mars probes succumbed to launch vehicle failures during ascent and never even made it into Earth orbit. Soviet engineers attempted it again with a pair of much more capable flyby probes of which only Mars 1 survived launch on November 1, 1962. Unfortunately, Mars 1 suffered a major failure in its attitude control system during its cruise and contact was lost three months before its encounter with Mars on June 21, 1963. As a result, there were no close-up IR observations of the Sinton bands at this time.
Image: The earliest Soviet Mars probes carried IR instrumentation to observe Sinton bands at close range including Mars 1 launched in November 1962. Credit: RKK Energia.
But even as the Soviet Union was struggling to reach Mars with their first interplanetary probes, the case for there being plant life on Mars and the Sinton bands being evidence for it was already beginning to unravel. Donald Rea, leading a team of scientists at the University of California – Berkeley, published the results of their work on Sinton bands in September 1963. They examined the IR spectra of a large number of inorganic and organic samples in the laboratory and could not find a match for the observed Sinton bands. While they could not find a satisfactory explanation for the bands, they found that the presence of carbohydrates as proposed by Dr. Sinton was not a required conclusion.
Another major blow was landed in a paper by another University of California – Berkeley team headed by chemist James Shirk which was published on New Year’s Day 1965. Their laboratory work suggested that the Sinton bands could be caused by deuterated water vapor – water where one or both of the normal hydrogen atoms, H, in H2O are replaced with the heavy isotope of hydrogen known as deuterium, D, to form HDO or D2O. Shirk and his team speculated that the deuterated water vapor was present in the Martian atmosphere with the implication that the D:H ratio of Mars greatly exceeded that of the Earth.
The final explanation for the Sinton bands came in a paper coauthored by Donald Rea and B.T. O’Leary of the University of California – Berkeley as well as William Sinton himself published in March of 1965. Based on a new analysis of Dr. Sinton’s data from 1958, observations of the solar IR spectrum from Earth’s surface and the latest laboratory results, it was found that the absorption features in the Martian spectrum now identified as being at 3.58 and 3.69 μm were the result of HDO in Earth’s atmosphere. The feature at 3.43 μm was, in retrospect, a marginal detection in noisy data and was probably spurious. The mystery of the Sinton bands was solved and, unfortunately, it had nothing to do with life on Mars.
Sinton bands were not the only causality of advances in technology and remote sensing techniques at this time. As more detailed ground-based observations of Mars were made during the 1960s and the first spacecraft reached this world, it was eventually found that all of the earlier observations that had been taken as evidence of life on Mars were either inaccurate or had non-biological explanations. After a half century of observations from space and on the surface, we now know that the Martian environment is simply too hostile to support even hardy lichen-like plants as had been widely believed before the Space Age.
This story about the rise and fall of the view that Mars harbors plant-like life forms should not be taken as an example of the failure of science. Instead, it is a perfect example of how the self-correcting scientific process is supposed to work. Observations are made, hypotheses are formulated to explain the observations and those hypotheses are then tested by new observations. In this case, the pre-Space Age view that Mars supported lichen-like plants was disproved when new data no longer supported that view. And our subsequent experience with the in situ search for life on Mars by the Viking landers in 1976 is further evidence not that Mars is necessarily lifeless, but that detecting extraterrestrial life is much more difficult than had been previously believed. These lessons need to be remembered as future instruments start to scan distant extrasolar planets and claims are made that life has been found because of the alleged presence of one compound or another. Past experience has shown that such interpretations can easily be incorrect especially when dealing with new observing techniques of distant worlds with unfamiliar environments.
A Laser ‘Comb’ for Exoplanet Work
It’s been years since I’ve written about laser frequency comb (LFC) technology, and recent work out of the Max Planck Institute of Quantum Optics, the Kiepenheuer Institute for Solar Physics and the University Observatory Munich tells me it’s time to revisit the topic. At stake here are ways to fine-tune the spectral analysis of starlight to an unprecedented degree, obviously a significant issue when you’re dealing with radial velocity readings of stars that are as tiny as those we use to find exoplanets.
Remember what’s happening in radial velocity work. A star moves slightly when it is orbited by a planet, a tiny change in speed that can be traced by studying the Doppler shift of the incoming starlight. That light appears blue-shifted as the star moves, however slightly, towards us, while shifting to the red as it moves away. The calibration techniques announced in the team’s paper show us that it’s possible to measure a change of speed of roughly 3 cm/s with their methods, whereas with conventional calibration techniques, the best measurement is roughly 1 m/s (although see the citations below for HARPS calibration of an LFC that reaches 2.5 cm/s). Detecting an Earth-mass planet in an Earth-like orbit around a solar-type star involves observing velocity changes of 10 cm/s or less, so we’re clearly entering the right range here.
Let’s back up and consider how a laser frequency comb works. Below is an image from the European Southern Observatory explaining the ‘comb’ analogy — as you can see, the graph resembles a fine-toothed comb, one built around short, equally spaced pulses of light created by a laser. The different colors of the pulsed laser light are separated based on their individual frequencies. Combining an ultrafast laser as a calibration tool with an external source of light allows scientists to measure the frequency of the external light to a high degree of precision.
Image: This picture illustrates part of a spectrum of a star obtained using the HARPS instrument on the ESO 3.6-metre telescope at the La Silla Observatory in Chile. The lines are the light from the star spread out in great detail into its component colours. The dark gaps in the lines are absorption features from different elements in the star. The regularly spaced bright spots just above the lines are the spectrum of the laser frequency comb that is used for comparison. The very stable nature and regular spacing of the frequency comb make it an ideal comparison, allowing the detection of minute shifts in the star’s spectrum that are induced by the motion of orbiting planets. Note that in this image, the colour range is for illustrative purposes only, as the real changes are much more subtle. Credit: ESO.
The laser frequency comb is, then, a standard ‘ruler’ that can measure the frequency of light to extreme precision. In the case of the recently announced findings, the researchers worked with sunlight averaged over the complete solar disk, as captured by the ChroTel solar telescope (located at the Vacuum Tower Telescope installation in Tenerife, Canary Islands). They combined this light with the light from the laser frequency comb, injecting both into a single optical fiber. The result was sent on to a spectrograph for analysis, with striking results. Lead author Rafael Probst (Max Planck Institute of Quantum Optics) comments:
“Our results show that if the LFC light and the sunlight are simultaneously fed through the same single-mode fibre, the obtained calibration precision improves by about a factor of 100 over a temporally separated fibre transmission. We then obtain a calibration precision that keeps up with the best calibration precision ever obtained on an astrophysical spectrograph, and we even see considerable potential for further improvement.”
Probst goes on to say that although the technique is currently restricted to solar spectroscopy, it should be workable even for faint astronomical targets as it is perfected. He comments in this news release from the Institute of Physics that a key aspect of the work is the clean and stable beam at the output that results from using single-mode fiber, a kind of fiber common in laser applications but relatively little used thus far in astronomy. The LFC at the Vacuum Tower Telescope is the first installation for astronomical use based on single-mode fiber.
These refinements of laser frequency comb technique point toward future measurements of Doppler shifts that will make detecting Earth-sized planets with radial velocity methods more likely. The laser frequency comb seems poised to become a major tool. “In astronomy, frequency combs are still a novelty and non-standard equipment at observatories,” the authors write in their conclusion. “This however, is about to change, and LFC-assisted spectroscopy is envisioned to have a flourishing future in astronomy.”
The paper is Probst et al., “Comb-calibrated solar spectroscopy through a multiplexed single-mode fiber channel,” New Journal of Physics Vol. 17 (February 2015) 023048 (abstract). See also this video abstract of the work. Laser frequency comb work at HARPS reaching into the cm/s range is reported in Wilken et al., “A spectrograph for exoplanet observations calibrated at the centimetre-per-second level,” Nature Vol. 485, Issue 7400 (May, 2012), 611-614 (abstract).
Soft Robotics for a Europa Rover
Approaching problems from new directions can be unusually productive, something I always think of in terms of Mason Peck’s ideas on using Jupiter as a vast accelerator to drive a stream of micro-spacecraft (Sprites) on an interstellar mission. Now Peck, working with Robert Shepherd (both are at Cornell University) is proposing a new kind of rover, one ideally suited for Europa. The idea, up for consideration at the NASA Innovative Advanced Concepts (NIAC) program, is once again to exploit a natural phenomenon in place of a more conventional technology. What Peck and Shepherd have in mind is the use of ‘soft robotics’ — autonomous machines made of low-stiffness polymers or other such material — to exploit local energy beneath Europa’s ice.
We’re at the edge of a new field here, with soft robotics advocates using principles imported from more conventional rigid robot designs to work with pliable materials in a wide range of applications, some of which tie in with the growth in 3D printing. The people working in this area are developing applications for everything from physical therapy to minimally invasive surgery, with energizing inputs from organic chemistry and soft materials science. If the average robot is modeled around metallic structures with joints based on conventional bearings, soft robotics looks to the natural world for models of locomotion through terrain and innovative methods of energy production.
Peck and Shepherd are proposing what they call a ‘soft-robotic rover with electromagnetic power scavenging,’ a device capable of moving in the seas of Europa that is anything but the submarine-like craft some have envisioned to do the job. The closest analog in the natural world is the lamprey eel, the soft robotics version of which would use electrodynamic tethers to scavenge energy. The rover moves by swimming, powered not by solar or nuclear power but by the use of expanding gases. The rover under the ice sends data to a Europa orbiter using VLF wavelengths, like a submarine. Let me quote from the NIAC proposal on this mechanism:
The electrical energy scavenged from the environment powers all rover subsystems, including one that electrolyzes H2O. Electrolysis produces a mixture of H2 and O2 gas, which is stored internally in the body and limbs of this rover. Igniting this gas expands these internal chambers, causing shape change to propel the rover through fluid or perhaps along the surface of a planetary body.
Power Beneath the Ice
This is the first time I have encountered locomotion based on electromagnetic power scavenging, with accompanying reliance on soft robotic structures that could change the way we look at designing probes that will operate in ocean environments like that on Europa. It’s interesting to take this one step further, as the proposal itself notes, and remember that a rover inspired by biology may here point toward astrobiology, in that electromagnetic energy is considered to be a possible source of energy for any native life under Europa’s ice.
Image: Water electrolyzer and gas generation subsystem. Credit: Mason Peck, Robert Shepherd.
We know from experience in Earth orbit that tethers work, a result of the fact that a conductor moving through a magnetic field experiences an induced current. The power available at Jupiter gives us interesting options:
In Jupiter’s orbit, where the magnetic field can be up to 10,000 times more powerful and the ionosphere denser than Earth’s, the power can be even higher. A NASA/MSFC study characterized the power available for an EDT [electrodynamic tether] near Jupiter. The authors report that at least 1W would be available in Europa’s orbit but did not consider the much higher conductivity of the ocean or whatever tenuous atmosphere may exist near the surface.
Because higher conductivity produces greater current, the authors argue that far more power should be available on Europa, and that tethers no more than meters long should be sufficient to power the rover. According to the NIAC proposal, one end of the tether would be attached to the rover’s power systems while the other would be kept above the rover by a gas-filled balloon to maintain the necessary configuration as the rover conducted operations, and to ensure a predictable level of current. The tether itself also serves as an antenna to relay science data (possibly through an umbilical) to the surface. The proposed Phase I study would investigate tether configurations and the ability of an EDT to produce the power for transmission.
So we have energy harvested (or scavenged) by electrodynamic tethers being used to power up an electrolyzer that can split water into gaseous H2 and O2, an efficient way to use local resources in a domain where solar and perhaps nuclear power would be unusable (remember in relation to nuclear options that NASA has cancelled development of the Advanced Stirling Radioisotope Generator – ASRG – technologies, although some testing at NASA Glenn is to continue). The gases produced by the electrolysis would then be stored for energy usage, tapped as a combustible fuel/oxidizer mixture, and as a pressurant.
A Natural Model for Locomotion
This last point deserves a second look. In previous work, Robert Shepherd has created pneumatically powered silicon-based robots that can move and navigate obstacles, using onboard air compressors and lithium battery packs. Work at MIT has demonstrated a pneumatically powered swimming robot with a soft tail using onboard compressed CO2. Shepherd has also demonstrated the use of hydrogen combustion to increase the range and speed of soft robots, a model he and Peck propose for further study in the Europa concept, one that might be used both below the ice and on the surface.
Image: Water jetting actuated by ignition of H2/O2 gas and subsequent shape change. Credit: Mason Peck, Robert Shepherd.
Here again the model from nature is instructive, for as the report notes, “[t]he design of the combustion powered hydro-jetting mechanism is analogous to the morphology of an octopus’ mantle cavity.” The report anticipates using jetting methods to allow the rover to range widely at long distances and also for precision operations over short distances. Peck and Shepherd are also hoping to study grasping operations that would be modeled on biology, using ‘an array of teeth-like grippers positioned around the water jet area (mouth) of the synthetic lamprey.’
This is fascinating work that offers us solutions for powering an underwater robot but also provides mechanisms for movement in this environment (one that we may find in other gas giant moons) through the use of a form of robotics that mimics the natural world. Solutions inspired by biology help us move beyond the use of solar arrays, nuclear power or batteries to keep our rover operational and to give it what would seem to be a robust and lengthy lifetime. I would say that getting soft robotics into the picture for future space operations is a very wise idea, certainly one that justifies continued study and investment as we look toward the outer planets.
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