Centauri Dreams
Imagining and Planning Interstellar Exploration
Juno: Looking Deep into Jupiter’s Atmosphere
We’re learning more about the composition of Jupiter’s atmosphere, and in particular, the amount of water therein, as a result of data from the Juno mission. The data come in the 1.25 to 22 GHz range from Juno’s microwave radiometer (MWR), depicting the deep atmosphere in the equatorial region. Here, water (considered in terms of its component oxygen and hydrogen) makes up about 0.25 percent of the molecules in Jupiter’s atmosphere, almost three times the percentage found in the Sun. All of this gets intriguing when compared to the results from Galileo.
You’ll recall that the Galileo probe descended into the Jovian atmosphere back in 1995, sending back spectrometer measurements of the amount of water it found down to almost 120 kilometers, where atmospheric pressure reached 320 pounds per square inch (22 bar). Unlike Juno, Galileo showed that Jupiter might be dry compared to the Sun — there was in fact ten times less water than expected — but it also found water content increasing even as it reached its greatest depth, an oddity given the assumption that mixing in the atmosphere would create a constant water content. Did Galileo run into some kind of meteorological anomaly?
A new paper in Nature Astronomy looks at the matter as part of its analysis of the Juno results, which also depict an atmosphere not well mixed:
The findings of the Galileo probe were puzzling because they showed that where ammonia and hydrogen sulfide become uniformly mixed occurs at a level much deeper (~10 bar) than what was predicted by an equilibrium thermochemical model. The concentration of water was subsolar and still increasing at 22 bar, where radio contact with the probe was lost, although the concentrations of nitrogen and sulfur stabilized at ~3 times solar at ~10 bar. The depletion of water was proposed to be caused by meteorological effects at the probe location. The observed water abundance was assumed not to represent the global mean water abundance on Jupiter, which is an important quantity that distinguishes planetary formation models and affects atmospheric thermal structure.
Now Juno has found water content greater than what Galileo measured. But the fact that Galileo showed a water concentration that was still increasing when the probe no longer could send data makes its results inconclusive. The matter is important for those interested in planet formation because as the likely first planet to form, Jupiter would have contained the great bulk of gas and dust that did not go into the composition of the Sun. Thus planet formation models are keyed to factors like the amount of water the young planet would have assimilated. Scott Bolton, Juno principal investigator at the Southwest Research Institute in San Antonio, comments:
“Just when we think we have things figured out, Jupiter reminds us how much we still have to learn. Juno’s surprise discovery that the atmosphere was not well mixed even well below the cloud tops is a puzzle that we are still trying to figure out. No one would have guessed that water might be so variable across the planet.”
Image: The JunoCam imager aboard NASA’s Juno spacecraft captured this image of Jupiter’s southern equatorial region on Sept. 1, 2017. The image is oriented so Jupiter’s poles (not visible) run left-to-right of frame. Credit: NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill.
The research team, led by Cheng Li (JPL/Caltech) used data from Juno’s first eight science flybys, focusing on the equatorial region first because the atmosphere appears to be better mixed there than in other regions. Juno’s microwave radiometer can measure the absorption of microwave radiation by water at multiple depths at the same time. Using these methods, Juno could collect data from deeper in the atmosphere than Galileo, where pressures reach about 480 psi (33 bar). The next move will be to compare this with other regions, giving us a picture of water abundance as Juno coverage extends deeper into Jupiter’s northern hemisphere. Of particular interest will be what Juno will find at the planet’s poles.
From the paper:
We have shown that the structure of Jupiter’s EZ [equatorial zone] is steady, relatively uniform vertically and close to a moist adiabat [a region where heat does not enter or leave the system]; from this we have derived its water abundance. The thermal structure outside of the equator is still ambiguous owing to the non-uniform distribution of ammonia gas, for which we do not know the physical origin. Deriving the thermal structure outside of the equator in the future not only hints about the water abundance on Jupiter at other latitudes but also places constraints on the atmospheric circulation model for giant planets in the Solar System and beyond.
Image: Thick white clouds are present in this JunoCam image of Jupiter’s equatorial zone. These clouds complicate the interpretation of infrared measurements of water. At microwave frequencies, the same clouds are transparent, allowing Juno’s Microwave Radiometer to measure water deep into Jupiter’s atmosphere. The image was acquired during Juno’s flyby of the gas giant on Dec. 16, 2017. Credit: NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill.
The authors add that Juno has already revealed a deep atmosphere that is surprisingly variable as a function of latitude, highlighting the need to tread cautiously before making any assumptions about the planet’s overall water abundance. Extending these observations into other regions of the planet will be useful because oxygen is the most common element after hydrogen and helium in Jupiter’s atmosphere, and as water ice may thus have been the primary condensable in the protoplanetary disk. Consider this a deep probe into planet formation.
The paper is Li et al., “The water abundance in Jupiter’s equatorial zone,” Nature Astronomy 10 February 2020 (abstract).
Trident: Firming up the Triton Flyby
It’s not a Triton, or even a Neptune orbiter, but Trident is still an exciting mission, a Triton flyby that would take a close look at the active resurfacing going on on this remarkable moon. Trident has recently been selected by NASA’s Discovery Program as one of four science investigations that will lead to one to two missions being chosen at the end of the study for development and launch in the 2020s.
These are nine-month studies, and they include, speaking of young and constantly changing surfaces, the Io Volcanic Observer (IVO). The other two missions are the Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy (VERITAS) mission, and DAVINCI+ (Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging Plus).
Each of these studies will receive $3 million to bring its concepts to fruition, concluding with a Concept Study Report, at which point we’ll get word on the one or two that have made it to further development and flight. The NASA Discovery program has been in place since 1992, dedicated to supporting smaller missions with lower cost and shorter development times than the larger flagship missions. That these missions can have serious clout is obvious from some of the past selections: Kepler, Dawn, Deep Impact, MESSENGER, Stardust and NEAR.
Active missions at the moment include Lunar Reconnaissance Orbiter and InSight, but we leave the inner system with Lucy, a Discovery mission visiting a main belt asteroid as well as six Jupiter trojans, and Psyche, which will explore the unusual metal asteroid 16 Psyche. Discovery missions set a $500 million cost-cap excluding launch vehicle operations, data analysis or partner contributions. The next step up in size is New Frontiers, now with a $1 billion cost-cap — here we can mention New Horizons, OSIRIS-REx and Juno as well as Dragonfly.
I assume that New Horizons’ success at Pluto/Charon helped Trident along, showing how much good science can be collected from a flyby. Triton makes for a target of high interest because of its atmosphere and erupting plumes, along with the potential for an interior ocean. The goal of Trident is to characterize the processes at work while mapping a large swath of Triton and learning whether in fact the putative ocean beneath the surface exists. A mid-2020s launch takes advantage of a rare and efficient gravity assist alignment to make the mission feasible. Louise Prockter, director of the Lunar and Planetary Institute in Houston, is principal investigator.
Image: Dr. Louise Prockter, program director for the Universities Space Research Association, as well as director of the Lunar and Planetary Institute, is now principal investigator for Trident. Credit: USRA.
We can thank Voyager 2 for providing our only close-up images of Triton, which was revealed to be a place where explosive venting blows dark material from beneath the ice into the air, material which falls back onto the surface to create new features. The terrain is varied and notable for the striking ‘cantaloupe’ pattern covering large areas. With its distinctive retrograde rotation, orbiting opposite to Neptune’s rotation, and high inclination orbit, Triton may well be an object captured from the Kuiper Belt, in an orbit where tidal forces likely lead to interior heating that could maintain an ocean. What we learn here could inform our understanding not just of KBOs, but also giant moons like Titan and Europa, and smaller ocean worlds like Enceladus.
This would be a flyby with abundant opportunities for data collection, as this precis from the 2019 Lunar and Planetary Science Conference makes clear:
An active-redundant operational sequence ensures unique observations during an eclipse of Triton – and another of Neptune itself – and includes redundant data collection throughout the flyby… High-resolution imaging and broad-spectrum IR imaging spectroscopy, together with high-capacity onboard storage, allow near-full-body mapping over the course of one Triton orbit… Trident passes through Triton’s thin atmosphere, within 500 km of the surface, sampling its ionosphere with a plasma spectrometer and performing magnetic induction measurements to verify the existence of an extant ocean. Trident’s passage through a total eclipse allows observations through two atmospheric radio occultations for mapping electron and neutral atmospheric density, Neptune-shine illuminated eclipse imaging for change detection since the 1989 Voyager 2 flyby, and high-phase angle atmospheric imaging for mapping haze layers and plumes.
Image: Global color mosaic of Triton, taken in 1989 by Voyager 2 during its flyby of the Neptune system. Color was synthesized by combining high-resolution images taken through orange, violet, and ultraviolet filters; these images were displayed as red, green, and blue images and combined to create this color version. With a radius of 1,350 kilometers (839 mi), about 22% smaller than Earth’s moon, Triton is by far the largest satellite of Neptune. It is one of only three objects in the Solar System known to have a nitrogen-dominated atmosphere (the others are Earth and Saturn’s giant moon, Titan). Triton has the coldest surface known anywhere in the Solar System (38 K, about -391 degrees Fahrenheit); it is so cold that most of Triton’s nitrogen is condensed as frost, making it the only satellite in the Solar System known to have a surface made mainly of nitrogen ice. The pinkish deposits constitute a vast south polar cap believed to contain methane ice, which would have reacted under sunlight to form pink or red compounds. The dark streaks overlying these pink ices are believed to be an icy and perhaps carbonaceous dust deposited from huge geyser-like plumes, some of which were found to be active during the Voyager 2 flyby. The bluish-green band visible in this image extends all the way around Triton near the equator; it may consist of relatively fresh nitrogen frost deposits. The greenish areas includes what is called the cantaloupe terrain, whose origin is unknown, and a set of “cryovolcanic” landscapes apparently produced by icy-cold liquids (now frozen) erupted from Triton’s interior.
Credit: NASA/JPL/USGS.
If it flies, Trident would launch in 2026 and reach Triton in 2038, using gravity assists at Venus, the Earth and, finally, Jupiter for a final course deflection toward Neptune. The current thinking is to bring the spacecraft, which will weigh about twice New Horizons’ 478 kg, within 500 kilometers of Triton, a close pass indeed compared to New Horizons’ 12,500 kilometer pass by Pluto. This is indeed close enough for the spacecraft to sample Triton’s ionosphere and conduct the needed magnetic induction measurements to confirm or refute the existence of its ocean. As this mission firms up, we’ll be keeping a close eye on its prospects in the outer system. Remember, too, the 2017 workshop in Houston examining a possible Pluto orbiter, still a long way from being anything more than a concept, but interesting enough to make the pulse race.
My friend Ashley Baldwin, who sent along some good references re Trident, also noted that Trident’s trajectory is such that the gravity assist around Jupiter could, at 1.24 Jupiter radii, provide a close flyby of Io. Interesting in terms of the competing Io Volcanic Observer entry.
A New Look at the ‘Pale Blue Dot’
The 30th anniversary of the famous ‘Pale Blue Dot’ image of Earth, which took place on February 14, is an appropriate occasion for the newly updated image below, which brings the latest methods to bear on the data Voyager 1 presented us. Our planet takes up less than a single pixel and for that reason is not fully resolved. The rays of sunlight due to scattering within the camera optics intersect with Earth, reminding us that from Voyager’s position 6 billion kilometers from home, the Earth/Sun separation was only a matter of a few degrees.
Image: For the 30th anniversary of one of the most iconic images taken by NASA’s Voyager mission, a new version of the image known as “the Pale Blue Dot.” Planet Earth is visible as a bright speck within the sunbeam just right of center and appears softly blue, as in the original version published in 1990. This updated version uses modern image-processing software and techniques to revisit the well-known Voyager view while attempting to respect the original data and intent of those who planned the images. Credit: NASA/JPL-Caltech.
What we’re looking at is a color composite that combines images taken through green, blue, and violet spectral filters with the Voyager 1 Narrow-Angle Camera. 34 minutes after this work was done, Voyager 1 powered off its cameras, there being no targets for future flybys. In any case, controllers needed to conserve power for what would become the Voyager Interstellar Mission, an undertaking that is still alive. The image also reminds us that images of the inner planets would have been dangerous if taken earlier in the mission, given the possibility of damaging the cameras due to their proximity to the Sun.
And here is the original, with JPL caption from 1996:
Image: This narrow-angle color image of the Earth, dubbed ‘Pale Blue Dot’, is a part of the first ever ‘portrait’ of the solar system taken by Voyager 1. The spacecraft acquired a total of 60 frames for a mosaic of the solar system from a distance of more than 4 billion miles from Earth and about 32 degrees above the ecliptic. From Voyager’s great distance Earth is a mere point of light, less than the size of a picture element even in the narrow-angle camera. Earth was a crescent only 0.12 pixel in size. Coincidentally, Earth lies right in the center of one of the scattered light rays resulting from taking the image so close to the sun. This blown-up image of the Earth was taken through three color filters — violet, blue and green — and recombined to produce the color image. The background features in the image are artifacts resulting from the magnification. Credit: NASA/JPL.
A series of 60 images went into making what the mission team called a Family Portrait of the Solar System, a sequence that captured six planets as well as the Sun. For readers of Centauri Dreams, I doubt I have to wax poetic here, as most of you have your own thoughts, remembering the first time you saw this breathtaking image and tried to fit it into your own perspective on the cosmos. It was Carl Sagan who came up with the idea for the image, and it’s fitting that the Carl Sagan Institute’s Lisa Kaltenegger (Cornell) should comment on it;
“The Pale Blue Dot image shows our world as both breathtakingly beautiful and fragile, urging us to take care of our home. We are living in an amazing time, where for the first time ever we have the technical means to spot worlds orbiting other stars. Could one of them be another pale blue dot, harboring life? That is what we are trying to find out at the Carl Sagan Institute.”
Image: This simulated view, made using NASA’s Eyes on the Solar System app, approximates Voyager 1’s perspective when it took its final series of images known as the “Family Portrait of the Solar System,” including the “Pale Blue Dot” image. Credit; NASA/JPL-Caltech.
Ed Stone, Voyager project scientist, called this final use of the spacecraft’s lenses ‘last light,’ in contrast to the initial imaging by a telescope, which is known in the trade as ‘first light.’ It’s remarkable to consider today that at the time, the idea of Voyager’s look back at the Solar System was dismissed by some as a stunt, and it’s worth remembering that there were those in the Voyager design days who advocated that the spacecraft carry no cameras at all.
Jim Bell writes about the matter in his book The Interstellar Age:
Fortunately, after the Neptune encounter, top NASA officials such as associate administrator for science Len Fisk and administrator Richard Truly shared Carl Sagan’s vision of the historic, aesthetic value of the solar-system family portrait. Ed Stone was also a strong supporter of the idea. He recalls a dinner at Caltech organized by Sagan and The Planetary Society just before the Voyager Neptune flyby in 1989, during which he, Sagan, Fisk, and Voyager project manager Norm Haynes talked about what it would take to make ‘the picture of the century’ happen. By this point in time it was essentially a budgetary issue, as Voyager’s funding was set to ramp down steeply right after Neptune. Happily, Fisk and Truly interceded to make sure the people and resources were made available for this one last Voyager mosaic…
We might not have had the ‘pale blue dot’ image at all if it had not been for this intervention, a reminder of the significance of decisions that at the time may seem small once examined in the context of history. It’s only right to quote Sagan’s famous words on the image to close, from his book Pale Blue Dot: A Vision of the Human Future in Space (1994):
“Look again at that dot. That’s here. That’s home. That’s us. On it, everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every ‘superstar,’ every ‘supreme leader,’ every saint and sinner in the history of our species lived there – on a mote of dust, suspended in a sunbeam.”
Boundary Conditions for Emergent Complexity Longevity
We usually think about habitability in terms of liquid water on the surface, which is the common definition of the term ‘habitable zone.’ But even in our own system, we have great interest in places where this is not the case (e.g. Europa). In today’s essay, Nick Nielsen begins with the development of complex life in terms not just of a habitable zone, but what some scientists are calling an ‘abiogenesis zone.’ The implications trigger SETI speculation, particularly in systems whose host star is nearing the end of its life on the main sequence. Are there analogies between habitable zones and the conditions that can lead not just to life but civilization? These boundary conditions offers a new direction for SETI theorists to explore.
by J. N. Nielsen
Recently a paper of some interest was posted to arXiv, “There’s No Place Like Home (in Our Own Solar System): Searching for ET Near White Dwarfs,” by John Gertz. (Gertz has several other interesting papers on arXiv that are working looking at.) Here is the abstract of the paper in its entirety:
The preponderance of white dwarfs in the Milky Way were formed from the remnants of stars of the same or somewhat higher mass as the Sun, i.e., from G-stars. We know that life can exist around G-stars. Any technologically advanced civilization residing within the habitable zone of a G-star will face grave peril when its star transitions from the main sequence and successively enters sub-giant, red giant, planetary nebula, and white dwarf stages. In fact, if the civilization takes no action it will face certain extinction. The two alternatives to passive extinction are (a) migrate away from the parent star in order to colonize another star system, or (b) find a viable solution within one’s own solar system. It is argued in this paper that migration of an entire biological population or even a small part of a population is virtually impossible, but in any event, far more difficult than remaining in one’s home solar system where the problem of continued survival can best be solved. This leads to the conclusion that sub-giants, red giants, planetary nebula, and white dwarfs are the best possible candidate targets for SETI observations. Search strategies are suggested.
There are a number of interesting ideas in the above. The first thing that strikes me about this is that it exemplifies what I call the SETI paradigm: interstellar travel is either impossible or so difficult that SETI is the only possibility for contact with other civilizations. [1]
The SETI paradigm is worth noting in this context because Gertz is considering these matters on a multi-billion year time scale, i.e., a cosmological scale of time, and not the scale of time at which we usually measure civilization. Taking our own case of civilization as normative, if terrestrial civilization endures through the red giant and white dwarf stages of our star, that means our civilization will endure for billions of years, and in those billions of years (in the Gertz scenario) we will not develop any of the technology that would allow us to make the journey to other stars, including those other stars that will come within less than a light year of our own star with some frequency over cosmological scales of time. [2] We will, however, according to this scenario, develop technologies that would allow us to migrate to other parts of our own planetary system. I find that this contrast in technological achievement makes unrealistic demands upon credulity, but this is merely tangential to what I want to talk about in relation to this paper.
What most interests me about the scenario contemplated in this paper is its applicability to forms of emergent complexity other than human civilization. What I mean by “other forms of emergent complexity” is what I now call emergent complexity pluralism, which I present in my upcoming paper “Peer Complexity during the Stelliferous Era.” The paper isn’t out yet, but you can see a video of my presentation in Milan in July 2019: Peer Complexity during the Stelliferous Era, Life in the Universe: Big History, SETI and the Future of Humankind, IBHA & INAF-IASF MI Symposium. (Write to me if you’d like a copy of the paper.) In brief, we aren’t the only kind of complexity that may arise in the universe.
The simplest case of an alternative emergent complexity, and the case most familiar to us, is to think of Gertz’s scenario in terms of life without the further emergent complexities that have come to supervene upon human activity, chiefly civilization. In the case of a planet like Earth, possessed of a biosphere that has endured for billions of years and which has produced complex forms of life, one could expect to see exactly what Gertz attributes to technological civilizations, though biology alone could be sufficient to account for these developments. However — and this is a big however — the conditions must be “just right” for this to happen. In other words, something like the Goldilocks conditions of the “Goldilocks Zone” (the circumstellar habitable zone, or CHZ) must obtain, though in a more generalized form, so that each form of emergent complexity may have its own distinctive boundary conditions.
A further distinction should be introduced at this point. The boundary conditions of the emergence of complexity (whether of life, or civilization, or something else yet) may be distinct from the boundary conditions for the further development of complexity, and especially for developments that involve further complexity emerging from a given complexity, in the way that consciousness and intelligence emerged from life on Earth, and civilization emerged in turn from consciousness and intelligence. This distinction has been captured in origins of life research by the distinction between the habitability zone (the CHZ, in its conventional use) and the abiogenesis zone. The former is the region around a star where biology is possible, whereas the latter is the region in which biology can arise.
In a 2018 paper, The origin of RNA precursors on exoplanets, by Paul B. Rimmer, Jianfeng Xu, Samantha J. Thompson, Ed Gillen, John D. Sutherland, and Didier Queloz, this distinction between conditions for the genesis of life and conditions for the development and furtherance of life is made, and the two sets of boundary conditions are shown to overlap, but not to precisely coincide:
“The abiogenesis zone we define need not overlap the liquid water habitable zone. The liquid water habitable zone identifies those planets that are a sufficient distance from their host star for liquid water to exist stably over a large fraction of their surfaces. In the scenario we consider, the building blocks of life could have been accumulated very rapidly compared to geological time scales, in a local transient environment, for which liquid water could be present outside the liquid water habitable zone. The local and transient occurrences of these building blocks would almost certainly be undetectable. The liquid water habitable zone helpfully identifies where life could be sufficiently abundant to be detectable.” [3]
The idea implicit in defining an abiogenesis zone distinct from a habitable zone can be extrapolated to other forms of complexity: boundary conditions of emergence may be distinct from boundary conditions for development and longevity; the conditions for the emergence of civilization may be distinct from the conditions for the longevity of civilization. But let us return to the scenario of life maintaining itself within its planetary system without the assistance of intelligence or technology.
Image: This is Figure 4 from the Rimmer et al. paper. Caption: A period-effective temperature diagram of confirmed exoplanets within the liquid water habitable zone (and Earth), taken from a catalog (1, 42, 43), along with the TRAPPIST-1 planets (3) and LHS 1140b (4). The “abiogenesis zone” indicates where the stellar UV flux is large enough to result in a 50% yield of the photochemical product. The red region shows the propagated experimental error. The liquid water habitable zone [from (44, 45)] is also shown. Credit: Rimmer et al.
Whereas the CHZ is usually defined in terms of a region of space around a star clement for life as we know it, the boundary conditions for alternative emergent complexities will be optimal relative to the emergent complexity in question. That is to say, the wider we construe “habitability” (i.e., the more diverse kinds of emergent complexity that might inhabit a planet or planetary system) the more CHZs there will be, as each form of emergent complexity will have boundary conditions distinctive to itself.
In a planetary system with a large number of rocky worlds spaced relatively close together, these worlds could serve as “stepping stones” for enhanced lithopanspermia. [4] At each stage in the life of the parent star of such a planetary system with life, the life would be distributed among the available planets, and it would flourish into a planetary-scale biosphere on the world with the most clement conditions. When the star began to swell into a red giant, the inner planets would become inhospitable to life, but life could then migrate outward to the cooler planets. And then, when the star cooled down again, life could once again planet-hop nearer to the now-cooler star.
We do not yet know if the boundary conditions for emergent complexity longevity obtain within our own solar system. Is Mars close enough that life, going extinct on Earth, could make the transition to this cooler world, and possibly also further out to the moons of the gas giants? In The Jovian Oceans [5] I suggested that, as the sun grows into a red giant, the outer regions of the solar system will become warmer and the subsurface oceans of some of the moons of Jupiter and Saturn may thaw out and become watermoons (in contradistinction to waterworlds). These regions of our solar system may be clement to life when Earth is no longer habitable, but if life cannot make the journey to these worlds, they may as well not exist at all. We still have a billion years for sufficiently hardy microorganisms to evolve, and for collisions with large bodies to blast microorganisms off the surface of Earth and into trajectories that would eventually result in their impacting on Mars. The chances for this strike me as marginal, but over a billion years we cannot exclude marginal scenarios.
As I have noted in Life: from Sea to Land to Space, the expansion of life from Earth into space (like the expansion of life from the oceans onto land) will open up a vastly greater number of niches to life than could exist on any one planet, so that the opportunities for adaptive radiation are increased by orders of magnitude. But this expansive scenario for life in space is contingent upon the proper boundary conditions obtaining; life must expand into an optimal environment in order for it to experience optimal expansion and adaptive radiation. [6] And as the boundary conditions for the emergence of emergent complexity may be distinct from the boundary conditions for the longevity of emergent complexity, emergent complexity (like a biosphere) may flourish and die on one planet without the opportunity to exploit the potential of other niches. [7]
There are also distinctive boundary conditions for the longevity of civilization. If a civilization is to employ technological means to extend its longevity, whether through journeying to other stars, or, according to Gertz’s scenario, shifting itself within its home planetary system (“sheltering in place”), then the conditions must first be right for a life to arise, and then for civilization to supervene upon life, and finally for civilization to pass beyond its planetary origins by technological means. These boundary conditions might include, for example, an adequate supply of fossil fuels for the civilization to make its original transition to industrialization, and, later, sufficient titanium resources to build spacecraft, and sufficient fissionables to supply nuclear power or to operate nuclear rockets.
It takes a “just right” planetary system for a technological civilization to successfully make a spacefaring breakout from its homeworld — just as being a space-capable civilization is a necessary condition for spacefaring breakout, coming to an initial threshold of technological maturity in the context of favorable boundary conditions is also a necessary condition for being a spacefaring civilization. It also takes a “just right” stellar neighborhood for a spacefaring civilization to make an interstellar breakout from its home system. The boundary conditions for interstellar civilization are subject to change over cosmological scales of time, because stars change their relationships to each other within the galaxy, but there will still be regions in the galaxy with more favorable conditions and regions in the galaxy with less favorable conditions.
As I have noted in other contexts, technology is a means to an end, and usually not an end in itself, so that there is a certain fungibility in the use of technologies: if the resources are unavailable for a particular technology, they may be available for some other technology that can serve in a similar capacity. A marginal technology in favorable boundary conditions, or a superior technology in unfavorable boundary conditions, might do the trick either way. However, there are limits to technological fungibility. The boundary conditions for the longevity of technological civilizations set these limits.
Notes
[1] I have written about the SETI paradigm in my Centauri Dreams post Stagnant Supercivilizations and Interstellar Travel, inter alia.
[2] I discussed interstellar travel by waiting for other planetary systems to pass near our own in the aforementioned Stagnant Supercivilizations and Interstellar Travel.
[3] “The origin of RNA precursors on exoplanets,” by Paul B. Rimmer, Jianfeng Xu, Samantha J. Thompson, Ed Gillen, John D. Sutherland, and Didier Queloz, Science Advances, 01 Aug 2018: Vol. 4, no. 8, DOI: 10.1126/sciadv.aar3302
[4] Cf. two papers on this, “Enhanced interplanetary panspermia in the TRAPPIST-1 system” by Manasvi Lingam and Abraham Loeb, and “Fast litho-panspermia in the habitable zone of the TRAPPIST-1 system”, by Sebastiaan Krijt, Timothy J. Bowling, Richard J. Lyons, and Fred J. Ciesla, and my post Emergent Complexity in Multi-Planetary Ecosystems.
[5] This post also noted two papers, then recent, on habitability zones around post-main sequence stars, “Habitable Zones Of Post-Main Sequence Stars” by Ramses M. Ramirez, et al., and “Habitability of Super-Earth Planets around Other Suns: Models including Red Giant Branch Evolution” by W. von Bloh, M. Cuntz, K.-P. Schroeder, C. Bounama, and S. Franck, both of which are relevant to Gertz’s argument.
[6] René Heller has introduced the concept of superhabitable worlds, i.e., worlds more clement for life than Earth, thus optimal for life (cf., e.g., “Superhabitable Worlds”, by René Heller and John Armstrong), which suggests a similar implicit distinction between merely habitable planetary systems and superhabitable planetary systems, merely habitable galaxies and superhabitable galaxies, and so on.
[7] Freeman Dyson argued for the value of life that can adapt to conditions distinct from the planetary endemism that characterizes life as we know it: “…planets compare unfavourably with other places as habitats. Planets have many disadvantages. For any form of life adapted to living in an atmosphere, they are very difficult to escape from. For any form of life adapted to living in vacuum they are death-traps, like open wells full of water for a human child. And they have a more fundamental defect: their mass is almost entirely inaccessible to creatures living on their surface.” (Dyson, F. J. 2003. “Looking for life in unlikely places: reasons why planets may not be the best places to look for life.” International Journal of Astrobiology, 2(2), 103-110) Dyson’s reasons for favoring life independent of planets does not alter the fact that a lot of interesting chemistry occurs on planets that does not occur elsewhere because other environments do have not large scale geomorphological processes; however, Dyson’s observations do point to the selective value of life that can adapt to habitats without planets.
A Nearby ‘Planet’ in Formation
330 light years from the Sun is the infant planet 2MASS 1155-7919 b, recently discovered in Gaia data by a team from the Rochester Institute of Technology. It’s a useful world to have in our catalog because we have no newborn massive planet closer to Earth than this one. Circling a star in the Epsilon Chamaeleontis Association, 2MASS 1155-7919 b is thought to be no more than 5 million years old, orbiting its host at roughly 600 times the Earth/Sun distance. A stellar association like Epsilon Chamaeleontis is a loose cluster, with stars that have a common origin but are no longer gravitationally bound as they move in rough proximity through space.
RIT graduate student Annie Dickson-Vandervelde is lead author on the discovery paper:
“The dim, cool object we found is very young and only 10 times the mass of Jupiter, which means we are likely looking at an infant planet, perhaps still in the midst of formation. Though lots of other planets have been discovered through the Kepler mission and other missions like it, almost all of those are ‘old’ planets. This is also only the fourth or fifth example of a giant planet so far from its ‘parent’ star, and theorists are struggling to explain how they formed or ended up there.”
Image: Artist’s conception of a massive planet orbiting a cool, young star. In the case of the system discovered by RIT astronomers, the planet is 10 times more massive than Jupiter, and the orbit of the planet around its host star is nearly 600 times that of Earth around the sun. NASA/JPL-Caltech/R. Hurt (SSC-Caltech).
So a star a thousand times younger than the Sun has produced a giant planet far enough from its star to challenge our models of gas giant formation. But is it actually a planet?
From the paper:
The origins of systems involving such wide-separation substellar objects are presently the subject of vigorous debate (Rodet et al. 2019, and references therein). Given that 2MASS 1155-7919 b is quite possibly the youngest massive planet within ~100 pc—i.e., closer to Earth than the aforementioned massive young planets, as well as nearby star-forming clouds—this object is richly deserving of followup spectroscopy and imaging aimed at confirming its spectral type, age, and luminosity, in order to better understand its nature and origin.
Despite its unusual interest, the paper also reminds us, 2MASS J1155-7919 b joins other systems with wide separation, such worlds as HD 106906 b, 1RXS 1609 b, CT Cha b, and DENIS1538-1038. The fact that the latter two are thought to be brown dwarf candidates highlights the idea that such objects may form like low-mass stars, although the physical processes at work in their accretion and development are poorly understood. Here I’m going to switch to a different paper, this one on DENIS1538?1038 (Nguyen-Thanh et al., citation below).
Our discovery of the 1-Myr old BD [brown dwarf] that exhibits sporadic accretion with low accretion rates supports a possible scenario for BD formation…where low-mass accretion rates at very early stages (possibly with high outlfow mass-loss rate-to-mass-accretion-rate ratios) prevent VLM [very low mass] cores from accreting enough gas to become stars, and thus these cores would end up as BDs.
The authors of the 2MASS J1155-7919 b discovery paper point out that their putative gas giant is only slightly below the boundary between brown dwarfs and massive planets, making the above brown dwarf formation scenario a possibility. In either case, we have much to learn about widely separated objects in the same system when we find them this early in their evolution. The intellectual ferment in this area is exciting to watch.
The paper is Dickson-Vandervelde et al., “Identification of the Youngest Known Substellar Object within ~100 pc,” Research Notes of the AAS Vol. 4, No. 2 (7 February 2020). Full text. The Nguyen-Thanh et al paper is “Sporadic and intense accretion in a 1 Myr-old brown dwarf candidate,” in process at Astronomy & Astrophysics (preprint).
A Heliophysics Gateway to Deep Space
Are missions to the Sun particularly relevant to our interstellar ambitions? At the current state of our technology, the answer is yes. Consider Solar Cruiser, which is the planned NASA mission using a solar sail that could maintain non-Keplerian orbits, allowing it to investigate the Sun’s high latitudes. And throw in the European Space Agency-led Solar Orbiter, which left our planet early Monday (UTC) on a United Launch Alliance Atlas V rocket, lifting off from Launch Complex 41 at Cape Canaveral Air Force Station in Florida. Herewith the gorgeous arc of ascent:
Image: Launch of the ESA/NASA Solar Orbiter mission to study the Sun from Cape Canaveral Air Force Station in Florida on Feb. 9, 2020. Credit: Jared Frankle.
Missions to the Sun allow us to explore conditions close to a star and, significantly, deep in its gravity well, where interesting things can happen. When we discuss one way of propelling a sail beyond the heliosphere, the irony is that an Oberth maneuver, which takes place at a few solar radii, can bring additional chemical propulsion online at perihelion to extract the maximum push. So in propulsive terms, we go to the Sun in order to get flung from the Sun at highest speed. If we want to get beyond the heliosphere fast and with today’s tools, the Sun is a major factor.
Solar Orbiter is not, of course, designed around interstellar matters, but the synchronicity here works well for us. The more data about conditions near the Sun, the better for what we will want to do in the future. Günther Hasinger is the European Space Agency’s director of science:
“As humans, we have always been familiar with the importance of the Sun to life on Earth, observing it and investigating how it works in detail, but we have also long known it has the potential to disrupt everyday life should we be in the firing line of a powerful solar storm. By the end of our Solar Orbiter mission, we will know more about the hidden force responsible for the Sun’s changing behavior and its influence on our home planet than ever before.”
And, I would add, we’ll know a great deal more about how spacecraft operate inside Mercury’s orbit. Moreover, think about all the interesting maneuvers that have to take place to make this happen. Three gravity assists come into play as Solar Orbiter goes for the Sun, two of them past Venus in late 2020 and August of 2021, and one past Earth in November of 2021. The first close pass of the Sun will be in 2022, at about a third of an AU, with the gravity of Venus being used to push Solar Orbiter up out of the ecliptic plane. Ulysses achieved an inclined orbit in 1990, but Solar Orbiter will be carrying cameras allowing us to directly image the Sun’s poles, a role for which Ulysses was not equipped. The spacecraft is to reach an inclination 17 degrees above and below the solar equator.
Solar Cruiser and Solar Orbiter have much to teach us about interstellar possibilities, as does, for that matter, the continuing Parker Solar Probe mission. Along the way we learn, in addition to the significant science return about the Sun itself, about how spacecraft cope with being subjected to the solar wind and the temperatures of passage near the Sun. We learn about heat shielding and how to minimize what is needed so as to maximize payload. Solar Orbiter will face temperatures of up to 500º C, 13 times that experienced by satellites in Earth orbit.
So if we’re thinking deep space today, we should also be thinking about heliophysics. Our best bet at getting a successor to the Voyager missions well beyond the heliosphere and at significantly higher speeds that Voyager 1 is a close solar pass and propulsive kick that will demand deep knowledge of conditions at perihelion. Solar Orbiter’s 10 scientific instruments will measure electric and magnetic fields, passing particles and waves, solar atmospheric conditions and the outflow of material.
All these are factors as we contemplate the close approaches that will fling solar sails into the Kuiper Belt. In not many years, we could build a ‘sundiver’ mission that would make for great heliophysics as well as data from deep space — two missions in one.
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