Extraterrestrial civilizations, if they exist, would pose a unique challenge in comprehension. With nothing in common other than what we know of physics and mathematics, we might conceivably exchange information. But could we communicate our cultural values and principles to them, or hope to understand theirs? It was Ludwig Wittgenstein who said “If a lion could speak, we couldn’t understand him.” True?
One perspective on this is to look not into space but into time. Traditional SETI is a search through space and only indirectly, through speed of light factors, a search through time. But new forms of SETI that look for technosignatures — and this includes searching our own Solar System for signs of technology like an ancient probe, as Jim Benford has championed — open up the chronological perspective in a grand way.
Now we are looking for conceivably ancient signs of a civilization that may have perished long before our Sun first shone. A Dyson shell, gathering most of the light from its star, could be an artifact of a civilization that died billions of years ago.
Image: Philosopher Ludwig Wittgenstein (1889-1951), whose Tractatus Logico-Philosophicus was written during military duty in the First World War. It has been confounding readers like me ever since.
Absent aliens to study, ponder ourselves as we look into our own past. I’ve spent most of my life enchanted with the study of the medieval and ancient world, where works of art, history and philosophy still speak to our common humanity today. But how long will we connect with that past if, as some predict, we will within a century or two pursue genetic modifications to our physiology and biological interfaces with computer intelligence? It’s an open question because these trends are accelerating.
What, in short, will humans in a few hundred years have in common with us? The same question will surface if we go off-planet in large numbers. Something like an O’Neill cylinder housing a few thousand people, for example, would create a civilization of its own, and if we ever launch ‘worldships’ toward other stars, it will be reasonable to consider that their populations will dance to an evolutionary tune of their own.
The crew that boards a generation ship may be human as we know the term, but will it still be five thousand years later, upon reaching another stellar system? Will an interstellar colony create a new branch of humanity each time we move outward?
Along with this speculation comes the inevitable issue of artificial intelligence, because it could be that biological evolution has only so many cards to play. I’ve often commented on the need to go beyond the conventional mindset of missions as being limited to the lifetime of their builders. The current work called Interstellar Probe at Johns Hopkins, in the capable hands of Voyager veteran Ralph McNutt, posits data return continuing for a century or more after launch. So we’re nudging in the direction of multi-generational ventures as a part of the great enterprise of exploration.
But what do interstellar distances mean to an artilect, a technological creation that operates by artificial intelligence that eclipses our own capabilities? For one thing, these entities would be immune to travel fatigue because they are all but immortal. These days we ponder the relative advantages of crewed vs. robotic missions to places like Mars or Titan. Going interstellar, unless we come up with breakthrough propulsion technologies, favors computerized intelligence and non-biological crews. Martin Rees has pointed out that the growth of machine intelligence should happen much faster away from Earth as systems continually refine and upgrade themselves.
It was a Rees essay that reminded me of the Wittgenstein quote I used above. And it leads me back to SETI. If technological civilizations other than our own exist, it’s reasonable to assume they would follow the same path. Discussing the Drake Equation in his recent article Why extraterrestrial intelligence is more likely to be artificial than biological, Lord Rees points out there may be few biological beings to talk to:
Perhaps a starting point would be to enhance ourselves with genetic modification in combination with technology—creating cyborgs with partly organic and partly inorganic parts. This could be a transition to fully artificial intelligences.
AI may even be able to evolve, creating better and better versions of itself on a faster-than-Darwinian timescale for billions of years. Organic human-level intelligence would then be just a brief interlude in our “human history” before the machines take over. So if alien intelligence had evolved similarly, we’d be most unlikely to “catch” it in the brief sliver of time when it was still embodied in biological form. If we were to detect extraterrestrial life, it would be far more likely to be electronic than flesh and blood—and it may not even reside on planets.
Image: Credit: Breakthrough Listen / Danielle Futselaar.
I don’t think we’ve really absorbed this thought, even though it seems to be staring us in the face. The Drake Equation’s factor regarding the lifetime of a civilization is usually interpreted in terms of cultures directed by biological beings. An inorganic, machine-based civilization that was spawned by biological forebears could refine the factors that limit human civilization out of existence. It could last for billions of years.
It’s an interesting question indeed how we biological beings would communicate with a civilization that has perhaps existed since the days when the Solar System was nothing more than a molecular cloud. We often use human logic to talk about what an extraterrestrial civilization would want, what its motives would be, and tell ourselves the fable that ‘they’ would certainly act rationally as we understand rationality.
But we have no idea whatsoever how a machine intelligence honed over thousands of millenia would perceive reality. As Rees points out, “we can’t assess whether the current radio silence that Seti are experiencing signifies the absence of advanced alien civilisations, or simply their preference.” Assuming they are there in the first place.
And that’s still a huge’ if.’ For along with our other unknowns, we have no knowledge whatsoever about abiogenesis on other worlds. To get to machine intelligence, you need biological intelligence to evolve to the point where it can build the machines. And if life is widespread — I suspect that it is — that says nothing about whether or not it is likely to result in a technological civilization. We may be dealing with a universe teeming with lichen and pond scum, perhaps enlivened with the occasional tree.
A SETI reception would be an astonishing development, and I believe that if we ever receive a signal, likely as a byproduct of some extraterrestrial activity, we will be unlikely to decode it or even begin to understand its meaning and motivation. Certainly that seems true if Rees is right and the likely sources are machines. A SETI ‘hit’ is likely to remain mysterious, enigmatic, and unresolved. But let’s not stop looking.
If we were to find a civilization at Proxima Centauri, the nearest star, it would either be a coincidence of staggering proportions — two technological cultures just happening to thrive around neighboring stars — or an indication that intelligent life is all but ubiquitous in the galaxy. ‘Ubiquitous’ could itself mean different things: Many civilizations, scattered in their myriads amongst the stars, or a single, ancient civilization that had spread widely through the galaxy.
If a coincidence, add in the time factor and things get stranger still. For only the tiniest fraction of our planet’s existence has been impinged upon by a tool-making species, and who knows what the lifetime of a civilization is? Unless civilizations can live for eons, how could two of them be found around stars so close? Thus the possibility that BLC1 — Breakthrough Listen Candidate 1 — was a valid technosignature at Proxima Centauri was greeted with a huge degree of skepticism within the SETI community and elsewhere.
Image: This picture combines a view of the southern skies over the ESO 3.6-metre telescope at the La Silla Observatory in Chile with images of the stars Proxima Centauri (lower-right) and the double star Alpha Centauri AB (lower-left) from the NASA/ESA Hubble Space Telescope. Proxima Centauri is the closest star to the Solar System and is orbited by the planet Proxima b, which was discovered using the HARPS instrument on the ESO 3.6-metre telescope. Credit: Y. Beletsky (LCO)/ESO/ESA/NASA/M. Zamani.
But it was a good idea to look, because you don’t know what you’re going to find until you take the data, which is why SETI happens. While a great deal of attention has focused on the Alpha Centauri stars as targets for a future space probe, little attention has been paid to them in SETI terms. The southern hemisphere sky was examined by Project Phoenix in the 1990s (202 main sequence stars) and a second search, conducted by David Blair and team, likewise used the Parkes radio telescope in Australia to cover another 176 bright southern stars in that decade. Neither of these searches took in Proxima Centauri, which is, after all, a very faint M dwarf.
The discovery of Proxima Centauri b, a habitable zone world, brought new attention to the dim star. When the Parkes Observatory was turned toward Proxima Centauri in 2019 as part of the activities of Breakthrough Listen, the system received a thorough examination. We learn in two new papers in Nature Astronomy just how thorough:
We searched our observations towards Prox Cen for signs of technologically advanced life, across the full frequency range of the receiver (0.7–4.0?GHz). To search for narrowband technosignatures we exploit the fact that signals from any body with a non-zero radial acceleration relative to Earth, such as an exoplanet, solar system object or spacecraft, will exhibit a characteristic time-dependent drift in frequency (referred to as a drift rate) when detected by a receiver on Earth. We applied a search algorithm that detects narrowband signals with Doppler drift rates consistent with that expected from a transmitter located on the surface of Prox Cen b…Our search detected a total of 4,172,702 hits—that is, narrowband signals detected above a signal-to-noise (S/N) threshold—in all on-source observations of Prox Cen and reference off-source observations. Of these, 5,160 hits were present in multiple on-source pointings towards Prox Cen, but were not detected in reference (off-source) pointings towards calibrator sources; we refer to these as ‘events.’
Image: This is Figure 4 from et al. (citations below). Caption: The signal of interest, BLC1, from our search of Prox Cen. Here, we plot the dynamic spectrum around the signal of interest over an eight-pointing cadence of on-source and off-source observations. BLC1 passes our coincidence filters and persists for over 2?h. The red dashed line, purposefully offset from the signal, shows the expected frequency based on the detected drift rate (0.038?Hz?s?1) and start frequency in the first panel. BLC1 is analysed in detail in a companion paper. Credit: Smith et al./Breakthrough Listen.
In other words, a technosignature at Proxima Centauri, if actually there, should disappear depending on whether or not the telescope is pointing directly at the target system, which is how all but 5,160 hits were eliminated. The BLC1 signal made it through subsequent examination partly because it did not lie within the frequency range of local radio-frequency interference. The authors like to refer to it as a ‘signal of interest’ rather than a ‘candidate’ signal, but accept the BLC1 nomenclature because it is so widely adopted in coverage of the event.
BLC1 was a narrowband signal, which screened out natural astrophysical sources, and intriguingly, it persisted for several hours, much longer than would be accounted for by a passing aircraft or satellite. Moreover, it showed a drift rate that one would expect from a transmitter not on Earth’s surface, one that changed smoothly over time, “as expected for a transmitter in a rotational/orbital environment.” So you can see why it merited a deeper look.
As presented in the two Nature Astronomy papers, that second look has proceeded in the hands of Sofia Sheikh and colleagues at the University of California Berkeley. The scientists have examined archival observations of the Proxima Centauri system using an analysis thoroughly explained in the second of the two papers, one that included, in addition to drift rate study, a search for reappearances of the signal on other days and at other frequencies.
That involved searching for other signals near 982 MHz, where BLC1 appeared, looking first for signals with the same frequency and drift, and moving on to other frequencies (I’m simplifying mercilessly here — see the paper for the intricacies of the analysis). We wind up with a population of BLC1 look-alike signatures that, unlike BLC1, also appear when the telescope is not directly pointed at Proxima Centauri. The authors find every one of these to be caused by radio frequency interference, and determine that BLC1 is consistent with this population of look-alikes in terms of absolute drift rate, frequency and signal to noise ratio. Thus:
Using this procedure, we find that blc1 is not an extraterrestrial technosignature, but rather an electronically drifting intermodulation product of local, time-varying interferers aligned with the observing cadence.
That’s a useful and not unexpected finding, but the value of BLC1 is apparent. It has allowed scientists to develop a set of procedures for the analysis of technosignatures which were fully deployed here and explored in the papers. Sheikh and team have developed a technosignature verification framework built around this, the first signal of interest from Breakthrough Listen that required exhaustive investigation to rule out an alien technology. The value of that for future SETI work should be obvious:
…this signal of interest also reveals some novel challenges with radio SETI validation. It is well understood within the community that single-dish, on–off cadence observing could lead to spurious signals of interest in the case in which the cadence matches the duty cycle of some local RFI. The blc1 signal provided the first observational example of that behaviour, albeit in a slightly different manner than expected (variation of signal strength over position and time, which changed for each lookalike within the set). This case study prompts further application of observing arrays, multi-site observing and multi-beam receivers for radio technosignature searches. For future single-dish observing, we have demonstrated the utility of a deep understanding of the local RFI environment. To gain this understanding, future projects could perform omnidirectional RFI scans at the observing site, record and process the data with instrumentation with high frequency resolution such as the various BL backends, and then use narrowband search software such as turboSETI to obtain a population with which to characterize the statistics (in frequency, drift, power, duty cycle and so on) of local RFI.
The 10-part technosignature verification framework appears at the end of the Sheikh paper and summarizes both what BLC1 taught us through this analysis, but also how we can proceed more efficiently with persistent, narrowband technosignature searches in the future. I would say that’s a good outcome, one that moves the field forward thanks to this unusual detection.
The papers are Smith et al., “A radio technosignature search towards Proxima Centauri resulting in a signal of interest,” Nature Astronomy 25 October 2021 (full text); and Sheikh et al., “Analysis of the Breakthrough Listen signal of interest blc1 with a technosignature verification framework,” Nature Astronomy 25 October 2021 (full text).
Back in 2013, Heath Rezabek began developing a series in these pages on a proposal he called Vessel, which he had first presented at the 100 Year Starship Symposium in September of 2012. A librarian and futurist, Rezabek saw the concept as a strategy to preserve both humanity’s cultural as well as biological heritage, with strong echoes of Greg Benford’s Library of Life, which proposed freezing species in threatened environments to save them. In Heath’s case, a productive partnership with frequent Centauri Dreams contributor Nick Nielsen led to articles by both, which produced a series of interesting discussions in the comments.
I noticed in Philip Lubin’s new paper, discussed here on Friday, an explicit reference to the idea of interstellar craft as possible backup devices for living systems. Lubin singled out the Svalbard Global Seed Vault (styled by some the ‘Doomsday Vault’), which preserves seed samples numbering in the millions, with the aim of keeping them safe for centuries. Here too we have the idea of protecting fragile living systems from existential risk in the form of what Lubin refers to as a ‘genetic ark,’ meaning that while his paper looks at tiny ‘wafer’ probes capable of carrying microorganisms, future iterations might develop into another kind of Earth backup system.
Is interstellar flight in future centuries to become the vehicle for preserving our planet’s heritage and scattering copies of ideas and organisms through the universe? It’s a persuasive thought. Here’s how Lubin and team describe it:
In addition to the physical propagation of life, we can also send out digital backups of the “blueprints of life”, a sort of “how-to” guide to replicating the life and knowledge of Earth. The increasing density of data storage allows for current storage density of more than a petabyte per gram and with new techniques, such as DNA encoding of information, much larger amounts of storage can be envisioned. As an indication of viability, we note the US Library of Congress with some 20 million books only requires about 20 TB to store. A small picture and letter from every person on Earth, as in the “Voices of Humanity” project, would only require about 100 TB to store, easily fitting on the smallest of our spacecraft designs. Protecting these legacy data sets from radiation damage is key and is discussed in Lubin 2020 and Cohen et al. 2020.
Image: How much can we ultimately preserve of Earth? And if we eventually can build large-scale arks, where will we send them? Credit: Adam Benton.
Protecting Planets Beyond Our Own
I’m heartened by two things in this paper. The first is, as I mentioned Friday, the consideration of how to use deep space technologies in the service of biology, a field usually discussed in the interstellar community only in terms of biosignatures from exoplanet atmospheres. If we are at the beginning of what may eventually become an interstellar expansion, we should be thinking practically about what future technologies can do to enhance both the preservation of and adaptation of biological systems to deep space. The need for this kind of study is already apparent as we contemplate the possibility of future off-world colonies on the Moon or Mars.
It’s also heartening to see the thread of knowledge preservation mixing with thinking on biological preservation in the event of future catastrophe. If something goes desperately wrong on our home world — plug in the scenario, from nuclear war to runaway AI or nanotech — we need to be able to save enough of our species to rebuild, either here or elsewhere. If here, then archival installations in nearby space could complement those on Earth. If elsewhere, we can hope to scatter knowledge and biological materials widely enough that some may survive.
This concept, however, runs into the question of planetary protection, given that we already have a deep concern about contaminating places we visit with our spacecraft. There are guidelines in place, as the Lubin paper notes, under Article IX of the Outer Space Treaty in the form of Committee on Space Research (COSPAR) regulations. At present, these extend only to Solar System bodies, and include the problem of contamination from Earth as well as contamination from other bodies via sample return materials brought back to our planet.
If we ever reach the point where realistic travel times to other stars become possible, we’ll confront the issue in exoplanet systems as well. It’s a big topic, too big to handle here in the time allowed, but it’s interesting how Lubin and colleagues discuss it in terms of the tiny probes they contemplate sending out beyond the heliosphere. The problem may be resolved within the mission profile. From the paper:
An object with a mass of less than ten grams accelerating with potentially hundreds of GW of power, will, even if it were aimed at a planetary protection target (for example Mars), enter its atmosphere or impact the solar system body with enough kinetic energy to cause total sterilization of the biological samples on board. The velocity of the craft would thus serve as an in-built mechanism for sterilization. The mission profile does not include deceleration, so this mechanism is valid for the entirety of the mission.
We can add to this the fact that Starlight envisions craft aimed at targets outside the ecliptic, significantly lowering the chances of impact with a planet. If current requirements call for demonstrating probabilities of 99% to avoid impact for 20 years and 95% to avoid impact for 50 years, these requirements seem to be met by the kind of craft Starlight contemplates. The kinetic energy of one of these wafer craft moving at a third of the speed of light is roughly 1 kiloton TNT per gram, according to the authors, which would vaporize craft and payload.
If we go interstellar, though, other issues emerge. All that kinetic energy falls into a different light if we imagine an interstellar flyby probe slamming inadvertently into a planetary atmosphere. If the effect would be little more than that of an arriving large meteorite, we still face the question of affecting an environment. There is more to contamination than a biological question, and it’s obvious that any future interstellar capability will demand a rethinking of regulations governing how our presence makes itself known to any local life forms. We have plenty of time to ponder these matters, but it’s good to see they’re already on the radar in some quarters.
On this score, Lubin and team point to a 2006 paper in Space Policy by C.S. Cockell and G. Hornbeck called “Planetary parks-formulating a wilderness policy for planetary bodies” (abstract). Here questions of planetary protection mingle with what the authors call “utilitarian and intrinsic value arguments.” The need to preserve an exoplanet’s pre-existing environment is a major theme in this work, one that I want to explore in a future post.
The paper on Starlight is Lantin et al., “Interstellar space biology via Project Starlight,” Acta Astronautica Vol. 190 (January 2022), pp. 261-272 (abstract).
If you could send out a fleet of small lightsails, accelerated to perhaps 20 percent of the speed of light, you could put something of human manufacture into the Alpha Centauri triple star system within about 20 years. So goes, of course, the thinking of Breakthrough Starshot, which continues to investigate whether such a proposal is practicable. As the feasibility study continues, we’ll learn whether the scientists involved have been able to resolve some of the key issues, including especially data return and the need for power onboard to make it happen.
The concept of beam-driven sails for acceleration to interstellar speeds goes back to Robert Forward (see Jim Benford’s excellent A Photon Beam Propulsion Timeline in these pages) and has been examined for several decades by, among others, Geoffrey Landis, Gregory Matloff, Benford himself (working with brother Greg) in laboratory experiments at JPL, Leik Myrabo, and Chaouki Abdallah and team at the University of New Mexico. At the University of California, Santa Barbara, Philip Lubin has advocated DE-STAR (Directed Energy Solar Targeting of Asteroids and Exploration), a program for developing a system of modular phased laser arrays that could be used for asteroid mitigation and as propulsion for deep space missions.
Out of this grew Directed Energy Propulsion for Interstellar Exploration (DEEP-IN), which homes in on using tiny wafer probes for interstellar travel. Also known as Starlight, the program is now the subject of a paper in Acta Astronautica that takes interstellar mission concepts into the realm of biology. Working with Stephen Lantin and a team of researchers at the UCSB Experimental Cosmology Group, Lubin is examining how we can use small relativistic spacecraft to place seeds and microorganisms for experimentation outside the Solar System.
Image: Initial interstellar missions will require a complete reevaluation and redesign of the space systems of today. The objective of the Wafer Scale Spacecraft Development program (WSSD) at the University of California Santa Barbara (UCSB) is to design, develop, assemble and characterize the initial prototypes of these robotic platforms in an attempt to pave a path forward for future innovation and exploration. This program, which is just one venture of the UCSB Experimental Cosmology Group’s Electronic and Advanced Systems Laboratory (UCSB Deepspace EAS), focuses on leveraging continued advances in semiconductor and photonics technologies to recognize and efficiently address the many complexities associated with long duration autonomous interstellar mission. Credit: UCSB Experimental Cosmology Group.
Biology Beyond the Heliosphere
The goal of Starlight is not to seed the nearby universe — that’s an entirely different discussion! — but to conduct what the paper calls ‘biosentinel’ experiments, sending them not to another star but to empty interstellar space to help us characterize how simple biological systems deal with conditions there. This could be seen as a step toward eventual human travel to other stars, but the paper doesn’t dwell on that prospect, wisely I think, because what we learn from such experiments is vital information in its own right and may teach us a good deal about abiogenesis and the possibility of panspermia as a way of scattering life throughout the cosmos.
We may, in other words, be able to characterize biological systems to the point where we understand how to protect human life on a future interstellar mission, but the goal of Starlight is to begin the experimentation that will tell us what is possible for living systems under a wide variety of conditions in deep space. Such missions are, in effect, scouts. Putting them onto interstellar trajectories could open up pathways to larger, more biologically complex missions depending on what they find.
I think the emphasis on biology here is heartening, for space research even in the near-term has been top-heavy in terms of propulsion engineering, obviously critical but sometimes neglectful of such critical matters as how to create closed loop life support. Even a destination as nearby as Mars forces us to ask whether humans can adapt long-term to sharply different gravity gradients, among a multitude of other questions. Thus the need for an orbital station dedicated to biology and physiology that would inform mission planning and spacecraft design.
But that kind of complex starts with humans and examines their response in nearby space. What Lubin and team are talking about is studying basic biological systems and their response to conditions outside the heliosphere, where we can begin experiments within the realm of interstellar biology. Any lifeforms we send to interstellar space will be exposed to conditions of zero-g but also hypergravity, as during the launch and propulsion phases of the flight. We would likewise be working with experiments that can be adjusted in terms of exposure to vacuum, radiation and a wide range of temperatures affecting a variety of sample microorganisms.
A Fleet of Interstellar Laboratories
How to experiment, and what to experiment on? Remember that we’re dealing with payloads of wafer size given our constraints on mass. The UCSB work has focused on the design of a microfluidics chamber that can provide suitable conditions for reviving and sustaining microorganisms on the order of 200 ?m in length. The paper refers to performing ‘remote lab-on-a-chip experiments,’ using new materials, discussed within, that are enhanced for biocompatibility. I don’t want to go too deeply into the weeds on this, but here’s the gist:
[New thermoplastic elastomers] can be manufactured with diverse glass transition temperatures and either monolithically prepared with an imprinter or integrated with other candidates, such as glass. Polymerase chain reaction (PCR) on a chip is another area that will evolve naturally over the next decade and is one of many biological techniques that could be incorporated in designs. It is also possible that enzymes could be stabilized in osmolytes to perform onboard biochemical reactions. For the study of life in the interstellar environment, it is necessary to include experimental controls (in LEO and on the ground) and the use of diverse genotypes and species to characterize a wide response space…
The biological species best suited for this kind of investigation will have to have key characteristics, the first of which is a low metabolic rate in a chamber where, due to mass restrictions, nutrients will be limited. Also critical is cryptobiotic capability, meaning the ability to go into hibernation with the lowest possible metabolic rate. A tolerance for radiation is obviously helpful, making certain species clear candidates for these missions, especially tardigrades (so-called ‘water bears’), which are known for being ferociously robust under a wide range of conditions and are capable of reducing biological activity to undetectable levels.
We know that tardigrades can survive high radiation as well as high pressure environments; they have been demonstrated to be capable of enduring exposure to space in low Earth orbit. A second outstanding candidate is C. elegans, a multicellular animal small enough that a teaspoon can hold approximately 100 million juvenile worms. Also helpful is the fact that C. elegans has a rapid life cycle of about 14 days, and can, like tardigrades,be placed into suspended animation for later recovery. Other prospects among organisms and cell types are examined in these pages, and the authors call for near-term experiments on mammalian cells to delve into their response to space conditions.
Clearly, a high radiation environment, as found between the stars, offers the chance to study how life began by varying the degree of radiation shielding to which prebiotic chemicals are exposed:
The high radiation environment of interstellar space provides an interesting opportunity to study the biochemical origins of life in spacecraft with low radiation shielding versus those equipped with protective measures to limit the effect of galactic cosmic radiation (GCR) on the prebiotic chemicals, possibly shedding light on the flexibility in the conditions needed for life to arise.
Thus a spacecraft of this sort can also become an experiment in abiogenesis, the formation of nucleic acids and other biological molecules having heretofore been recreated only under laboratory conditions on Earth or in low-Earth orbit. Designing the equipment to make such experiments possible is part of the ongoing developmental work for Starlight.
An interstellar experimental biology probes the factors that make some organisms better adapted for space conditions, but it does hold implications for human expansion:
Selecting for species that are physiologically better fit for interstellar travel opens up new avenues for space research. In testing the metabolism, development, and replication of species, like C. elegans, we can see how biological systems are generally affected by space conditions. C. elegans and tardigrades are inherently more suited to space flight as opposed to humans due to factors like the extensive DNA protection mechanisms some tardigrades have for radiation exposure or the dauer larva state of arrested development C. elegans enter when faced with unfavorable growth conditions. However, as there is overlap between our species, like the human orthologs for 80% of C. elegans proteins, we can begin to make some predictions on the potential for human life in interstellar space.
Any time we ponder sending life forms into space, including simple bacteria that may have contaminated lander probes on other planets, we run into serious issues of planetary protection. I want to look at the paper’s discussion of those in the next post, along with a consideration of space-based biology in the context of concepts that could offer backup systems for Earth.
The paper is Lantin et al., “Interstellar space biology via Project Starlight,” Acta Astronautica Vol. 190 (January 2022), pp. 261-272 (abstract).
Juno has worked wonders for our knowledge of Jupiter, but we continue to rely on Hubble observations and still helpful imagery from Galileo as we study the giant planet’s intriguing moon Europa, anticipating the arrival of Europa Clipper and the European Space Agency’s Jupiter Icy Moons Explorer (JUICE) at the end of the decade. The great question is whether life can exist beneath the ice, and the evidence of plume activity, first found in 2013 in Hubble data, is encouraging in some ways more than others. A mission to Saturn’s moon Enceladus can rely on geysers in a particular place — the Tiger Stripes at the south pole — and on geysers that are frequent. At Europa, predicting where and when a plume will burst forth is all but a black art. How do we sample a Europan plume?
Image: Will we one day see Europa through the eyes of a (well-shielded) lander? Credit: NASA.
While contemplating that question, we push ahead with other analyses that help us characterize this unusual place. A team led by Lorenz Roth (KTH Royal Institute of Technology, Space and Plasma Physics, Sweden) has produced evidence from Hubble that a persistent fraction of water vapor exists within Europa’s atmosphere, what little of it there is (this is an atmosphere with one-billionth the surface pressure of the atmosphere on our planet). Here we are talking not about plumes but processes including sublimation, where surface ices transition directly to vapor. It’s a finding that feeds into planning for those upcoming Jupiter missions and also informs us more deeply about how icy moons react to this high radiation environment.
The paper on this work, which appeared in Geophysical Research Letters (citation below), reinforces the contrast between plume activity and persistent vapor:
A key finding of this study is the consistency in the detection of the reduced oxygen emission ratio on the trailing hemisphere disk center and the overall stability of the ratio profiles in all images with similar geometry. In particular, the oxygen emission ratios in center and limb regions in the four trailing side visits, which were obtained in 1999, 2012, and 2015 and are all consistent within uncertainties. This means, they are diagnostic for persistent atmospheric properties, in stark contrast to the apparent transient nature of detected features that were interpreted to relate to H2O plumes (Paganini et al., 2019; Roth et al., 2014b).
The transition of ice to water vapor raises questions. Water vapor on Europa seems to be limited to the moon’s trailing hemisphere, the part of the moon that remains opposite to its direction of motion along its orbit. The cause of the asymmetry between leading and trailing hemispheres is intriguing given that we find the same phenomenon on Ganymede as well.
The paper also notes that the ratio of H2O relative to O2 is similar on the trailing hemispheres of both Europa and Ganymede. I should note before going further that the detection of water vapor on Ganymede came from Roth’s use of the technique he here uses on Europa, as discussed in Nature Astronomy earlier this year.
Image: Since Europa is tidally locked, Europa’s terrain maintains the same orientation relative to Jupiter (center image). The notional Jupiter centered plot viewed from Jupiter’s north pole in the rotating frame with the y-axis points to the Sun, z-axis normal to the Jupiter orbit plane, and x-axis completing the right rule. Also shown are maps of Europa showing the fourteen roughly equal-area regions delineated by the Europa Science Definition Team (SDT) and the surface illumination around Jupiter. Credit: Lam et al., from “A Robust Mission Tour for NASA’s Planned Europa Clipper Mission,” 7 January 2018 / AIAA Space Flight Mechanics Meeting 2018.
How fully do we understand the production of water vapor in the Europan environment? Consider this snip from the paper. It points not only to sublimation, but also to the phenomenon known as ‘sputtering,’ in which surface particles are bombarded by energetic plasma. Sputtering is considered an alternative source for at least some of the H2O found on Europa’s trailing hemisphere, especially given the modeling of plasma discussed in this paper:
The trailing hemisphere [of Europa] also coincides with the plasma upstream hemisphere, where most of the thermal plasma impinges on Europa’s surface according to modeling (Cassidy et al., 2013; Pospieszalska & Johnson, 1989). In addition, the sputtering yield (amount of neutrals ejected per incident charged particle flux) also increases with surface temperature (Famá et al., 2008), further favoring the trailing hemisphere independent of the distribution of the incident flux…. For both sources [Europa and Ganymede], however, modeled H2O abundances are often significantly lower than our derived value on the trailing side.
Roth’s datasets were produced in Hubble ultraviolet observations using the instrument’s Space Telescope Imaging Spectrograph (STIS). There is still much for the next two spacecraft to investigate in terms of water vapor at Europa as we examine the difference between leading and trailing day-side hemispheres, but the exact mechanism remains unresolved:
Putting the main results in a nutshell, oxygen emission ratios found in HST observations suggest a persistent H2O atmosphere above Europa’s trailing hemisphere, but the source of the water vapor cannot [be] unambiguously identified.
The paper is Roth, “A Stable H2O Atmosphere on Europa’s Trailing Hemisphere From HST Images,” Geophysical Research Letters Vol. 48, Issue 20 (28 October 2021). Published online 13 September 2021. Full text. The paper on Ganymede is Roth et al., “A sublimated water atmosphere on Ganymede detected from Hubble Space Telescope observations,” Nature Astronomy 5 (26 July 2021), 1043-1051 (abstract),