Centauri Dreams
Imagining and Planning Interstellar Exploration
On SETI, International Law, and Realpolitik
When Ken Wisian and John Traphagan (University of Texas at Austin) published “The Search for Extraterrestrial Intelligence: A Realpolitik Consideration” (Space Policy, May 2020), they tackled a problem I hadn’t considered. We’ve often discussed Messaging to Extraterrestrial Intelligence (METI) in these pages, pondering the pros and cons of broadcasting to the stars, but does SETI itself pose issues we are not considering? Moreover, could addressing these issues possibly point the way toward international protocols to address METI concerns?
Ken was kind enough to write a post summarizing the paper’s content, which appears below. A Major General in the USAF (now retired), Dr. Wisian is currently Associate Director of the Bureau of Economic Geology, Jackson School of Geosciences at UT. He is also affiliated with the Center for Space Research and the Center for Planetary Systems Habitability at the university. A geophysicist whose main research is in geothermal energy systems, modeling, and instrumentation & data analysis, he is developing a conference on First Contact Protocols to take place at UT-Austin this fall, a follow-on to his recent session at TVIW 2019 in Wichita.
by Ken Wisian
The debate over the wisdom of active Messaging to ExtraTerrestrial Intelligence (METI), has been vigorously engaged for some time. The progenitor of METI and the more accepted passive Search for ExtraTerrestrial Intelligence (SETI) has been largely assumed to be of little or no risk. The reasons for this assumption appear to be:
1. It does not alert ETI to our existence and therefore we should not face a threat of invasion or destruction from aliens (if it is even practical to do so over interstellar distances)
2. The minor Earthbound threat from extremists (of various possible persuasions) who might not like the possibility of ETI’s existence conflicting with their “world view” would be no more than an annoyance.
Implicit in the above is the underlying assumption that the only realistic threat that could arise from METI or SETI is that from a hostile ETI. In other words, the threat is external to humanity. What this too simple reasoning overlooks is human history, particularly international affairs, conflicts and war. [1]
SETI as used here is the passive searching for electromagnetic signals from ETI. This is currently primarily considered to be in the form of radio or laser signal, deliberately sent to somewhere. The search for non-signal evidence (e.g. inadvertent laser propulsion leakage, etc) is not considered here, though it could tie in to the discussion in a distant, indirect manner. Note: an ETI artifact (e.g. a spaceship) could have similar import as a SETI detection discussed here.
So what harm could SETI do? Looking at current and historical international affairs, particularly great-power competition, the answer is readily apparent – competition for dominance. In international affairs, nations compete, sometimes violently, for position in the world. This can be for economic or military advantage, more land or control over the seas, or merely survival. Witness the South China Sea today, stealing the secrets to nuclear weapons in the 1940’s and 1950’s, or the Byzantine Empire engaging in industrial espionage to steal the secret to making silk from China.
Now contemplate the potential technology advances that could come with a SETI detection. This could range from downloading the “Encyclopedia Galactica” to establishing two-way dialogue that includes sharing technology. With the potential for revolutionary science and technology leaps, whether directly destructive or not, to say the great & super powers would be “interested” is a monumental understatement.
Now think about the potential advantage (read as domination-enabling) that could accrue to one country if they were the only beneficiaries of said technology advances. “How?” you ask. “Anyone can point a radio telescope to the sky” Not so fast. Unless the signal comes from within our own galactic back yard, most likely within the Solar System, it will take a relatively large, complicated industrial complex (physical plant) with very specialized personnel to run, in order to send and receive interstellar communications. This is the key fact that could lead to SETI/METI being the next “Great Game” [2] of international affairs.
Large, specialized complexes & associated personnel are limited in number and therefore subject to physical control. For the sake of argument, let’s say there are a dozen such facilities in the world. This is far less than the number of critical infrastructure sites the US and coalition forces decided had to be taken out in Iraq in the Gulf Wars in order to reduce their military capability – a very manageable target set size. Now you begin to see the problem; superpowers, seeing a potentially world-dominating advantage in monopolizing the ETI communication channel, might also see as feasible preserving their access to ETI while at the same time denying the same to all other countries.
While “Great Games” like this can sometimes be kept in the purely political realm, that is relatively rare. Competition of this sort often includes violent espionage or proxy wars and occasionally can escalate to direct super-power competition. Thus, an actual SETI detection could lead rapidly to the first true information war – a superpower war (with all the existential risk that carries) fought purely for control of knowledge.
Monopolizing communication with ETI could be the trigger for the first information-driven world war.
Realization of the risk that even passive SETI presents should drive further actions:
1. The development of realistic and binding international treaties on the handling of first contact protocols – admittedly a long-shot. The existing post-detection protocol is a very small and completely non-binding first step in this direction.
2. Formation of deliberately international SETI facilities with uninterruptible data sharing to partner countries (and/or the UN). These would also have interleaved internal chains of command from member countries. While this would be somewhat inefficient, the offset to risk is well worth the effort. A phase 2 to this would be a similar arrangement for METI. This would implicitly force the adoption of international standards and provide a process for METI.
3. Further (renewed?) discussion and research into SETI risk. This should bring in many disciplines that are often not involved deeply in the SETI/METI fields, from government policy to history to psychology and many others. In staring so hard at the very obvious risk of METI, we missed the risk from SETI alone. We need to turn around and explore that road before proceeding further down the highway to METI.
Notes
[1] What I am getting at here is that unfortunately, this is a stereotypical “ivory tower” point of view, too idealistic and disconnected from messy, illogical human affairs. I say this reluctantly as a “card-carrying” (i.e. Ph.D.) member of the academic world.
[2] I am definitely abusing the term “Great Game” in multiple ways. The term refers to the 19th competition between the British and Russian empires for control of Central and South Asia. It was a deadly serious and deadly game in actuality, but the term captures well the feeling of being in a fierce competition.
PG: Let me insert here this excerpt from the paper highlighting the question of international law and the issues it raises:
The potentially enormous value to nation states of monopolizing communication with ETI, for the purpose of technological dominance in human affairs, is a significant factor in understanding possible scenarios after a confirmed contact event and bears further thinking by scholars and policy specialists. History shows that in circumstances that states perceive as vital they will likely act in their perceived best interest in accordance with principles of realpolitik thinking. In these circumstances, international law is frequently not a strong constraint on the behavior of governments and a protocol developed by scientists on how to handle first contact is unlikely to be of any concern at all. This risk needs to be acknowledged and understood by the larger international community to include scientists active in SETI in addition to political leaders and international relations scholars.
The paper is Wisian and Traphagan, “The Search for Extraterrestrial Intelligence: A Realpolitik Consideration,” Space Policy May, 2000 (abstract).
Astrobiological Science Fiction
I had never considered the possibilities for life on Uranus until I read Geoffrey Landis’ story “Into the Blue Abyss,” which first ran in Asimov’s in 1999, and later became a part of his collection Impact Parameter. Landis’ characters looked past the lifeless upper clouds of the 7th planet to go deep into warm, dark Uranian oceans, his protagonist a submersible pilot and physicist set to explore:
Below the clouds, way below, was an ocean of liquid water. Uranus was the true water-world of the solar system, a sphere of water surrounded by a thick atmosphere. Unlike the other planets, Uranus has a rocky core too small to measure, or perhaps no solid core at all, but only ocean, an ocean that has actually dissolved the silicate core of the planet away, a bottomless ocean of liquid water twenty thousand kilometers deep.
It would be churlish to give away what turns up in this ocean, so I’m going to direct you to the story itself, now available for free in a new anthology edited by Julie Novakova. Strangest of All is stuffed with good science fiction by the likes of David Nordley, Gregory Benford, Geoffrey Landis and Peter Watts. Each story is followed by an essay about the science involved and the implications for astrobiology.
Although I’ve been reading science fiction for decades, our discussions of it in these pages are generally sparse, related to specific scientific investigations. That’s because SF is a world in itself, and one I can cheerfully get lost in. I have to tread carefully to be able to stay on topic. But now and then something comes along that tracks precisely with the subject matter of Centauri Dreams. Strangest of All is such a title, downloadable as a PDF, .mobi or .epub file. I use both a Kindle Oasis and a Kobo Forma for varying reading tasks, and I’ve downloaded the .epub for use on the Forma, but .mobi works just fine for the Kindle.
What we have here is a collaborative volume, developed through the European Astrobiology Institute, containing work by authors we’ve talked about in these pages before because of their tight adherence to physics amidst literary skills beyond the norm. The quote introducing the volume still puts a bit of a chill down my spine:
“…this strangest of all things that ever came to earth from outer space must have fallen while I was sitting there, visible to me had I only looked up as it passed.”
That’s H. G. Wells from The War of the Worlds (1898), still a great read since the first time I tackled it as a teenager. What Novakova wants to do is use science fiction to make astrobiology more accessible, which is why the science commentaries following each story are useful. Strangest of All looks to be a classroom resource for those who teach, part of what the European Astrobiology Institute plans to be a continuing publishing program in outreach and education. We’ve talked before about science fiction’s role as a career starter for budding physicists and engineers.
Gerald Nordley’s “War, Ice, Egg, Universe” takes us to an ocean world with a frozen surface on top, a place like Europa, where the tale has implications for how we approach the exploration of Europa and Enceladus, and perhaps Ganymede as well. In fact, with oceans now defensibly proposed for objects ranging from Titan to Pluto, we are looking at potential venues for astrobiology that defy conventional descriptions of habitable zones as orbital arcs supporting liquid water on the surface. Referring to characters in the story, the EAI essay following Nordley’s tale comments:
Chyba (2000) and Chyba & Phillips (2001) tried to work even with these unknowns and calculate the amount of energy for putative Europan life, and to describe what ecosystems might potentially thrive there. According to these estimates, even a purely surface radiation-driven ecosystem might yield cell counts of over one cell per cubic centimeter; perhaps even a thousand cells per cubic centimeter in the uppermost ocean layers. Putative hydrothermal vents, of course, would create a different source of energy and chemicals for life (albeit one much more difficult to discover – in contrast, life near the icy shell might erupt into space in the geysers and be discovered by “simple” flybys). Any macrofauna, though, seems highly improbable given the energy estimates. Since Loudpincers was about eight times larger than the human Cyndi, by his own account, we’ll really have to look for his civilization elsewhere, perhaps on a larger moon of some warm Jupiter.
You see the method — the follow-up essay explores the ideas, but goes beyond that to provide references for continuing the investigation in the professional literature. This essay also speculates about Ganymede, where a liquid water ocean may be caught between two layers of ice. The ‘club sandwich’ model for Ganymede posits several layers of oceans and ice, which would make Ganymede perhaps the most bizarre ocean-bearing world in the Solar System, one with incredible pressures bearing down on high-pressure ice at the bottom (20 times the pressure of the bottom of the Mariana Trench on Earth).
Gregory Benford’s “Backscatter” is likewise ice-oriented, this time in the remote reaches of the Kuiper Belt and the Oort Cloud beyond. From the essay following the story:
Although it’s difficult to imagine a path from putative simple life in early water-soaked asteroids heated by the radioactive aluminum to vacflowers blooming on the surface of an iceteroid, life in the Kuiper Belt, the Oort Cloud and beyond cannot be ruled out – and we haven’t even touched the issue of rogue planets, which might have vastly varying surface conditions stemming from their size, mass, composition, history and any orbiting bodies.
The essay gives us an overview of the science that, as in Benford’s story, conceives of possible life sustained by sparse inner heat and the presence of ammonia and salts, perhaps with tidal heating thrown in for good measure. Cold brines would demand chemical and energy gradients to sustain life, a difficult thing to discover or measure unless cryovolcano activity coughs up evidence of the ocean below the ice. Some silicon compounds may support a form of life in ice as far out as the Oort, or perhaps in liquid nitrogen. Usefully, the essay on “Backscatter” runs through the scholarship.
The European Astrobology Institute has put together a project team around “Science Fiction as a Tool for Astrobiology Outreach and Education,” out of which has come this initial volume. The references in the science essays make Strangest of All valuable even for those of us who have encountered some of these stories before, for the fiction has lost none of its punch. Thomas Bucknell’s “A Jar of Goodwill” looks at new forms of plant metabolism on a world dominated by chlorine and a key question in addressing alien life: Will we know intelligence when we see it? Peter Watts’ “The Island” looks at Dyson spheres in an astrobiologically relevant form that Dyson himself never thought of (well, he probably did — I bet it’s somewhere in his notebooks).
All told, there are eight stories here along with the essays that explore their implications, an easy volume to recommend given the EAI’s willingness to make the volume available at no cost to readers. See what you think about the Fermi paradox as addressed in D. A. Xiaolin Spires “But Still I Smile.” Plenty of material here for discussion of the sort we routinely do here on Centauri Dreams!
The Odds on Intelligent Life in the Universe
If we could somehow rewind time to the earliest days of the Solar System and start over again, would life — and intelligence — reappear? It’s an experiment science fiction authors are able to try, but it defies real world science. Nonetheless, we can make approaches to the problem through the analysis of probabilities. In particular, we can use statistics, and the technique known as Bayesian inference, which weighs probabilities updated by new evidence.
This is a helpful exercise given that so often I hear people referring to the idea that intelligent life must be everywhere because the universe is so vast and there are so many opportunities for it to arise. But does life inevitably emerge on what we might consider habitable worlds?
What if this process of abiogenesis is rare? The question points to the fact that we have absolutely no idea what the likelihood is, and therefore assumptions about intelligent life based solely on numerical opportunity are nothing but speculations.
Enter Columbia University’s David Kipping, whose work has been featured often in these pages. Kipping tackles the question of the likelihood of life and the development of intelligence in a new paper in Proceedings of the National Academy of Sciences. He has a chronology to work with, one involving life’s earliest appearance as found in fossils, the emergence of humans, and the habitability constraints of our planet’s surface conditions, using it to draw inferences on how quickly life can arise, and how unusual intelligence may be.
The scenarios — derived as what in Bayesian terms are called ‘objective priors’ — are relatively straightforward, each of them worth examining in light of the fact that we have no observational evidence for life beyond the Earth. Our planet is our data point, and with all the disadvantages that produces, we can still draw inferences about life elsewhere from the constraints we can establish here. We know that abiogenesis is possible because we are here to write about it. But Kipping applies statistical methods based on Bayesian mathematics to consider the odds.
The first scenario (and the one I favor): Life is common, but rarely develops intelligence. I suspect we’re going to find evidence for simple life all over the Orion Arm as we extend our technologies outward, but little evidence for technologies. But other scenarios exist: Life is common and so is intelligence. And perhaps abiogenesis is rare. In that case, we may find life unusual but intelligence a common consequence when it does happen. Finally, life may be as rare as intelligence.
Image: Are we alone in the universe? A new study uses Bayesian statistics to weigh the likelihood of life and intelligence beyond our solar system. Credit: Shutterstock/Amanda Carden.
Which scenario to choose? Bayesian techniques involve testing a position against new evidence that can be applied to the question, which allows estimates to get better as they are refined. Bayesian mathematical formulae tackle how to model one scenario against another. And I found the result Kipping arrived at encouraging. Let me quote him on the matter:
“In Bayesian inference, prior probability distributions always need to be selected. But a key result here is that when one compares the rare-life versus common-life scenarios, the common-life scenario is always at least nine times more likely than the rare one.”
Drawing on our single data point — Earth — Kipping points out that we know life emerged quickly. We have to factor in the impact with the Mars-sized “Theia” some 4.51 billion years ago (leading to the formation of the Moon), but mineralogical evidence from zircons points to an atmosphere and liquid water present on Earth’s surface by roughly 4.4 billion years ago. The earliest evidence for life is found in 4.1 billion year old zircon deposits in the form of depleted carbon inclusions, a controversial datapoint, but undisputed evidence for life turns up in microfossils found in 3.465 billion year old rocks in western Australia.
We can come up, then, with the length of time for which Earth is expected to persist as habitable for intelligent beings, factoring in the growing luminosity of the Sun and the increased rate of weathering of silicate rocks on Earth and eventual depletion of carbon dioxide in the atmosphere. Kipping arrives at a habitable ‘window’ of 5.304 billion years. That’s from the beginning of life to its likely end, and it shows how significant is the question of how fast abiogenesis happens. If it takes too long, life would never emerge under the conditions most planets would face as their star continued to evolve. 900 million years from now, Earth will be a hostile place indeed.
But back to the key result — and I have to send the reader to the paper for the complex Bayesian mathematics involved — Kipping draws on a 2012 paper from Spiegel and Turner to refine the Bayesian formalism produced there for interpreting life’s early emergence on Earth. He considers it against the broader context of the habitable ‘window.’ From the Kipping paper:
The early emergence of life on Earth is naively interpreted as meaning that if we reran the tape, life would generally reappear quickly. But if the timescale for intelligence is long, then a quick start to life is simply a necessary byproduct of our existence—not evidence for a general rapid abiogenesis rate. Using our objective Bayesian framework, we show that the Bayes factor between a fast versus a slow abiogenesis scenario is at least a factor of 3—irrespective of the prior or the timescale for intelligence evolution. This factor is boosted to 9 when we replace the earliest microfossil evidence… with the more disputed 13C-depleted zircon deposits…
Thus the common life scenario gains odds, and markedly so. As for intelligence, Kipping’s analysis precludes the possibility that it emerges quickly (in less than billions of years), while the idea that intelligence is rare remains viable. Even so, he finds betting odds of only 3:2 that intelligence rarely emerges — this slight preference for rare intelligence is consistent with our lack of SETI results but leaves the question of searching for intelligence elsewhere wide open. Life is likely to emerge on other worlds, in other words, but our one data point — Earth — tells us that intelligence emerges only with time and difficulty. We have no outstanding way to choose here one way or another. Let me quote from the paper on this:
…our work supports an optimistic outlook for future searches for biosignatures…The slight preference for a rare intelligence scenario is consistent with a straightforward resolution to the Fermi paradox. However, our work says nothing about the lifetime of civilizations, and indeed the weight of evidence in favor of this scenario is sufficiently weak that searches for technosignatures should certainly be a component in observational campaigns seeking to resolve this grand mystery.
Life commonly found, the prevalence of intelligence still a mystery. Keep looking, but you now have some insight into where to place your chips in your next trip to the astrobiological casino.
If you’re interested in learning about Bayesian inference and the surprising successes of Bayesian analysis, I recommend Sharon McGrayne’s book The Theory That Would Not Die: How Bayes’ Rule Cracked the Enigma Code, Hunted Down Russian Submarines, and Emerged Triumphant from Two Centuries of Controversy (Yale University Press, 2011) as an excellent backgrounder.
The paper is Kipping, “An objective Bayesian analysis of life’s early start and our late arrival,” Proceedings of the National Academy of Sciences 18 May 2020 (full text). You can see Kipping’s lively video presentation on this work at https://www.youtube.com/watch?v=iLbbpRYRW5Y.
TRAPPIST-1: Orbital Alignment Among Rocky Worlds
You would think that the orbits of planets would align closely with the spin of their star, since they emerged from the same primordial disk. Many planets do just that, and in our own system, the orbits of the planets are aligned within 6 degrees of the Sun’s rotation. But the numerous cases of star-planet orbital misalignment around other stars cause us to question whether these systems formed out of alignment or were influenced by later perturbations. A massive companion in a wide orbit could do the trick, and other mechanisms to tilt the orbital or spin axes are discussed in the literature.
To examine the question, the Rossiter-McLaughlin effect comes into play. Discovered by studying binary stars, the effect is named after the two University of Michigan graduate students who figured it out back in the 1920s. They realized that as a star rotates, part of it seems to be coming toward the observer, creating a blueshift, while the other side seems to be moving away, producing a redshift. As a transiting planet blocks part of the background stellar disk, it creates an observable effect in the redshift that flags the direction of the planet’s rotation.
Thus we tease out yet further information about planets we cannot directly see. Now a team of astronomers using the Subaru Telescope has been able to deploy the Rossiter-McLaughlin effect to measure the obliquity, or spin-orbit angle, of three of the planets in the intriguing TRAPPIST-1 system. Here we have an M-dwarf orbited by seven small planets, evidently rocky, with three in or near the habitable zone as defined by the possibility of liquid water on the surface.
What we learn is that the three transiting planets the team observed on August 31, 2018 (two of them near the habitable zone) have an obliquity that is near zero. This marks the first time that stellar obliquity has been measured in a system around a very low-mass star like TRAPPIST-1. Led by Teruyuki Hirano (Tokyo Institute of Technology), the astronomers used the InfraRed Doppler (IRD) spectrograph, a new instrument on the Subaru Telescope, to make the measurement. Says Hirano:
“The data suggest alignment of the stellar spin with the planetary orbital axes, but the precision of the measurements was not good enough to completely rule out a small spin-orbit misalignment. Nonetheless, this is the first detection of the effect with Earth-like planets and more work will better characterize this remarkable exoplanet system.”
Image: Artist’s impression of the TRAPPIST-1 exoplanet system.(Credit: NAOJ).
Learning about spin-orbit misalignment (or the lack of same) is useful as we try to understand how planets around low mass stars evolve. The lack of larger worlds around this star, or the presence of a nearby star, means that the planetary orbits here are probably located close to where the planets first formed, so the orbits offer a window into the early days of the system. Rossiter-McLaughlin analyses have hitherto been restricted to planets of at least Neptune mass, so we’re pushing into new territory as we take advantage of IRD’s high spectral resolution.
Assuming the orbits of the TRAPPIST-1 system are coplanar, the authors offer their take on the system’s evolution:
Our result supports the idea that the known planets in the TRAPPIST-1 system achieved their compact configuration through convergent migration, and did not experience any substantial misaligning torques from processes such as planet-planet scatterings or long-term gravitational perturbations from a massive outer companion on an inclined orbit. It is unlikely that any primordial obliquity has been erased by tidal realignment between the star and these low-mass planets.
And on that assumption of a coplanar system, the authors add that they were unable to test it by measuring the mutual inclinations between the planets:
The mutual inclinations might be measurable in the future using repeated observations of Doppler transits to give a higher S/N. The mutual inclination between two planetary orbits might also be measured by observing the photometric effect of a planet-planet eclipse during a double transit event (Hirano et al. 2012). This would be another important clue to understand the architecture and dynamical history of the TRAPPIST-1 system.
No equivalent studies have been performed on a star this small, but the window is clearly opening on methods for studying the orbital architectures of such planetary systems.
The paper is Hirano et al., “Evidence for Spin-Orbit Alignment in the TRAPPIST-1 System,” Astrophysical Journal Letters Vol. 890, No. 2 (25 February 2020). Abstract.
More Evidence for Plumes on Europa
Were deviations in Jupiter’s magnetic field, recorded by Galileo’s magnetometer during a flyby of Europa in 2000, an indication of a cryovolcanic eruption? The data on this event have been evaluated by several independent groups in Europe and the US, an indication of how much interest such a plume would generate. If, like Enceladus, Europa occasionally blows off material from below the surface, we would have the possibility of collecting water from its ocean without having to drill through kilometers of ice.
Now a team of European Space Agency scientists led by Hans Huybrighs, working with colleagues at the Max Planck Institute for Solar System Research (MPS), has gone to work on the question, this time through the measurements made by Galileo’s Energetic Particles Detector (EPD), an instrument with roots both at MPS and the Applied Physics Laboratory of Johns Hopkins University (APL). EPD recorded significantly fewer fast-moving protons near Europa than expected during the same flyby. This adds weight to the conclusions of those finding evidence for a plume in the magnetometer data.
Jupiter’s magnetic field is twenty times stronger than Earth’s, extending far enough into space that Europa orbits within it. What Huybrighs and company set out to do was to simulate conditions during the flyby, modeling high-energy proton movements that correspond with what the EPD recorded.
The question raised by the EPD instrument is why these energetic protons were disappearing during the E26 flyby, an observation earlier considered to be caused by Europa itself obscuring the detector, making the measurement unreliable. But the paper makes the case that some of the proton depletion could only be explained by a plume of water vapor that disrupted Europa’s tenuous atmosphere and perturbed magnetic fields in the region. Indeed, the simulations are only successful under the assumption of a plume, whose effects are added into those caused by the atmosphere. Evidence for a plume noted by earlier researchers is thus strengthened by an analysis drawn from an independent dataset.
Image: Color image data from the Galileo mission recorded between 1995 and 1998 were used to create this depiction of Europa’s cracked and icy surface. The inset shows dark reddish, disrupted regions dubbed Thera and Thrace. Credit: Galileo Project, Univ. Arizona, JPL, NASA.
The authors’ conclusion also highlights how much we do not know about the environment at Europa, while pointing to a helpful tool for planning purposes on future missions:
Large uncertainties remain in the properties (density profile, 3D structure, temporal variability…) of Europa’s tenuous atmosphere and plumes (Plainaki et al., 2018). This study emphasizes that energetic ions are an important tool that can contribute to the detection and characterization of Europa’s tenuous atmosphere and plumes and probe its moon-magnetosphere interaction, independently of other methods. This is in particular relevant for the upcoming JUICE mission (Grasset et al., 2013), which has the instrumentation to detect both energetic ions and ENAs, using the Particle Environment Package (PEP)…
The ability of JUICE to detect energetic charged and neutral particles near Europa will help us spot future plumes. Note this ESA description of the Particle Environment Package:
A plasma package with sensors to characterise the plasma environment in the Jovian system.
PEP will measure density and fluxes of positive and negative ions, electrons, exospheric neutral gas, thermal plasma and energetic neutral atoms in the energy range from <0.001 eV to >1 MeV with full angular coverage. The composition of the moons’ exospheres will be measured with a resolving power of more than 1000.
For more on the science payload of this mission, with 10 instruments onboard, click here. JUICE launches in 2022 in a mission aimed at Europa, Callisto and eventual orbit at Ganymede.
The paper is Huybrighs et al., “An active plume eruption on Europa during Galileo flyby E26 as indicated by energetic proton depletions,” Geophysical Research Letters 12 May 2020 (abstract).
Ryugu: An Asteroid’s Interactions with the Sun
The near-Earth asteroid Ryugu is only about a kilometer wide, but it’s telling us a good deal about its own history and that of the Solar System itself thanks to the two touchdowns of Hayabusa2, in February and July of 2019. The geological changes so clear on Earth, the bombardments from objects creating craters here and elsewhere, all mark the evolution of large bodies, but the asteroids take us back to the system’s earliest days with little change. They’re bundles out of the deep freeze of time.
Now we wait for the sample return, currently on its way back to Earth, with arrival in December of this year. Aboard will be surface materials collected during both touchdowns, which will complement the data on the chemical and physical composition of the asteroid already gathered. A team led by Tomokatsu Morota (University of Tokyo) has been using Hayabusa2’s onboard ONC-W1 and ONC-T imaging instruments to analyze the dusty matter kicked up by the spacecraft’s engines during the two touchdowns.
Fine grains of dark-red colored minerals turned up in this analysis. The team believes these were produced by solar heating, indicating close passage by the Sun at some point in Ryugu’s past. This view is reinforced by the distribution of the dark red-matter. Spectral examination shows the material can be found at specific latitudes on the asteroid, corresponding to areas that would have received the most solar radiation in the asteroid’s past. Adds Morota:
“From previous studies we know Ryugu is carbon-rich and contains hydrated minerals and organic molecules. We wanted to know how solar heating chemically changed these molecules. Our theories about solar heating could change what we know of orbital dynamics of asteroids in the solar system. This in turn alters our knowledge of broader solar system history, including factors that may have affected the early Earth.”
Image: False color map of the surface of Ryugu with craters marked by circles. Credit: © 2020 Morota et al.
The distribution of surface materials suggests a history of disruption by impacts and what the paper describes as thermal fatigue and mass wasting. Let me quote from this section:
The thickness of the mixed layer of redder and bluer materials is estimated to be a few meters, derived from the minimum crater size (~10 m in diameter) that penetrates to the underlying blue materials. The presence of ejecta rays with a length of a few tens of meters that consist primarily of redder materials implies that the redder material layer originally had a minimum thickness of a few tens of centimeters. Solar heating is more likely than space weathering to be the source of the reddening of Ryugu’s surface, because space weathering typically affects only a thin layer of ~100 nm, whereas the diurnal and annual thermal skin depths (the depth at which temperature variations decay to 1/e of their value at the surface) are ≲10 cm and ~1.5 m, respectively.
It’s encouraging to hear from Morota’s group that their spectral studies and examination of Ryugu’s albedo indicate that both the dark-red material once heated by the Sun as well as blue unheated materials would have been collected in Hayabusa2’s two forays to the surface. Also on the agenda in coming months is a close look at the distribution of Ryugu’s craters and boulders. The surface craters hold information about the characteristics of the asteroid’s rocks and the history of small impacts. They also greatly complicated the search for a safe landing site.
What produced the differences between the two types of materials remains unclear:
Two distinct types of material are present on the surface with different colors: bluer material distributed at the equatorial ridge and in the polar regions and redder material in the mid-latitude regions. However, the cause of these spectral variations is not understood.
The paper is Morota et al., “Sample collection from asteroid (162173) Ryugu by Hayabusa2: Implications for surface evolution,” Science Vol. 368, Issue 6491 (8 May 2020), pp. 654-659 (abstract).
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