by Paul Gilster | Mar 9, 2012 | Culture and Society |
Physicist Al Jackson, who is the world’s greatest dinner companion, holds that title because amongst his scientific accomplishments, he is also a fountainhead of information about science fiction. No matter which writer you bring up, he knows something you never heard of that illuminates that writer’s work. So it was no surprise that when the subject of self-replicating probes came up in these pages, Al would take note in the comments of Philip K. Dick’s story “Autofac,” which ran in the November, 1955 issue of H. L. Gold’s Galaxy. A copy of that issue sits, I am happy to say, not six feet away from me on my shelves.
This is actually the first time I ever anticipated Al — like him, I had noticed “Autofac” as one of the earliest science fiction treatments of the ideas of self-replication and nanotechnology, and had written about it in my Centauri Dreams book back in 2004. If any readers know of earlier SF stories on the topic, please let me know in the comments. In the story, the two protagonists encounter a automated factory that is spewing pellets into the air. A close examination of one of the pellets reveals what we might today consider to be nanotech assemblers at work:
The pellet was a smashed container of machinery, tiny metallic elements too minute to be analyzed without a microscope…
The cylinder had split. At first he couldn’t tell if it had been the impact or deliberate internal mechanisms at work. From the rent, an ooze of metal bits was sliding. Squatting down, O’Neill examined them.
The bits were in motion. Microscopic machinery, smaller than ants, smaller than pins, working energetically, purposefully — constructing something that looked like a tiny rectangle of steel.
Further examination of the site shows that the pellets are building a replica of the original factory. O’Neill then has an interesting thought:
“Maybe some of them are geared for escape velocity. That would be neat — autofac networks throughout the whole universe.”
The Ethically Challenged Probe
Just how ‘neat’ it would be remains to be seen, of course, assuming nanotech assemblers can ever be built and self-replication made a reality (an assumption many disagree with). Carl Sagan, working with William Newman, published a paper called “The Solopsist Approach to Extraterrestrial Intelligence” (reference below) that made a reasonable argument: Self-reproducing probes are too viral-like to be built. Their existence would endanger their creators and any species they encountered as the probes spread unchecked through the galaxy.
Sagan and Newman were responding to Frank Tipler’s argument that extraterrestrial civilizations do not exist because we have no evidence of von Neumann probes (it was von Neumann whose studies of self-replication in the form of ‘universal assemblers’ would lead to the idea of self-replicating probes, though von Neumann himself didn’t apply his ideas to space technologies). Given the time frames involved, Sagan and Newman argued, probes like this should have become blindingly obvious as they would have begun to consume a huge fraction of the raw materials available in the galaxy.
The reason we have seen no von Neumann probes? Contra Tipler, Sagan and Newman say this points to extraterrestrials reaching the same conclusion we will once we have such technologies — we can’t build them because to do so would be suicidal. Freeman Dyson, writing as far back as 1964, referred to a “cancer of purposeless technological exploitation” that should be observable. All of this depends, of course, on the assumed replication rates of the probes involved (Sagan and Newman chose a much higher rate than Tipler to reach their conclusion). Robert Freitas picked up on the cancer theme in a 1983 paper, but saw a different outcome:
A well-ordered galaxy could imply an intelligence at work, but the absence of such order is insufficient evidence to rule out the existence of galactic ETI. The incomplete observational record at best can exclude only a certain limited class of extraterrestrial civilisation – the kind that employs rapacious, cancer-like, exploitative, highly-observable technology. Most other galactic-type civilisations are either invisible to or unrecognisable by current human observational methods, as are most if not all of expansionist interstellar cultures and Type I or Type II societies. Thus millions of extraterrestrial civilisations may exist and still not be directly observable by us.
Self-Replication Close to Home
We need to talk (though not today because of time constraints) about what mechanisms might be used to put the brakes on self-replicating interstellar probes. For now, though, I promised a look at what the human encounter with such a technology would look like. Fortunately, Gregory Benford has considered the matter and produced a little gem of a story called “Dark Sanctuary,” which originally ran in OMNI in May of 1979 but is now accessible online. Greg told me in a recent email that Ben Bova (then OMNI‘s editor) had asked him for a hard science fiction story, and von Neumann’s ideas on self-replicating probes were what came to mind.
Mix this in with some of Michael Papagiannis’ notions about looking for unusual infrared signatures in the asteroid belt and you wind up with quite a tale. In the story, humans are well established in the asteroid main belt, which is now home to the ‘Belters,’ people who mine the resources there to boost ice into orbits that will take it to the inner system to feed the O’Neill habitats that are increasingly being built there. One of the Belters is struck by a laser and assumes an attack is underway, probably some kind of piracy on the part of rogue elements nearby. A chase ensues, but what the Belter eventually sees does not seem to be human:
I sat there, not breathing. A long tube, turning. Towers jutted out at odd places — twisted columns, with curved faces and sudden jagged struts. A fretwork of blue. Patches of strange, moving yellow. A jumble of complex structures. It was a cylinder, decorated almost beyond recognition. I checked the ranging figures, shook my head, checked again. The inboard computer overlaid a perspective grid on the image, to convince me.
I sat very still. The cylinder was pointing nearly away from me, so radar had reported a cross section much smaller than its real size. The thing was seven goddamn kilometers long.
“Dark Sanctuary” is a short piece and I won’t spoil the pleasure of reading it for you by revealing its ending, but suffice it to say that the issues we’ve been raising about tight-beam laser communications between probes come into play here, as does the question of what beings (biological or robotic) might do after generations on a starship — would they want to re-adapt to living on a planetary surface even if they had the opportunity? Would we be able to detect their technology if they had a presumed vested interest in keeping their existence unknown? Something tells me we’re not through with the discussion of these issues, not by a long shot.
The papers I’ve been talking about today are Sagan and Newman, “The Solipsist Approach to Extraterrestrial Intelligence”, Quarterly Journal of the Royal Astronomical Society, Vol. 24, No. 113 (1983), and Tipler, “Extraterrestrial Beings Do Not Exist”, Quarterly Journal of the Royal Astronomical Society, Vol. 21, No. 267 (1981). I’ve just received a copy of Richard Burke-Ward’s paper “Possible Existence of Extraterrestrial Technology in the Solar System,” JBIS Vol. 53, No 1/2, Jan/Feb 2000 — this one may also have a bearing on the self-replicating probe question, and I’ll try to get to it in the near future.
by Paul Gilster | Mar 8, 2012 | Astrobiology and SETI |
Let’s imagine for a moment that John Mathews (Pennsylvania State University) is right in theorizing that space-faring civilizations will use self-reproducing probes to expand into the galaxy. We’ve been kicking the issues around most of this week, but the SETI question continues to hang in the background. For if there really are extraterrestrial civilizations in the nearby galaxy, how would we track down their signals if they used the kind of communications network Mathews envisions, one in which individual probes talked to each other through tight-beam laser communications designed only for reception by the network itself?
One problem is that the evidence we’re looking for would most likely come in the form of spread-spectrum signals, a fact Jim Benford pointed out in a comment to my original post on Mathews, and one that also pointed to recent work by David Messerschmitt (UC-Berkeley). The latter makes a compelling case for spread-spectrum methods as the basis for interstellar communication because such signals are more robust in handling radio-frequency interference (RFI) of technological origin. In SETI terms, RFI is a major issue because it mimics the interstellar signal we are hoping to find, and Messerschmitt assumes an advanced civilization, having experienced RFI issues in its own past, will use the best tools to minimize them.
Image: 3D map of all known stellar systems in the solar neighbourhood within a radius of 12.5 light-years. Can we build self-reproducing probes that could explore these systems over the course of millennia? If other civilizations did the same, could we detect them? Credit: ESO/R.-D.Scholz et al. (AIP).
Spread-spectrum techniques spread what would have been narrowband information signals over a wide band of frequencies. Think of the kind of ‘frequency hopping’ deployed in World War II, where a transmitter would work at multiple frequencies and the receiver would need to tune in to each of the transmitted frequencies. In addition to being resistant to interference, the method allows you to resist enemy jamming of your communications or to conceal communications in what would otherwise seem to be white noise. Actress Hedy Lamarr and composer George Antheil developed a frequency hopping technique that made radio-guided munitions much harder for enemy forces to jam, a spread-spectrum story entertainingly told in a recent book (Richard Rhodes’ Hedy’s Folly: The Life and Breakthrough Inventions of Hedy Lamarr, the Most Beautiful Woman in the World, 2011). Lamarr and Antheil’s system used 88 different carrier frequencies.
Messerschmitt isn’t talking about probes but about one civilization trying to reach another — he works from the perspective of the transmitter designer trying to reach a receiver about which little can be known. From the paper:
The transmitter can…explicitly design a transmit signal that minimizes the effect of RFI on the receiver’s discovery and detection probabilities in a robust way; that is, in a way that provides a constant immunity regardless of the nature of the RFI. It is shown that the resulting immunity increases with the product of time duration and bandwidth, and that the signal should resemble statistically a burst of white noise. Intuitively this is advantageous because RFI resembles such a signal with a likelihood that decreases exponentially with time-bandwidth product. Both a transmitter and receiver designer using this optimization criterion and employing the tools of elementary probability theory will arrive at this same conclusion. Although the context is different, variations on this principle inform the design of many modern widely deployed terrestrial digital wireless communication systems, so this has been extensively tested in practice and is likely to have a prominent place in the technology portfolio of an extraterrestrial civilization as well.
We’ve learned a great deal about dealing with RFI, especially given the rapid growth of wireless communications here on Earth, and we’ve also learned more about how radio signals propagate in the interstellar medium, thanks in large part, Messerschmitt notes, to advances in pulsar astronomy. Couple this with the ever-quickening pace of electronics and computer development and the search technologies in play are expanded so that we can accommodate the problem of natural noise sources as well as our own RFI. And we would have to assume that any extraterrestrial civilization would employ RFI mitigation techniques in its own communications.
In the case of accidental interception of a signal beamed between two intelligent probes, we can also look at the issue in terms of our detection algorithms. Early SETI work involved the so-called Fourier Transform to search for comparatively narrowband signals, and moved in the 1960s to the Fast Fourier Transform as the tool of choice. But as François Biraud noted as early as 1982, our terrestrial move from narrow-band to broader bandwidth communications presents a new challenge, breaking information into chunks carried by frequency-shifting carrier waves. Claudio Maccone has long argued that FFT methods are inappropriate for this kind of signal.
Enter the Karhunen-Loève Theorem (KLT) that Maccone continues to champion, a way of improving our sensitivity to an artificial signal that can dig tricky spread spectrum signals out of the background noise. Whether or not KLT algorithms are put to work with new installations like the Square Kilometer Array remains to be seen, but arguments like Messerschmitt’s point to the viability of spread-spectrum methods as a prime choice for interstellar communications. The point, then, is that spread-spectrum modulation is a factor we can deal with, allowing us to incorporate Messerschmitt’s ideas into our SETI toolkit even as we ponder the circumstances that might lead an extraterrestrial civilization to deploy a network of self-reproducing probes.
The Messerschmitt paper is “Interstellar Communication: The Case for Spread Spectrum” (preprint), while the Mathews paper is “From Here to ET,” Journal of the British Interplanetary Society 64 (2011), pp. 234-241. I have more to say about all this, and particularly about the ethical issues raised by self-reproducing technologies, but I’m running out of time this morning. The discussion continues tomorrow, when I’ll ponder how a civilization like ours might accidentally run into a network of extraterrestrial probes, and what that encounter might look like.
by Paul Gilster | Mar 7, 2012 | Astrobiology and SETI |
It was back in the 1980s when Robert Freitas came up with a self-reproducing probe concept based on the British Interplanetary Society’s Project Daedalus, but extending it in completely new directions. Like Daedalus, Freitas’ REPRO probe would be fusion-based and would mine the atmosphere of Jupiter to acquire the necessary helium-3. Unlike Daedalus, REPRO would devote half its payload to what Freitas called its SEED package, which would use resources in a target solar system to produce a new REPRO probe every 500 years. Probes like this could spread through the galaxy over the course of a million years without further human intervention.
A Vision of Technological Propagation
I leave to wiser heads than mine the question of whether self-reproducing technologies like these will ever be feasible, or when. My thought is that I wouldn’t want to rule out the possibility for cultures significantly more advanced than ours, but the question is a lively one, as is the issue of whether artificial intelligence will ever take us to a ‘Singularity,’ beyond which robotic generations move in ways we cannot fathom. John Mathews discusses self-reproducing probes, as we saw yesterday, as natural extensions of our early planetary explorer craft, eventually being modified to carry out inspections of the vast array of objects in the Kuiper Belt and Oort Cloud.
Image: The Kuiper Belt and much larger Oort Cloud offer billions of targets for self-reproducing space probes, if we can figure out how to build them. Credit: Donald Yeoman/NASA/JPL.
Here is Mathews’ vision, operating under a System-of-Systems paradigm in which the many separate systems needed to make a self-reproducing probe (he calls them Explorer roBots, or EBs) are examined separately, and conceding that all of them must be functional for the EB to emerge (the approach thus includes not only the technological questions but also the ethical and economic issues involved in the production of such probes). Witness the probes in operation:
Once the 1st generation proto-EBs arrive in, say, the asteroid belt, they would evolve and manufacture the 2nd generation per the outline above. The 2nd generation proto-EBs would be launched outward toward appropriate asteroids and the Kuiper/Oort objects as determined by observations of the parent proto-EB and, as communication delays are relatively small, human/ET operators. A few generations of the proto-EBs would likely suffice to evolve and produce EBs capable of traversing interstellar distances either in a single “leap” or, more likely, by jumping from Oort Cloud to Oort Cloud. Again, it is clear that early generation proto-EBs would trail a communications network.
The data network — what Mathews calls the Explorer Network, or ENET — has clear SETI implications if you buy the idea that self-reproducing probes are not only possible (someday) but also likely to be how intelligent cultures explore the galaxy. Here the assumption is that extraterrestrials are likely, as we have been thus far, to be limited to speeds far below the speed of light, and in fact Mathews works with 0.01c as a baseline. If EBs are an economical and efficient way to exploring huge volumes of space, then the possibility of picking up the transmissions linking them into a network cannot be ruled out. Mathews envisages them building a library of their activities and knowledge gained that will eventually propagate back to the parent species.
A Celestial Network’s Detectability
Here we can give a nod to the existing work on extending Internet protocols into space, the intent being to connect remote space probes to each other, making the download of mission data far more efficient. Rather than pointing an enormous dish at each spacecraft in turn, we point at a spacecraft serving as the communications hub, downloading information from, say, landers and atmospheric explorers and orbiters in turn. Perhaps this early interplanetary networking is a precursor to the kind of networks that might one day communicate the findings of interstellar probes. Mathews notes the MESSENGER mission to Mercury, which has used a near-infrared laser ranging system to link the vehicle with the NASA Goddard Astronomical Observatory at a distance of 24 million kilometers (0.16 AU) as an example of what is feasible today.
Tomorrow’s ENET would be, in the author’s view, a tight-beam communications network. In SETI terms, such networks would be not beacons but highly directed communications, greatly compromising but not eliminating our ability to detect them. Self-reproducing probes propagating from star to star — conceivably with many stops along the way — would in his estimation use mm-wave or far-IR lasers, communicating through highly efficient and highly directive beams. From the paper:
The solar system and local galaxy is relatively unobscured at these wavelengths and so these signaling lasers would readily enable communications links spanning up to a few hundred AUs each. It is also clear that successive generations of EBs would establish a communications network forming multiple paths to each other and to “home” thus serving to update all generations on time scales small compared with physical transit times. These various generations of EBs would identify the locations of “nearby” EBs, establish links with them, and thus complete the communications net in all directions.
Working the math, Mathews finds that current technologies for laser communications yield reasonable photon counts out to the near edge of the Oort Cloud, given optimistic assumptions about receiver noise levels. It is enough, in any case, to indicate that future technologies will allow networked probes to communicate from one probe to another over time, eventually returning data to the source civilization. An extraterrestrial Explorer Network like this one thus becomes a SETI target, though not one whose wavelengths have received much SETI attention.
On Ethics and Possibilities
In any case, there is no reason why an exploring extraterrestrial culture would necessarily want its activities to be noticed. Rather than eavesdropping on leakage from an extremely efficient communications network, a more likely SETI outcome would involve human expansion through gradually more autonomous probes, with the chances of finding evidence for ET expanding as our own sphere of exploration widens. Getting a positive SETI result might thus involve centuries if not millennia.
It may also be the case that reproducing probes are severely restricted out of ethical concerns. Runaway propagation poses many dilemmas, so that few if any cultures build them. A null result might also indicate that their development is more difficult and expensive than anticipated, particularly in terms of finding needed energy sources.
How would we track narrow-beam communications systems in the mm-wave/IR region? As some commenters on yesterday’s post have already noted, they would likely be spread-spectrum, but there are tools for handling such signals. More on this, and on the ethics issues as well, tomorrow. Here again is the citation for the Mathews paper: “From Here to ET,” Journal of the British Interplanetary Society 64 (2011), pp. 234-241. For more on Robert Freitas’ REPRO ideas, see his paper “A Self-Reproducing Interstellar Probe,” JBIS 33 (July 1980), pp. 251-264.
by Paul Gilster | Mar 6, 2012 | Missions |
Imagine a future in which we manage to reach average speeds in the area of one percent of the speed of light. That would make for a 437-year one-way trip to the Alpha Centauri system, too long for anything manned other than generation ships or missions with crews in some kind of suspended animation. Although 0.01c is well beyond our current capabilities, there is absolutely nothing in the laws of physics that would prevent our attaining such velocities, assuming we can find the energy source to drive the vehicle. And because it seems an achievable goal, it’s worth looking at what we might do with space probes and advanced robotics that can move at such velocities.
How, in other words, would a spacefaring culture use artificial intelligence and fast probes to move beyond its parent solar system? John Mathews ( Pennyslvania State) looks at the issue in a new paper, with a nod to the work of John von Neumann on self-reproducing automata and the subsequent thoughts of Ronald Bracewell and Frank Tipler on how, even at comparatively slow (in interstellar terms) speeds like 0.01c, such a culture could spread through the galaxy. There are implications for our own future here, but also for SETI, for Mathews uses the projected human future as a model for what any civilization might accomplish. Assume the same model of incremental expansion through robotics and you may uncover the right wavelengths to use in observing an extraterrestrial civilization, if indeed one exists.
Image: The spiral galaxy M101. If civilizations choose to build them, self-reproducing robotic probes could theoretically expand across the entire disk within a scant million years, at speeds well below the speed of light. Credit: STScI.
But let’s leave SETI aside for a moment and ponder robotics and intelligent probes. Building on recent work by James and Gregory Benford on interstellar beacons, Mathews likewise wants to figure out the most efficient and cost-effective way of exploring nearby space, one that assumes exploration like this will proceed using only a small fraction of the Gross Planetary Product (GPP) and (much later) the Gross Solar System Product (GSSP). The solution, given constraints of speed and efficiency, is the autonomous, self-replicating robot, early versions of which we have already sent into the cosmos in the form of probes like our Pioneers and Voyagers.
The role of self-replicating probes — Mathews calls them Explorer roBots, or EBs — is to propagate throughout the Solar System and, eventually, the nearby galaxy, finding the resources needed to produce the next generation of automata and looking for life. Close to home, we can imagine such robotic probes moving at far less than 0.01c as they set out to do something targeted manned missions can’t accomplish, reaching and cataloging vast numbers of outer system objects. Consider that the main asteroid belt is currently known to house over 500,000 objects, while the Kuiper Belt is currently thought to have more than 70,000 100-kilometer and larger objects. Move into the Oort and we’re talking about billions of potential targets.
A wave of self-reproducing probes (with necessary constraints to avoid uninhibited growth) could range freely through these vast domains. Mathews projects forward not so many years to find that ongoing trends in computerization will allow for the gradual development of the self-sufficient robots we need, capable of using the resources they encounter on their journeys and communicating with a growing network in which observations are pooled. Thus the growth toward a truly interstellar capability is organic, moving inexorably outward through robotics of ever-increasing proficiency, a wave of exploration that does not need continual monitoring from humans who are, in any case, gradually going to be far enough away to make two-way communications less and less useful.
[Addendum: By using ‘organic’ above, I really meant to say something like ‘the growth toward a truly interstellar capability mimics an organic system…’ Sorry about the confusing use of the word!]
From the paper:
The number of objects comprising our solar system requires autonomous robotic spacecraft to visit more than just a few. As the cost of launching sufficient spacecraft from earth would quickly become prohibitive, it would seem that these spacecraft would necessarily be or become self-replicating systems. Even so, the number of robots needed to thoroughly explore the solar system on even centuries timescales is immense. These robots would form the prototype EBs (proto-EB) and would ultimately explore out to the far edge of the Oort Cloud.
The robotic network is an adjunct to manned missions within the Solar System itself, but includes the capability of data return from regions that humans would find out of reach:
These proto-EBs would also likely form a system whereby needed rare resources are mined, processed, and transported inward while also providing the basis for our outward expansion to the local galaxy. EB pioneering activities would also likely be used to establish bases for actual human habitation of the solar system should economics permit. Additionally, this outward expansion would necessarily include an efficient and cost effective, narrow-beam communications system. It is suggested that any spacefaring species would face these or very similar issues and take this or a similar path.
Note that last suggestion. It’s gigantic in its consequences, but Mathews is trying to build upon what we know — civilizations with technologies that allow them to operate outside this paradigm are an illustration of why SETI must necessarily cast a wide net. Even so, EB networks offer an area of SETI spectrum that hasn’t been well investigated, as we’ll see in tomorrow’s post.
To analyze how a robotic network like what the paper calls the Explorer Network (ENET) might be built and what it would need to move from the early proxy explorers like Voyager to later self-reproducing prototypes and then a fully functional, expansive network, Mathews explores the various systems that would be necessary and relates these to what an extraterrestrial civilization might do in a similar exploratory wave. In doing this, he reflects thinking like Frank Tipler’s, the latter having argued that colonizing the entire galactic disk using these methods would involve a matter of no more than a million years. Note that both Mathews and Tipler see the possibility of intelligence spreading throughout the galaxy with technologies that work well within the speed of light limitation. Extraterrestrial civilizations need not be hyper-advanced. “In fact,” says Mathews, “it seems possible that we have elevated ET far beyond what seems reasonable.”
This is an absorbing paper laced with ingenious ideas about how a robotic network among the stars would work, including thoughts on propulsion and deceleration, the survival of electronics in long-haul missions, and the ethics and evolution of our future robot explorers. Tomorrow I want to continue with Mathews’s concepts to address some of these questions and their implications for the Fermi paradox and SETI. For now, the paper is Mathews, “From Here to ET,” Journal of the British Interplanetary Society 64 (2011), pp. 234-241.
by Paul Gilster | Mar 5, 2012 | Uncategorized |
We recently looked at biosignatures as part of a discussion about using polarized light to examine exoplanet atmospheres. As if on cue, we now get a reminder of how carefully the biosignature hunt must proceed. It’s not enough, for example, to find one or two interesting gases in a distant atmosphere, for natural processes can account for potential biomarkers, which is why we need to find gases like ozone and methane, oxygen and carbon dioxide existing simultaneously. The most recent discovery from Cassini data puts an exclamation point on the matter with the discovery of molecular oxygen ions in the thin atmosphere of Dione, one of Saturn’s 62 moons.
With a radius of no more than 560 kilometers, Dione is evidently composed of a layer of water ice surrounding a rocky core. We are not, obviously, talking about a thick atmosphere around a world this small. Cassini and its CAPS instrument (Cassini Plasma Spectrometer) closed to within 503 kilometers of the surface in April of 2010, finding one oxygen ion for every 11 cubic centimeters of space in a gaseous envelope thin enough to be called an ‘exosphere.’
Image: The ragged surface of Saturn’s moon Dione. Credit: NASA/JPL-Caltech.
Robert Tokar (Los Alamos National Laboratory), lead author of the paper on this work, notes that the concentration of oxygen in Dione’s atmosphere is the equivalent of what we would find at an altitude of about 480 kilometers in Earth’s atmosphere. Adds Tokar:
“We now know that Dione, in addition to Saturn’s rings and the moon Rhea, is a source of oxygen molecules. This shows that molecular oxygen is actually common in the Saturn system and reinforces that it can come from a process that doesn’t involve life.”
The process is thought to work like this: During Dione’s 2.7-day orbit of Saturn, the moon is struck by charged particles produced by the planet’s inner magnetosphere, causing molecular oxygen ions to be displaced into the tenuous atmosphere, after which they are again stripped by the planet’s magnetosphere. The process of molecular oxygen displacement is called ‘sputtering,’ and while the paper notes some uncertainties in its calculations — surface temperature variations on Dione can be significant — it emphasizes the core finding:
…what is not uncertain is we report here the first in situ detection of a component of Dione’s thin sputter produced atmosphere by collecting the pick-up ions. Since the pick-up ion density is directly related to the atmospheric densities, we have also obtained a rough estimate of the atmospheric O2 density. This is consistent with the earlier observations of oxygen products trapped in the surface ice and places Dione in a category with Europa, Ganymede, Rhea and Saturn’s main rings all of which have oxygen atmospheres.
The exosphere around Rhea was detected in 2010 and is similar to Dione’s, with an oxygen density at the surface of some 5 trillion times less than what would be found at the Earth’s surface. Data from Cassini’s ion and neutral mass spectrometer from a later flyby are also under investigation. The Dione finding is not completely without interest for astrobiologists, given that molecular oxygen might be able to combine with carbon in the sub-surface lakes of gas giant moons like Europa.
The paper is Tokar et al., “Detection of exospheric O2+ at Saturn’s moon Dione,” Geophysical Research Letters Vol. 39 (2012), L03105 (abstract).
