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The Snows of Enceladus

Once again it’s time to catch up with Enceladus, the little moon that has such a huge impact on the planetary system it moves through. We’re learning, for example, how much water vapor is erupting from the features in the moon’s south polar region known as the ‘tiger stripes.’ Cassini measurements (using the Ultraviolet Imaging Spectrograph aboard the spacecraft) had pegged the rate of discharge at 200 kilograms of water vapor every second. New measurements from ESA’s Herschel space observatory match up closely to these findings. Saturn’s E-ring, formed from plume particles, would dissipate in a few hundred years without discharges like these.

You may recall that back in June, Herschel results were announced that showed a huge torus of water vapor circling Saturn itself, one that appeared to be the source of water found in Saturn’s upper atmosphere. More than 600,000 kilometers across and 60,000 kilometers thick, the enormous cloud was produced by Enceladus and picked up by Herschel’s infrared detectors. Water had previously been detected by both Voyager and Hubble in Saturn’s upper clouds, and also spotted by ESA’s Infrared Space Observatory in 1997. These earlier detections had raised the question of how water molecules were entering Saturn’s atmosphere from space.

Studying Herschel’s cloud of water vapor and running computer models that incorporated what we know of Enceladus’ plumes helped researchers put the pieces of the puzzle together. It turns out that most of the water in the torus is lost to space, but enough falls through the rings to enter the planet’s atmosphere to account for the amount of water observed there. Tim Cassidy (University of Colorado, Boulder) is one of those who worked on the data:

“What’s amazing is that the model, which is one iteration in a long line of cloud models, was built without knowledge of the observation. Those of us in this small modeling community were using data from Cassini, Voyager and the Hubble telescope, along with established physics. We weren’t expecting such detailed ‘images’ of the torus, and the match between model and data was a wonderful surprise.”

More in this NASA news release. Meanwhile, we have the announcement at the EPSC-DPS Joint Meeting 2011 in Nantes that ice particles from the plumes also fall back onto the surface of Enceladus, building up areas blanketed by super-fine snow whose present state tells us that the plumes have been active for tens of millions of years or more. We already knew that modeling particle trajectories from the plumes produced accumulation on Enceladus itself — this work was done by Sasha Kempf (Max Planck Institute) and Juergen Schmidt (University of Potsdam) in 2010. The new work, by Paul Schenk (Lunar and Planetary Institute, Houston) relies on a painstaking examination of high resolution images in areas of suspected accumulation.

The result: Smooth terrain with topographic undulations suggestive of buried fractures and craters, and changes in slope along the rims of deeper fractures, all consistent with material coating the top of solid crustal ices. The researchers have been able to apply models of deposition showing that the rate of accumulation of these ice particles is less than a thousandth of a millimeter per year. Because the average layer is 100 meters deep in the area studied, the team calculates tens of millions of years would be needed to accumulate the entire amount.

Image: Perspective view of “snow” covered slopes of Enceladus. This heavily fractured terrain lies north of the edge of the active south polar region. The largest of these fractures in the foreground is roughly 1 kilometre wide and 300 meters deep (0.6 miles wide and 1000 feet deep). The fainter dimples on the plateaus are actually older craters and fractures that appear to be covered by thick accumulations of fine particulates, sub-millimetre sized ice grains falling to the surface from the giant plumes to the south. At 12 meters per pixel (~40 feet) this view is one of the highest resolution images Cassini has obtained of Enceladus. Perspective rendering of the surface is derived from colour imaging a stereo topography of Cassini images, produced by D. Paul Schenk (Lunar and Planetary Institute, Houston). See also this ESA news release.

And so we can now talk about the ‘snows of Enceladus.’ They’re important, for their steady accumulation tells us that the heat source that drives the plumes and maintains any liquid water found under the ice crust must have been there for a long time. Schenk says the particles found here are roughly a micron or two across, making them finer than talcum powder that “would make for the finest powder a skier could hope for.” A pleasing thought, but Centauri Dreams assumes the scientist is even happier with the prospect that Enceladus’ snows will help us understand the internal heating mechanism that drives the plumes. What we need now is more high resolution imagery of a world few would have suspected would turn out to be so compelling.

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Comments on this entry are closed.

  • Alex Tolley October 11, 2011, 14:32

    “…heat source that drives the plumes and maintains any liquid water found under the ice crust must have been there for a long time. ”

    Long enough to be the equivalent of a deep ocean trench and thus a possible abode of life?

    More importantly for astrobiology, the plumes must occasionally loft organic materials and even life onto the surface. Thus a surface probe looking for molecules with consistent chirality is a possibility, without having to drill beneath the surface.

  • Michael October 11, 2011, 14:47

    Could it be that the heat source is just friction caused by the ice shell rocking back and forth over a rocky core driven by Saturns gravity tugging on the ice shell more easily than the heavy rocky core.

  • andy October 11, 2011, 15:29

    I’ve heard that the bright reflective ices of Saturn’s rings are evidence for a young age for the ring system. Would the water output from Enceladus also help to replenish the icy surfaces of the ring particles, enabling a primordial ring system to still be highly reflective?

  • Connie McManus October 11, 2011, 18:18

    All I could think of when I saw the image was what would it be like to ski that!?

  • Mephane October 12, 2011, 3:23

    @Michael:

    That’s what I’ve also heard so far as a possible heat source, but as far as I know no definitive answer has been found yet.

  • Martin J Sallberg October 12, 2011, 4:48

    I have read about another study which shows almost no sodium in Enceladean geyser ice crystals, and that it means the geysers are not connected to a liquid sea at all, but merely “spits” small particles of cracked ice. Or has that stud been debunked? Anyway, in such a very exotic environment as a under-ice sea it is possible that evolution may have shaped an ecosystem where different organisms have different chiralities. Some biochemical characteristics are shared by vastly different lineages in the same type of environment, such as mostly omega 3 fat in almost all sea animals (cetaceans have fat composition more like their fish “fellow convergents” than like their mammalian cousins). This implies that biochemical evolution is very shaped by its physical environment, so you cannot expect all biospheres to be “monochiralic”, especially not if the biospheres in question are very differnt from Earth.

  • Alex Tolley October 12, 2011, 9:22

    At Martin S,

    “…so you cannot expect all biospheres to be “monochiralic”, especially not if the biospheres in question are very differnt from Earth.”

    That, in itself, would be an interesting finding. If we found a “shadow biosphere” on earth with different chiralities on key molecules, I would be surprised and delighted.

    We’ll see whether Enceladus has any signs of biomolecules or other biosignatures sometime in the future. My hope is that the plumes prove to be a feature that allows us to find out sooner, with low costs.

  • Tobias Holbrook October 12, 2011, 13:17

    “All I could think of when I saw the image was what would it be like to ski that!?”
    Pretty rubbish, I’d imagine – but maybe Osmium skis, to boost your wieght massively, would do the trick… :)

    Enceladus is a very interesting moon… if it has a long term ocean (and it seems to), we could possibly engineer a biosphere for it, if it doesn’t have one already. Maybe a creature using the magnetic field to provide electricity for synthesis is a possibility?

  • Eniac October 13, 2011, 23:13

    …, so you cannot expect all biospheres to be “monochiralic”, especially not if the biospheres in question are very differnt from Earth.

    I think you can very much expect just that, for the same reasons that there is only one chirality on Earth. Not to mention the many other, less fundamental, aspects of biochemistry that are arbitrary, but universally shared.

  • Martin J Sallberg October 15, 2011, 1:21

    @Eniac: Do you know how many aspects of biology that has traditionally been considered arbitrary has later been proved to be non-arbitrary and functional? Having no more than two sexes was considered arbitrary… until it turned ot that combining more sets of genomes would be tricky and very awkward for evolution. Our chirality was considered arbitrary… until it turned out that it was more resilient to cosmic radiation than the opposite chirality (that is why I do not expect perfectly shielded environments such as sub-ice oceans to be monochiralic). That midsize to large animals have four legs while tiny animals have six or eight was considered arbitrary… until it turned out that many legs are awkward on one hand but good for grip on the other. Indeed, without the cosmic radiation pressure to keep monochiralic, it is likely that varying chirality would be favoured by evolution as a means of keeping oneself inedible to predators (that also means I recommend advanced chemical testing devices for any future fishermen on such worlds). It has also been discovered that prokaryotes easily swap genomes over vast species differences (Thermotoga maritima is half bacterium and half archaea), the “tree of life” actually looks more like a chicken-wire than like a tree, some cyanobacteria have an extra amino acid that acts as a poisonous protection from being eaten, the history of life on Earth is so incredibly complex and non-parsimonous that I expect ALL “universally” (or rather globally) shared traits to be functional and non-arbitrary.

  • Eniac October 15, 2011, 14:38

    Martin, your suggestion that there is a dependence on chirality in resistance to cosmic radiation is very interesting, not to say unbelievable. I was not able to dig up any research on this. Could you please let us know who found this out and how?

  • Martin J Sallberg October 16, 2011, 2:21

    @Eniac: Have you tried googling exact phrases with different formulations? By the way, what is unbelievable with it? Differences in the three-dimensional structure of molecules often make their resilience to various forms of stress different. And why are you only criticizing select parts of my comments and never explicitly agree with any parts of them?

  • Eniac October 16, 2011, 16:05

    I was not asking for advice on how to google, but for a reference. Given your evasive response, and lack of finding anything relevant using my own style of googling, I will now conclude that there isn’t one.

    Differences in the three-dimensional structure of molecules often make their resilience to various forms of stress different.

    Yes, but chirality is handedness, which means there is not supposed to be a physically relevant difference. It would be very surprising if the resilience of a chemical compound in the face of radiation were dependent on handedness, and I have never seen this suggested before.

    And why are you only criticizing select parts of my comments and never explicitly agree with any parts of them?

    I did not mean to criticize, just find out more about this particular idea, which I would find intriguing if there was any substance behind it.

    I do not explicitly agree with parts of your comments because I cannot find many I could agree with, and I do not criticize them all because there is not enough time.

  • Martin J Sallberg October 17, 2011, 6:52

    I know I have read an article about it, I just cannot find it. Also, think about the fact that many organisms evolve poisons to make themselves unedible. Therefore it is just logical to think that if there was no very good reason to keep basic biochemistry in one particular way, different organisms would start evolving deep biochemical incompatibilities to protect themselves from being eaten, so it is your belief in “aspects of biochemistry that are arbitrary, but universally shared” that is the extraordinary claim. Apart from the already mentioned extra amino acid in some cyanobacteria (known to cause Alzheimer and some forms of color-blindness, validating the claim that it evolved as a poison, protection from being eaten), there is also the fact that anaerobic Lorificerans have evolved gradually from aerobic ancestors, falsifying Stephen Jay Gould´s belief in an absolute divide between diversity and “disparity”.

  • Eniac October 18, 2011, 12:08

    I also found no evidence of an “extra” amino acid in cyanobacteria, in the sense that it is genetically coded. As a synthesized chemical, of course there are many non-standard amino acids synthesized by all sorts of organisms, as a tiny part of the very diverse universe of biochemicals.

  • Martin J Sallberg October 19, 2011, 2:38

    @Eniac: Of course you could not find Internet evidence of that, since those cyanobacteria genomes are not mapped yet. But the fact that the extra amino acid is specific to some strains of cyanobacteria suggests that it is indeed genetically coded. And you completely ignored the evolutionary argument against shared arbitrary biochemistry, based on protection against being eaten and the evidence contrary to claims of a divide between diversity and “disparity”. Distinguishing between diversity and “disparity” is like the creationist distinction between “microevolution” and “macroevolution”.

  • ljk December 22, 2011, 10:19

    More radar images of icy moons from Cassini: Iapetus, Enceladus, and Rhea

    The Planetary Society Blog

    Dec. 21, 2011 | 11:10 PST | 19:10 UTC

    When I posted about the really cool Cassini SAR images of Enceladus a few weeks ago, I initially wrote that this was the first-ever SAR image of an icy moon other than Titan. Several people (some readers and two members of the Cassini science team!) corrected that statement: Cassini has performed SAR imaging of other icy moons (including Enceladus) before, though none had the resolution of the November 6, 2011 Enceladus swath.

    If you’re not sure what SAR imaging is, it stands for Synthetic Aperture Radar, and it is a method of bouncing radio waves off a planetary surface and analyzing the timing and frequencies of their echoes to produce images. It’s a method that is most often used for worlds with cloudy or hazy atmospheres, like Venus, Earth, and Titan, because the clouds are transparent to radio waves. But Cassini has been employing the imaging method to good effect on other worlds, and I’ll show you those neat images in this post.

    Bob Pappalardo sent me a paper published last year in Icarus that covers all kinds of radio data that Cassini has acquired on Saturn’s icy moons. It’s a special paper, he said, because it is the last paper published by radar scientist Steve Ostro; in fact, the paper was submitted posthumously. Bob said he considers the Enceladus swath the “Ostro memorial observation.” Ostro is keenly missed by the radar science and Cassini communities.

    I’m afraid I find the paper a bit difficult to wrap my head around, but here’s what I can summarize from it. For all the larger moons except Hyperion, there is a very strong correlation between the brightness of the surface as seen by the RADAR instrument and the brightness of the surface as seen by the cameras and spectrometers. By contrast, when Arecibo’s giant radar dish observes these moons, there’s no such correlation.

    These and other measurements and some “if-this-then-that” statements led Ostro and his coauthors to conclude that Iapetus’ radar properties can be explained by there being ammonia present in its surface, with higher concentrations as you go downward. The exception of Hyperion is interesting — in radar it’s much brighter than you would expect. They suggest that Hyperion’s dark coloration is only at the uppermost surface, beneath which it’s nearly pure ice.

    With some help from Jason Perry, here is the SAR image of Iapetus, and then an animation that blinks back and forth between the SAR image and a composite map of photos. I’m not going try to explain the comparison between the two, because I’m not confident of the science — instead, I invite you to watch the animation blink back and forth, and note what’s similar and what’s different. Enjoy!

    Full article and images here:

    http://www.planetary.org/blog/article/00003310/