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The Planetary Society BlogBy Emily LakdawallaPossible dry plumes on EnceladusDec. 21, 2006 | 09:15 PST | 17:15 UTC
Last week there was a paper published in Science magazine proposing a novel mechanism to explain the spectacular fountains erupting from the south pole of Enceladus: "A Clathrate Reservoir Hypothesis for Enceladus' South Polar Plume." First, a review. The plumes were discovered last year by nearly all of Cassini's instruments acting in concert. First, the south polar atmosphere was detected by the fields and particles instruments as well as the UVIS spectrometer; then the south pole was found to be young and colorful. The CIRS spectrometer found the surface at the south pole to be a relatively "hot" 85 Kelvin, 15 Kelvin warmer than expected. The plumes themselves were spotted in camera images several months later. In a paper published in Science earlier this year, the camera team suggested that the plumes may originate in reservoirs of liquid water just below the surface.
So the authors asked what you'd need to do to make water carry these other gases. Their answer is to propose a different form of water: a clathrate. Clathrate is, according to the article, "ice with a cage-like structure in which water ice traps other volatile components." That cage-like structure is well-known on Earth for being able to contain prodigious quantities of methane; Google "methane clathrate" and you'll find lots of articles about this stuff in the oceans. Clathrate wouldn't be stable on the surface at Enceladus, but it could be found at some depth beneath the surface. In fact, its instability near the surface is key to the new model. Basically, they propose that there could be clathrate ice trapping methane, carbon dioxide, nitrogen, and other gases at some depth beneath Enceladus' surface. Tectonic activity flexes the surface and can cause it to crack, exposing a thin channel down to the clathrate, which may be a few to a few tens of kilometers below the surface. What happens next depends upon a bewildering array of variables, but the key fact is that the clathrate would decompose violently when exposed to a vacuum, spewing forth water and other gas molecules. In the paper, they wrote, "Construction of detailed models is neither possible nor warranted by the data available. We could, however, look at two extreme cases: decomposition into a vacuum and into a network of cracks and fissures." The extreme case of exposing the clathrate directly to space would produce a pretty dramatic plume: "For decomposition of the mixed clathrate at [a temperature of] 190 K and [a pressure of] 0.5 MPa, the limiting rate [of decomposition] would be 900 kg s-1 m-2." To put that in words, that's one ton of material coming from every square meter of crack every second. The paper goes on to say "An eruption of this magnitude would pose a notable problem to a spacecraft in low orbit --" -- ouch! Imagine flying a spacecraft through a discharge of vapor coming out at many tons per second! -- "but is unlikely to be maintained long enough for Cassini to have encountered such an event." They go on to work through the math of the case of clathrate decomposition into a network of cracks leading eventually to the surface, and come up with numbers that agree well with the discharge rates observed at Enceladus. The analysis in this paper is rigorous and makes an excellent case for the possibility of plumes at Enceladus driven by an essentially dry process rather than by the presence of near-surface liquid water. I sent emails out to both Sue Kieffer and John Spencer (who was the fourth author on the paper) asking questions about the plausibility of this model. I asked John about how sure they are that clathrates exist in icy satellites. Since they would decompose violently when exposed to a vacuum, they can't be seen on the surface of the satellites using spectrometers. "Clathrates are a very plausible component of satellite interiors but we don't have any direct evidence for their presence -- it's just one of several possible mechanisms for plume generation." Kieffer was even more cautious, saying "I can't add much, and don't want to speculate beyond my area of expertise. We've been very cautious to limit our comments to our areas of expertise; unfortunately, we can't control the headline writers." In other words, they're not claiming to have disproven the presence of liquid water at Enceladus, and are very very carefully avoiding the implication that they're picking a fight with the people who advanced the liquid water explanation, that is, the Cassini imaging team. Rather, they've proposed another possible mechanism for the plumes, so now there are two published competing hypotheses. Headline writers do like to try to make this kind of publication into a fight between two competing groups, but in fact this is just science moving forward the way it's supposed to. There is a set of observations at Enceladus that need explanation, and now there are two proposed explanations. The next thing to do is to test both hypotheses by designing new sets of observations that Cassini can perform while it's still flying around the Saturn system, observations that will be designed to discern between the two competing theories -- or any others that arise in the meantime. Still, there's a lot of interest in framing this publication in the context of a heated debate, because of course Enceladus has recently been advanced as a possible destination for the next flagship outer planet mission. That would be Enceladus in lieu of Europa or even Titan, on the strength of its evident geologic activity (implying a ready source of energy) and the possible liquid water present at its south pole, both thought to be requirements for life. The authors of this paper are clearly mindful of that debate. Kieffer told me, "I'm one of those who believes that there's a lot of exciting science beyond astrobiology and that whether or not there's liquid water, there is so much else that we don't understand about this crazy planet, I'd love to see another mission to Enceladus. Anytime there's an object that mysterious in the solar system, it should pique our interest -- we're bound to discover lots of new things!" My opinion is that we currently have an excellent spacecraft in the Saturn system to continue our study of Enceladus (and Titan) and to try to understand what makes them tick. I, too, would love to see a followup mission to Enceladus -- but frankly, if we can go back to Saturn, I'd rather see a followup mission to Titan first. As fascinating as all these places are, we have to prioritize, and Titan appears to be a world as complex and fascinating as Earth. |
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