Robert Owen (email@example.com)
Fri, 17 Sep 1999 17:27:09 -0400
Larry Klaes wrote:
> From: "Bruce Moomaw"
> To: "Icepick Europa Mailing List"
> Subject: Europa turned upside down?
> Date: Sat, 11 Sep 1999 12:41:04 -0700
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> Earlier I promised to present my views of the abstracts just released for the upcoming Division of Planetary Sciences meeting ( <http://www.aas.org/dps99>http://www.aas.org/dps99 ) -- and, as I said, those dealing with Europa are lulus. When combined with earlier papers, they suggest that we may be approaching the question of looking for life on Europa completely wrong -- that, instead of inspecting the floor of Europa's (possible) ocean for life, we should be looking at its ceiling.
> As I mentioned in the two articles on Europan life I did for "SpaceDaily" back in June, there is a lot of skepticism among scientists as to whether Europa even has any hydrothermal vents -- and certainly whether it has them in numbers capable of supporting life. Europa's total heat flow per km from its interior today (including both radioactive and tidal heating) seems to be, at most, about the same as Mars' -- about 1.5 times that of the Moon -- which means that volcanic activity on its ocean floor must be quite weak. And in one of those DPS abstracts (No. 66.01, "Quantitative Models of Tidal Dissipation and Heat Transport in Europa"), William Moore and Gerald Schubert say flatly that it can have none: "The results [of our analysis of Europa's internal structure] indicate that, while sufficient heat may exist to keep water liquid under favorable conditions, convective heat transfer in the silicate mantle is at all times efficient enough to prevent silicate melting. Thi!
> precludes the possibility of hydrothermal activity akin to the mid-ocean ridge vents of Earth, and demonstrates that there is more to finding habitable regions in the solar system than finding liquid water."
> Even if they're wrong, hydrothermal activity on Europa is certainly weak -- and in a widely-praised article in the 8/25/98 Journal of Geophysical Research ("The Biological Potential of Mars, the Early Earth, and Europa") --Bruce Jakosky and Everett Shock say that there simply is not enough geochemical energy available on Europa to support more than a very small biosphere. And Eric Gaidos et al., in the 6/4/99 "Science" ("Life in Ice-Covered Oceans"), say that, because Europa beyond any doubt whatever doesn't have enough heat for crustal tectonics like Earth's, the minerals of its mantle must be highly depleted in oxygen -- which means that whatever hydrothermal vents it does have will belch forth only hydrogen, methane and sulfides, and not CO2, SO2 or elemental sulfur (which are by far the most important energy sources for chemosynthetic bacteria on Earth, including those at our vents). As they say, this doesn't rule out chemosynthetic bacteria -- but it does mean that
> they must be even scarcer on Europa's seafloor than we had assumed.
> So far, so bad. But now consider Chris Chyba's DPS abstract (No. 66.09, "Free Energy for Life on Europa") -- which I quote in full:
> "Prospects for life in the possible subsurface ocean of Europa depend on sources of free energy. But these may be hard to find on a perpetually ice-covered world [Chyba quotes Gaidos' article]. However, both simple organics and oxidants should be synthesized in the uppermost layer of Europa's ice crust via disequilibrium radiation chemistry. If melt-through events then occur, such as those which may have created Connemara Chaos and other similar (though in many cases much smaller) regions on Europa, these molecules would be mixed into the ocean, where they could serve as the redox couples to drive a Europan biosphere. Quantative estimates based on terrestrial analogues sugggest that over one million kg of microbial biomass could bloom in a single Connemara-scale event. Life on Europa would migrate from melt-through event to melt-through event, somewhat analogous to the way life moves among hydrothermal vents on Earth as old vents shut down and new ones arise. In the Eur!
> case, however, sources of disequilibrium redox reaction couples would lie at the top of the ocean, rather than (or perhaps in addition to) its bottom. Candidate energy sources such as these can be no more than plausibility arguments, but they emphasize the importance of in situ exploration to investigate the question of life on Europa."
> Well. Ironically, in that same science fiction story by Dr. Geoffrey Landis I recently quoted in which an expensive search for life on Europa proves futile, life is eventually discovered in the very deep, superhot ocean on Uranus, powered by oxidants which are produced in Uranus' upper atmosphere by radiation and then rain down into the ocean. For Uranus and Neptune, whose internal water-ammonia oceans are extremely high-pressure and extremely hot, this is a terrible long shot; but if Chyba is right, something analogous may be happening on Europa -- and our Hydrobot should poke around the ice ceiling of Europa's ocean rather than its floor. I've simply got to get in touch with Chyba (which is very hard to do; he's a very busy man) to ask him about this and a variety of other important questions.
> Still more of the DPS abstracts, though, throw still another curve at us. Apparently there is still substantial doubt that those substances detected by "Galileo" and mixed with Europa's surface ice are magnesium and sodium sulfate salts after all. In abstract No. 62.02, Robert W. Carlson -- the head of the Galileo NIMS team -- proposes that they're something a lot less friendly: concentrated sulfuric acid, coming either from Europa's internal sulfur or from Io's volcanoes. (Apparently I was premature in saying that the latter could have no effect on Europa's prospects for life.) "Sulfuric acid... is a major surface component, along with water ice. The sulfuric acid concentration correlates spatially with Europa's visually dark material, which we identify as radiolytically altered sulfur polymers. Radiolysis by incident Jovian plasma continually cycles sulfur between three chemical reservoirs: sulfuric acid, polymerized sulfur, and sulfur dioxide, with the acid being abo!
> 50 times more abundant than the other forms because of the stability of the sulfate anion under irradiation." In papers 62.01 and 62.04, James Dalton and Thomas McCord (the latter being the chief proponent of the belief that the stuff is sulfate salts) agree that Carlson may -- repeat, may -- be correct. As Carlson says, any Europan ocean may be battery acid. Not good (although, if I remember correctly, there's at least one kind of Earth bacteria that prospers in concentrated sulfuric acid).
> What all this really proves, once again, is that it's very urgent to get a chemical analysis of Europa's ice from a lander. As I've said before (quoting the JPL team), even if Europa doesn't have any ocean now, it certainly had one during its early days, and any ancient life forms are likely to be frozen and well-preserved in it. Europa is almost equally interesting biologically even if it doesn't have an ocean today -- and so the importance of flying the Europa Orbiter before a surface lander still seems to me questionable. Something else I have to ask Chyba about.
> Bruce Moomaw
-- ======================= Robert M. Owen Director The Orion Institute 57 W. Morgan Street Brevard, NC 28712-3659 USA =======================
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