Cripes, where have you guys been? Judging from the questions that have been asked since May 30, no one remembers that during the Golden Age of this webgroup we already discussed all these issues -- frequently ad nauseam. To provide a brief run-through:
(1) The gravitational measurements of Europa's density make it clear that the total depth of its ice/water layer must be 100-150 km -- probably 120-130 km. Since ordinary Ice I (the only form in which Europa's ice layer can exist) has a density 92% that of water, the pressure at the base of the ocean must be slightly less than if the layer was composed entirely of liquid water. (The other crystalline forms of ice -- whose density is actually greater than for liquid water -- require pressures much higher than Europa's layer can provide; and, contrary to J.H. Byrne and Robert Crawley, Europa's low gravity can't produce ice in any lower-density form than Ice I. But Europa's 0.135-G surface gravity means that, even at absolute deepest, the pressure at the bottom of the ocean must be only about 1.8 times that at the bottom of the Marianas Trench -- so pressure alone will not be a great problem in designing vehicles capable of reaching it. (2) JPL is homing in on a preliminary design for a Europa Cryobot, called "CHIRPS" ( see http://techreports.jpl.nasa.gov/1999/99-2051.pdf . It would weigh about 30 kg, be a meter long and 12 cm wide, and use 1 kilowatt of heat to melt down through the ice at an average speed of 6 km/year (accelerating a bit when it gets below the cryogenically cold thin upper layer). It would sometimes use active hot-water jets to accelerate the melting and to wash away rocky sediemnt embedded in the ice, and would have sonar to sense rocky obstacles ahead and an ability to swerve slightly to the side to avoid them. Needless to say, there would an attempt first to find shallower spots in the ice layer (or cracks or pockets filled with liquid water) with the Europa Orbiter, and considerable analysis of the surface ice with earlier landers -- both to look for evidence of biological remnants frozen into the ice (which could very well have been transported to the surface over periods of a few tens of millions of years, even if Paul Schenk's thick-ice model is correct) and to test the physical properties of the ice to obtain further data for the proper design of the deep Cryobot. It's quite a safe bet that there would also be shallower Cryobots -- probing only a few hundred meters deep -- for similar scientific studies before we tried for something as ambitious as an all-out descent through the ice to the ocean. (3) Electrolysis isn't any more efficient for burning through the ice than simple melting is -- in fact, it's a lot less efficient than simply using the heat from plutonium-238 in the nose and also utilizing some of that to power an onboard RTG for electrical power. The meltwater would very quickly refreeze behind the Cryobot -- in fact, it appears that some of the heat vents must be on the Cryobot's upper part to keep it from refreezing there. (4) JPL has given up on carrying a communications tether -- it would be hopelessly bulky, and vulnerable to horizontal ice-layer creep, just as Tom Green suggests. Instead, the projected design carries a stack of hockey puck-like radio transceivers -- powered themselves by tiny nuclear batteries -- and drops them off at intervals of about 1 km on the way down to provide a radio link all the way down through the ice. (5) Any Europa lander -- like any Europa orbiter -- must, in any case, be resistant enough to Jupiter's radiation to survive at least a month or so in Europa's immediate vicinity even before landing; however, the Cryobot would be completely safe from it when no more than a meter deep. (6) To Mickey Schmidt: your proposal to use the structure of Europa's natural big impact craters to gauge the depth of its ice layer are exactly what Paul Schenk (and, separately, Elizabeth Turtle) have done, in several abstracts printed over the past few years even before Schenk's article in the May 23 "Nature". Their data is the solidest evidence for the ice layer being quite thick -- and they've both reached the conclusion, judging from the size of the biggest craters that didn't rupture the layer, that it's about 19 km thick. There are quite a few other lines of evidence suggesting that the "thick ice" model (of which Robert Pappalardo is perhaps the most famous advocate) is indeed the correct one, at least in this current age. But there's also a growing suspicion that the thickness of the ice layer may increase and decrease -- possibly all the way to a brief complete melt-through event -- over a cycle of a few tens of millions of years, and that right now it's in the upper range of its thickness. This would explain why Europa's surface cratering suggests that its surface isn't any older than that, and also why the nature of its surface features since then seems to have changed from ridges and cracks that formed in a thin ice layer to more recent "chaotic" terrain and "lenticular" domes that seem to have been produced by the extremely slow convective flow of ice itself when it's near 0 deg C (which should be about the temperature of all of Europa's ice save for a cryogenically cold, brittle top layer only a few km thick). No less than three different physical mechanisms have been proposed which would be likely to produce such dramatic cyclical changes in the thickness of Europa's ice. It's also this slow, convective churning -- over periods of a few hundred thousand years -- that can transport biological material from the top of the ocean all the way to Europa's surface even if the thick ice model is true; the moanings in the press about the difficulty of looking for Europan life if Schenk is correct are totally unwarranted, for this question has been discussed ever since the thick-ice model was proposed. (The thick ice model also allows -- indeed, usually calls for -- occasional pockets of very briny liquid water quite near the surface.) (7) Gary McMurtry is quite right in fearing that the super-brininess of Europa's water layer and ice -- maybe far higher than that of Earth's oceans-- could produce problems for the Cryobot. To quote Jeffrey Kargel's article in the Nov. 2000 "Icarus": "The likelihood that salts are major constituents of the [ice] crust poses formidable challenges to any plans to melt through the shell... Although the salts are soluble in water, the tendency might be for a melting device to induce partial (incongruent) melting, whre by a less hydrated solid salt is generated along with liquid brine. The less-hydrated salt could tend to encrust the melting device. Additional heating would not only continue the process of dehydration and partial melting, but would not likely remove the encrusing salt reside, which may eventually accumulate into an impenetrable mass. Extremely high temperatures might be required to completely melt this encrustation. The complex thermochemical dynamics of a melting device operating in a very salt-rich, heterogeneous crust have not been studied to our knowledge, but must be evaluated rigorously; testing of such a device in pure polar ice or even somewhat impure sea ice is not a sufficient engineering test. It may be that a mechanical digger or a combined melter-digger would be a better approach than melting by itself." Frank Carsey -- the head of the CHIRPS development team, who has also proposed a first-generation model to melt several hundred meters down through Mars' north polar ice cap as a candidate for the 2007 Mars Scout mission -- agrees; he told me that he's confident that CHIRPS' hot-water jet system could wash its way down through the rock dust mixed into Mars' ice layer, but has much less confidence that by itself it could handle the kind of brine buildup it might encounter in Europa's ice. (8) All this being said, however, Europa seems to me simply to grow in biological interest as time goes on. The most recent news on that subject -- which can only be called electrifying -- comes from a growing suspicion that wholly nonbiological processes (hydrolysis of Europa's surface ice by Jupiter's intense radiation belts) may actually be transporting enough free oxygen into its ocean (through that slow solid-ice convection) to support, not just microbes, but large animal life! To quote the new DPS White Paper on Europa exploration (which will be officially published in July; I've seen an advance copy): "Upper limits given by Cooper et al (2001) allow for large (i.e., terrestrial) concentrations of oxygen of ~ 20 mM (millimoles of O2 per liter of water) within the entire 100-km thick ocean, while lower concentration limits of ~ 0.2 mM could arise now from continuous O2 production [simply] by decay of the radioisotope potassium-40 within salts in crustal ice and ocean water [even if the theory that it's formed by Jovian radiation is completely wrong]. The peak dissolved oxygen density in the photic zone (depth < 200 meters) of earth's oceans is ~ 0.3 mM, while macrofauna in the deep oceans survive at 0.03 mM (Chyba and Hand 2001). Therefore even the lower limits on O2 production leave much room for substantial O2 losses in transit from the radiolytic source at the irradiated surface to a living ocean." Intriguing, no? If this new concept (initiated by Chris Chyba a few years ago) is true, Europa's ocean could be much RICHER in O2 than Earth's ocean -- and it was the buildup of oxygen in Earth's atmosphere and oceans (produced by early photosynthetic cyanobacteria) 2-3 billion years ago that triggered the evolution of multicellular life. Chyba and Hand also envision Jovian radiolysis producing large traces of formaldehyde in the surface ice, which is then transported downwards in the same way -- and many bacteria use formaldehyde as food. (In fact, its production is the intermediate stage in photosynthesis in green plants.) It's awfully easy for me, now, to visualize a Europan oceanic ecosystem in which dense bacterial mats accumulate on the lower surface of the ice layer, feeding on the radiation-produced goodies, and serving in turn as a food source for more complex organisms. There's still the problem of how nutrients produced in the upper few cm of Europa's ice by Jovian radiation can get down through that upper few km of nonconvecting supercold ice to reach the convecting lower ice layer -- but Pappalardo and Amy Barr have also proposed a mechanism that could efficiently do that (centering around meltwater formed in Europa's ridge regions by ice friction and/or compression), and other mechanisms are possible. At any rate, Europa is very worthwhile indeed as a source for biological study -- but this will be hard, and the full-scale Cryobot will be the end of an exploration process probably taking decades. Even much simpler near-surface probes, however, could with luck detect proof of Europan life. == You are subscribed to the Europa Icepick mailing list: [EMAIL PROTECTED] Project information and list (un)subscribe info: http://klx.com/europa/
