O

o layer (ice shell and water ocean) is probably composed of three sub-layers: an JE C

outer, brittle/elastic ice layer, an underlying ductile layer of potentially convecting ice, and a lower layer of liquid. The densities of these layers are expected to be greater for the lower layers, although some low-density pockets (briny ice/water or temperature-driven density anomalies) may occur throughout [409].

Melosh et al., argue that most of Europa's ocean is at the temperature of maximum density and that the bulk of the vigorously convecting ocean is separated from the bottom of the ice shell by a thin "stratosphere" of stably stratified water which is at the freezing point, and as a result, buoyant. Europa's overall density indicates it is mainly composed of silicates, not ice, and therefore contains radioactive heat-producing elements. This is enough to keep water beneath the ice shell stirred by vigorous thermal convection [410].

There has been much speculation that Europa's ocean might harbor life or, at least, prebiotic conditions. According to Kargel et al., just as life on Earth requires liquid water, chemical disequilibria, and elemental building blocks, we may find that these same stipulations are met on Europa and may have existed continuously or sporadically since Europa's origin. Molecular and isotopic studies of gas hydrates in lightless chemosynthetic communities of the Gulf of Mexico provide evidence that bacteria can directly oxidize hydrate-bound methane. Hydrocarbons from decomposing gas hydrates appear to drive complex biogeochemical processes in sediments that surround gas hydrate extrusion features helping to support complex chemosynthetic microorganisms. Metabolic processes such as methanogen-esis, sulfur reduction, and iron oxide reduction have been suggested for Europa. Excreted byproducts and decaying material from chemoautotrophic organisms could serve as the basis for a Europan ecosystem by supplying carbon and energy sources to heterotrophic life forms at higher trophic levels. Photosynthesis would be unnecessary in this scenario. According to some geologic interpretations of Europa, there could in fact be enough chemical energy available on Europa to drive the development of a biomass. Calculations indicate that there may also be enough organic carbon to allow the existence of a dense, well-developed ecosystem. Even in the unlikely absence of endogenic sources of organic carbon, impacts can deliver organics, including amino acids. While temperature can certainly play a part in the development of life in an ocean, the exact temperature of an ocean can depend upon pressure and solutes. And, at any given location, the local temperature could be warmer near a hydrothermal vent. Thus, there could be conditions in Europa's ocean similar to conditions on Earth. Where Europan and terrestrial environments overlap in their physiochemical nature, examples from Earth reveal that life can cope and grow under conditions as extreme as the range of conditions expected for Europa. It is certainly possible that there are conditions on Europa that are far colder and of higher pressure than on Earth, but it is not certain that either of these conditions would be prohibitive to life as long as liquid water is present to allow organisms to grow. On Earth hypersaline, acidic, alkaline, and metal-rich environments harbor tenacious microbial communities. Therefore, it appears that £ organisms are able to exist and thrive under an extraordinary range of conditions te wherever fresh liquid water or brines are present. Likewise, brine-filled cracks n ys in sea ice or in the lithosphere provide a suitable habitat for microorganisms.

O ^ Because life survives and grows in all of the coldest environments on Earth where vite liquid is present, even colder conditions may allow biological growth as long

J5 IjS as liquid water is stabilized by solutes. Environments of even more extreme con-

q © ditions may be surmountable if such conditions were stable. The adaptive strate-

O gies of microorganisms on Earth reveal that most every physiological stress can be overcome so long as the environment contains liquid water. Thus, it is possible that these types of environments could offer hope for life in the ice and rocky interior of Europa [411].

Seeming to agree with Kargel et al., Lipps and Rieboldt assert that Europa may harbor life because of the presence of a briny ocean, energy sources, and nutrient supplies. And, they point out that Europa's ocean probably contains far more water than oceans on Earth [412].

While some scientists continue to consider the existence of an ocean on Europa to be debatable, the interpretation by some scientists of the Galileo experiment using its magnetometer proved almost beyond doubt that Europa does indeed have an ocean beneath its frozen surface! Does this ocean hold some form of life? Only time and future exploration can tell us that. Perhaps a future orbiter and lander will give us the data we need [413, 414].

6.1.3 Ganymede

One astronaut who landed on the Moon described the terrain he saw there as 'Magnificent desolation!' Scientists, first seeing Ganymede's surface in detail, could easily have used the same description. Ganymede's surface is a tangle of geographic features! It has been pushed, pulled, and racked with unimaginable tectonic stress, bombarded with impacts, and then frozen in place (Fig. 6.27).

The surface temperature on Ganymede is 132-143 K [415], it is 5,268 km in diameter with a mean density of 1.936 ± 0.022 g cm-3 [416], and completes an orbit of Jupiter in 7.155 days [417]. Ganymede is the largest satellite in the solar system, even larger than the planet Mercury.

Low resolution views of Ganymede obtained with Voyager show a surface divided into dark regions that are heavily cratered, and lighter regions organized into massive bands of grooved terrain that is much less heavily cratered. Higher

Fig. 6.27. A Galileo spacecraft view of Ganymede in natural color. The dark areas are the older, more heavily cratered regions and the light areas are younger tectonically deformed regions. The brownish-gray color is due to mixtures of rocky materials and ice. Bright spots are geologically recent impact craters and their ejecta. North is to the top of the picture. (Credit: NASA/JPL/Caltech)

Fig. 6.27. A Galileo spacecraft view of Ganymede in natural color. The dark areas are the older, more heavily cratered regions and the light areas are younger tectonically deformed regions. The brownish-gray color is due to mixtures of rocky materials and ice. Bright spots are geologically recent impact craters and their ejecta. North is to the top of the picture. (Credit: NASA/JPL/Caltech)

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