Searching for Life in the Solar System ... And Beyond
A Research Discussion Meeting
London, UK - 31 October 1996

Icy satellites as possible abodes of life

David A. Rothery
Department of Earth Sciences,
The Open University, Milton Keynes, MK7 6AA

Hot vents on Earth's deep ocean floors are sites where bacteria and bacteria-like organisms (the hyperthermophilic archea) feed off the supply of chemical energy, and are entirely independent of energy from the Sun. Life on Earth may have begun in such a setting, perhaps at bubble membranes between alkaline spring-water and acidic sea-water. Many of the icy satellites of the outer planets probably once hosted comparable environments, and a few may still do so today. The icy satellites have rocky cores overlain by thick icy mantles. Given an adequate source of heat, we would expect water to be drawn down into the rocky core, to emerge elsewhere as a hot saline fluid, bearing many constituents dissolved from the rock. These could sustain life, in the same way as hot vents on the Earth's deep ocean floors. We know that Europa (a large satellite of Jupiter only slightly smaller than our own Moon) is heated by tidal forces as present, and this is the likeliest place elsewhere in the Solar System to find life today. Europa has a thinner icy mantle than other satellites. Even this is about 100 km thick, so it would be difficult to study the life directly. However, billions of years of organic processes may have produced tell-tale signs in the chemistry of the surface ice, which would could be revealed by in situ analysis or sample return. Several other icy bodies have experienced tidal heating, and moreover would have had hotter interiors in the distant past when radioactive decay was a more potent heat source. Indirect evidence that hydrothermal reactions have occurred comes from the apparent absence of carbon monoxide and the preponderance of methane in outer solar system ices. This could have been caused by the oxidation of rock, and may have been organically mediated. There is a growing body of evidence that life was established on Earth within the first 500 million years, and so there is little reason to doubt that life could have got going within many of these icy bodies. Titan, Saturn's largest satellite, has a rich, dense atmosphere which allow life to survive at the surface. In other icy satellites, any life is likely to be concentrated deep down, close to the sources of chemical energy.