Callisto, ocean and possible life
Cutaway diagram of Callisto, showing possible ocean beneath the icy surface.
The first indication that Jupiter's moon Callisto might be of astrobiological interest came in October 1998, with the publication of a paper proposing that Callisto, like Europa, might have an underground ocean of water and perhaps the basic ingredients for life. This suggestion followed from measurements of Callisto's magnetic field by the Galileo spacecraft. Krishnan Khurana of the University of California, Los Angeles, and colleagues found that Callisto's magnetic field fluctuates in time with Jupiter's rotation, suggesting that Jupiter's powerful magnetic field generates electric currents in the moon which, in turn, give rise to the fluctuations. In trying to understand how currents could flow within the body of Callisto, Khurana et al hypothesized the existence of a subterranean ocean of salty water which would effectively act like a giant battery. This idea was given further credence by Galileo data which showed that the electric currents within Callisto flow in different directions at different times.
Whereas Europa is heated internally both by tidal stresses and the decay of radioactive elements, Callisto is only significantly heated by the latter. Therefore, the outermost Galilean moon may not be quite as good a prospect in the search for extraterrestrial life as its Jovian neighbor. Nevertheless, it seems certain now that future probes will scrutinize both Europa and Callisto for signs of subsurface water and prebiological or biological activity. One possibility is that, if primitive life has evolved on these worlds, it may be similar to the halophiles found in highly saline environments on Earth. Plans to investigate the astrobiological potential of the Galilean moons have also been given a boost by the discovery of a large lake deep under the ice of Antarctica, Lake Vostok, which may serve as a test-bed for instrumentation.
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