ocean on Europa
The history of this idea can be traced back to a 1971 theoretical paper in which the author, the American astronomer John S. Lewis, argued that " ... the Galilean satellites of Jupiter and the large satellites of Saturn, Uranus, and Neptune very likely have extensively melted interiors."
Speculation about a sub-ice ocean and hypothetical Europan life (see Europa, life on) stepped up following Voyager 2's close encounter with Europa in 1979,1, 2 while by the time the Galileo spacecraft arrived at Jupiter in 1995, the ocean model was well established as a plausible hypothesis. What the new probe found only strengthened its case. High-resolution images provided clear evidence of near-surface melting and the movements of large blocks of icy crust, similar to those of icebergs or ice rafts on Earth.3 Galileo's cameras also revealed very few impact craters – a sure sign that resurfacing has taken place in geologically recent times, no more than 30 million years ago. Various sources of heat have been discussed by planetary astronomers over the years as the means by which Europa's ice shelf might be kept molten from below, but the principal mechanisms are now thought to be tidal distortions caused by the shifting gravitational pulls of Jupiter and the other large Jovian moons and the internal decay of radioactive elements.
Similar tidal heating may have created an ocean on Callisto, drives volcanoes on Io, and may give rise to water geysers on Europa, though none have yet been observed.
Related category• ASTROBIOLOGY
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