ocean on Europa
Cutaway view of the possible internal structure of Europa. A metallic (iron, nickel) core (gray) is drawn to the correct relative size. This is surrounded by a rock shell (brown), which in turn is surrounded by a shell of water in ice or liquid form (blue and white). The surface layer of Europa is shown as white to indicate that it may differ from the underlying layers.
Artist's conception of an underground ocean on Europa. If the moon's heat – possibly from volcanic activity in its rocky mantle – is intense enough and the ice shell is thin enough, the ice shell can directly melt, causing regions of what appear to be broken, rotated and tilted ice blocks on the surface. Credit: Michael Carroll/NASA/JPL.
Great interest surrounds the likelihood that an ocean of liquid water exists beneath the icy surface of Europa. This ocean may be as much as 150 kilometers deep, which would mean it contained twice as much water as exists in all of Earth's oceans.
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.
1. Reynolds, R. T., Squyres, S. W., Colburn, D. S., and McKay, C. P.
"On the Habitability of Europa," Icarus, 56, 246 (1983).
2. Ross, M. N., and Schubert, G. "Tidal Heating in an Internal Ocean Model on Europa," Nature, 325, 133 (1987).
3. McKinnon, W. B. "Sighting the Seas of Europa," Nature, 386, 765 (1997).