Move over, Mars. Jupiter's moon, Europa, may be the most promising site for extraterrestrial life in our solar system.

Features on the surface of Jupiter's moon, Europa, indicate that an icy shell floats on an ocean of liquid water, and that material has circulated between the surface and the ocean in geologically recent times. Many scientists think that these characteristics make Europa an excellent place to look for life. (For more information, please see the labeled and unlabeled versions of this graphic in our gallery.)

There are two worlds in the entire solar system that, we think, consist largely of rocky interiors covered in deep seawater: Earth and Jupiter's moon, Europa. On Earth, those conditions may have been responsible for the emergence of life. Could the same be true of Europa? Scientists are eager to find out.

NASA has adopted a "follow the water" strategy in its search for extraterrestrial life. On Earth, to the best of our knowledge, all life requires liquid water and life exists everywhere that water does. So it seems reasonable to seek life on other worlds by looking for liquid water.

The Galileo spacecraft, which orbited Jupiter from 1995 to 2003, appears to have found a plentiful supply at Europa and its neighboring moons, Callisto and Ganymede. Beneath their icy crusts, each of those three worlds is thought to contain more liquid water than all of Earth's oceans combined.

But only at Europa is the ocean thought to be in direct contact with a rocky mantle, which can provide nutrients to the water. And only at Europa does the icy shell appear to have been circulating material between the surface and the ocean in geologically recent times, and perhaps still today. That provides an additional route for nutrients-in this case, organic materials delivered by comets and oxygen produced by the effect of Jupiter's radiation on water ice-to enter the ocean, and also for possible ocean-dwelling organisms to rise to the surface where we might find them.

A subsurface ocean is thought to be in contact with Europa's rocky mantle and with its icy shell, providing possible routes for organic materials to enter the ocean.

Scientists and engineers at JPL have designed a mission concept called "Europa Explorer" as a potential next step in investigating the possibility of life on Europa. Where Galileo conducted a series of flybys of Europa while orbiting Jupiter, Europa Explorer would orbit Europa (after spending a year or more orbiting Jupiter and conducting flybys of all four Galilean satellites). According to JPL icy-moons specialist Bob Pappalardo, such a mission would be able to confirm the existence of a subsurface ocean and provide important information about its habitability. If the findings indicate that Europa is a habitable environment, a future lander might search for evidence of past or existing life. If those results are positive, Pappalardo noted, it would be a turning point for humanity.

Referring to the great astronomer for whom the recent spacecraft was named, Pappalardo said, "It was 400 years ago that Galileo's observations of these moons of Jupiter, including Europa, changed our perception of where we are, our place in the universe. No longer were we the center of the universe-we were one planet going around a sun in the very extensive cosmos. Once again, one of those same moons, Europa, if we were to find evidence of life there, could change our understanding of our place in the cosmos."

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Last Updated: 3 February 2011