No other body in the solar system, except of course for Earth, has shown evidence of liquid water. Life as we know it, carbon-based life, requires this, says Stanford University chemist Richard Zare. ``Not steam, not ice, but liquid water.''
Zare is on the team of scientists that last summer raised the possibility of life on Mars, announcing discovery of what they believed were microscopic fossil organisms embedded in a Martian meteorite.
Though the surface of Mars is so cold that no liquid water can exist, the landscape is scarred with dry rivers and streams, signs that water did flow earlier in the planet's history, said Zare.
But Zare and other scientists are still locked in debate over whether the famous Mars rock really houses fossil life, rather than some misleading geologic formation.
Europa is more of a mystery than Mars.
Though scientists made some hyperbolic comments last week about a life-incubating ocean on Europa, they haven't actually seen the water, let alone the life, in the images they received from the spacecraft Galileo.
It was the Voyager mission 20 years ago that raised the first hints that an ocean might flow beneath the thick ice covering this body, one of four large moons orbiting Jupiter, the fifth planet from the sun.
Voyager sent back pictures of a smooth ice surface, the smoothness hinting of an underlying liquid, which would keep the surface relatively warm and melt any ridges or mountains that might form. Dark lines appeared to be cracks in a thin ice shell.
Much sharper pictures were made public last week from Galileo, which has been exploring Jupiter and its moons since the end of 1995. The Galileo images showed a jumble of ``icebergs'' and flat ice chunks that appeared to have been broken apart and rearranged - as if by the flow of some underlying fluid.
