Redefining "Habitable" Liquid water is still the prerequisite for any form of life that we have so far encountered. For a planet or moon to have sufficient heat to maintain liquid water, scientists once believed that it had to orbit within an optimal distance of its home star. Galileo now brings evidence that the habitable zone may not be defined solely by an orbiting body's distance from its sun. It has revealed that three of Jupiter's rocky inner moons--Europa, Ganymede, and Callisto--may harbor liquid water beneath their icy crusts. The tugging and pulling these moons endure in their gravitational dance with Jupiter and with one another may generate enough internal heat to keep water liquid despite the lack of solar warmth. And some scientists believe primitive life-forms may dwell at the base of Martian polar ice caps, where the planet's internal heat has melted the permafrost.	In 1976 the Viking 1 lander showed early morning water frost or snow on Martian rocks. Mars may have regions of permafrost where water ice has been locked in the soil for millions of years.
A close-up of Europa's surface shows blue areas thought to be pure water ice. Darker mineral-laden water or slush from underground appears to have percolated to the surface through cracks in the crust.	The surface of Callisto (right), which should be scarred by meteoritic craters, is instead relatively smooth and blanketed by something loose and fine grained. Upwelling ice may sublimate, leaving a residue of loose dirt, just as a snowman melts and leaves behind a little pile of soil.

puddle or tide pool. Some researchers think that Earth had an added advantage as a cradle for life: the Moon. Tides may have stirred the organic stew along Earth's early seashore, helping the first cells assemble and gain a foothold. Perhaps a large nearby satellite is another factor that nudges a planet toward habitability.

What about other places in our solar system? Venus also lies within the Sun's habitable zone. However, its out-of-control greenhouse effect has pushed temperatures too high (page 125). Mars appears cold and lifeless today. Yet its surface shows the dry remains of countless meandering riverbeds, deltas, and floodplains. There's little doubt that liquid water flowed on Mars in the distant past. We don't yet know how long that wet era lasted. Conceivably, life could have appeared on Mars before the planet became a vast Sahara. Earthlings were stunned in 1996 when researchers claimed that a potato-sized chunk of Mars called ALH 84001 contained fossilized evidence of primitive life. The rock struck Antarctica 13,000 years ago as a meteorite. It contains minerals that may have been altered chemically by microbes, plus tiny structures that resemble ultrasmall "bacteria." However, other scientists have disputed those claims. It now seems that the initial announcement about ancient life on Mars may have been premature. The final word on whether tiny extraterrestrials once colonized the Red Planet must await future missions there.

As we venture farther out into the solar system, it appears that all hope is lost for remaining in the Sun's habitable zone. Temperatures plunge to ­150 degrees Fahrenheit or colder, and light dwindles to the intensity of moonlight. Yet we have discovered that liquid water can exist in such harsh settings. The most intriguing case is Europa, Jupiter's fourth-largest satellite. Tidal forces from Jupiter's intense gravitational pull flex Europa with relentless to-and-fro motions as the moon orbits the planet. Those motions create a potent source of internal heat. Images from the Galileo

explorer suggest that giant slabs of ice cover an ocean of liquid water or possibly slush. This ocean may lie under at least 5 miles of ice, far too deep for sunlight to penetrate. But the energy from Jupiter's tidal forces may be sufficient to sustain life on the ocean floor, just as warmth from Earth's interior supports dark colonies of life along its volcanic midocean ridges. Astronomers envision future space missions in which robots drill through Europa's icy shield to search for such organisms.