The Moon, imaged by Mariner 10.

Earth's own Moon divulged no trace of water to NASA astronauts who explored six landing sites more than 25 years ago. But all those sites were far from the poles. In the 1990s, the Clementine and Lunar Prospector robotic spacecraft each found indications that the Moon may hold supplies of water ice in permanently shaded areas near its poles. Like the water ice on Mars, those supplies could become useful for future exploration. Water is not only a vital resource in itself, but it can also be split into oxygen and hydrogen for breathing and for rocket fuel for return trips or journeys beyond the Moon.

If the Moon has stashed some water ice in its polar shadows, comets probably delivered the stuff. Comets are largely ice, and they have been colliding into the Earth, the Moon and the rest of the solar system for billions of years.

Artist's concept of Stardust studying a comet's environment, made of dust and ice.

The planets are believed to have formed about 4.5 billion years ago from clumping together of smaller pieces that were orbiting the Sun in what's called a protoplanetary disc. Comets are probably bits of the disc material that were left over, so they make a tempting target for NASA spacecraft to study for learning more about the solar system's evolution. Deep Space 1 is heading for a close examination of Comet Borrelly in September 2001. Another spacecraft, Deep Impact, is planned as a way to knock some surface material off a comet for a better look at the pristine icy interior. The Stardust spacecraft is on its way to collect samples of the gas and dust being shed by Comet Wild 2 in 2004, then deliver them back to Earth in 2006.

Saturn's rings offer a scaled-down analogy to the protoplanetary disc. One of the many research goals for NASA's Cassini orbiter after it reaches Saturn in 2004 is to examine the distribution and behavior of icy chunks that make up the rings.

These and other missions of solar system exploration in coming years should yield better understanding about the roles of water on other worlds, for deeper appreciation of it on Earth, even in your kitchen.

Europan Evidence