Oceans, Oceans, Everywhere

Mars has held a place in the human imagination as a possible home of alien life for more than a century. The presence of water, scientists assure us, is essential for any life that we understand. We've known for some time that the polar caps of Mars contain water ice as well as dry ice. Recently, by establishing ancient shorelines using photographs taken by probes in Martian orbit, planetologists have shown that Mars once had two oceans, one in the northern hemsphere and one in the southern.

Evidence suggests the oceans once covered roughly a third of the planet's surface. That's less than half the extent of Earth's iceans, but still substantial bodies of water. Some of that water probably escaped into space because of Mars' relatively low gravity, but scientists speculate most of that water may still exust on Mars in underground reservoirs.

So, in one solar system, we have evidence of oceans existing, at one time or another, on Earth, Mars, under the ice of Jupiter's moon Europa, and possibly also under the ice of Jupiter's other huge moons, Ganymede and Callisto. Even with all that water, most of the water in the Solar System is likely in the atmospheres of the giant planets Jupiter, Saturn, Neptune, and probably Uranus.

We know water is abundant in the universe. We have an idea of how water might be distributed in a solar system by studying our own neighborhood. If life is in some sense a product of water, could life be abundant in the universe? By exploring Mars and Europa in upcoming decades, we may be able to answer that question.