Many scientists believe that the blue history of Europa, one of Jupiter's moons, is still being written. Europa circles Jupiter every few days, and this rapid orbit generates friction that heats up the moon's interior. For that reason, some feel that an enormous salty ocean still exists beneath Europa's frozen surface, containing perhaps twice as much liquid as all the Earth's oceans combined.
Though the search for life on Mars has diverted attention and resources from Europa, the icy moon offers many indications that life could thrive there, including the presence of oxygen, hydrated salt and perhaps photosynthesis. Algae, bacteria and even animals exist in similar conditions in Antarctica, often living under ice shelves.
"If we made Europa a high priority and thought carefully about where to land, I think there's a good chance we'd find signs of life there," says planetary scientist Richard Greenberg of the University of Arizona. "If there was past life on Europa, I don't see why it wouldn't still be there. It's extremely active."
Because Europa is bombarded by radiation, Earth-like organisms could not live on the surface. But they might exist just several feet below in visible cracks. In recent papers and talks, Jere Lipps of the University of California, Berkeley, has outlined several ways in which life on Europa, or its remains, might be exposed to the surface—and likewise to rovers or orbiters sent to study the moon. These include places where ice has cracked and refrozen with life trapped inside; blocks of ice that have broken off, flipped over and now face the surface; and debris lodged in ridges or deep crevices.
Such exposures mean explorations to Europa could spot life without potentially difficult landing-and-digging missions. "Europa is active in the sense that its body is continually being reshaped," says Greenberg. "Ice is cracking, opening, closing. There's a good chance that oceanic substances regularly emerge to the surface."
While Europa and other sites near Earth, such as Saturn's moon Titan, remain promising places to find water, some scientists have set their sights far beyond this solar system. Recently, Travis Barman of Lowell Observatory in Flagstaff, Arizona, detected water in the atmosphere of a planet some 150 light years away—the first such evidence for a planet outside Earth's cluster.
The planet, known as HD 209458b, resides in the constellation Pegasus and is made entirely of gas. As seen from Earth, HD 209458b passes in front of its star every few days. During this stage, the planet's atmosphere blocks a certain amount of starlight, enabling Barman to model the atmospheric components. When he compared his models to images of HD 209458b from the Hubble telescope, those that included water in the atmosphere proved accurate, he reports in the June 1 Astrophysical Letters.
A couple weeks later, a team of European researchers announced another breakthrough outside this solar system: the discovery of a planet incredibly similar to Earth. The planet, some 20 light years away and five times the mass of Earth, circles the star Gliese 581. Several years ago, scientists found another planet—this one similar to Venus—orbiting this same star.
The new planet is much closer to Gliese than Earth is to the Sun, completing its orbit in about two weeks. But because Gliese is smaller than the Sun, the temperature on this planet's surface could be amenable to liquid water, the researchers report in an upcoming issue of Astronomy & Astrophysics. "The planet is the closest Earth twin to date," they write.
In the end, though, watery conditions, or even water itself, can only tell so much of the story of life beyond Earth. The conclusion must wait until more powerful tools or more precise explorations turn mere suggestion into solid proof.