To find out what water might look like on alien worlds, a group of researchers decided to see how Earth's oceans would appear from afar, as if from another planet.

Using the Deep Impact/EPOXI spacecraft, currently headed for a rendezvous next year with Comet Hartley 2, they peered back at Earth from more than 30 million miles (50 million kilometers) away, tracking the way reflected light changes as oceans rotate in and out of view.

From that distance, Earth's surface features were blurred [see photo at left], but the presence of water passing through the spacecraft's view increased the planet's blueness. Landmasses, on the other hand, lent a reddish hue. The team of researchers was able to assemble from those color variations a rough map of the liquid and land boundaries on Earth, presented in a paper set to be published in the Astrophysical Journal.

Such investigations may soon become useful, as the race is on to find potentially habitable planets outside of the solar system. NASA's Kepler spacecraft last month began its search for such extrasolar planets, scanning a patch of sky in the hopes of turning up Earth-like worlds that could sustain water oceans.

At the moment, astronomers are not able to image extrasolar planets with anything near the resolution enjoyed by the study's authors. (The spacecraft, which photographed Earth from approximately the vantage point of Venus at its closest to Earth, was far closer than any exoplanet.) But the researchers say such an analysis is not merely science fiction. Lead author Nicolas Cowan, a graduate student in astronomy at the University of Washington, said in a statement that the research was carried out "as if we were aliens looking at Earth with the tools we might have in 10 years."

One notable limitation of the new technique, which would likely also apply to a similar approach proposed in April to track a rotating planet's mirrorlike reflectance, is a dependence on the distribution of water on the planet. "We could erroneously see the planet as a desert world if it had a nearly solid band of continents around its equator and oceans at its poles," Cowan said in a separate statement.

Earth and the moon, as photographed by Deep Impact/EPOXI last year from a distance of 30 million miles (50 million kilometers), courtesy of NASA/JPL-Caltech/UMD/GSFC