The search for life in the solar system took a turn this week with the announcement that Europa, a moon of Jupiter first discovered by Galileo, shows signs of water geysers erupting from its south pole. This could be the best evidence yet that Europa has a subsurface ocean. With liquid water and energy from the moon's internal tidal forces, Europa could fit the scientific definition of a habitable world, a place where life could exist, dark and chilly though that existence might be.

THE TIPOFF: The hidden ocean has long been suspected, but scientists have never seen anything as dramatic as plumes of water vapor more than 100 miles high. If this finding holds up, it will boost Europa even further as a target for robotic exploration. The Hubble discovery was published online by the journal Science.

HOW IT WORKS: The implication is that tidal forces within the moon — created by Jupiter's immense gravity — cause Europa to contract and expand. When the crust decompresses slightly, liquid water squeezes through a crack and squirts into the cold vacuum of space. The water would quickly change form, freezing and then sublimating into vapor. Alan Stern, principal investigator for New Horizons, a probe on its way to Pluto, said of the discovery: "I think it's game-changing." Washington Post