Earth. Our world is the poster child for solar-powered planets. Most flora and fauna on Earth – with some important exceptions such as the bacteria that live in deep rock – are ultimately animated by the roaring nuclear fusion taking place in the Sun’s heart. On Earth, it’s usually chlorophyll that converts this radiant energy into chemical compounds to energize our existence (or bulge our waistlines).
Venus. Despite the fact that Venus, our sister planet, has been described as purgatory personified, there are some researchers who still hold out hope for life there.
Mars. While Mars’ highly reactive and powder-dry landscape is practically guaranteed to be sterile, there is indirect evidence for watery aquifers a few hundred feet beneath the surface. If these liquid reservoirs exist, life may have found refuge within.
Titan. This large moon of Saturn, revealed in detail by NASA’s Cassini mission, and subject to shameless examination by the Huygens probe, is far too cold for liquid water. But its air is thick with hydrocarbons. David Grinspoon has suggested that the Sun’s weak ultraviolet light might rip apart some of these atmospheric compounds, producing acetylene. Falling into the liquid lakes of methane and ethane below, this gas (best known for firing blowtorches on Earth) could serve as a food for microscopic life. Unlikely? Yes. Impossible? No.
The best known of tidally heated satellites are:
Europa. There’s good evidence, mostly from its changing magnetic field, that this ice-covered world orbiting Jupiter has an ocean lying 10 miles or so beneath its crusty exterior. At the bottom of this vast, cryptic sea, volcanic vents might be spewing nutrients and hot water into a cold, dark abyss, providing both the food and energy for simple life.
Ganymede and Callisto. Both of these jovian moons show magnetic field variations similar to those of Europa, suggesting that they, too, might be hiding large, watery oceans. Given their thicker ice skins, finding that life – if it exists – would be even more daunting than for Europa.
Enceladus. In the news recently, this Saturnian satellite seems to be a giant Slurpee – an icy moon that, thanks to tidal heating, is spouting geysers of water into space. An unexpected entry in the horse race of habitability, Enceladus is the first other world for which we have convincing evidence of liquid
Credit:Seth Shostak,SETI Institute. More: 8 Worlds where life might exist