Saturn's Earthlike Moon

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Searching for signs of life on other planets—at least in our own solar system—has always been a bit of a fool's errand. Other than Mars, which may have been teeming with biology long ago but has dried to dust since, the planets look decidedly barren. But the moons may be another story. Some of the smaller bodies that circle our sibling planets are wet, hazy, and dense with organic chemistry and could be little cosmic nurseries. Of all of them, few top Saturn's giant moon Titan for pure biological potential.

Astronomers analyzing Titan's chemically rich atmosphere had long believed the moon could be an organically astonishing place, but until the Huygens space probe parachuted down onto the moon's surface last January, nobody knew for sure. This week, a flurry of seven papers were published on the website of the journal Nature, analyzing the data the ship was able to send home in the few hours it stayed alive on the bitterly cold world. That data confirmed everything astronomers had hoped might be true about the oddly Earth-like place.

One thing investigators knew from the start was that nothing remotely like the life we know could exist on Titan. Its paralyzingly cold, -290* Fahrenheit (179* Celsius) climate ruled that out entirely. But the orange, opaque haze that shrouds the moon—and hid its face completely from the Voyager 1 and 2 spacecraft that ventured out that way in 1980 and 1981 —suggested that the place may swirl with water and hydrocarbons such as ethane and methane, the same chemical bricks that allowed life to emerge on Earth. Scientists have thought Titan could serve as a sort of flash-frozen version of the Earth in its pre-biotic days, just the kind of place you'd want to go to study how living things did succeed in emerging here.

Huygens was designed to conduct those studies and it succeeded, spectacularly so. The descending spacecraft detected a multilayered atmosphere around Titan in which winds blow at a savage 280 mph (450 kph) at high altitudes and slow to a pleasant walking speed on the surface. Water is abundant, though the temperature is far too cold for it to exist in a liquid state. The atmosphere is nonetheless humid with methane which freezes, thaws, evaporates and rains out precisely as water does on Earth. The Titanian air is also rich in nitrogen, which is the dominant element in Earth's atmosphere too. If Titan were loosed from its orbit around Saturn and could somehow fly closer to the warmth of the Sun, it would likely—eventually—bloom into life.

Mottled markings on Titan's surface originally suggested that the moon might be home to methane or ethane lakes, but Huygens did not detect evidence of that. It did see rock-strewn highlands, drainage channels, lowlands and basins that look for all the world like earthly riverbeds. These too would have been the result of flowing ethane or methane, not water. Titan's surface—at least on the spot where the spacecraft landed—was neither frozen solid nor entirely yielding. Rather, it had the consistency of heavy slush or wet clay, leading some researchers to believe it might have landed in a spot where a methane tide had just receded.

Most tantalizing of all, the Huygens probe detected bursts of energy throughout the atmosphere that had the telltale electromagnetic signature of lightning. It's lightning, many biologists believe, that first kick-started the process on Earth that converted inert hydrocarbons to early life. On Titan too that process could be trying to get under way, though the bitter cold will likely never let it happen.

Still, if Titan is forever fated to be Earth's stillborn sister, it is its sister all the same. The two worlds came from a common chemical incubator and began to evolve in common ways. Even if Titan never becomes the loamy, living place our planet is, it can still teach us a lot about how we took that fortunate turn.