麻豆淫院

On Saturn鈥檚 giant moon Titan, it is so cold that water is frozen as hard as granite. And yet there is a complete liquid cycle of methane and ethane. Scientists wonder whether there could also be life.

New discoveries have a way of messing with old definitions. Take, for example, the concept of a habitable world.

The standard definition of a 鈥渉abitable world鈥� is a world with liquid water at its surface; the 鈥�habitable zone鈥� around a star is defined as that Goldilocks region - not too hot, not too cold - where a watery planet or moon can exist.

And then there鈥檚 Titan. Saturn鈥檚 giant moon Titan lies about as far from the standard definition of habitable as one can get. The temperature at its surface hovers around 94 degrees Kelvin (minus 179 C, or minus 290 F). At that temperature, water is a rock as hard as granite.

And yet many scientists now believe life may have found a way to take hold on Titan. Water may all be frozen solid, but methane and ethane are liquids. In the past few years, instruments on NASA鈥檚 Cassini spacecraft and images captured by ESA鈥檚 Huygens probe have revealed an astonishing world with a complete liquid cycle, much like the hydrologic cycle on Earth, but based on methane and ethane rather than on water.

鈥淲hat Cassini actually found on Titan, from 2004 onwards, was a methane-ethane cycle that very much echoes the kind of hydrologic cycle we see on the Earth,鈥� says Jonathan Lunine, currently at the University of Rome Tor Vergata while on leave from the University of Arizona. Cassini has revealed rivers and lakes of methane-ethane, the lakes evaporating to form clouds, the clouds raining hydrocarbons back down onto the surface, flowing through rivers and collecting in lakes. It is the only world in our solar system other than Earth where a liquid cycle like this takes place. There鈥檚 just no water.

But there are plenty of hydrocarbons. Methane and ethane are the simplest hydrocarbon molecules. By themselves, they are of limited biological interest. But hydrocarbons are versatile: they can assemble themselves into fantastically complex structures. Indeed, complex hydrocarbons form the basis of what we call life. So one has to wonder: has hydrocarbon chemistry on Titan crossed the threshold from inanimate matter to some form of life?

One thing is for certain: if there is life on Titan, it is not life as we know it. There is no way that terrestrial life could have originated or could survive on Titan. 鈥淒NA and RNA,鈥� says Lunine, 鈥渇orm out of compounds that require oxygen and phosphorus, and there鈥檚 very little oxygen in the Titan system.鈥� And the very structure of DNA depends on liquid water. 鈥淒NA forms a helix because of its water-loving and water-repellant ends.鈥� So life on Titan 鈥渨ould have to find other molecules that carry information.鈥� Moreover, because Titan is so cold, the amount of energy available for building complex biochemical structures is limited. But as Lunine points out, that鈥檚 not necessarily a showstopper. 鈥淲e don鈥檛 have a lot of experience with the chemistry that might go on at these temperatures.鈥� We don鈥檛 know what鈥檚 possible.

The chance to discover a form of life with a different chemical basis than life on Earth has led some researchers to consider Titan the most important world on which to search for extraterrestrial life. In a recent paper in the journal Astrobiology, Robert Shapiro, a professor of chemistry at New York University, and Dirk Shulze-Makuch of Washington State University rated Titan a higher-priority target for investigation than even Mars.

On Mars, and on Jupiter鈥檚 moon Europa and Saturn鈥檚 moon Enceladus as well, astrobiological efforts center on the hunt for water-based life. But such life, even if it is found, could have shared a single origin with life on Earth, getting started on one world and being transferred by meteorites to others. Not so for Titan. If there is life on Titan, it arose separately from life on Earth.

Not everyone agrees that Titan is the priority, though. NASA and ESA recently gave the nod to a Jupiter-system mission that will explore Europa as the next flagship mission to the outer solar system. It may be decades before another major mission flies to Saturn and Titan.

But a smaller-scale and less-expensive lander known as the Titan Mare Explorer (TiME) could launch as early as 2015, arriving in 2022 or 2023. Ellen Stofan of Proxemy Research in Rectortown, Va., the principal investigator for the TiME mission, described the lander as a buoy-shaped capsule that would splash down in one of Titan鈥檚 northern lakes and float across its surface for a minimum of two Titan days (sixteen Earth days).

鈥淲e have a number of instruments on board. The most important from a pure scientific point of view is a mass spectrometer,鈥� Stofan said. 鈥淲e鈥檒l take basically a sip of [the lake] liquids, several times, and analyze them to really nail down their chemical compositions. We know there鈥檚 methane, we know there鈥檚 ethane,鈥� but TiME would inventory more complex organic (hydrocarbon) compounds, as well.

If there is life on Titan, it may be difficult to detect. 鈥淚 don鈥檛 expect you to go to these lakes and see beautiful filamentary structures made of cells that are macroscopic in size or easily seen,鈥� says Lunine, who is a co-investigator on the proposed TiME mission. The clues could be subtle. 鈥淲e would have to look for particular peculiarities in composition, hydrocarbons that are lacking that should be there, others that are more abundant鈥� than expected.

No-one knows 鈥渨hat happens to organic chemistry in [Titan鈥檚] environment,鈥� Lunine adds. 鈥淒oes it go to a kind of a chemistry that we can call life but works in liquid hydrocarbons? We don鈥檛 know the answer to that. But the answer is profound.鈥� Because if the answer is yes, it means that the origin of life has taken place more than once. 鈥淚f the answer is yes, then it says that life 鈥� must be a common outcome of planetary processes in the cosmos.鈥�

If the answer is yes, it means we are not alone.