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How Titan Got Its Atmosphere

The presence of methane in Titan’s atmosphere has puzzled astronomers for decades. Now they think they know where it came from.

Methane doesn’t last long in sunlight. The sun’s rays rapidly break it down into other organic molecules. So the discovery of methane anywhere in the solar system causes a frisson of excitement among astronomers.

And understandably so. The methane cannot have been there for long (otherwise it would have been broken down by sunlight). So it must have been recently released into the atmosphere. On Earth, most methane in the atmosphere is produced by the ongoing, unstoppable farting of living things.

That’s why the recent discovery of methane in small amounts on Mars caused such excitement. Could it be that methane-farting Martians are responsible? Probably not. Many commentators ignore the fact that methane on Earth is released also by volcanoes, hydrothermal vents, and in some reactions between rocks and water.

However, the 800-pound gorilla is Saturn’s moon, Titan, which has a dense nitrogen atmosphere with a sizable fraction of methane. The question is how this methane gets there, if it is constantly being replaced as it is broken down by sunlight.

There are two suggested answers (ignoring the wild suggestion that some kind of farting organisms could be responsible).

The first possibility is an ongoing reaction beneath Titan’s surface between iron or magnesium silicates, water, and carbon dioxide to produce methane. This is called serpentization and occurs on Earth in various places, such as the Precambrian rocks beneath parts of Canada.

The second is that methane ice was incorporated in Titan’s interior when the moon formed in the early solar system and that the atmosphere is constantly refreshed by huge methane belches from below as this ice melts and escapes.

An international group of planetary geologists say they know the answer. Recent measurements of the ratio of hydrogen to deuterium in Titan’s methane, they argue, cannot be explained by serpentization reactions, since the water involved would have an improbably strange mix of these isotopes.

On the other hand, primordial methane may well have had a mix of hydrogen and deuterium that is closer to what we see on Titan today. And the difference can be explained by the way photolysis prefers one isotope to the other.

Interestingly, the researchers suggest a way of testing their idea. They say that another of Saturn’s moons, Enceladus, must have been formed from the same primordial methane. Enceladus seems to occasionally burp this stuff into orbit around Saturn. A measurement of the isotopic ratio of this methane could settle the question, or at least strongly back the argument.

And who could do such a measurement? Over to the team at the Saturn-orbiting spacecraft, Cassini.

Ref: arxiv.org/abs/0908.0430: A Primordial Origin for the Atmospheric Methane of Saturn’s Moon Titan

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