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.
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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.
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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.
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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