Keeping Titan Old

As the Cassini orbiter makes its 103rd close pass by Titan, have long-agers found ways to keep it billions of years old?

Space.com announced the 103rd flyby of Titan by Cassini, and it’s a daring one: just 2,274 miles above the surface at 13,000 miles per hour; PhysOrg announced the next day that it was successful.  With about 40% of the surface mapped by radar and other instruments, enough should be known by now to explain its youthful appearance in terms of the assumed age of the solar system (4.5 billion years).

Astrobiology Magazine reported that JPL chemists are getting closer to reproducing Titan’s peculiar smog, but the fit isn’t perfect yet.  That article did not address the age conundrum: how long can Titan’s atmosphere create these complex molecules, and are they reversible?

Jeff Hecht at New Scientist was the only reporter recently to address the age question.  First, he laid out the problem:

Scientists have puzzled over Titan’s atmospheric methane because sunlight causes the molecule to react readily with other chemicals in the air, producing the moon’s dense smog. Calculations suggest that the amount of methane now found in Titan’s atmosphere should have been used up within tens of millions of years – a blip in the moon’s roughly 4-billion-year lifetime….

Adding to the mystery, the methane reactions create hydrocarbon compounds that rain over the surface. If fresh methane emissions steadily replaced used-up methane over the course of Titan’s history, this process would happen constantly, and Titan would be covered not by lakes, but by a global ocean hundreds of metres deep.

Such an ocean, of course, was not found, sending theoreticians scrambling for answers.  Hecht’s favorite answer is the snowball cycle proposed by Caltech’s Michael Wong.  Periodically, the atmosphere collapses, then reappears.  This happens because the methane provides only a marginal greenhouse effect to keep the nitrogen in a gas phase.  In the cycle scenario, the methane gets used up, the nitrogen freezes out, and the atmosphere turns to ice on the surface – until enough new methane outgasses to warm it up again.   There are several “if’s” in this scenario:

Methane levels may rise and fall if the gas is periodically released from inside the moon, which would explain how Titan has so much in its atmosphere today. If at some point the methane dropped by a factor of 100, temperatures would fall, and surface liquids would ice over. The haze produced by methane reactions would also freeze out, leaving the atmosphere clear and exposing the snowy surface.

For evidence, Wong points to several “Snowball Earth” episodes on our home planet (which Titan is supposed to resemble in some respects).

But how would this explain the moon’s missing ocean? This clear atmosphere would produce a different mix of molecules, so the cold snap would leave the moon’s surface covered in lots of compounds called nitriles, which would be solid rather than creating an ocean. The absence of deep oceans suggests that Titan spends more time as a snowball than as a smogball.

Any way to test this theory?  “Getting good enough readings of the surface composition to check this would require a future mission to Titan,” Hecht writes, meaning the test will not likely be possible in our lifetime, even if a proposed sampling mission gains approval (PhysOrg).  Perhaps the New Horizons mission next year will reveal similar processes operating on Pluto.  Even so, Pluto is not Titan, nor is Earth; the “Snowball Earth” theory, furthermore, is not without its skeptics (9/02/13).

The short article did not address whether evidence exists for a methane reservoir under Titan’s surface, or if it’s there, how it would escape (4/16/13, 4/09/11).  It also seems that nitriles would be produced in the current regime, not just during the freezing periods.  Others, like Sushil Atreya, have said that the entire atmosphere (mostly nitrogen) would collapse if the methane were depleted.

As we have noted before, this is an ad hoc theory rescue operation in progress.  A clear prediction was made in the 1990s that spacecraft would find Titan covered in a global ocean of liquid hydrocarbons; that prediction was falsified even before the Huygens Probe landed with a thud in January, 2005.  There is no evidence that Titan’s methane is being replenished.

Assume, for the sake of argument, Wong’s theory works: there are cycles of methane collapse and resurrection.  Let’s grant an underground reservoir of methane that outgasses into the atmosphere.  Since the observed methane budget would be spent in a few tens of millions of years, how many cycles could repeat in 4.5 billion years?  A generous assumption of 100,000 years per cycle would produce 45 cycles of atmospheric collapse and resurrection.  A more reasonable 10,000 years would produce 450 cycles.  Is that reasonable?

What we observe is a fairly dry moon with few impact craters, abundant sand dunes, and significant river channels, and a few large lakes.  This fits a scenario of steadily depleting methane.  We think science should stick with observations, not manufacture implausible scenarios to favor one’s preferred timeline.  If Titan is young, let it be young.