Titan’s Lakeside the Key?

The lakes on Saturn’s moon may hold the secret to its entire climate system…

Titan is the most infamous of all the Saturnian moons.  The largest of its kin, it bears a thick atmosphere and is the only other body in the Solar System known to exhibit stable liquid lakes on its surface.  Admittedly, these lakes are made of hydrocarbons rather than water, and the surface temperature is a chilly -180 degrees C, so hardly a holiday destination.  Even so, Titan may be one of the few places in our neighbourhood where life (and we, with a little technology) could exist.

But what would life be like on a moon orbiting a gas giant? It’s difficult to know, but if Titanian life needs a solvent to begin in (e.g. the hydrocarbon lakes), then the past, present and future of those lakes will be extremely important.  Today’s edition of Nature Geoscience carries some interesting research on the distribution of lakes on Titan.  NASA’s Cassini spacecraft has been taking radar data since its arrival in 2004, and the findings are curious – the Northern Hemisphere of Titan seems to have more lakes than the Southern Hemisphere.  Why should this be? The North and South are pretty similar as hemispheres go.

What if it’s a seasonal effect?  The Titanian year lasts about 30 Earth years – this means that every 15 years the North and the South switch between summer and winter. The seasonal changes bring increased rainfall, filling the lakes on one hemisphere while drying the other out.  This might drain some shallow lochs, but not Titan’s hundred metre deep lakes, according to the study’s lead author Oded Aharonson of Caltech.

His solution is ingenious.  Let’s remember that Titan is a moon, circling Saturn.  Saturn’s orbit is slightly eccentric, with a period of (you guessed it) 29 years.  This means that Titan will come 12% closer to the Sun during one of the hemisphere’s summers (the Southern in this case).  The southern summers are therefore hotter than the northern summers, forcing more evaporation in the South and drying out the lakes asymmetrically.

This is an example of astronomical factors directly influencing the climate.  We have such cyclical variations on Earth, too: we call them Milankovitch cycles – they appear to be responsible for the motion of glaciers around the globe, and they cause Ice Ages.  And it looks like we’ve detected evidence for Milankovitch-esque cycles on Titan.  What else will we find on this unassuming moon? We’ll have to let Cassini work a little longer to find out!


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