It’s Raining CH4

The sky is overcast, hazy and orange, there’s a light wind. It’s a tad chilly too, -170°C. Water’s frozen as hard as steel. And yet it appears to be raining. Large globules of liquid slowly float like snowflakes to the surface.

We are ofcourse not in some bizarre and crazy sci-fi TV series, but on the surface of Saturn’s moon, Titan.

Saturn's Rings, the small moon Epimetheus and Titan

Titan’s a pretty exciting place. It’s the only moon in the entire Solar System with an appreciable atmosphere (it’s thicker than the Earth’s!) and it’s bigger than the planet Mercury!

The moon has remained elusive and confusing for quite some time. The Voyagers flew past back in the 70’s but we really didn’t get to know a lot until Cassini arrived in the Saturnian system back in 2004 and the Huygens probe descended through the atmosphere of Titan that we really got to know the place.

The atmosphere is made of complex hydrocarbons (molecules containing hydrogen and carbon) to which Carl Sagan once said ‘It’s a moon covered in complex organic molecules like those that in the earliest history of our planet, led to the origin of life’. Titan’s in deep, deep freeze though.

It’s so cold that water-ice doesn’t actually behave like ice (as we know it). This ice is so cold it behaves like rock here on Earth.

So how on Earth, sorry, how on Titan is it raining?

On Earth we have a very special, delicate, amazing atmosphere. Beautifully thin and elegant. It allows water to exist in 3 states at the same time. A liquid, a gas and a solid. On Titan, it being to cold for this equilibrium of water, there’s something else in this delicate balance – Methane. On Titan it exists as a solid, a liquid and a gas. The temperature and pressure are just right to allow this. So on Earth we have the hydrological cycle, on Titan it’s the methanological cycle.

True Colour image of the top of Titans Atmosphere

We have river valleys here on Earth, carved by the effects of the rain. Titan has river valleys, they too are carved by the effects of the rain.

Since Cassini arrived at Saturn, we’ve known about lakes of methane and rains in the north and south poles of the moon. In fact some amazing radar images have been taken that show these lakes. But it how long been thought, well in fact known, that the equatorial regions of Titan are your ice cold equivalent of a desert. Not any more. Cassini has noticed something rather exciting in Titans atmosphere. This here photo!

Clouds on Titan

The surface of the moon has been seen to darken slightly. The explanation for this, huge methane rainstorms. Like on the Earth, Titan has seasons too, although on a much, much longer timescale (it takes Saturn 27 years to go around the Sun) and the equivalent season Titan is in at the moment is April. April showers! This hadn’t been seen before precisely because it takes Saturn such a darn long time to go around the Sun. Cassini’s mission ends in 2017, so there’s still plenty of time to see any other exciting stuff that might be going on.

Any terrestrial body with an atmosphere I find amazing. That’s really just the Earth, Mars and Titan, but the processes that go on in these atmospheres staggers me. Weather is cool.

No wonder then that Titan is Brain Cox’s favourite wonder!

Press release from the Jet Propulsion Laboratory

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