The methane rain falls mainly on the plain

NASA’s Cassini space probe has been orbiting Saturn (that’s the pretty planet with the rings) taking pictures of it and its moons since 2004. In particular it’s focused on Saturn’s largest moon, Titan; in fact, Cassini travelled to Saturn accompanied by a lander called Huygens, which actually went down to Titan’s surface.

Titan is so intriguing for scientists because it’s large enough to have an atmosphere. And recently Cassini was fortunate enough to catch pictures of methane rain around Titan’s equator.

Image from NASA's Cassini spacecraft, showing a huge arrow-shaped methane storm blows across the equatorial region of Saturn's largest moon, Titan (click to embiggen)
A huge arrow-shaped methane storm blows across Saturn's largest moon, Titan (Image: NASA/JPL/SSI)

Along with its parent planet, Titan is over 9 times further from the Sun than the Earth is, so naturally it’s a lot colder. Its average surface temperature is −179.5 °C, which is of course well below the freezing point of water.

But methane (chemical formula CH4), which is a gas on Earth, is liquid between -182.5 °C and -161.6 °C. This means that Titan can experience the same types of weather events – rain, storms, etc. – as us. Only with methane instead of water.

Over the years, Cassini and Huygens have gathered plenty of evidence to show this is actually the case. But these new pictures are the first time that tropical rains have been observed on Titan. Actually, this storm occurred in Titan’s spring, so it’s really a very large spring shower.

Why is this so important? If nothing else, this is a great opportunity for Earth scientists to study another world’s climate – until now we’ve only had a sample size of one to work with. But Titan gives us a chance to compare, and see how “typical” Earth’s weather really is.

You can see more about Titan, Saturn and the other planets on the excellent BBC TV series Wonders of the Solar System, which has been screening in Australia on SBS television. Episodes are still available to view on the SBS website, so catch it while you can.

This program is hosted “rock star physicist” Brian Cox, who is all over the media at the moment because, unlike a lot of “rock star physicists”, he is an actual rock star. OK, pop star. Or maybe, dance-synth-fusion star. Specifically, he was the keyboard player of the British band D:Ream:

Which goes to show: just because you have a number one dance hit, it needn’t hold you back from having a career in particle physics too.


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