It has recently rained on Titan, Saturn’s largest moon

Credits : NASA/JPL/University of Arizona/University of Idaho

A recent image taken by the Cassini probe shows signs of precipitation on Titan, Saturn’s biggest moon.  This rain is the first signal that summer has arrived in the moon’s northern hemisphere. 

It might have taken it’s time but it has finally arrived.  Summer arrived in the northern hemisphere of Titan, Saturn’s largest moon, a couple of months ago. Thanks to the moon’s gradient we know that seasons on Titan last more than seven years.  Given this knowledge, astronomers in charge of the Cassini mission expected to see the summer move to the northern hemisphere in 2017. However their expectations did not go as planned. For several months, nothing seemed to suggest a change of season which was a little disconcerting researchers. However evidence of precipitation has recently been detected on Titan, which suggests the arrival of the summer season.

Raining in the north

“The whole Titan community has been looking forward to seeing clouds and rains on Titan’s north pole, indicating the start of the northern summer, but despite what the climate models had predicted, we weren’t even seeing any clouds,” explained Rajani Dhingra, one of the people responsible for the mission. Researchers were relying on the presence of clouds to assess whether or not rain could fall on Titan.  However this is where the problem lies, as clouds are not the only indicator.

Cassini’s team recently discovered an anomaly on one of the images taken by Cassini on June 7, 2016. Much of the surface suddenly became reflective. However this was not the case on photographs taken before or afterward. It is a bit like the pavement being dry on your way home, then very wet and reflective when leaving your house again – proof that it has just been raining. For astronomers, reflective spots prove that precipitation has actually occurred in the north.

Titan Saturne Cassini
Reflective marks prove there has been recent rain on Titan. Credits : NASA

Hydrocarbon water droplets

Although it seems as though it has been raining on Titan, remember it is not water that is falling from the sky.  The sea, lakes and rivers are made up of liquid methane and ethane. (It is very cold on the moon around – 180 degrees C.  In very cold temperatures like this these gases turn into their liquid state.) After analysing the reasons behind the reflections on the images returned by Cassini, it appears that the rain in falling on a rocky and uneven ground rather than on a smooth surface.

It appears that Titan’s weather forecast was wrong but summer has in fact arrived. For researchers, the next step will be to evaluate the reasons for this discrepancy.  “We want our model predictions to match our observations,” stated Dhingra. “This rainfall detection proves Cassini’s climate follows the theoretical climate models we know of. Summer is happening. It was delayed, but it’s happening. We will have to figure out what caused the delay, though.”


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