NASA’s Cassini spacecraft just spotted a mysterious ice cloud over Saturn’s largest moon, Titan, and its appearance challenges everything we thought we knew about the moon’s atmosphere.

First spotted decades ago by NASA’s Voyager 1 spacecraft, the cloud has reappeared for the second time, and it’s somehow made up of compounds that barely exist in Titan’s atmosphere. So where did it come from?

“The appearance of this ice cloud goes against everything we know about the way clouds form on Titan,” said lead researcher Carrie Anderson from NASA’s Goddard Space Flight Centre.

Back when Voyager 1 first spotted this cloud in Titan’s stratosphere during its 1980-81 Saturn flyby, scientists determined that it was formed from a compound of carbon and nitrogen called dicyanoacetylene (C4N2).

C4N2 is a key compound for Titan, because it’s part of a unique ‘chemical cocktail’ that gives the moon its hazy, burnt-orange atmosphere.

But there was a problem – up in the stratosphere where the C4N2 cloud had formed, scientists detected less than 1 percent of the C4N2 gas needed for the cloud to condense and form.

In other words, there simply isn’t enough C4N2 in Titan’s stratosphere to facilitate cloud formation, according to our current understanding of the laws of thermodynamics.

Fast-forward to now, and NASA’s Cassini spacecraft has just sent back data from its latest Titan flyby to reveal that the same type of cloud is there, up in the giant moon’s stratosphere, and it’s still made from an ‘impossible’ amount of C4N2.

Titan is one of the most exciting places in our Solar System, because it’s basically like a frozen version of Earth, with mountain chains and rolling dunes on its surface, protected by a thick, smoggy atmosphere.

Composite image of Titan. Credit: NASA
Composite image of Titan. Credit: NASANA

Cassini’s mission coming to a close next year, may shed some light on this phenomenon. Let’s hope the hardest working spaceprobe in the Solar System can give us some answers before then.

The research has been published in Geophysical Research Letters.