Oil slicks on Titan’s seas

Titan’s seas are permanently submitted to an influx of large organic structures produced in the atmosphere by photochemistry and condensation. We have shown that part of this atmospheric material stays by capillarity at the surface of the seas. The damping effect of the resulting slicks is an explanation to the flatness of the seas as seen by the radar measurements of the Cassini space probe.

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Apr 22, 2019
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What happens when large condensed atmospheric molecules reach Titan’s seas? Will they float or will they sink? This question may sound simple, but when it was expressed frankly at a workshop on Titan’s surface in Paris in 2016, a surprising absence of answer aroused from the group of experts on Titan gathered. It was understood that day that the difficulty came from the location of the phenomenon: an interface!

A strong community is working on Titan and is organized in two main domains. A first group is working on the surface and is addressing issues on Titan’s seas as important as their depth, their incredible flatness or the main composition of these cryo-liquid extents. From this point of view, the large particles from Titan’s atmosphere are a diluted and may look as an insignificant contributor to the main composition of the seas. The second group is on the contrary interested in the complexity of Titan’s atmospheric chemistry and dynamics. The composition of the permanent photochemical haze is far from being understood. Ice clouds have been observed at various altitudes, latitudes and with different spectral signatures revealing condensation processes depending on meteorological parameters. So for this second group of experts, Titan’s seas are beautiful objects but far away from their pressing scientific questioning.

The sea/atmosphere interface was therefore a new field of investigation that required gathering both communities. So the two co-authors began discussing on that question at that meeting. One is expert on Titan’s seas, and the other on Titan’s atmospheric aerosols. They probed and modelled one by one the interactions between the condensed particles and the cryo-seas. They were rapidly able to discard the simple Archimede buoyancy process, as the falling particles are expected to be denser than the cryo-solvents composing Titan’s seas. Capillarity was much more than a challenge as the wettability of Titan’s organic particles with the cryo-seas is a missing data. However, given the knowledge on laboratory analogues of Titan’s aerosols and their solubility properties, the authors predicted that a significant part of the falling material would stay at the surface of Titan’s seas. Then the authors realized the consequences of such a slick: the damping effect could be the reason for the incredible flatness of the seas, thus proposing an innovative answer to a major question of the community. See the results in in Nature Geoscience : https://www.nature.com/articles/s41561-019-0344-4

Poster image: Artwork (c) Ron Miller

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Nathalie Carrasco

Professor, University of Versailles Saint Quentin

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