Currents and tides in the seas of Titan

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A new field of science is born: exo-oceanography. As the seas of Titan are mapped in ever more details by the Cassini probe, Lorenz et al. (2014) boldly venture into studying the currents and tides in the sea of Kraken.

Methane and ethane are abundant in the atmosphere of Titan and the temperature is sufficiently low for their precipitation. Although the Hyugens probe has landed on a dry spot, we now know from accumulated data from the Cassini mission that lakes are abundant on Titan, particularly in the polar regions. They show up as very dark regions on images and smooth surfaces on radar data (see e.g. this post).

The largest body of liquid hydrocarbons on Titan is named Kraken Mare, and Lorenz et al. present gloriously detailed maps from the new Cassini data. The Sea of the Kraken (see map below) turns out to consist of two distinct seas, connected by a large channel. To place it in an Earthly context, Kraken Mare is the size of the Caspian Sea, and the channel has the length and width of the Strait of Gibraltar.

Kraken Mare

Map of Kraken Mare from the Cassini data, showing the channel connecting the two sides. The scale bar is in kilometres. [Lorenz et al. Figure 3]

Lorenz et al. speculate that tides will cause large currents through the channel, re-distributing liquid from one lobe of the sea to the other. Titan is in synchronized rotation around Saturn (its rotation and orbital periods are identical), so that it does not undergo the kind of tides that the Moon causes on Earth’s oceans. But it is subjected to weaker tides due to orbital eccentricity (the orbit as an eccentricity of 2.8%). An earlier study of Ligeia Sea (Lorenz et al. 2010) suggested that tides on Titan could raise the surface of the seas by up to 80 cm.

Tides cause turbulent currents through straits like Gibraltar on Earth, with effects that can be noticed from space under favourable conditions. The Strait of Gibraltar is 36 miles long and 8 miles wide at the narrowest (58 and 13 km), the channel between the two sides of Kraken Lacus is 40 by 17 km.

Radar Map of the Gibraltar Straights. Currents from the Atlantic Ocean (left) into the Mediterranean cause visible differences in smoothness.

Radar Map of the Gibraltar Straights. Currents from the Atlantic Ocean (left) flowing into the Mediterranean cause detectable differences in smoothness. [Figure 6 from the paper, original source ESA]

Could we measure the currents in Kraken Mare? The Cassini probe has a radar able to measure the smoothness of Titan’s surface. A strong current flowing through the Kraken channel could cause enough ripples on the surface to be detectable as a change of smoothness.

Beyond the specific interest of tides and currents in hydrocarbon seas, isn’t it marvellous that we can even talk about ripples on alien seas with precise space data to back up our speculations?

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About Author

I am a professor of planetary science at the University of Exeter. My specialty is the study of exoplanets, in particular the observation and modelling of exoplanet atmospheres. I have done my PhD a the University of Geneva and worked in Chile, France and Switzerland.