A whole new world : topographic map of Titan

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Not often do you get an entirely new World Map. The detailed altimetry map of Mars from the MOLA instrument on Mars Global Surveyor was a revolution. Now Ralph Lorenz and collaborators present a complete altitude map of Titan (Saturn’s largest moon), reconstructed from radar data from the Cassini spacecraft, with a resolution of 1×1 degree (a degree of latitude is about 45 km on Titan).

An outline of the paper: A GLOBAL TOPOGRAPHIC MAP OF TITAN, by Ralph Lorenz et al. (Icarus 225)

It must be said, though, that the present altimetry data covers only about 35% of the planet. Inferring a global map from these data requires a lot of interpolation, and the result is a first approximation. As an illustration of the potential of the reconstruction, the authors provide an altitude map of the Earth built with the same sampling as the Titan data. The main continents and seafloors can be identified, but with a lot of distortions.

Earth topography, as it would be recovered with the same sampling as the Titan data

Earth topography, as it would be recovered with the same sampling as the Titan data

Detailed studies of the Cassini and Huygens data have shown that Titan has lakes of methane and ethane, which are fed by river systems and can grow or shrink within a Titan season. The lakes are at present clustered in the polar regions, particularly in the northern hemisphere. The planet also shows regions of “evaporates” that are thought to correspond to former lakes, and “blandlands”, terrains without clear features whose origin is still unclear.

According to the radar scans, the landscape of Titan is quite flat. Individual scans typically show less than 50m slopes over several hundred of kilometers (see Zebker et al. 2009 for a discussion). Even the local elevations detected are quite smooth, rising a few hundred meters over hundreds of kilometers (something like the Massif Central in France). The authors note that if the origin of this topography is volcanic, it would indicate shield volcanism (built almost entirely of fluid lava flows) with a high viscosity.

This is the global map of Titan:

Global topographic map of Titan

Global topographic map of Titan

The dominant global feature is that the equatorial regions are more elevated than the polar ones, an important feature to include in atmospheric circulation models of Titan.

The authors also point out the presence of flat lowlands in the South polar regions, that could be the remains of former giant lakes. Most of Titan’s lakes are now located near the North Pole, but that might be due to the shape of the orbit. The Milankovich cycles mean that about 50’000 years ago (“when humans were migrating out of Africa for the first time”, as Lorenz et al. point out), the lakes might have been in the South, and they might have formed the flat lowlands when drying.

The authors also compare the global map to the pattern of flow channels in some areas that have been imaged in detail, and find a satisfactory correspondence.

The terrain near an elevation in the topography data, showing flow channels compatible with drainage downhill [from Fig. 11 of the paper]

The terrain near an elevation in the topography data, showing flow channels compatible with drainage downhill [from Fig. 11 of the paper]

This map will help Titan in joining the selected club of Solar System bodies for which detailed circulation and climate models can be run.

 

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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.