March 1, 2012

The dunes's build-up on Titan

With her partners from ESA and ASI, Alice Le Gall, investigator of LATMOS-IPSL, just showed using the radar data of NASA's Cassini that the dunes' dimensions on Titan (one of the Saturn's moons) vary according to the latitude and altitude. Because dunes do build-up only under certain conditions and their aspects reflect the environment in which they evolve, this information brought new reflection and response elements on the Titan's climatic and geological history.
Update: 03/01/2012

Titan is the biggest Saturn's moon and the second biggest one of the solar system. Dunes cover around 13% of Titan's surface, i.e. 10 millions square kilometres (the surface area of the United States). Plains cover the rest of the moon's landscape. According to the environment in which the dunes evolve, they can measure several hundreds kilometres length, 1 to 2 kilometres wide and around 100 metres high. They build-up only under certain conditions: strong enough wind and mobile sediments.

In terms of altitude, the higher they are, the thinner and more scattered. The Cassini's radar seems to show shortfalls between dunes which could be indicative of a thinner sand deposit in places. This suggests that the sand used to build-up dunes comes from Titan's plains. Scientists believe that the sand of Titan is not made from silica, like on Earth, but from solid hydrocarbons which had precipitated outside the atmosphere.

In terms of latitude, Titan's dunes are limited to the equatorial regions in a 30 degrees wide ridge, both North and South. Nevertheless, those dunes are bigger in the South, probably because of the Saturn's elliptic orbit.

Titan is in orbit around Saturn and its seasons are determined by the path of Saturn around the Sun. As Saturn takes around 30 years to achieve a complete orbit, each Titan's season lasts approximately seven years. The slightly elliptic orbit of Saturn implies that in the Southern hemisphere, summers are shorter but more intense. So the southern regions are probably drier, implying less soil humidity. The drier the grains of sand, the easier they are to carry by the wind to build-up dunes. According to Alice Le Gall "the soil humidity certainly increases in the north, so the sand particles are less mobile and the dunes development is more difficult."

Lakes and seas are not symmetrically spread in terms of latitude. Those liquid methane and ethane resources are principally located in the north. The idea of a more humid soil and less easily wind-transported grains of sand is then reinforced.

According to Nicolas Altobelli, scientist on the Cassini-Huygens project at ESA, "How do the dunes build-up? Why this shape? This size? This repartition on the Titan's surface? Those questions are primordial to understand the climate of Titan and its geology because the dunes are a significant atmosphere-surface exchange interface. Being built-up from frozen atmospheric hydrocarbons, dunes could provide important evidences on the methane/ethane cycle which remains a mystery on Titan, even if it is comparable in many aspects to the water cycle on Earth."

Article références

A. Le Gall, and al., "Latitudinal and altitudinal controls of Titan's dune field morphometry", 2012, Icarus, 217, 231-242

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