From dunes to canyons, the variety of terrain on Saturn's satellite is astonishing. Scientists have explained how soft organic dust 'fuses' into grains of sand and rock, while winds and rain form dunes and carve canyons.
Titan, Saturn's satellite, is the only known body in the solar system other than Earth that has a seasonal cycle of liquids. Scientists at Stanford and the Jet Propulsion Laboratory have shown how this cycle of fluids, together with the cycle of sedimentary rocks, explains Titan's diverse landscape. Their paper is published in Geophysical Research Letters.
From afar, Titan looks like Earth. It is flanked by dunes at the equator, plains dominate at mid-latitudes and lakes and canyons at the poles. Thanks to a cocoon of dense atmosphere, rain falls and winds blow. Except that liquid methane flows on Titan instead of water and nitrogen winds collect dunes of hydrocarbon sand.
The mechanical properties of this sand are very different from those of the silicate sedimentary rocks that cover Earth and Mars, which we are accustomed to. The main difference is that hydrocarbon particles are much less hard and have to be quickly crushed into dust while dunes require relatively coarse granules. How do these rudimentary organic compounds maintain their size?
On Earth, silicate rocks and minerals are eventually broken down into smaller particles by the forces of nature. Wind and water currents carry these particles to sedimentary areas where, under the influence of pressure, groundwater and sometimes heat, the particles 'fuse' back into the stones.
Scientists speculate that a similar cycle must exist on Titan. Obviously, the forces of nature are grinding the rocks down to sand and dust there too. What is not clear is how the stones and sand for the dunes are obtained from the dust again.
Ooids, small spherical granules that are often found in the shallow waters of tropical seas, have helped to find the answer to this question. They are unique in that they are formed by chemical precipitation: calcium carbonate from water is deposited in layers onto the grains of ordinary sand. The particles rub against each other, break apart, and the calcium carbonate is released back into the water and deposited again. The processes of breaking down and building up compensate each other and the particles maintain their size. The researchers hypothesised that, in the same way, dust particles on Titan also 'fuse' with each other to form sand grains and rocks which are then ground up by the wind.
The team used this hypothesis to model the sediment migration cycle using atmospheric and climatic data collected by the Cassini mission. It turned out that the process of 'fusion', wind patterns and precipitation explain the diversity of the landscape.
Winds are strongest in the equatorial belt, so the sediment there is constantly breaking up and forming dunes. At mid-latitudes, the winds are much weaker, but the humidity is higher, and particles 'fuse' into the dense rocks that form the plains. At the poles, rains and rivers carve deep canyons and labyrinths in the rock. Their earthly counterpart is karsts.
"Our overarching model allows us to understand how these sedimentary landforms interact," comments Mathieu Lapôtre, a geologist at Stanford University. - If we understand how these puzzle pieces fit together and their mechanics, then by studying the topography formed by these sedimentary processes, we can make assumptions about Titan's climate and geological history - and their impact on the likelihood of life on Titan.
