Laboratory experiments on the interaction between ice and hydrocarbons: implications for the formation of lakes on Titan (Invited)

Mathematics – Logic

Scientific paper

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[0714] Cryosphere / Clathrate, [0746] Cryosphere / Lakes, [5422] Planetary Sciences: Solid Surface Planets / Ices, [5462] Planetary Sciences: Solid Surface Planets / Polar Regions

Scientific paper

Laboratory experiments have been conducted to aid understanding of the global carbon cycle on Titan. In particular, we aim to understand how liquid hydrocarbons flow on and under its surface. A chamber has been built in order to simulate the conditions on Titan’s surface. There temperatures vary between 90 K at the winter pole to 94 K at the equator. At these temperatures both methane and ethane are in the liquid phase at pressures above the triple point. Although no rain has been observed by either the Cassini spacecraft or the Huygens probe, the presence of hydrocarbon clouds at the pole, along with river channels and lakes, suggest that liquids condense, fall and run on the surface. Our experiments reproduce these conditions. A pot of liquid hydrocarbons is maintained at a temperature of 92 K and drops form at the tip of a capillary. These drops fall onto an ice target placed on a stage whose temperature is controlled by the flow of liquid nitrogen. Different ice targets have been used: polycrystalline ice with different porosities, as well as single ice crystals. We will present the results of these experiments, which show that water ice can rapidly absorb a large amount of hydrocarbons. These experiments also suggest that the ice must be saturated in hydrocarbons before lakes start forming. During these experiments, no formation of methane or ethane clathrates has been observed, although the experiments are conducted within their stability field. The kinetics of clathrate formation is poorly known, but other experiments performed at higher pressure suggest that they would form rapidly on geological timescales. Titan’s upper crust could therefore be composed of ice saturated with liquid hydrocarbons lying on top of a layer of clathrates. The presence of this layer would influence Titan’s interior temperature profile since the conductivity of clathrates can be up to one order of magnitude smaller than that of water ice. This layer could be destabilized during volcanic or tectonic events that have affected Titan’s surface in a geologically recent past, providing a source for Titan’s atmospheric methane. This work has been carried out at the Jet Propulsion Laboratory-California Institute of Technology, under contract with NASA.

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