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Liquid ethane is present in the lakes and seas observed on Titan's surface by the Cassini-Huygens mission. While interaction between liquid hydrocarbons and water ice is expected to result in the formation of clathrate hydrates, such reaction (and its kinetics) has not yet been demonstrated for cryogenic liquids under relevant planetary conditions. In this paper, we report the first experimental evidence for rapid formation of clathrates upon direct contact of liquid ethane with water ice at 1 bar using micro-Raman spectroscopy. Kinetics experiments conducted in the temperature range 150-173 K yield an activation energy of 14.8 +/- 2.2 kJ/mol, which is suggestive of a diffusion-controlled mechanism for clathrate formation. This implies that a clathrate reservoir can form within seasonal time scales on Titan if liquid ethane comes into contact with a pre-existing icy bedrock, which may hold important implications for the structure and dynamics of Titan's crust and its global evolution.

Plain Language Summary Saturn's moon Titan has a dense atmosphere and liquid lakes (of methane and ethane) on its surface. These organic compounds are the main constituents of natural gas on Earth. Water is completely frozen and comprises most of Titan's bedrock materials. Under the right conditions, it is common for water ice to form new solids that trap natural gas in their structures, called clathrate hydrates. On Earth, clathrate hydrates are often found in the permafrost and deep ocean. However, Titan is so much colder that these natural gases exist as liquids. We do not yet know if clathrates can form under such conditions, and if so, how quickly. In this study, we prepared small grains of water ice, bathed them in liquid ethane, and monitored the rate at which clathrate hydrates appear. Our data confirm that clathrates would indeed form on Titan from an ethane lake or rain event onto water ice, and they would do so very quickly (in just a few Earth years). This suggests that clathrate hydrates can be an important material on Titan that may be encountered by future missions there.