Saturn’s largest moon, Titan, hosts liquid hydrocarbon lakes and seas on its surface. During the last close encounter with Titan (22 April 2017), the Cassini spacecraft used its RADAR as a sounder to probe the depth of several lakes in the north polar terrain. This was the first time that Titan’s lakes, as opposed to its seas, have been viewed in a sounding configuration. Here, we show that these lakes can exceed 100 m depth and their transparency at the 2.17 cm radar wavelength indicates that they have a methane-dominated composition. This composition differs significantly from that of Ontario Lacus, the only major lake in Titan’s southern hemisphere, which is more ethane rich. If the methane-rich north polar lakes, perched hundreds of metres above the major seas, are formed by a karstic-type process, then they may drain by subsurface flow at rates between 0.001 and 1 m yr−1 (Titan year). Subsurface reservoirs and flows therefore may be an important element of the Titan geochemical system.
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The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. Long Burst Data Record products are available from NASA Planetary Data System (https://pds-imaging.jpl.nasa.gov/data/cassini/cassini_orbiter).
Journal peer review information: Nature Astronomy thanks Alice Le Gall and the other anonymous reviewer(s) for their contribution to the peer review of this work.
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M.M. and R.S. acknowledge support from Italian Space Agency (ASI) grant 2014-041-R.0.; M.M., A.G.H. and V.P. acknowledge support from NASA CDAP grant NNX15AH10G; J.I.L. is grateful for the ministrations of the Cassini mission in supporting his research. R.L. acknowledges the support of NASA OPR Grant NNX13AK97G. We appreciate the efforts of the Cassini TOST (Titan Orbiter Science Team) and RADAR Team in planning and executing these observations.
Supplementary Figures 1–2, Supplementary Table 1
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Nature Reviews Physics (2019)