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# Deep and methane-rich lakes on Titan

## Abstract

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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## Data availability

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.

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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## Acknowledgements

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.

## Author information

M.M. led the data analysis, conceived the main conceptual ideas and wrote the manuscript. V.P. made a significant contribution to the analysis and interpretation of data, performed some of the numerical calculations, contributed to the writing of the manuscript and prepared figures. J.I.L. participated in the calculations of the lake compositions and co-wrote the interpretation in terms of geological mechanisms. A.G.H. and R.L. participated in the interpretation of geological mechanism, contributed to drafting the article and revising it for intellectual content. R.S. and G.M. contributed to data analysis and interpretation and contributed to drafting the article.

### Competing interests

The authors declare no competing interests.

Correspondence to M. Mastrogiuseppe.

## Supplementary information

1. ### Supplementary Information

Supplementary Figures 1–2, Supplementary Table 1