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The case for seasonal surface changes at Titan’s lake district

This article has been updated

Abstract

Titan, Saturn’s largest moon, hosts lakes and seas of liquid hydrocarbons at its poles1. General circulation models demonstrate that regional evaporation and precipitation rates of methane are likely to change with the seasons (Titan’s year is 29.5 Earth years) and evolve on a geological timescale (~105 Earth years)2,3,4. Cassini observations suggest shoreline recession at a few south polar lakes during local summer5, but similar seasonal changes have yet to be observed at the north pole where lakes are larger and more numerous6,7. We present three ‘phantom lakes’ that appear to be north polar surface liquids in winter observations by Cassini RADAR but that are inconsistent with lakes in infrared images obtained up to seven years later, after vernal equinox, suggesting that the liquids were removed in between. If this were the case, the phantom lakes could be interpreted as shallow ponds, with either a pure methane composition or a regolith porous enough to remove the less volatile ethane. These phantom lakes provide observational constraints on removal timescales for surface liquids at Titan’s north pole. The location, size and longevity of surface liquid reservoirs affect sediment processing7, seasonal weather8, climate evolution9, and even, perhaps, their habitability10. As solubility of the possible non-polar mixtures is generally low, short-lived lakes might be nutrient-poor10 and thus have low astrobiological potential.

Fig. 1: Cassini images of Titan’s north pole.
Fig. 2: Sequence of observations of the phantom lakes, outlined in each dataset.
Fig. 3: Normalized radar cross-sections (σ0) from SAR swaths as a function of incidence angle for the phantom lakes A, B and C.
Fig. 4: Behaviour of phantom lakes relative to other VIMS spectral units within the atmospheric windows.

Data availability

All data presented here are available from the NASA Planetary Data System (https://pds-imaging.jpl.nasa.gov/portal/cassini_mission.html), except the NLDSAR swaths that are maintained by A.L. (http://cssnldsar.geophysx.org/). The data that support the plots within this paper are also available from the corresponding author upon reasonable request.

Change history

  • 01 May 2019

    In the version of this Article originally published, the following ‘Journal peer review information’ was missing: “Nature Astronomy thanks Marco Mastrogiuseppe and the other anonymous reviewer(s) for their contribution to the peer review of this work.” This statement has now been added.

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Acknowledgements

This work was supported under the NASA Earth and Space Science Fellowship Program grant NNX14AO30H to S.M.M. J.W.B. acknowledges support from NASA Cassini Data Analysis Program NNX15AI77G. A.L. and S.R. acknowledge the financial support of the UnivEarthS Labex program at Sorbonne Paris Cité (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02). S.R. is also supported by the French National Research Agency (ANR-APOSTIC-11-BS56-002, ANR-12-BS05-001-3/EXO-DUNES). Part of this work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. Government sponsorship is acknowledged.

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S.M.M. led the procuring and analysis of the data and wrote the manuscript. J.W.B. assisted in all aspects of the analysis and writing. J.D.H. and S.P.D.B. contributed to the analysis of RADAR data. M.M.H. helped develop scattering models. A.L. wrote the code to produce NLDSAR and provided these data. All authors contributed to the discussion of these results. S.R. provided the radiative transfer model and contributed to their analysis. E.P.T. assisted in the calibration of ISS data and planned the ISS observations. C.S. planned the VIMS Titan observations.

Corresponding author

Correspondence to Shannon M. MacKenzie.

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Journal peer review information: Nature Astronomy thanks Marco Mastrogiuseppe and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Supplementary Figures 1–6, Supplementary Text, Supplementary References

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MacKenzie, S.M., Barnes, J.W., Hofgartner, J.D. et al. The case for seasonal surface changes at Titan’s lake district. Nat Astron 3, 506–510 (2019). https://doi.org/10.1038/s41550-018-0687-6

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