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Thermally anomalous features in the subsurface of Enceladus’s south polar terrain

Nature Astronomy volume 1, Article number: 0063 (2017) Cite this article

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Abstract

Saturn’s moon Enceladus is an active world. In 2005, the Cassini spacecraft witnessed for the first time water-rich jets venting from four anomalously warm fractures (called sulci) near its south pole1,2. Since then, several observations have provided evidence that the source of the material ejected from Enceladus is a large underground ocean, the depth of which is still debated3–6. Here, we report on the first and only opportunity that Cassini’s RADAR instrument7,8 had to observe Enceladus’s south polar terrain closely, targeting an area a few tens of kilometres north of the active sulci. Detailed analysis of the microwave radiometry observations highlights the ongoing activity of the moon. The instrument recorded the microwave thermal emission, revealing a warm subsurface region with prominent thermal anomalies that had not been identified before. These anomalies coincide with large fractures, similar or structurally related to the sulci. The observations imply the presence of a broadly distributed heat production and transport system below the south polar terrain with ‘plate-like’ features and suggest that a liquid reservoir could exist at a depth of only a few kilometres under the ice shell at the south pole. The detection of a possible dormant sulcus further suggests episodic geological activity.

The Cassini spacecraft (NASA (National Aeronautics and Space Administration), the European Space Agency and the Italian Space Agency) has explored the Saturnian system for the past 12 years. The detection of water-rich jets erupting from four large fractures (also called sulci, or informally, the ‘tiger stripes’) near the south pole of Enceladus is one of the major discoveries of the mission1,2. These jets are evidence for ongoing internal activity at Enceladus, whose south polar terrain (SPT) is anomalously warm and radiates an endogenic power of up to ~16 GW as indicated by thermal infrared data9,10. The detection of sodium salts3 in the icy grains ejected from Enceladus, together with topography and gravity data4,5, strongly suggest that the moon hides a reservoir of liquid water beneath its surface. The recent detection of a large physical libration6 provides the first evidence for a global ocean underneath a relatively thin ice shell, potentially as thin as 5–10 km in the SPT. This ocean may harbour hydrothermal activity on its floor11 and Enceladus is therefore one of the most promising environments suitable for extant life in the Solar System.

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Figure 1: Stereographic polar projection of active and passive RADAR observations of Enceladus’s SPT acquired during the closest approach of flyby E16.
Figure 2: Thermal anomalies along the E16 RADAR track.
Figure 3: Possible scenario for the heat production and transport in the subsurface of Enceladus’s SPT.

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Acknowledgements

The authors wish to thank the Cassini-Huygens team for the design, development and operation of the mission. The Cassini-Huygens mission is a joint endeavour of NASA, the European Space Agency (ESA) and the Italian Space Agency (ASI), and it is managed by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Most of the authors of this work are members or associate members of the Cassini RADAR Team. A.L.G. gratefully acknowledges the support of the French Space Agency, CNES, and the Université de Versailles Saint-Quentin (UVSQ) (Chair CNES/UVSQ). R.L. acknowledges the support of the NASA grant, NNX13AH14G ‘Cassini RADAR Science Support’. A.L. acknowledges the financial support of the UnivEarthS Labex programme at Sorbonne Paris Cité (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02).

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Authors and Affiliations

  1. LATMOS/IPSL, UVSQ Université Paris-Saclay, UPMC Université de Paris 06, CNRS, 78280 Guyancourt, France,

    A. Le Gall & A. Stolzenbach

  2. LESIA, Observatoire de Paris, PSL-Research University, CNRS, Univ

    C. Leyrat

  3. ersité, Pierre et Marie Curie Paris 06, Université Paris-Diderot, 92195 Meudon France

    C. Leyrat

  4. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA

    M. A. Janssen, C. Sotin & R. D. West

  5. Laboratoire de Planétologie et Géodynamique de Nantes, Université Nantes Atlantique, 44322 Nantes Cedex 3, France

    G. Choblet, G. Tobie, O. Bourgeois, C. Sotin & M. Massé

  6. Laboratoire Astrophysique, Instrumentation et Modélisation, CNRS-UMR 7158, Université Paris-Diderot, CEA-Saclay, Gif sur Yvette, 91191, France

    A. Lucas

  7. Institut Physique du Globe de Paris, Université Paris-Diderot, USPC, 75205 Paris Cedex 13, France

    A. Lucas

  8. Southwest Research Institute, Boulder, 80302, Colorado, USA

    C. Howett

  9. US Geological Survey, Flagstaff, 86001, Arizona, USA

    R. Kirk

  10. Johns Hopkins Applied Physics Laboratory, Laurel, 20723, Maryland, USA

    R. D. Lorenz

  11. Center for Radiophysics and Space Research, Cornell University, Ithaca, 14853, New York, USA

    A. H. Hayes

  12. Lunar and Planetary Laboratory, University of Arizona, Tucson, 85721, Arizona, USA

    L. Bonnefoy

  13. CNRM-GAME, Météo-France, and CNRS, 31057 Toulouse Cedex 1, France

    G. Veyssière

  14. American Public University System, Charles Town, West Virginia 25414, USA

    F. Paganelli

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  1. A. Le Gall
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Contributions

A.L.G. led the analysis and the writing of the article. C.L. developed the thermal model described in the Supplementary Information and used for the analysis of the data. M.J. contributed to the data acquisition and calibration. G.C., G.T., O.B., A.L., C.S. and M.M. contributed to the geodynamical interpretation of the results. C.H. calibrated and mapped the CIRS observation of the SPT shown in Fig. 2d. R.K. conducted the radarclinometry analysis presented in the Supplementary Information. All authors contributed to the discussions and commented on the manuscript.

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Correspondence to A. Le Gall.

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Supplementary information

Supplementary Information

Supplementary Figures 1–11, Supplementary Table 1 and Supplementary References. (PDF 1178 kb)

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Le Gall, A., Leyrat, C., Janssen, M. et al. Thermally anomalous features in the subsurface of Enceladus’s south polar terrain. Nat Astron 1, 0063 (2017). https://doi.org/10.1038/s41550-017-0063

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