Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

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

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 debated36. 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.

This is a preview of subscription content, access via your institution

Access options

Rent or buy this article

Prices vary by article type

from$1.95

to$39.95

Prices may be subject to local taxes which are calculated during checkout

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.

Similar content being viewed by others

References

  1. Porco, C. C. et al. Cassini observes the active south pole of Enceladus. Science 311, 1393–1401 (2006).

    Article  ADS  Google Scholar 

  2. Spitale, J. N. & Porco, C. C. Association of the jets of Enceladus with the warmest regions on its south polar fractures. Nature 449, 695–697 (2007).

    Article  ADS  Google Scholar 

  3. Postberg, F. Sodium salts in E-ring ice grains from an ocean below the surface of Enceladus. Nature 459, 1098–1101 (2009).

    Article  ADS  Google Scholar 

  4. Collins, G. C. & Goodman, J. C. Enceladus’ south polar sea. Icarus 189, 72–82 (2007).

    Article  ADS  Google Scholar 

  5. Iess, L. et al. The gravity field and interior structure of Enceladus. Science 344, 78–80 (2014).

    Article  ADS  Google Scholar 

  6. Thomas, P. C. Enceladus’s measured physical libration requires a global subsurface ocean. Icarus 264, 37–47 (2016).

    Article  ADS  Google Scholar 

  7. Elachi, C. RADAR: the Cassini Titan radar mapper. Space Sci. Rev. 115, 71–110 (2004).

    Article  ADS  Google Scholar 

  8. Janssen, M. A. Titan’s surface at 2.2-cm wavelength imaged by the Cassini RADAR radiometer: calibration and first results. Icarus 200, 222–239 (2009).

    Article  ADS  Google Scholar 

  9. Spencer, J. R. et al. Cassini encounters Enceladus: background and the discovery of a south polar hot spot. Science 311, 1401–1405 (2006).

    Article  ADS  Google Scholar 

  10. Howett, C. J. A., Spencer, J. R., Pearl, J. & Segura, & M. High heat flow from Enceladus’ south polar region measured using 10–600 cm−1 Cassini/CIRS data. J. Geophys. Res. 116, E03003 (2011).

    Article  ADS  Google Scholar 

  11. Hsu, H.-W. Ongoing hydrothermal activities within Enceladus. Nature 519, 207–210 (2015).

    Article  ADS  Google Scholar 

  12. Keihm, S. J. Interpretation of the lunar microwave brightness temperature spectrum: feasibility of orbital heat flow mapping. Icarus 60, 568–589 (1984).

    Article  ADS  Google Scholar 

  13. Bondarenko, N. V., Head, J. W. & Ivanov, M. A. Present-day volcanism on Venus: evidence from microwave radiometry. Geophys. Res. Lett. 37, L23202 (2010).

    Article  ADS  Google Scholar 

  14. Lorenz, R. D., Le Gall, A. & Janssen, M. A. Detecting volcanism on Titan and Venus with microwave radiometry. Icarus 270, 30–36 (2016).

    Article  ADS  Google Scholar 

  15. Paillou, P. Microwave dielectric constant of Titan-relevant materials. Geophys. Res. Lett. 35, L18202 (2008).

    Article  ADS  Google Scholar 

  16. Leyrat, C., Lorenz, R. D. & Le Gall, A. Probing Pluto’s underworld: ice temperatures from microwave radiometry decoupled from surface conditions. Icarus 268, 50–55 (2016).

    Article  ADS  Google Scholar 

  17. Ostro, S. J. Cassini RADAR observations of Enceladus, Tethys, Dione, Rhea, Iapetus, Hyperion, and Phoebe. Icarus 183, 479–490 (2006).

    Article  ADS  Google Scholar 

  18. Ries, P. A. & Janssen, M. A. A large-scale anomaly in Enceladus’ microwave emission. Icarus 257, 88–102 (2015).

    Article  ADS  Google Scholar 

  19. Black, G. J., Campbell, D. B. & Nicholson, P. D. Icy Galilean satellites: modeling radar reflectivities as a coherent backscattering effect. Icarus 151, 167–180 (2001).

    Article  ADS  Google Scholar 

  20. Janssen, M. A., Le Gall, A. & Wye, L. C. Anomalous radar backscatter from Titan’s surface? Icarus 212, 321–328 (2011).

    Article  ADS  Google Scholar 

  21. Hapke, B. Coherent backscatter and the radar characteristics of outer planet satellites. Icarus 88, 407–417 (1990).

    Article  ADS  Google Scholar 

  22. Nahm, A. L. & Kattenhorn, S. A. A unified nomenclature for tectonic structures on the surface of Enceladus. Icarus 258, 67–81 (2015).

    Article  ADS  Google Scholar 

  23. Schenk, P. M. Plasma, plumes and rings: Saturn system dynamics as recorded in global color patterns on its midsize icy satellites. Icarus 211, 740–757 (2011).

    Article  ADS  Google Scholar 

  24. Bland, M. T., McKinnon, W. B. & Schenk, P. M. Constraining the heat flux between Enceladus’ tiger stripes: numerical modeling of funiscular plains formation. Icarus 260, 232–245 (2015).

    Article  ADS  Google Scholar 

  25. Čadek, O. Enceladus’s internal ocean and ice shell constrained from Cassini gravity, shape, and libration data. Geophys. Res. Lett. 43, 5653–5660 (2016).

    Article  ADS  Google Scholar 

  26. Crow-Willard, E. N. & Pappalardo, R. T. Structural mapping of Enceladus and implications for formation of tectonized regions. J. Geophys. Res. Planets 120, 928–950 (2015).

    Article  ADS  Google Scholar 

  27. Tobie, G., Mocquet, A. & Sotin, C. Tidal dissipation within large icy satellites: applications to Europa and Titan. Icarus 177, 534–549 (2005).

    Article  ADS  Google Scholar 

  28. Nimmo, F., Spencer, J. R., Pappalardo, R. T. & Mullen, M. E. Shear heating as the origin of the plumes and heat flux on Enceladus. Nature 447, 289–291 (2007).

    Article  ADS  Google Scholar 

  29. Rey, P., Vanderhaeghe, O. & Teyssier, C. Gravitational collapse of the continental crust: definition, regimes and modes. Tectonophysics 342, 435–449 (2001).

    Article  ADS  Google Scholar 

  30. Gioia, G., Chakrobarty, P., Marshak, S. & Kieffer, S. W. Unified model of tectonics and heat transport in a frigid Enceladus. Proc. Natl Acad. Sci. USA 104, 13578–13581 (2007).

    Article  ADS  Google Scholar 

  31. Souček, O., Hron, J., Běhounková, M. & Čadek, O. Effect of the tiger stripes on the deformation of Saturn’s moon Enceladus. Geophys. Res. Lett. 43, 7417–7423 (2016).

    Article  ADS  Google Scholar 

  32. Bland, M. T., Singer, K. N., McKinnon, W. B. & Schenk, P. M. Enceladus’ extreme heat flux as revealed by its relaxed craters. Geophys. Res. Lett. 39, L17204 (2012).

    Article  ADS  Google Scholar 

Download references

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).

Author information

Authors and Affiliations

Authors

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.

Corresponding author

Correspondence to A. Le Gall.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary Information

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

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

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

Download citation

  • Received:

  • Accepted:

  • Published:

  • DOI: https://doi.org/10.1038/s41550-017-0063

This article is cited by

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing