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JWST molecular mapping and characterization of Enceladus’ water plume feeding its torus

Nature Astronomy volume 7, pages 1056–1062 (2023)Cite this article

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Abstract

Enceladus is a prime target in the search for life in our Solar System, having an active plume that is likely to be connected to a large liquid water sub-surface ocean. Using the sensitive near-infrared spectograph instrument on board the James Webb Space Telescope, we searched for organic compounds and characterized the plume’s composition and structure. The observations directly sample the fluorescence emissions of H2O and reveal an extraordinarily extensive plume (up to 10,000 km or 40 Enceladus radii) at cryogenic temperatures (25 K) embedded in a large bath of emission originating from Enceladus’ torus. Intriguingly, the observed outgassing rate (300 kg s−1) is similar to that derived from close-up observations with Cassini 15 years ago, and the torus density is consistent with previous spatially unresolved measurements with Herschel 13 years ago, which indicates that the vigour of gas eruption from Enceladus has been relatively stable over decadal timescales. This level of activity is sufficient to maintain a derived column density of 4.5 × 1017 m−2 for the embedding equatorial torus, and establishes Enceladus as the prime source of water across the Saturnian system. We performed searches for several non-water gases (CO2, CO, CH4, C2H6, CH3OH), but none were identified in the spectra. On the surface of the trailing hemisphere, we observe strong H2O ice features, including its crystalline form, yet we do not recover CO2, CO or NH3 ice signatures from these observations. As we prepare to send new spacecraft into the outer Solar System, these observations demonstrate the unique ability of the James Webb Space Telescope to provide critical support for the exploration of distant icy bodies and cryovolcanic plumes.

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Fig. 1: Enceladus’ surface geometric albedo and detected water vapour emissions.
Fig. 2: Water emission is observed across the whole FOV, revealing an immense water plume and an extended torus.

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Data availability

The data used in this analysis are publicly available at the Space Telescope Science Institute (STScI) JWST archive (https://mast.stsci.edu/), programme no. 1250.

Code availability

The retrieval software package used in this study is the Planetary Spectrum Generator, which is free and available online at https://psg.gsfc.nasa.gov (refs. 29,30), with the data-reduction scripts available at https://github.com/nasapsg. Figures were made with Matplotlib version 3.2.1 (ref. 60), available under the Matplotlib license at https://matplotlib.org/.

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Acknowledgements

This work was supported by NASA’s Goddard Astrobiology Program, Goddard’s Fundamental Laboratory Research (FLaRe) and the Sellers Exoplanet Environments Collaboration (SEEC). It is based on observations made with the NASA/ESA/CSA JWST. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with programme no. 1250. C.R.G. was supported by Southwest Research Institute Internal Research & Development grant 15-R6248 and NASA Astrobiology Institute grant NNN13D485T. K.P.H. acknowledges support from the NASA Astrobiology Program (award no. 80NSSC19K1427) and the Europa Lander Pre-Project, managed by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA.

Author information

Authors and Affiliations

  1. NASA Goddard Space Flight Center, Greenbelt, MD, USA

    G. L. Villanueva, S. N. Milam, V. Kofman, S. Faggi & N. Rowe-Gurney

  2. Association of Universities for Research in Astronomy, Washington, DC, USA

    H. B. Hammel

  3. American University, Washington, DC, USA

    V. Kofman & S. Faggi

  4. Southwest Research Institute, San Antonio, TX, USA

    C. R. Glein

  5. Carl Sagan Center for Research at the SETI Institute, Mountain View, CA, USA

    R. Cartwright

  6. KTH Royal Institute of Technology, Stockholm, Sweden

    L. Roth

  7. Jet Propulsion Laboratory, Pasadena, CA, USA

    K. P. Hand

  8. NASA Headquarters, Washington, DC, USA

    L. Paganini

  9. Southwest Research Institute, Boulder, CO, USA

    J. Spencer & S. Protopapa

  10. Space Telescope Science Institute, Baltimore, MD, USA

    J. Stansberry & B. Holler

  11. Department of Astronomy & CRESST II, University of Maryland, College Park, MD, USA

    N. Rowe-Gurney

  12. INAF-Osservatorio Astrofisico di Catania, Catania, Italy

    G. Strazzulla

  13. Università degli Studi della Basilicata, Potenza, Italy

    G. Liuzzi

  14. Universite Paris-Saclay, Orsay, France

    G. Cruz-Mermy

  15. Cornell Center of Astrophysics and Planetary Sciences, Ithaca, NY, USA

    M. El Moutamid

  16. University of Idaho, Moscow, ID, USA

    M. Hedman & K. Denny

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  1. G. L. Villanueva
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Contributions

G.L.V., H.B.H., S.N.M., K.P.H., L.P., J. Spencer, J. Stansberry and G.S. designed the observations and prepared the observational plans. G.L.V., V.K., S.F., R.C., J. Stansberry, B.H., S.P., G.L., M.H. and K.D. analysed the data, extracted calibrated spectra, produced maps and performed retrievals. C.R.G., L.R., N.R.-G., G.C.-M. and M.E.M. assisted with the interpretation of the results and provided context to related mission and astronomical investigations. All of the authors contributed to the preparation, writing and editing of the manuscript.

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Correspondence to G. L. Villanueva.

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Nature Astronomy thanks Sascha Kempf and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Searches for organic species in the plume of Enceladus.

Residual spectra for three spectral regions in which CH3OH, C2H6, CH4, CO2 and CO molecular emission were searched. The spectra were integrated across the plume region. The models are at the 3s level in number of molecules and were computed assuming a rotational temperature of 25 K as for water.

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Villanueva, G.L., Hammel, H.B., Milam, S.N. et al. JWST molecular mapping and characterization of Enceladus’ water plume feeding its torus. Nat Astron 7, 1056–1062 (2023). https://doi.org/10.1038/s41550-023-02009-6

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