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Surface water-ice deposits in the northern shadowed regions of Ceres

Nature Astronomy volume 1, Article number: 0007 (2017) Cite this article

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Abstract

Ceres, a dwarf planet located in the main asteroid belt, has a low bulk density1, and models predict that a substantial amount of water ice is present in its mantle and outer shell2–4. The Herschel telescope and the Dawn spacecraft5 have observed the release of water vapour from Ceres6,7, and exposed water ice has been detected by Dawn on its surface at mid-latitudes8. Water molecules from endogenic and exogenic sources can also be cold-trapped in permanent shadows at high latitudes9–11, as happens on the Moon12,13 and Mercury14,15. Here we present the first image-based survey of Ceres’s northern permanent shadows and report the discovery of bright deposits in cold traps. We identify a minimum of 634 permanently shadowed craters. Bright deposits are detected on the floors of just 10 of these craters in multi-scattered light. We spectroscopically identify one of the bright deposits as water ice. This detection strengthens the evidence that permanently shadowed areas have preserved water ice on airless planetary bodies.

Ceres has an obliquity of 4.028° ± 0.01° (see Methods) and as a result, parts of impact crater interiors and depressions located at polar latitudes remain shaded over the course of a Ceres year (1,682 days). The Dawn spacecraft acquired images for the periods 27–37 days prior to and 27–37 days after the northern summer solstice (22 July 2015; Supplementary Fig. 1). At the solstice, shadows are at a minimum in the northern polar region. Thus, clear (panchromatic) and colour images (wavelength range 0.4–1.0 μm) from the Dawn spacecraft’s Framing Camera16 at this season can be used to determine the location and extent of permanently shadowed regions and to investigate whether shadowed localized bright deposits in crater interiors — as observed in multi-scattered light and partially in illumination — are indeed water-ice deposits as predicted from simulated illumination conditions and exosphere model calculations10,11,17. Cold traps are conventionally defined by H2O loss rates of less than 1 m Gyr–1, which corresponds to a temperature of less than about 110 K (ref. 18). We define permanently shadowed regions (PSRs) for those sites where the disk of the Sun never crosses the horizon (that is, terrain in the umbra but not penumbra; see details on image processing and analysis in the Methods).

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Figure 1: Composite binary-averaged mosaic of the north polar region of Ceres.
Figure 2: Craters hosting permanent shadows and bright deposits.
Figure 3: Craters hosting bright deposits.
Figure 4: Water-ice signatures within crater 2 in VIR spectra.

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Acknowledgements

We acknowledge the outstanding work of the Dawn design, engineering and operations team. We thank NASA, the Max Planck Society, and the German and Italian Space Agencies for support for this investigation.

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Authors and Affiliations

  1. Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, Göttingen, 37077, Germany

    T. Platz, A. Nathues, M. Schäfer, G. S. Thangjam, M. Hoffmann, P. Gutierrez-Marques, W. Dietrich & J. Ripken

  2. University of Hawaii at Manoa, Institute for Astronomy, 2680 Woodlawn Drive, Honululu, 96822, Hawaii, USA

    N. Schorghofer

  3. German Aerospace Center (DLR), Institute of Planetary Research, Rutherfordstrasse 2, Berlin, 12489, Germany

    F. Preusker, S. E. Schröder & K.-D. Matz

  4. NASA Goddard Space Flight Center, Greenbelt, 20771, Maryland, USA

    E. Mazarico

  5. The University of Arizona, Lunar and Planetary Laboratory, 1629 East University Boulevard, Tucson, 85721, Arizona, USA

    S. Byrne & M. E. Landis

  6. Freie Universität Berlin, Planetary Sciences and Remote Sensing, Malteserstrasse 74-100, Berlin, 12249, Germany

    T. Kneissl & N. Schmedemann

  7. Bear Fight Institute, 22 Fiddler’s Road, PO Box 667, Winthrop, 98862, Washington, USA

    J.-P. Combe

  8. University of California Los Angeles, Institute of Geophysics and Planetary Physics, Los Angeles, 90024, California, USA

    C. T. Russell

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  1. T. Platz
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Contributions

T.P. designed the concept of this study, developed the threshold methodology, performed the image processing and prepared the manuscript. A.N., N.Scho., F.P., E.M., S.E.S., M.S., M.H., P.G.-M., W.D., J.R., K.-D.M. and C.T.R. contributed to the development of data analysis and interpretation. S.B. and M.E.L. performed the temperature calculations. Crater analysis and impact simulation were performed by T.K. and N.Schm., respectively. J.-P.C. and G.S.T. analysed the VIR data. All authors discussed and contributed to the preparation of this manuscript.

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Correspondence to T. Platz.

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Supplementary information

Supporting Information

Supplementary Figures 1–8 with captions, Supplementary Table 1 with caption, captions for Supplementary Videos 1 and 2. (PDF 2318 kb)

Supplementary Video 1

Supplementary Video 1 (GIF 14067 kb)

Supplementary Video 2

Supplementary Video 2 (MOV 34209 kb)

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Platz, T., Nathues, A., Schorghofer, N. et al. Surface water-ice deposits in the northern shadowed regions of Ceres. Nat Astron 1, 0007 (2017). https://doi.org/10.1038/s41550-016-0007

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