Bright carbonate deposits as evidence of aqueous alteration on (1) Ceres

Abstract

The typically dark surface of the dwarf planet Ceres is punctuated by areas of much higher albedo, most prominently in the Occator crater1. These small bright areas have been tentatively interpreted as containing a large amount of hydrated magnesium sulfate1, in contrast to the average surface, which is a mixture of low-albedo materials and magnesium phyllosilicates, ammoniated phyllosilicates and carbonates2,3,4. Here we report high spatial and spectral resolution near-infrared observations of the bright areas in the Occator crater on Ceres. Spectra of these bright areas are consistent with a large amount of sodium carbonate, constituting the most concentrated known extraterrestrial occurrence of carbonate on kilometre-wide scales in the Solar System. The carbonates are mixed with a dark component and small amounts of phyllosilicates, as well as ammonium carbonate or ammonium chloride. Some of these compounds have also been detected in the plume of Saturn’s sixth-largest moon Enceladus5. The compounds are endogenous and we propose that they are the solid residue of crystallization of brines and entrained altered solids that reached the surface from below. The heat source may have been transient (triggered by impact heating). Alternatively, internal temperatures may be above the eutectic temperature of subsurface brines, in which case fluids may exist at depth on Ceres today.

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Figure 1: Occator crater.
Figure 2: Spectra of bright and dark areas in Occator.
Figure 3: Spatial distribution and scatter plots of different absorption intensities.
Figure 4: Spectral fits of the Occator bright material spectrum.

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Acknowledgements

We thank the following institutions and agencies which supported this work: the Italian Space Agency, the National Aeronautics and Space Administration (NASA, USA) and the Deutsches Zentrum für Luft- und Raumfahrt (DLR, Germany). The VIR was funded and coordinated by the Italian Space Agency and built by SELEX ES, with the scientific leadership of the Institute for Space Astrophysics and Planetology and the Italian National Institute for Astrophysics, and is operated by the Institute for Space Astrophysics and Planetology, Italy. A portion of this work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, USA, under contract to NASA. We also thank the Dawn Mission Operations team and the Framing Camera team.

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Authors

Contributions

M.C.D.S., A.R., E.A. and F.G.C. performed data analysis and calibration. M.C. provided optical constants from reflectance spectra. M.C.D.S., C.M.P. and B.L.E. contributed to the spectral interpretation of the data. All authors contributed to the discussion of the results and to writing the paper.

Corresponding author

Correspondence to M. C. De Sanctis.

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Competing interests

The authors declare no competing financial interests.

Additional information

The VIR calibrated data will be made available through the PDS Small Bodies Node website (http://sbn.pds.nasa.gov/).

Reviewer Information Nature thanks V. Reddy, A. S. Rivkin and M. M. Zolotov for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 Occator floor spectrum.

Spectral fit (red) of the reflectance spectrum of the floor of the Occator crater (black) using the same end-members as discussed for the average surface2. The computed χ2 for the mixture is given in the plot. Error bars for the Occator floor spectrum are calculated taking into account a mean absolute deviation of the calibration uncertainties along the 256 samples.

Extended Data Figure 2 Spectral fit with hexahydrite.

Modelled spectrum (red) of a mixture of hexahydrite (MgSO4·6H2O) (30 vol%) with the average for Ceres (70 vol%). Strong absorptions bands due to H2O are visible at 1.4 μm, 1.95 μm, 2.45 μm and 3 μm in the modelled spectrum, which are not observed in the Occator bright spectrum (black). Error bars for the Occator spectrum are calculated taking into account a mean absolute deviation of the calibration uncertainties along the 256 samples.

Extended Data Figure 3 Spectral fit with water ice.

Spectral fit (red) of the reflectance spectrum of the bright spot in Occator (black) using the same end-members discussed for the average surface2 and adding water ice (Extended Data Table 1). Resulting parameters are reported in Extended Data Table 2. Several absorptions present are poorly fitted, and several absorptions predicted by the best-fit model are absent. The computed χ2 for the mixture is given in the plot. Error bars for the Occator spectrum are calculated taking into account a mean absolute deviation of the calibration uncertainties along the 256 samples.

Extended Data Figure 4 Comparison of Occator bright material spectrum with carbonates.

a, Continuum-removed spectrum of the Occator bright material compared to natrite15, sodium bicarbonate (see http://psf.uwinnipeg.ca), calcite35, and dolomite35. b, A scatter plot of the longest-wavelength continuum-removed absorption band centres for different carbonates shows that Ceres data from Occator bright areas are similar to data from natrite and are distinct from data from other parts of the planet, which plot near magnesite. The 3.9-μm absorption is strong in both the Occator bright areas and in dark floor material. The 3.4-μm absorption is strong in the bright areas but broader; its centre is challenging to define over most of the Ceres surface owing to the presence of other optically active phases. Spectral sampling of laboratory data varied, but in all cases was <0.01 μm. Error bars for the Occator spectrum are not reported in the plot.

Extended Data Figure 5 Comparison of Occator bright material spectrum with carbonaceous chondrites.

Comparison of spectra from two carbonate- and organic-bearing carbonaceous chondrites (MAC 02606 (CM2) and Ivuna (CI))36 and the Occator bright material. The spectra have been normalized to 1 at 2.62 μm. The Occator spectrum is not reported in the plot.

Extended Data Figure 6 Spectra of candidate materials fitting the observed 2.20–2.22-μm absorption in Occator bright materials.

The shaded grey area indicates the expected bandwidth of aluminium phyllosilicates35 (see the PSF web site at http://psf.uwinnipeg.ca and the RELAB database at http://www.planetary.brown.edu/relab/), and the shaded red area indicates the expected bandwidth of NH4Cl (ref. 15). The Occator spectrum is 20× contrast-enhanced. The magnesium-exchanged montmorillonite is heated to 300 °C (ref. 37). Spectra of ammoniated salts NH4Cl, (NH4)2CO3 and NH4HCO3 (ref. 15) are also plotted. Dotted lines correspond to ammonium absorptions near 2 μm.

Extended Data Figure 7 Spectral fit without NH4 salt.

Spectral fit (red) of reflectance spectrum of the bright spot in Occator (black) using the same end-members as discussed for the natrite, montmorillonite, illite and dark material (Extended Data Table 1). Resulting parameters are reported in Extended Data Table 2. The computed χ2 for the mixture is given in the plot. Error bars for the Occator spectrum are calculated taking into account a mean absolute deviation of the calibration uncertainties along the 256 samples.

Extended Data Table 1 End-members used in evaluating mixing model results
Extended Data Table 2 Combination of end-members used to produce the best fit

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De Sanctis, M., Raponi, A., Ammannito, E. et al. Bright carbonate deposits as evidence of aqueous alteration on (1) Ceres. Nature 536, 54–57 (2016). https://doi.org/10.1038/nature18290

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