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Impact-driven mobilization of deep crustal brines on dwarf planet Ceres

Abstract

Ceres, the only dwarf planet in the inner Solar System, appears to be a relict ocean world. Data collected by NASA’s Dawn spacecraft provided evidence that global aqueous alteration within Ceres resulted in a chemically evolved body that remains volatile-rich1. Recent emplacement of bright deposits sourced from brines attests to Ceres being a persistently geologically active world2,3, but the surprising longevity of this activity at the 92-km Occator crater has yet to be explained. Here, we use new high-resolution Dawn gravity data to study the subsurface architecture of the region surrounding Occator crater, which hosts extensive young bright carbonate deposits (faculae). Gravity data and thermal modelling imply an extensive deep brine reservoir beneath Occator, which we argue could have been mobilized by the heating and deep fracturing associated with the Occator impact, leading to long-lived extrusion of brines and formation of the faculae. Moreover, we find that pre-existing tectonic cracks may provide pathways for deep brines to migrate within the crust, extending the regions affected by impacts and creating compositional heterogeneity. The long-lived hydrological system resulting from the impact might also occur for large impacts in icy moons, with implications for creation of transient habitable niches over time.

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Fig. 1: Topography and isostatic gravity of the Hanami Planum region (dashed white line).
Fig. 2: Cross-sections of biaxial ellipsoids showing representative low-density regions in the Hanami Planum region and in the local area around Occator crater consistent with the gravity inversion, and their gravity signatures.
Fig. 3: Evolution of the temperature field in Occator crater at t = 0, 1, 5 and 10 Myr after the crater-forming impact.
Fig. 4: Evolution of the melt chamber beneath Occator and scenario of brine emplacements.

Data availability

The data that support the plots within this paper, and conclusions of this study are available from the PDS Small Bodies Node website (https://sbn.psi.edu/pds/resource/dawn) or from the corresponding author upon reasonable request.

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Acknowledgements

This research was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. We thank the Dawn operations team for the development, cruise, orbital insertion and operations of the Dawn spacecraft at Ceres. M.A.H. was supported by National Science Foundation (NSF) grant DMS-1720349. US Government sponsorship is acknowledged.

Author information

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Authors

Contributions

C.A.R. led the preparation of the manuscript and the geophysical data analysis and interpretation with A.I.E. (MCMC modelling), J.C.C.-R. and M.A.H (thermal modelling), and S.M. and B.C.J. (scaled iSALE results). J.E.C.S. analysed dome morphology, D.L.B., H.G.S. and P.M.S. contributed to geomorphologic interpretation and J.T.K. illustrated the evolution scenario. A.N., R.S.P., T.H.P., L.C.Q., M.D.R. and C.T.R. contributed to the preparation of the manuscript.

Corresponding author

Correspondence to C. A. Raymond.

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The authors declare no competing interests.

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Peer review information Nature Astronomy thanks Douglas Hemingway 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 Isostatic gravity anomaly for degrees n=1 to local degree strength4 including XM2 high-resolution data.

The outline of Hanami Planum in white dashed curve.

Extended Data Fig. 2 Degree strength and isostatic anomaly maps through Hanami Planum.

The degree strength (left plot) is overplotted over the isostatic anomaly in the right panel. Degree strength equals 20, 30 and 40 for the blue, green and red contours, respectively.

Extended Data Fig. 3 Posterior distributions resulting from MCMC modeling of gravity field from n=1 to local degree strength beneath Hanami planum.

Top: Longitude (λ), latitude (ϕ) and total mass deficit (M) are well-constrained. Bottom: Source aspect ratio (i.e., ratio of horizontal ellipsoid semi-axis a to the vertical semi-axis c), depth to source (Ztop) and density contrast (Δρ) show broader probable ranges. The 2.5 – 97.5 % confidence intervals are shown on the top of each histogram.

Extended Data Fig. 4 Posterior distribution resulting from MCMC modeling of gravity field from n=1 to local degree strength beneath the Occator crater region.

The average field in an annulus between 20 and 30 degrees from the center of anomaly A was removed to suppress the large long-wavelength negative isostatic anomaly before the inversion was performed. Top: longitude (λ), latitude (ϕ) and total mass deficit (M) are well-constrained. Bottom: Source aspect ratio (i.e., ratio of horizontal ellipsoid semi-axis a to the vertical semi-axis c), depth to source (Ztop) and density contrast (Δρ) show broader probable ranges. The 2.5 – 97.5 % confidence intervals are shown on the top of each histogram.

Extended Data Fig. 5 Effect of the impact on the region predicted by hydrocode model.

Maps of post-impact temperature field a, and damage b, with depth. Temperature field was scaled from iSALE hydrocode modelling by25 using updated thermal evolution models that reflect the current understanding based on Dawn data23 (See Methods). The damage region plot is from the modeling of25. Intact material has a damage parameter, D=0, while thoroughly fractured material has D=143.

Extended Data Fig. 6 Thermal model for the Hanami planum region assuming a thermal gradient of 2.73 K/km.

The solid black line at ~420 km indicates the boundary between the ice- and hydrate-rich crust (above) and the rocky mantle (below). See Methods for details.

Extended Data Table 1 Results of MCMC modelling.

The median values are given with 2.5th to 97.5th percentile ranges.

Extended Data Table 2 Bounds on the model parameters for the Hanami Planum MCMC runs.

Lower and upper bounds are given for the semiaxes’ lengths, central depth and density contrast of the anomolous ellipsoid. Note that the geographic coordinates of the center of the anomaly were constrained to be within a 5° small circle of the specificed location. In addition, the top point on the vertical ellpsoid axis was constrained to be the shallowest point of the anomalous ellipsoid.

Extended Data Table 3 Bounds on the model parameters for the Occator SE MCMC runs.

Lower and upper bounds are given for the semiaxes’ lengths, central depth and density contrast of the anomolous ellipsoid. Note that the geographic coordinates of the center of the anomaly were constrained to be within a 5° small circle of the specificed location. In addition, the top point on the vertical ellpsoid axis was constrained to be the shallowest point of the anomalous ellipsoid.

Extended Data Table 4 The model parameters for the Hanami Planum and Occator SE representative models shown in Figure 2.

These model parameters represent the median values and do not correspond to the maximum-likelihood solution.

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Raymond, C.A., Ermakov, A.I., Castillo-Rogez, J.C. et al. Impact-driven mobilization of deep crustal brines on dwarf planet Ceres. Nat Astron 4, 741–747 (2020). https://doi.org/10.1038/s41550-020-1168-2

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