NASA’s Dawn mission revealed a partially differentiated Ceres that has experienced cryovolcanic activity throughout its history up to the recent past. The Occator impact crater, which formed ~22 Myr ago, displays bright deposits (faculae) across its floor whose origins are still under debate: two competing hypotheses involve eruption of brines from the crust–mantle transition boundary (remnants of an ancient ocean) or alternatively from a shallow impact melt chamber. Here we report new constraints on the history of Occator that help in testing the hypotheses of its formation. We used high-resolution images of the Dawn Framing Camera obtained close to the end of the mission. We found a long-lasting and recent period of cryovolcanic activity, which started ≤9 Myr ago and lasted for several million years. Several resurfacing events, affecting the faculae and some (dark) solidified impact melt units, are shown to have occurred millions of years after crater formation and the dissipation of the impact-generated heat. These findings are indicative of a deep-seated brine source. Extensive volatile-driven emplacement of bright material occurred in the central floor, causing its subsidence due to mass loss at depth. Finally, a thick (extrusive) dome of bright material was raised in the central depression. The derived chronostratigraphy of Occator is consistent with a recently geologically active world, where salts play a major role in preserving liquid in a heat-starved body.
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The Framing Camera data is available through the PDS Small Bodies Node website (https://pds-smallbodies.astro.umd.edu/). Higher data products that support the findings of this study are available from the corresponding author upon reasonable request.
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We thank the Dawn operations team, especially M. Rayman (JPL), C. Polanskey (JPL) and S. Joy (UCLA), for the development, cruise, orbital insertion, and operations of the Dawn spacecraft at Ceres. In addition, we thank the FC operations team, especially P. Gutierrez-Marques, I. Büttner and J. Ripken at MPI for Solar System Research. The FC project is financially supported by the Max Planck Society and the German Space Agency, DLR. Part of this work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract to NASA. O.R. is supported by an appointment to the ESA Research Fellow Program at the European Space and Technology Center (ESTEC). D.A.W. was supported by the Dawn Project via a NASA grant to UCLA, subcontract to ASU #2090SSA473.
The authors declare no competing interests.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Upper Panel: Sketch to illustrate our methodologic approach. Lower Panel: Colour-coded height difference with respect to the average surface level of the proximal crater floor. Shape data taken from the LAMO DEM (provided by DLR-PF) referenced to the Ceres ellipsoid. The volume inside the central depression rim (black solid line) is 12.6 km³, excluding the volume of the central dome. The corresponding central depression area is about 40 × 106 m². The volume of the dome inside the central depression (black dashed line) is 0.57 km³ and the corresponding size of that area is about 3 × 106 m². The small inset (lower left) shows the area covered by the colour-coded panel.
Average cross-section of Occator crater and its central depression computed from 360 cross-sections of crater and depression; each offset by 1°. The central depression’s cross-section is enlarged in the displayed inset. The central elevation (inset) resembles the average dome dimension, the maxima the average dimension of the central depression. Measurements have been performed using a LAMO Occator DTM provided by DLR-PF.
a, Overview of the central depression (Cerealia Facula). Its centre hosts a dome (Cerealia Tholus). The dome’s body (cp. main text Fig. 2) likely consists entirely of bright material, whose surface is heavily fractured (panel (b)). Fractures are up to 100 m deep and only penetrate bright material. A mesa (Pasola Facula, panel (d)), coated with a layer of bright material is seen in the south-western part of the central depression. Pasola Facula is elevated above the dome and shows similar reflectances to the central bright deposit. Freshly exposed material moves downslope on its eastern flank. Other regions, showing thinner bright deposits are displayed in panels (c) and (e). The outskirts of the central depression are heavily fractured (panel (a) and (e)). The often diffuse appearing bright deposits show small bright centres (panel (c) and (e)), which could be small vents, although some are likely fresh impact craters.
The diameter of the dome is about 3 km. Bright and dark material contact zones are marked by pointed open triangles (in downslope direction) and filled black double arrowed ridge symbols (ridge crest indicated by dashed line, attached triangles point downslope). The simple dashed line indicates the lowest area. Lobe fronts (indicated by black arrows) are found on the north-western dome collar. These surface features could represent landslide aprons or flow fronts of extruded material. CSFD analyses reveal ages of ≤ 2 Myr for these mass wasting/flows (see Methods and Supplementary Table 1).
This figure shows the border between dark and bright material (from upper left to lower right) at the western central bright material deposit of Cerealia Facula. The bright material in the northeast portion of the image is partly covered with dark patches. These patches are likely boulders in different preservation stages that moved downhill over the dark scarp below Pasola Facula (bright area on the lower left). A context image is shown in the upper right.
Extended Data Fig. 6 Digital terrain model of the inner floor overlain with the XM2 clear filter mosaic.
All profiles (three sub-panels below) reveal Cerealia Tholus and the central depression. Eastern and western remnants of the central peak are shown in profile AA’. Profile BB’ passes through elevated areas in the northern and southern part of the central depression. Profile CC’ passes through Pasola Facula and the east mesa, which are diagnostic for the nature of Cerealia Facula. The central bright deposit of Cerealia Facula is interrupted by steep (dark) scarps at the eastern and western slopes of the central depression. A continuous bright layer at Cerealia Facula is expected to have formed by effusive events. Thus, the height differences of areas with continuous bright deposits along profile CC’ are interpreted as a result of subsidence in the central depression area.
The central depression is surrounded by a number of elevated terrains, which are likely remnants of a collapsed central peak. A part of the easternmost remnant (peak height is about 800 m above the foreground terrain) is shown here in a perspective view from high-resolution XM2 orbit data. Stereo-photogrammetric processing of images FC21A0095119 and FC21A0096539, utilizing the Ames Stereo-Pipeline 2.6.0 and visualized with ArcScene 10.5.1 without exaggeration. Inset shows context.
Stereo-photogrammetric processing of images FC11B0005586 and FC21B0094735, utilizing the Ames Stereo-Pipeline 2.6.0 and visualized with ArcScene 10.5.1 without exaggeration. Pasola Facula is located as a high-standing mesa on the western edge of the central pit, about 1 km above its base.
The displayed area has an extent of about 3.7 km (from lower left to upper right) and the inset shows the context of the scene. The east mesa is marked by a solid arrow. Stereo-photogrammetric processing of images FC11B0005922 and FC21B0093319, utilizing the Ames Stereo-Pipeline 2.6.0 and visualized with ArcScene 10.5.1 without exaggeration.
Overall view (a) and cluster of bright material/spots in Vinalia Faculae and enlarged views (b-i) of individual spots. The most prominent bright deposit/spot with the greatest thickness is the spot in panel (d), followed by panel (h) and (f). Isolation of individual spots is evident from spatial distribution and morphology ((b), (c), (g), (i)). A few of the spots show obvious association with a dome-like and/or pit-like and/or depression-like feature (for example, (d), (f), (h), (i)). Association between fractures (a) and the most prominent bright spots ((d), (h)) and others ((g), (i)) is observed. The high-resolution FC imagery suggests the following stratigraphy in the Vinalia Faculae area: (1) lobate deposits up to a few hundred metres in thickness, (2) thin layer of superposing dark material about ~10 to ~20 m thick, and (3) bright carbonate salt deposits, at most a few metres thick, and regionally discontinuous. Outside of Vinalia Faculae, the bright salt deposits are absent on the lobate deposits. Interestingly, some resurfacing events affecting the lobate deposits coincide with Cerealia Facula formation (main text Fig. 4).
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Nathues, A., Schmedemann, N., Thangjam, G. et al. Recent cryovolcanic activity at Occator crater on Ceres. Nat Astron 4, 794–801 (2020). https://doi.org/10.1038/s41550-020-1146-8
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