Post contributed by Noé Le Becq, PhD candidate, Laboratoire de Planétologie et Géosciences, Nantes (France)
Ceres was discovered in 1801 by Giuseppe Piazzi and is located in the main asteroid belt between the orbits of Mars and Jupiter. In the mid-2000s, Hubble Space Telescope observations revealed that Ceres was ice-rich (McCord and Sotin, 2005), making it the closest icy world to Earth! Therefore, the NASA Dawn spacecraft explored it between 2015 and 2018, unveiling a dwarf planet with a crust made of a mixture of ice and rocks (Ermakov et al., 2017), and a complex surface showing signs of recent or possibly ongoing geological activity (Zambon et al., 2017; Scully et al., 2019). The absence of atmosphere makes water ice unstable when exposed to Ceres’ surface. Yet sublimation of the ice contained in the crust could be at the origin of certain morphologies observed (Sizemore et al., 2019), and could more specifically have an important role in the degradation of impact craters over time (Image 1A). After an impact, the freshly exposed ice-rich material on the crater walls sublimates, leading to its fragmentation and the formation of large talus deposits underneath (Image 1B). This is an important process in the evolution of the topography of impact craters on Ceres, very different to what is observed on rocky moons and planets of the inner Solar System.
Image 1: On the left, Ceres as seen by the NASA Dawn spacecraft during its approach in 2015. Occator crater, shown in 3D on panel A, is one of the youngest craters at the surface of Ceres and has large talus deposits along its walls. Panel B is a zoomed 3D view (with no vertical exaggeration) of the North-eastern wall of Occator crater, where the talus can be observed. Ceres approach image (PIA19558) and Occator 3D view (PIA21913) are from NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
In Image 1B, we show a 3D view of one of the talus slopes on the north-eastern wall of Occator crater (shown in its entirety on Image 1A). This region boasts images with a resolution of 3 m/pixel, revealing a wealth of detail. We observe distinct, cohesive outcrops at the top of the talus. Here, these outcrops are very dissected, but in other younger craters, they can be particularly prominent, forming an imposing overhanging cliff. Under the outcrops, is a long smooth talus slope, with few superposed impact craters compared to the crater floor, indicating a very young age. The same observations can be made in many other cerean craters suggesting that the degradation of the outcrops, feeding a growing talus slope underneath is an important process in the degradation of impact craters on Ceres. Moreover, it has also been observed that talus slopes are active longer and have significantly more outcrops when located on pole-facing hillslopes, indicating a sensitivity to illumination, and supporting the hypothesis of the role of ice sublimation in their formation.
Image 2: Schematics of the proposed new model of impact crater degradation on Ceres, driven by the ice sublimation within the outcrops, and the formation of talus slopes. The 3D blocks diagrams show the schematic evolution of a crater wall from impact to the disappearance of the talus deposit, and images below show the different stages of evolution of the crater wall as actually observed on Ceres.
The proposed new crater degradation scenario is shown in Image 2. The retreat of the crater wall due to the fragmentation of the outcrops and the formation of the talus is the prevalent process during the first tens of million years after the impact, and could then continue for several hundred million years on the slopes facing the pole. This new scenario of impact crater degradation on Ceres driven by the ice sublimation is very different to what has been observed on other atmosphere-less bodies such as the Moon or Mercury. Indeed, on rocky planets and moons, the prevailing process of impact crater degradation is topographic diffusion, mostly driven by impact gardening. Topographic diffusion results in sharp topographical features being gradually smoothed out over time by the random and continuous flux of small impactors redistributing the regolith. On Ceres, although this process dominates after the talus becomes inactive once the sublimation of all the ice in the outcrops is complete, the first tens of millions of years after impact are dominated by degradation with ice sublimation and the formation of talus. If we compare similar size impact craters on the Moon and on Ceres, as shown on Image 3, this difference of degradation processes is quite obvious. The lunar crater on panel A has very smoothed edges and a very regular bowl-shape, whereas the cerean crater shows very sharp edges at the top and the bottom of the walls, with large talus deposits and big outcrops on the pole-facing wall.
Image 3: Comparison between similar size impact craters on the Moon (Panel A) and on Ceres (Panel B). The lunar crater has smooth edges due to topographic diffusion. The cerean crater has sharp edges and smooth slopes due to the outcrop fragmentation and the formation of talus.
Finally, this new process gives us a better understanding of the role of ice in the evolution of planetary landscapes. Studying the surface of Ceres with Dawn’s high-resolution data offers a window of opportunity to explore the effects of ice on surface morphologies, in anticipation of future missions (JUICE, Europa Clipper) to the icy moons of the outer Solar System (Castillo-Rogez et al. 2020). Understanding the processes at work at the surface of Ceres then forms a bridge between the exploration of the rocky inner solar system and the icy outer Solar System.
Further Reading:
Castillo-Rogez, J. (2020), Future exploration of Ceres as an ocean world. Nature Astronomy 4, 732–734.
Ermakov, A. I. et al. (2017), Constraints on Ceres’ Internal Structure and Evolution From Its Shape and Gravity Measured by the Dawn Spacecraft. Journal of Geophysical Research: Planets 122, 2267–2293.
McCord, T. B. & Sotin (2005), C. Ceres: Evolution and current state. Journal of Geophysical Research: Planets 110.
Sam, L. & Bhardwaj, A. (2022), A Remote Sensing Perspective on Mass Wasting in Contrasting Planetary Environments: Cases of the Moon and Ceres. Remote Sensing 14, 1049.
Scully, J. E. C. et al. (2019), Ceres’ Occator crater and its faculae explored through geologic mapping. Icarus 320, 7–23.
Sizemore, H. G. et al. (2019), A Global Inventory of Ice-Related Morphological Features on Dwarf Planet Ceres: Implications for the Evolution and Current State of the Cryosphere. Journal of Geophysical Research: Planets 124, 1650–1689.
Zambon, F. et al. (2017), Spectral analysis of Ahuna Mons from Dawn mission’s visible-infrared spectrometer. Geophysical Research Letters 44, 97–104.
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