The Icy Boulders of Ceres

Boulders on small Solar System bodies provide a window into the interior. They may be created by spallation during large impacts and therefore are typically found in and around fresh craters. Boulders typically survive for millions of years, until they are gradually eroded into dust by exposure to the space environment. From its vantage point in the lowest mapping orbit, the NASA Dawn spacecraft was able to distinguish boulders on the surface of Ceres at a resolution of 35 m. We study the properties of Ceres’ global boulder population: distribution over the globe, spectral properties, and size-frequency distribution. A positive identification of boulders requires at least 3 pixels, so the boulders in our sample are larger than 105 m. Boulders of such large size are also known as megaclasts or superblocks (Bruno & Ruban, 2017). We compare the Ceres boulder population with that of Vesta, Dawn’s previous target (Schröder et al., 2020).

We identified a total of 4423 boulders on the surface of Ceres with a diameter larger than 3 image pixels (105 m). All boulders are associated with impact craters. The number of boulders per crater is only weakly correlated with crater size, mostly because the largest craters in our sample have fewer boulders than expected. These craters, Azacca, Ikapati, Occator, show evidence of large scale flows that may have destroyed or obscured the majority of their boulders. The number of boulders for craters of the same size is larger on Ceres than on Vesta. Another difference with Vesta is that the boulder density on Ceres decreases dramatically towards the equator. Complicating matters is the fact that boulders may be better visible towards Ceres’ poles because of the higher solar incidence angle. We carefully evaluated the influence of incidence angle on visibility, and found that it cannot explain the large differences in boulder density. Our finding can be understood if the erosion rate of boulders is linked to the degree of solar insolation, which scales with the cosine of the latitude and is therefore minimal at the poles. We also evaluated the average boulder lifetime, by comparing the density of boulders in and around craters with their age, as estimated by crater counting. The typical lifetime of meter-sized boulders on Vesta and Ceres should be similar, based on the expected impactor velocity and density distributions (Basilevsky et al., 2015). Instead, we find that boulders appear to live shorter on Ceres than on Vesta, suggesting that they erode faster. On Ceres, water ice appears to be abundant just meters below the surface (Prettyman et al., 2017; Schmidt et al., 2017). We hypothesize that Ceres boulders contain a significant fraction of water ice, which makes them more susceptible to erosion by solar insolation than rocky boulders.

References
Basilevsky, A. T., Head, J. W., Horz, F., & Ramsley, K. (2015) Survival times of meter-sized rock boulders on the surface of airless bodies. Planetary & Space Science 117, 312-328, doi: 10.1016/j.pss.2015.07.003
Bruno, D. E., & Ruban, D. A. (2017) Something more than boulders: A geological comment on the nomenclature of megaclasts on extraterrestrial bodies. Planetary & Space Science 135, 37-42, doi: 10.1016/j.pss.2016.11.006.
Prettyman, T. H., N. Yamashita, M. J. Toplis, H. Y. McSween, N. Schörghofer, S. Marchi, W. C. Feldman, J. Castillo-Rogez, O. Forni, and D. J. Lawrence (2017) Extensive water ice within Ceres’ aqueously altered regolith: Evidence from nuclear spectroscopy. Science, 355: 55–59. doi: 10.1126/science.aah6765.
Britney E. Schmidt, Kynan H. G. Hughson, Heather T. Chilton, Jennifer E. C. Scully, Thomas Platz, Andreas Nathues, Hanna Sizemore, Michael T. Bland, Shane Byrne, and Simone Marchi (2017) Geomorphological evidence for ground ice on dwarf planet Ceres. Nature Geoscience, 10:338–343, doi: 10.1038/ngeo2936.
Schröder, S. E., Carsenty, U., Hauber, E., Schulzeck, F., Raymond, C. A., & Russell, C. T. (2019) The boulder population of asteroid 4 Vesta. Submitted to Earth & Space Science.