The surface of Phobos is covered by many small craters (down to several meters or less in diameter) and the relationship between their morphology and the regolith remains uncertain. Several of them exhibit concentric, flat-floored, or central-mound geometries [Thomas et al., 2000], which suggests the presence of a loose regolith layer overlying a more cohesive substrate [Hemmi and Miyamoto, 2020; Oberbeck and Quaide, 1967]. However, post-crater geological events, such as landslides, seismic shaking, the emplacement of other crater ejecta [Basilevsky et al., 2014], could have partially modified or obliterated small craters, which accounts for their irregular shapes. In either case, the absence of their topography data hampers the accurate measurements of their morphometric parameters and their degradation states. Recently, the author has generated 20-m digital terrain model (DTM) and revealed that their crater depth-to-diameter ratios (d/D) are 0.037 and 0.174 (mean value = 0.089, median = 0.093) for thirty-five sub-kilometer craters, and their morphologies indicate the depositional events filling materials into their interiors. Here, in this study, the author aims to quantitatively analyze the morphology of craters smaller than 100 m in diameter by using the highest resolution digital terrain model (DTM). In this study, by generating the new ~6.5-m/post DTM of Phobos from the latest shape-from-shading technique [Alexandrov and Beyer, 2018], the topographic profiles of the small craters down to ~30 m in diameter were measured. Their d/D values range between 0.002 and 0.2, and well below 0.1 for the craters less than 100 m in diameter, which suggests that the crater degradations (decrease in crater depth) had continued to the recent past despite the low-gravity environment of Phobos.
References
Alexandrov, O., and R. A. Beyer (2018), Multiview Shape-From-Shading for Planetary Images, Earth and Space Science, 5(10), 652-666, doi:10.1029/2018EA000390.
Basilevsky, A. T., C. A. Lorenz, T. V. Shingareva, J. W. Head, K. R. Ramsley, and A. E. Zubarev (2014), The surface geology and geomorphology of Phobos, Planetary and Space Science, 102, 95-118, doi:10.1016/j.pss.2014.04.013.
Hemmi, R., and H. Miyamoto (2020), Morphology and Morphometry of Sub-kilometer Craters on the Nearside of Phobos and Implications for Regolith Properties, TRANSACTIONS OF THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES, 63(4), 124-131, doi:10.2322/tjsass.63.124.
Oberbeck, V. R., and W. L. Quaide (1967), Estimated thickness of a fragmental surface layer of Oceanus Procellarum, Journal of Geophysical Research (1896-1977), 72(18), 4697-4704, doi:10.1029/JZ072i018p04697.
Thomas, P. C., J. Veverka, R. Sullivan, D. P. Simonelli, M. C. Malin, M. Caplinger, W. K. Hartmann, and P. B. James (2000), Phobos: Regolith and ejecta blocks investigated with Mars Orbiter Camera images, Journal of Geophysical Research: Planets, 105(E6), 15091-15106, doi:10.1029/1999JE001204.
