Evaluating Volatile Induced Surface Features on Vesta and Ceres

Kurzfassung

This work evaluates volatile induced surface features on Vesta and Ceres, two of the largest asteroids present within the asteroid belt. Both the planetary objects have similar surface acceleration but different regolith nature. Vesta is a relatively dry body whereas Ceres is rich with water ice. Direct measurement of volatiles is challenging due to harsh space conditions. However, when they are mixed with regolith, it produces peculiar landforms due to melting and/or sublimation and affects the overall evolution of a planetary body. Therefore, in this study the surface features which have direct or indirect link to ice and/or volatiles are examined in order to understand the volatile distribution. For this, regional and global scale investigations related to ponded deposits, pit chains and mass wasting analysis were conducted on Vesta and Ceres. In the vicinity of Marcia and Cornelia impact craters of Vesta, two types of pond deposits were observed. Type 1 melt ponds have smooth, shallow deposits (depth <100 m) and are produced from the downslope movement of volatile bearing impact melt material. In contrast, type 2 dust ponds deposit consist of rough surface with ~200 m depth. These deposits are produced from the mobility of granular dust via infrequent high-amplitude seismic diffusivity and/or short-lived volatile outgassing activity. Due to low amounts of volatiles, the dusty material did not achieve kinetic sieving and thus do not attain typical smooth pond morphology. The findings of this study strongly support the hypothesis related to presence of low amounts of volatiles within Vesta’s regolith. To understand the volatile distribution on Ceres, the analysis of pit chains is carried out within three impact craters namely; Occator, Azacca and Urvara. Radial pit chain pattern of Occator is related to subsurface laccolith swelling of volatile rich cryomagmatic material. Linear pit chain clusters at floors of Azacca and Urvara are attributed to seasonal thermal contraction of ice layer present near the surface. Additionally, based on the pit chains depth the depicted average minimum thickness of regolith within Azacca, located at equator is ~200 m. On the contrary, within Occator and Urvara, the localized thickness is 30 m and 800 m, respectively, which is attributed to their distinct subsurface condition. Hence, this investigation favors the presence of ice layer within the subsurface layer and reveals that it is not distributed homogeneously on Ceres. Lastly, the global scale comparative examination of the mass wasting process on Vesta and Ceres shows few common and some distinct characteristics. In general, granular sliding on Vesta and flow-like movements on Ceres are observed as dominant population. Further, slides and slumping features are restricted to mid-latitudes on Ceres which implies ice-rock fractionation at regional scale. Additionally, the volatile concentration also influences the deposit mobility on Vesta and Ceres and is analyzed by estimating height, width and effective coefficient of friction; H/L. The outcome suggests that deposits become immobile at shorter distances on Vesta in comparison to Ceres (avg. distance 4.5 km and 11.2 km, respectively). The difference in morphology and mobility is related to contrast in the amounts of volatiles present within regolith of both the bodies. While comparing the effective coefficient of friction of Vesta and Ceres with planetary objects in outer solar system, the examination shows that lower temperature may have more influence on mobility. Together, all the above-mentioned studies summarize the volatile induced surface landforms and provide evidences related to their distribution on Vesta and Ceres. This work also presents the first-time comparative investigation that reveals the influence of volatile content on the morphological characteristics of Vesta and Ceres.