The near-Earth carbonaceous asteroid 162173 Ryugu is thought to have been produced from a parent body that contained water ice and organic molecules. The Hayabusa2 spacecraft has obtained global multi-color images of Ryugu. Geomorphological features present include a circum-equatorial ridge, east/west dichotomy, high boulder abundances across the entire surface, and impact craters. Age estimates from the craters indicate a resurfacing age of ≲106 years for the top 1-meter layer. Ryugu is among the darkest known bodies in the Solar System. The high abundance and spectral properties of boulders are consistent with moderately dehydrated materials, analogous to thermally metamorphosed meteorites found on Earth. The general uniformity in color across Ryugu’s surface supports partial dehydration due to internal heating of the asteroid’s parent body.

C-type asteroids are among the most pristine objects in the solar system, but little is known about their interior structure and surface properties. Telescopic thermal infrared observations have so far been interpreted in terms of a regolith covered surface with low thermal conductivity and particle sizes in the centimeter range. This includes observations of C-type asteroid (162173) Ryugu, for which average grainsizes of 3-30 mm have been derived 1,2,3. However, upon arrival of the Hayabusa2 spacecraft at Ryugu, a regolith cover

The structure of the upper layer of a comet is a product of its surface activity. The Rosetta Lander Imaging System (ROLIS) on board Philae acquired close-range images of the Agilkia site during its descent onto comet 67P/Churyumov-Gerasimenko. These images reveal a photometrically uniform surface covered by regolith composed of debris and blocks ranging in size from centimeters to 5 meters. At the highest resolution of 1 centimeter per pixel, the surface appears granular, with no apparent deposits of unresolved sand-sized particles. The thickness of the regolith varies across the imaged field from 0 to 1 to 2 meters. The presence of aeolian-like features resembling wind tails hints at regolith mobilization and erosion processes. Modeling suggests that abrasion driven by airfall-induced particle "splashing" is responsible for the observed formations.

Comets can be regarded as active planetary bodies because they display evidence for nearly all fundamental geological processes, which include impact cratering, tectonism, and erosion. Comets also display sublimation-driven outgassing, which is comparable to volcanism on larger planetary bodies in that it provides a conduit for delivering materials from the interior to the surface. However, in the domain of active geological bodies, comets occupy a special niche since their geologic activity is almost exclusively driven by externally supplied energy (i.e. solar energy) as opposed to an internal heat source, which makes them "seasonally-active" geological bodies. During their active phase approaching the Sun, comets also develop a transient atmosphere that interacts with the surface and contributes to its evolution, particularly by transporting materials across the surface. Variations in solar energy input on diurnal and seasonal scale cause buildup of thermal stresses within consolidated materials that lead to weathering through fracturing, and eventually mass-wasting. The commonly irregular shapes of comets also play a major role in their evolution by leading to (1) non-uniform gravitational forces that affect material movement across the surface, and

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