You can clearly see boulders on comets and asteroids. What are these?

Comet 67P/Churyumov-Gerasimenko is half the density of water. This density implies a high porosity in the range of 70 to 80 percent, depending on the assumed dust-to-ice ratio. (You can use 0.9 gram per centimeter for ice and 1.5 to 2 grams per centimeter density for the dust.) Other comets show densities similar to 67P, but those have been less accurately determined during high-velocity spacecraft flybys and via the plume ejected by the impactor of NASA’s Deep Impact mission. Thus, we can infer that cometary material is made of weakly bonded icy dust aggregates with high porosity and low strength.

Images of Comet 67P’s cliffs taken with the European Space Agency’s Rosetta spacecraft show a heated and cracked surface with long linear features. This material looks sintered — compacted by heating ice without melting it — by bonds among these aggregates, perhaps strengthened by organic compounds. The thermal processing causes cliff collapses, as seen at various places on 67P, and can release single boulders that follow the gravitational downslope and come to rest. In one particular place — the Hapi region between the small and big lobe of Comet 67P — the boulders seem to form a line by the gravitational attraction from both lobes.

Boulders also might be moved in eruptions when subsurface gas pressure is released. Such explosions can likely push house-sized boulders basically to any place on the surface.