Possible sublimation and dust activity on primitive NEAs: Example of (162173) Ryugu

Because of relatively high surface temperatures and quick evolution of orbital parameters of near-Earth asteroids (NEAs) of primitive types, the possibility of retention of icy materials (predominately H2O ice) remains in question. Yet, based on summarized results of spectroscopic observations of Apollonian (162173) Ryugu (Cg-type) being a target of Japanese Hayabusa 2 space mission, we suspected a transient sublimation activity on the asteroid. However, unlike some main-belt primitive asteroids demonstrating sublimation of ices close to their perihelion distances and, respectively, at the highest subsolar temperatures, the effect on Ryugu was apparently observed near aphelion. To explain the difference, we calculated the subsolar temperature depending on Ryugu heliocentric distance and performed some analytical estimations related to internal structure and thermo-physical parameters of this and similar NEAs. Presumed temporal sublimation and dust activity of Ryugu is an indication of existence of a residual frozen core in its interior which could point to a relatively recent transition of the asteroid from the main asteroid belt to the near-Earth region.