  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.
