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

TL;DR

This study investigates the potential for sublimation and dust activity on the primitive near-Earth asteroid Ryugu, suggesting residual ice and recent transition from the main belt based on spectroscopic and thermal analyses.

Contribution

It presents evidence of possible sublimation activity on Ryugu near aphelion, indicating the presence of residual ice and recent orbital transition, which is novel for NEAs.

Findings

Possible sublimation activity observed near aphelion.

Residual frozen core likely exists inside Ryugu.

Indicates recent transition from main belt to near-Earth orbit.

Abstract

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.