Ejecta emplacement as the possible origin of Ryugu's equatorial ridge

TL;DR

This study proposes that ejecta from craters during rapid rotation can form Ryugu's equatorial ridge, with formation timescales consistent with asteroid collisional lifetimes, explaining surface composition variations.

Contribution

It introduces a numerical model showing ejecta deposition as a plausible origin for Ryugu's equatorial ridge during fast rotation periods.

Findings

Ejecta from craters can deposit on the equator during rapid rotation.

Ridge formation timescale is compatible with asteroid collisional lifetimes.

Model explains younger, blue material on the ridge.

Abstract

The Japanese spacecraft Hayabusa 2 visited the asteroid (162173) Ryugu and provided many high-resolution images of its surface, revealing that Ryugu has a spinning-top shape with a prominent equatorial ridge, much like the shapes reported for some other asteroids. In this study, through dozens of numerical calculations, we demonstrate that during a period of fast rotation, ejecta from craters formed at lower and mid-latitudes can be deposited on the equatorial ridge. Assuming a rotation period of 3 h, we estimate that an equatorial ridge with a height of 50 m can be generated in 128(+47 / -27) My for a main-belt asteroid, or 3.1(+4.2 / -1.2) Gy for a near-Earth asteroid. Therefore, an equatorial ridge can form within the average mean collisional lifetime of a km-sized asteroid within the main belt, but not for near-Earth asteroids. Furthermore, our model may explain why blue (younger) material occurs on the equatorial ridge.