# The Western Bulge of 162173 Ryugu Formed as a Result of a Rotationally Driven Deformation Process

## TL;DR

This study investigates the formation of Ryugu's western bulge through a finite element model, revealing a past deformation process driven by rotational forces at specific spin periods and low cohesive strength.

## Contribution

It introduces a structural deformation model explaining Ryugu's western bulge formation based on finite element analysis and spin period thresholds.

## Key findings

- Western bulge is structurally relaxed compared to other regions.
- Deformation likely occurred at spin periods between 3.0 and 3.75 hours.
- Cohesive strength of surface materials estimated between 4 and 10 Pa.

## Abstract

162173 Ryugu, the target of Hayabusa2, has a round shape with an equatorial ridge, which is known as a spinning top-shape. A strong centrifugal force is a likely contributor to Ryugu's top-shaped features. Observations by Optical Navigation Camera onboard Hayabusa2 show a unique longitudinal variation in geomorphology; the western side of this asteroid, later called the western bulge, has a smooth surface and a sharp equatorial ridge, compared to the other side. Here, we propose a structural deformation process that generated the western bulge. Applying the mission-derived shape model, we employ a finite element model technique to analyze the locations that experience structural failure within the present shape. Assuming that materials are uniformly distributed, our model shows the longitudinal variation in structurally failed regions when the spin period is shorter than ~3.75 h. Ryugu is structurally intact in the subsurface region of the western bulge while other regions are sensitive to structural failure. We infer that this variation is indicative of the deformation process that occurred in the past, and the western bulge is more relaxed structurally than the other region. Our analysis also shows that this deformation process might occur at a spin period between ~3.5 h and ~3.0 h, providing the cohesive strength ranging between ~4 Pa and ~10 Pa.