Japan Aerospace Exploration Agency

Rikkyo University

The University of Tokyo

Chiba Institute of Technology

Kochi University

Maebashi Institute of Technology

Hokkaido University of Education

Tokai National University Organization

Nagoya University

Summary

A rocky mass with ultra-high porosity that is so light that it floats on water was discovered by analyzing high-resolution images taken by the Mid-Infrared Camera (TIR) and the Optical Navigation Camera (ONC) during the Hayabusa2's approach operation to the asteroid Ryuguu.

Ryuguu is thought to be a re-accumulation of materials broken up by subsequent collisions after thermal evolution, compression, etc., of a fluffy object called a micro-planet, which was a collection of dust in the early solar system. On the other hand, no one has ever seen a microplanet yet, and one of the biggest challenges in the planet formation process is whether they really existed and what they looked like.

The rocky mass discovered in this research is thought to be the material that most strongly preserves the appearance of the microplanets that triggered the birth of planets in the solar system. In addition, by examining the surface of Ryuguu using data from all scientific instruments onboard Hayabusa2, we found that fragments of the same material as the ultra-high porosity rock mass are distributed globally on the Ryuguu surface, and there is a possibility that some of them are contained in the samples collected by Hayabusa2 from Ryuguu.

If the most primitive material with ultra-high porosity discovered this time can be found in the collected samples, it will not only clarify the formation and evolutionary history of Ryuguu's parent body, but also demonstrate the initial stage of the solar system formation process of microplanet formation.

Figure: TIR (a, b) and ONC (c, d) images of the discovered ultra-high porosity rock mass (hot spot, circled in red). The right image is an enlargement of the left image, and the ONC image in (d) has a white line outlining the rock masses near the hotspot, indicating that these rock masses may be ultra-high porosity. (Modified from ©Sakatani et al., 2021)

The research results were published in the British online journal Nature Astronomy on May 24, 2021 (May 25, Japan Standard Time).

Professor Seiji Sugita of Department of Earth and Planetary Science, Professor Shogo Tachibana of UTokyo Organization for Planetary and Space Science, Associate Professor Tomokatsu Morota of Department of Earth and Planetary Science, Assistant Professor Yuichiro Head of Department, and Kosei Yumoto (1st year PhD student) participated in this research achievement.

For more information, please visit the website of the Japan Aerospace Exploration Agency (JAXA).