The Deficiency of Fine Regolith on Asteroid (162173) Ryugu

• Applicant: Dr. Maximilian Hamm
• Subject Area: Astrophysics and Astronomy ; Geodesy, Photogrammetry, Remote Sensing, Geoinformatics, Cartography
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 497966340

The Japanese Hayabusa2 mission investigated the near-Earth asteroid (162173) Ryugu. As part of this mission, the European lander "MASCOT" was placed on the surface of Ryugu to study it in detail. Contrary to what was suspected from telescopic observations, Hayabusa2 has shown that the surface of this asteroid is completely covered with decimeter- to meter-sized rocks. Millimeter to centimeter-sized particles are much less frequent than expected. Furthermore, global measurements by the Hayabusa2 spacecraft's infrared camera and infrared measurements by the MASCOT lander's MARA radiometer showed no sign of dust. Dust appears to be either completely absent or accumulating in a few places, such as cracks or pores. Similar observations were made on asteroids (25143) Itokawa and (101955) Bennu. All three asteroids are "rubble-piles", i.e. agglomerates of rock fragments loosely held together by gravity, lacking fine regolith, and all three are smaller than 1 km in diameter. The processes that may be responsible for the absence of dust on these asteroids are currently unknown and controversial.In this proposed project, we want to systematically investigate different possible processes using Ryugu as an example. The large data set available to us from the Hayabusa2 mission provides a unique opportunity for this. The project is divided into four sections: First, we want to use the infrared data to estimate the maximum amount of dust that can be held on the surface. Then we want to simulate the plasma environment around Ryugu and estimate whether dust particles on Ryugu can be charged and lofted off the surface by photoemission and interaction with the solar wind. This process is often postulated as a reason for the absence of dust on Ryugu, but has not yet been studied for Ryugu. Next, we will use a thermomechanical model to investigate the stresses to which the surface material is subjected by the vastly different daytime and nighttime temperatures. These stresses in the material can lead to thermo-mechanical fatigue, which can be a major source of small regolith fragments. Last, we plan to extend the simulation to other orbits, asteroid sizes, and material properties to draw conclusions about asteroids in different regions of the solar system.Our project is complementary to most previous work, which has focused on the modification of Ryugu's surface by impacts from other asteroids and interplanetary dust grains. Rather than a purely theoretical investigation, we will use the unique dataset gathered by Hayabusa2 for our study. By answering the question of why so little dust and fine regolith was found on Ryugu, we hope to contribute to the understanding of the processes that shape asteroid surfaces. This understanding is important to relate the present observable state of the asteroids to their formation in the early solar system.

• DFG Programme: Research Grants
• International Connection: Japan, USA
• Cooperation Partners: Hsiang-Wen Hsu, Ph.D.; Dr. Jamie Molaro; Hiroki Senshu, Ph.D.