Thermal inertia measurements of airless planetary surface and its applications to planetary remote sensing

2018 Fiscal Year Final Research Report

• Project/Area Number: 26287108
• Research Category: Grant-in-Aid for Scientific Research (B)
• Allocation Type: Partial Multi-year Fund
• Section: 一般
• Research Field: Solid earth and planetary physics
• Research Institution: Japan Aerospace EXploration Agency
• Principal Investigator: Okada Tatsuaki 国立研究開発法人宇宙航空研究開発機構, 宇宙科学研究所, 准教授 (30321566)
• Research Collaborator: Tanaka Satoshi ; Fukuhara Tetsuya ; Taguchi Makoto ; Arai Takehiko ; Sakatani Naoya ; Shimaki Yuri ; Senshu Hiroki ; Demura Hirohide ; Helbert Jorn ; Mueller Thomas G.
• Project Period (FY): 2014-04-01 – 2019-03-31
• Keywords: 熱慣性 / 熱撮像 / 空隙率 / はやぶさ２ / 小惑星 / 惑星形成 / 熱赤外カメラ / 岩塊

Outline of Final Research Achievements

This study is to construct a thermophysical model by performing the laboratory experiments and the numerical simulation for the atmosphere-free planetary surfaces covered with soil or granule regolith, and to apply the model to the analysis for thermal infrared imaging of planetary surfaces. The Japanese asteroid explorer Hayabusa2 arrived at the near-earth C-type asteroid Ryugu in June 2018, and started its remote sensing observations. The first set of high-resolved global thermal images of an asteroid has been obtained in history. The data implies that the surface materials have much lower thermal inertia than the typical carbonaceous chondrite meteorites, indicating the highly porous materials. This is completely different from our original thought, so that we started to reconstruct the thermophysical model, by considering the porosity and roughness in order to account for the brightness temperature and its diurnal profile observed by thermal infrared imager on Hayabusa2.

Free Research Field

惑星探査科学

Academic Significance and Societal Importance of the Research Achievements

本研究では主に小惑星の表層熱物性を示す熱物理モデルを実験および数値シミュレーション的に構築し、「はやぶさ２」熱赤外撮像装置TIRによる観測結果と比較することを意図していたが、TIRの観測結果によって、小惑星表層の状態が想定と大きく乖離していることが判明した。即ち砂礫質のレゴリスでは覆われておらず、高空隙な岩石で覆われており、熱物理モデルは完璧に再構築をする必要が生じた。岩塊は母天体の内部構造であり、それが高空隙であったという事実は小天体の圧密度が低いことを示唆する。これは現在の地球に衝突の恐れのある地球近傍小惑星や、原始太陽系の微惑星の機械的性質が脆弱であることを示す、重要な成果である。