Frontier Discovery—Using an infrared camera to reveal the high porosity of the original asteroid

Frontier Discovery—Using an infrared camera to reveal the high porosity of the original asteroid

Carbonaceous (C-type) asteroids are relics of the early solar system. Since their formation about 4.6 billion years ago, they have preserved the original material, and the surface is covered with millimeter-level particles. They may be analogues of carbonaceous chondrites and are essential for understanding the process of planet formation. However, their physical properties are still little known, because carbonaceous chondrite meteorites often fail to enter the Earth’s atmosphere and display weak or uncharacteristic reflection spectra. Due to the physical properties of the surface material, most impact ejections may escape due to low gravity. Therefore, their surface physical state requires in-depth study.

Recently, the Japanese Academy of Aerospace Sciences used the Hayabusa 25 spacecraft equipped with a thermal infrared imager to take a circle of C-type asteroid 162173 Ryugu's rotation thermal imaging map published in the top academic journal Nature online, through thermal imaging to study the physical state of the planet's surface. From the thermal inertia, the particle size, porosity, boulder abundance, and surface roughness of the asteroid can be obtained. It can be remotely sensed without physical contact, even for porous or loosely bonded surfaces.

The picture shows the thermal imaging of the asteroid

Each pixel obtained by analysis is about 18m or spans about 50 pixels, covering its high-resolution infrared thermal image rotating in 6° steps, indicating that the asteroid boulder and its surrounding environment have similar temperatures, and Heat energy is obtained, inertia of about 300Jm-2s-0.5K-1 (300 tiu). Contrary to the prediction that the surface is composed of heavy boulders and dense boulders, this low thermal inertia indicates that the boulders are more porous than typical carbonaceous chondrites and are surrounded by porous debris with a diameter greater than 10 cm.

The close-up thermal image confirms the presence of this porous debris, and the flat day-night temperature curve indicates a strong surface roughness effect. Scientists also observed in the close-up thermal image that the boulders in the daytime are colder and the thermal inertia exceeds 600 tiu, which is equivalent to a dense boulder similar to a typical carbonaceous chondrite.

Reference materials:

Tatsuaki Okada, Tetsuya Fukuhara, Satoshi Tanaka, et al. Highly porous nature of a primitive asteroid revealed by thermal imaging[J]. Nature. 579, 518-522, 2020.