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Trip To An Asteroid: Hayabusa 2 Is Now Traveling Toward C-type Asteroid Ryugu

Still, in the 21st century, we don’t have the answer to this big question: How our Earth, life, and this solar system were made? The key is given by organic materials and water in space. Asteroids mainly located between Mars and Jupiter are primitive bodies: they have escaped large impacts or destruction and luckily preserve their ingredients until now.

C-type asteroids, one of the major asteroid groups, are considered to be enriched in organic materials and hydrated minerals. C-type asteroids are thought to be parent bodies of some meteorites based on the similarity of reflectance spectra obtained by telescopic observations. The meteorites are enriched in carbonaceous materials and hydrous minerals and classified as “carbonaceous chondrites.”

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Fortunately, we can get such valuable meteorites while staying on the ground. We can indirectly estimate information about C-type asteroids and primitive small bodies: mineralogy, chemical composition, spectral properties.

Next step is clarifying a direct connection between C-type asteroids and carbonaceous chondrites. To do this we need to go to the C-type asteroid, and bring the particles from its surface, and come back to the Earth. The asteroid explorer Hayabusa 2 spacecraft of the Japan Aerospace Exploration Agency (JAXA) was launched on December 3, 2014, to do just that.

To understand the surface property of target C-type asteroid, Ryugu (竜宮, the beautiful castle under the sea in a Japanese fairy tale), spectral observation is performed by the Optical Navigation Camera telescope (ONC-T), the Near-Infrared Spectrometer (NIRS3), and thermal Infrared Imager (TIR) onboard the Hayabusa 2 spacecraft.

NIRS3 has a detectable absorption band around 3 µm in wavelength, “the 3-µm band”, which can be attributed to the hydroxyl of SiOH in sheet silicates, and also the terrestrial adsorbed and rehydrated H2O. The recent paper investigated the performance of NIRS3 and confirmed that the NIRS3 has sufficient ability to obtain spectral features of hydrous minerals on the surface of Ryugu. For this investigation, several carbonaceous chondrites heated at their parent bodies and unheated chondrites were selected.

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The measured spectra of the chondrites were correlated and show clear 3-¾m bands, whose shape and depth reflect the state of hydrous minerals in each chondrite. The 3-¾m band of unheated chondrites is deep, on the other hand, the band of the heated chondrites shallow, which reflects the heating and dehydration degrees of hydrous minerals of the chondrites. This corresponds with the mineralogical results obtained by X-ray diffraction analyses. Moreover, the relationship among reflectance values at several wavelengths of the 3-¾m band can be used to examine the heating degree of the carbonaceous chondrites. Recent observation studies indicate that Ryugu may have been heated and have heterogeneity at its surface. Hayabusa 2 on its outbound flight is expected to grab the key to the evolutionary process the of the early solar system, and our history.

This article is a compilation of work by Ms. Matsuoka and others. One recent published paper on the topic, An evaluation method of reflectance spectra to be obtained by Hayabusa2 Near-Infrared Spectrometer (NIRS3) based on laboratory measurements of carbonaceous chondrites was published the journal Earth, Planets, and Space.

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