Using the infrared satellite AKARI, a research team has detected the existence of water in the form of hydrated minerals in some asteroids for the first time. This discovery will contribute to our understanding of the distribution of water in our solar system, the evolution of asteroids, and the origin of water on Earth.

Using the infrared satellite AKARI, a Japanese research team has detected the existence of water in the form of hydrated minerals in some asteroids for the first time. This discovery will contribute to our understanding of the distribution of water in our solar system, the evolution of asteroids, and the origin of water on Earth.

The findings were made by the team led by the Project Assistant Professor Fumihiko Usui (Graduate School of Science, Kobe University), the Associate Senior Researcher Sunao Hasegawa, the Aerospace Project Research Associate Takafumi Ootsubo (Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency), and Professor Emeritus Takashi Onaka (Graduate School of Science, University of Tokyo). The results were published on December 17 in the online Advanced Access edition of Publications of the Astronomical Society of Japan.

Our Earth is an aqua-planet and is the only planet in our solar system where the presence of water on the planet surface has been confirmed. We are, however, not yet sure how our Earth acquired water. Recent studies have shown that other celestial bodies in our solar system have, or used to have, water in some form. Asteroids are considered to be one of the candidates that brought water to Earth. 

Asteroids

Note that the liquid water is not flowing on the surface of asteroids, but water is retained in asteroids as hydrated minerals, which were produced by chemical reactions of water and anhydrous rocks that occurred inside the asteroids, that is, aqueous alteration. Hydrated minerals are stable even above the sublimation temperature of water ice. Thus, by looking for hydrated minerals, we can investigate whether asteroids have water.

Infrared wavelengths contain characteristic spectral features of various substances, such as molecules, ice, and minerals, which cannot be observed at visible wavelengths. Therefore, it is indispensable to see at infrared wavelengths for the study of solar system objects.

Hydrated minerals exhibit diagnostic absorption features at around 2.7 micrometers. The absorption of water vapor and carbon dioxide in the terrestrial atmosphere prevents us from observing this wavelength with ground-based telescopes.

It is necessary to make observations from outside of the atmosphere, that is, in space. However, observations with space-borne telescopes have been scarce; the Infrared Space Observatory (ISO), launched in 1995, did not have a sufficient sensitivity to make spectroscopy of faint asteroids and the Spitzer Space Telescope, launched in 2003, did not have coverage of this wavelength range. For this reason, it has not fully been understood how much water is contained in asteroids.