Japanese scientists have detected evidence of water in 17 asteroids for the first time using data from the infrared satellite AKARI. 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, researchers said.
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. However, scientists are 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.
Researchers from Japan Aerospace Exploration Agency (JAXA) and University of Tokyo found that water is retained in asteroids as hydrated minerals, which were produced by chemical reactions of water and anhydrous rocks that occurred inside the asteroids. Hydrated minerals are stable even above the sublimation temperature of water ice, according to the study which appears in the Publications of the Astronomical Society of Japan. By looking for hydrated minerals, scientists 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 observe at infrared wavelengths for the study of solar system objects. Hydrated minerals exhibit diagnostic absorption features at around 2.7 micrometres. The absorption of water vapour and carbon dioxide in the terrestrial atmosphere prevents us from observing this wavelength with ground-based telescopes.
It is absolutely 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 a coverage of this wavelength range. For this reason, it has not fully been understood how much water is contained in asteroids.
The Japanese infrared satellite AKARI, which was launched in February 2006 and ended operations in 2011, was equipped with the Infrared Camera (IRC) that allowed us to obtain spectra at near-infrared wavelengths from two to five micrometres. Using this unique function, the spectroscopic observations of 66 asteroids were carried out and their near-infrared spectra were obtained. This provides the first opportunity to study the features of hydrated minerals in asteroids at around the wavelength of 2.7 micrometres. The observations detected absorption, which were attributed to hydrated minerals for 17 C-type asteroids.
C-type asteroids, which appear dark at visible wavelengths, were believed to be rich in water and organic material, but the present observations with AKARI are the first to directly confirm the presence of hydrated minerals in these asteroids. The heating energy could be supplied by the solar wind plasma, micrometeorite impacts, or the decay heat from radioactive isotopes in the rocks. This trend had been predicted by meteorite measurements, but this is the first time that it has been confirmed in asteroids.
Many C-type asteroids display this trend, suggesting that C-type asteroids were formed by the agglomeration of rocks and water ice, then aqueous alteration occurred in the interior of asteroids to form hydrated minerals, and finally C-type asteroids were heated and dehydrated.