Japanese researchers have detected evidence of water in 17 asteroids for the first time using data from the infrared satellite 'AKARI'.
According to researchers, the discovery will contribute to the understanding of the distribution of water in our solar system, the evolution of asteroids and the origin of water on Earth. The study had appeared in the Publications of the Astronomical Society of Japan.
• Earth is an aqua-planet, 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 the Earth acquired water.
• The recent studies have revealed that even other celestial bodies in our solar system have or used to have water in some form. Asteroids were considered to be one of the possible candidates that brought water to Earth.
• Researchers from the Japan Aerospace Exploration Agency (JAXA) and the 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.
• The hydrated minerals are stable even above the sublimation temperature of water ice. By looking for hydrated minerals, scientists can investigate whether asteroids have water.
• The 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 the researchers from observing this wavelength with ground-based telescopes.
• However, it is necessary to make the observations from outside of the atmosphere, that is, in space and 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.
The Spitzer Space Telescope, launched in 2003, did not have the coverage of the required wavelength range.
Hence, researchers were not able to fully understand how much water is contained in the asteroids.
The Japanese infrared satellite, which was launched in February 2006 and ended operations in 2011, was equipped with the Infrared Camera (IRC) that allowed the researchers 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.
The information provided 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 was attributed to hydrated minerals for 17 C-type asteroids.
The 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.