Space Telescope Detects Hydrated Minerals in Numerous Asteroids

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A Kobe University research team has detected the existence of water in a number of asteroids for the first time. Water is retained in the asteroids as hydrated minerals that were produced by chemical reactions occurring within the asteroid.

AKARI space telescope, Kobe University.
By using a space-borne telescope, the team was able to successfully detect the presence of water in many asteroids. Courtesy of Kobe University.

The spectroscopic survey was conducted using the infrared camera (IRC) on board the Japanese IR satellite AKARI, launched in February 2006. In the warm mission period of AKARI, observations were performed for 66 asteroids for wavelengths from 2.5 to 5 μm. Observed results showed most C-complex asteroids to have clear absorption features related to hydrated minerals at a peak wavelength of approximately 2.75 μm, while S-complex asteroids were shown to have no significant features in this wavelength range.

IR wavelengths contain characteristic spectral features of substances such as molecules, ice, and minerals that cannot be observed at visible wavelengths. Hydrated minerals, such as those found on the asteroids, exhibit diagnostic absorption features at around 2.7 μm, and the absorption of water vapor and carbon dioxide in the terrestrial atmosphere prohibits observation of this wavelength with ground-based telescopes. It is therefore necessary to make observations from space-borne telescopes.

Space-borne telescope detects water in asteroids, Kobe University.

his shows six examples for both C-type and S-type asteroids. You can clearly see the absorption at wavelengths of around 2.7 μm (indicated by the green arrows) attributed to hydrated minerals. You can also see signatures of water ice or ammonia-rich material at around 3.1 μm (indicated by the blue arrows). The data shown in this figure are the reflected spectra of the sunlight by the surface of asteroids. Courtesy of Kobe University.

The present observations from AKARI are the first to directly confirm the presence of hydrated minerals in these asteroids, the researchers said. They further said that spectra of the observed asteroids show common patterns. To fully understand the observed patterns, it will be necessary to accumulate observations of more asteroids and compare space-borne measurements with the measurements of meteorites collected on Earth. Professor Fumihiko Usui said, “By solving this puzzle, we can make a significant step toward identifying the source of Earth’s water and unveiling the secret of how life began on Earth.” 


The research was published in Publications of the Astronomical Society of Japan ( 

Published: December 2018
Infrared (IR) refers to the region of the electromagnetic spectrum with wavelengths longer than those of visible light, but shorter than those of microwaves. The infrared spectrum spans wavelengths roughly between 700 nanometers (nm) and 1 millimeter (mm). It is divided into three main subcategories: Near-infrared (NIR): Wavelengths from approximately 700 nm to 1.4 micrometers (µm). Near-infrared light is often used in telecommunications, as well as in various imaging and sensing...
The scientific observation of celestial radiation that has reached the vicinity of Earth, and the interpretation of these observations to determine the characteristics of the extraterrestrial bodies and phenomena that have emitted the radiation.
An afocal optical device made up of lenses or mirrors, usually with a magnification greater than unity, that renders distant objects more distinct, by enlarging their images on the retina.
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