Previous analysis has revealed that asteroid Ryugu is composed of material that has experienced severe aqueous alteration. However, in-situ observations by the Hayabusa2 spacecraft were interpreted as the ancestor body of asteroid Ryugu having experienced dehydration by thermal metamorphism (intense heating) after aqueous alteration, or that Ryugu was in an orbit that passed close to the Sun so that the asteroid's surface was dehydrated to a depth of about 1m by solar irradiation.
This study from the Hayabusa2 Initial Analysis Team "Sandy Materials Analysis" resolves this apparent contradiction between the laboratory and in-situ measurements, finding that the surface of Ryugu has been altered by space weathering to produce a dehydrated skin.
The surfaces of airless bodies such as asteroids are gradually altered due to several causes, including micrometeoroid impacts and irradiation by the solar wind. The gradual alteration is called "space weathering", and it was found by this study that the surface of Ryugu has been altered by this process.
Space weathering has been previously investigated using the samples retuned from the Moon and the S-type (rich in silicate materials) asteroid Itokawa. However, asteroid Ryugu contains abundant hydrated phyllosilicates (a kind of clay), which are basically absent in the Moon and asteroid Itokawa. Due to these abundant phyllosilicates, space weathering that was found on the surface of Ryugu grains is unique, differing from the waterless Moon and Itokawa. So to speak, every airless body experiences different types of "sun burn" according to the character of each body.
The most characteristic feature of the space weathering on Ryugu is a higher abundance of grains covered by a few-µm thick silicate melt. Due to the melting by micrometeoroid impact, phyllosilicates on the surfaces of grains were dehydrated. The surface of Ryugu reflected the sunlight as if the entire body had experienced global thermal metamorphism due to a high abundance of grains covered by silicate melts on the surface of Ryugu. The near-infrared spectrometer equipped on the Hayabusa2 spacecraft therefore detected a weak 2.7-µm absorption band indicating the low abundance of hydroxyl groups on the surface.
There are many C-type asteroids in the main-belt of the asteroid belt. It is possible that main-belt C-type asteroids that show a weak 2.7-µm band may also have experienced dehydration induced by space weathering rather than global thermal metamorphism. This study will play an important role in clarifying the actual state of space weathering and interpreting the reflectance spectra of water-bearing asteroids, including C-types.
Title：A dehydrated space-weathered skin cloaking the hydrated interior of Ryugu
For details, please see the JAXA press release.