Credit: JAXA
Announcement point
- Studies by other research groups have already shown that asteroid Ryugu consists of something that has undergone extensive reactions with liquid water. However, Hayabusa2 on-site observations suggest that when the asteroid Ryugu was part of a larger celestial body, the temperature inside it was high, or it was formerly in a close orbit to the Sun. As a result of the intense heating up to a depth of about 1 meter from the surface, we have obtained data that can be interpreted as the loss of most of the water on the surface of the celestial body or of the entire celestial body into space. This research aims to resolve the contradiction between the two.
- The surface of a celestial body without an atmosphere is gradually changing due to various factors (called space weather). This study showed that the surface material of asteroid Ryugu is also undergoing space erosion. However, asteroid Ryugu contains a large amount of hydrated silicate minerals (a type of clay) that neither the Moon nor Itokawa contain, so it has a unique feature that is different from the waterless Moon and asteroid Itokawa. In other words, celestial bodies without an atmosphere will tan differently depending on their individuality.
- Space weathering on Ryugu is characterized by the melting of several microns of stone (or sand) surfaces due to heating caused by micrometeoroid impacts. This melting dehydrates the clay and it is found to reflect sunlight as if the entire celestial body had undergone intense heating.
- C-type asteroids, to which Ryugu belongs, are most abundant in the main belt, where the asteroids are concentrated. This study sheds light on the current state of space weathering of C-type asteroids for the first time and is expected to greatly advance the interpretation of the reflection spectra of water-containing asteroids (how the asteroid reflects sunlight).
overview
As mentioned earlier in the point, the surface of the celestial body without the atmosphere of the solar system is exposed to the collision of micrometeoroids at a speed exceeding 10 kilometers per second, the irradiation of the solar wind, which is the flow of plasma from the sun, the sun and the galaxy, is in a very hostile environment where it is constantly exposed to cosmic rays. These effects are known to gradually change the chemical composition, structure and optical properties of the surface of astronomical objects without an atmosphere (and magnetic field). This change is called spatial alteration.
Asteroid Ryugu belongs to the C-type asteroids, which are the most abundant small bodies in the inner solar system. The return of samples from Ryugu by Hayabusa2 provided the first opportunity to study the characteristics of this C-type asteroid in the laboratory. Changes in the surface of the samples due to space erosion have so far been studied in samples from the Moon and the S-type asteroid Itokawa brought back to Earth. These samples are basically composed of hydroxyl groups (OH) and water molecules (H2It consists of anhydrous minerals that do not contain O).
On the other hand, Ryugu, a C-type asteroid, accumulated minerals, organic matter, and ice when its original body formed, and then the ice melted and the minerals reacted with water on a large scale. This celestial body was destroyed by collisions with other celestial bodies and the debris gathered to form the current Ryugu. Because of this, the material on Ryugu contains a large amount of clay minerals called layered silicate minerals. The crystalline structure of layered silicate minerals is destroyed in the Ryugu particles (in this study “sand-sized” samples were analyzed, so the samples are referred to as particles) for which spatial weathering was detected. where the layered silicate minerals were partially dissolved. In both cases, the trivalent iron ions contained in the layered silicate minerals were reduced to divalent iron ions.
Also, the hydroxyl group contained in the layered silicate mineral has been lost. This means that water has been removed from the surface of the Ryugu particle. In particular, the dehydration reaction was significant when the layered silicate minerals were partially molten.
These results indicate that dehydration of layered silicate minerals present on the surface of asteroid Ryugu greatly contributes to space weathering on C-type asteroids. The reflection spectrum of the asteroid, which weakly absorbs light with a wavelength of 2.7 microns, measured by Hayabusa2 indicates that there are few hydroxyl groups. For C-type asteroids in general, bodies with a weak absorption band at 2.7 microns may indicate a degree of dehydration caused by space erosion, rather than a loss of volatiles throughout the body.
title:The true face of the water-rich asteroid Ryugu hidden in the sun
Original title:A dehydrated skin from space weather enveloping Ryugu’s hydrated interior
Publishing magazine:Nature astronomy
DOI：10.1038/s41550-022-01841-6
For more information,JAXA press releasePlease refer to.
