When And How Asteroid Ryugu Lost Its Water? Remote Sensing Data Explains

Science always wonders us with a new discovery every day. Recently, Japan’s spacecraft Hayabusa 2 brought rocks from a near-Earth asteroid called Ryugu. The analysis of these rocks is going on. Meanwhile, other data collected by spacecraft is giving out additional details about the asteroid and its past.

The study recently published in Nature Astronomy has focused on the fact why Ryugu isn’t as rich in water-bearing minerals as a few other asteroids. Research claims that the parent body(ancient body) of Ryugu would’ve likely dried out in some kind of a heating event which left Ryugu drier than it’d be.

Ralph Milliken, a planetary scientist at Brown University and study co-author, said that they were doing their research on the fact of distribution of water in the early solar system and on a way of finding how water may have been delivered to earth. We hope to understand better by studying the properties of such water-bearing asteroids which are thought to have a role in the water on earth.

Milliken added one reason why Ryugu was chosen was that Ryugu belongs to a class of asteroids which are dark and suspected to have water-bearing minerals and organic compounds. Researchers believe that these asteroids are possible parent bodies for, dark, water- and carbon-bearing meteorites found on Earth known as carbonaceous chondrites.

These meteorites are found to be substantial sources of information, but it is not possible to determine with certainty which asteroid a given carbonaceous chondrite meteorite have come from.

The Hayabusa 2 mission is the first time one of these asteroids has been collected directly and brought to earth. Initial observations made by Hayabusa 2 when flying around asteroid Ryugu suggested that it may not be as water-rich as expected. As of now, there are many ideas on how Ryugu lost some of its water.   

The asteroid Ryugu is a rubble pile, which means it is a loose conglomeration of rock held together by gravity. It is believed to be formed from debris left over when larger and more solid asteroids are broken apart by a large impact event.

So, it is fairly possible that the water signature seen on Ryugu is all remains of a previously rich parent asteroid which dried because of some heating event.

Another idea could be that Ryugu dried out due to a catastrophic disruption and reformation as a rubble pile. One more reason could be that Ryugu had a few close spins past the sun in its past, which could have heated it up and dried out its surface.

The Hayabusa 2 spacecraft had equipment that would help scientists. The spacecraft had fired a small projectile onto the asteroid’s surface, creating a crater and helping them compare the water content with the help of infrared spectrometer.

Milliken explained that usually high-temperature heating from the sun would occur mostly at the surface and not too deep into the subsurface. The research, however, found that water content in both the surface and subsurface is similar, thus bringing us to an idea that Ryugu’s parent was altered.

However, more research has to be done in order to find out the exact details regarding this.   

“The excavated material may have had a smaller grain size than what’s on the surface,” said Takahiro Hiroi, a senior research associate at Brown and study co-author. Hiroi added that these grain size effect could make it appear darker and redder than its coarser counterpart on the surface. It’s hard to rule out that grain-size effect with remote sensing.

The mission isn’t limited to studying samples remotely. Hayabusa2’s success is surely a starting point for more such expeditions. The research is certainly exciting and also a bit of surprise, and we’re sure to learn a lot more about the links between meteorites and their parent asteroids.

Journal Reference:
K. Kitazato, R. E. Milliken, T. Iwata, M. Abe, M. Ohtake, S. Matsuura, Y. Takagi, T. Nakamura, T. Hiroi, M. Matsuoka, L. Riu, Y. Nakauchi, K. Tsumura, T. Arai, H. Senshu, N. Hirata, M. A. Barucci, R. Brunetto, C. Pilorget, F. Poulet, J.-P. Bibring, D. L. Domingue, F. Vilas, D. Takir, E. Palomba, A. Galiano, D. Perna, T. Osawa, M. Komatsu, A. Nakato, T. Arai, N. Takato, T. Matsunaga, M. Arakawa, T. Saiki, K. Wada, T. Kadono, H. Imamura, H. Yano, K. Shirai, M. Hayakawa, C. Okamoto, H. Sawada, K. Ogawa, Y. Iijima, S. Sugita, R. Honda, T. Morota, S. Kameda, E. Tatsumi, Y. Cho, K. Yoshioka, Y. Yokota, N. Sakatani, M. Yamada, T. Kouyama, H. Suzuki, C. Honda, N. Namiki, T. Mizuno, K. Matsumoto, H. Noda, Y. Ishihara, R. Yamada, K. Yamamoto, F. Yoshida, S. Abe, A. Higuchi, Y. Yamamoto, T. Okada, Y. Shimaki, R. Noguchi, A. Miura, N. Hirata, S. Tachibana, H. Yabuta, M. Ishiguro, H. Ikeda, H. Takeuchi, T. Shimada, O. Mori, S. Hosoda, R. Tsukizaki, S. Soldini, M. Ozaki, F. Terui, N. Ogawa, Y. Mimasu, G. Ono, K. Yoshikawa, C. Hirose, A. Fujii, T. Takahashi, S. Kikuchi, Y. Takei, T. Yamaguchi, S. Nakazawa, S. Tanaka, M. Yoshikawa, S. Watanabe, Y. Tsuda. Thermally altered subsurface material of asteroid (162173) Ryugu. Nature Astronomy, 2021; DOI: 10.1038/s41550-020-01271-2

Press Release: Brown University