A container containing soil samples collected by Hayabusa 2 from the asteroid Ryugu. It fell in the Australian desert on December 6, 2020./NASA
Analysis results showed that the samples brought back from the asteroid Ryugu by Japan’s asteroid probe were quite different in terms of composition from the meteorites known to be the purest among the meteorites that fell to Earth. Scientists assumed that the meteorite that fell to Earth came from the home of an asteroid and estimated its composition, but this study confirmed that there are limitations to the asteroid analysis method.
The international research team, centered around the Department of Earth Sciences at Tohoku University in Japan, compared samples from the asteroid Ryugu and CI chondrites from the meteorite that fell to Earth using ‘VisNIR’ spectroscopy and published the results in the international academic journal ‘Science Advances’. ‘ announced on the 7th.
The sample used in the study was about 5.4 g of dust and small stones taken from the surface of the asteroid probe Hayabusa2, launched by the Japan Aerospace Exploration Agency (JAXA) in 2014, while exploring the carbonaceous C-type asteroid Ryugu. Hayabusa2 arrived at the asteroid Ryugu in 2018, four years after launch, and returned to Earth with samples in December 2020. It is a journey spanning 5 billion kilometers over 6 years.
What the research team compared with the Ryugu sample was a meteorite called CI chondrite. It is a 700g meteorite that fell in the Ivuna region of Tanzania in 1938. It is so rare that out of more than 60,000 meteorites that fell on the Earth’s surface, only 9 remained on the ground without burning. In particular, it contains information about the creation of the solar system because its chemical composition is similar to the surface of the sun.
The University of Tokyo research team, which first announced the results of Ryugu sample analysis, announced last year that Ryugu had similar composition to CI chondrite. CI chondrite has a higher ratio of heavy elements such as sodium and sulfur than calcium, and the samples collected from Ryugu are similar. Ryugu is believed to have been formed only about 5 million years after the formation of the solar system began, and it is highly likely that the CI chondrite has a similar parent body to Ryugu.
An illustration of Hayabusa 2 collecting soil samples from asteroid Ryugu./JAXA
However, the research results presented this time by the research team show that there are differences between the Ryugu sample and the CI chondrite. As a result of examining visible light and near-infrared (Vis-NIR) spectroscopy, the research team confirmed that the Ryugu sample had a lower visible light reflectance and higher wavelength absorption than CI chondrite. This is because CI chondrite has higher water, iron, and silicate contents than the Ryugu sample.
The cause is the location where Ryugu samples and CI chondrites were collected. Unlike the Ryugu samples collected from space by the asteroid probe, CI chondrites were affected by weather for a long time after falling to Earth. As it reacted with oxygen in the air, oxidation occurred and the iron, silicate, and sulfate contents increased. Also, moisture was retained due to the periodic rain. Until now, CI chondrites were thought to be most similar to the solar surface. However, as a result of this study, it was confirmed that CI chondrites are insufficient to reveal the secrets of the universe because a kind of ‘globalization’ has occurred.
Rather, the Ryugu sample had the most similar spectrum to CI chondrite heated in vacuum at 300 degrees Celsius for 100 hours. When the CI chondrite was heated, the iron and sulfate in the silicate decreased and most of the water evaporated, and the visible and near-infrared spectra of the heated CI chondrite were similar to those of the Ryugu sample. However, the research team explained that the heating experiment environment is not the same as the asteroid environment.
The significance of this study is that it revealed the limitations of meteorites used to estimate the characteristics of asteroids. Asteroids are broadly classified into C-type, S-type, and M-type depending on the spectrum of their constituent materials. The decision is made by observing asteroids and then analyzing and comparing the spectra of meteorites that have fallen to Earth. However, through this study, we once again recognized the stark differences between space and terrestrial environments.
The most accurate way to study asteroids is to obtain samples directly from space. This also means that if technology related to asteroid exploration is not secured, we may fall behind in the competition necessary to understand the universe.
The research team said, “The comparison of asteroid Ryugu and CI chondrites shows that the impact on the ground is more severe than in space,” and “A simple comparison of spectra between chondrites and asteroids leads to incomplete and inaccurate estimates of the composition distribution of the solar system.” “It can be calculated,” he emphasized.
References
Science Advances(2023), DOI: https://doi.org/10.1126/sciadv.adi3789
