The history of one of the oldest objects in the Solar system unveiled

23-09-2022

An international team of scientists have unveiled details of the history of the asteroid Ryugu, a truly ancient object in the Solar system, after the Hayabusa2 mission brought samples from this asteroid back to Earth. The ESRF was one of the institutes involved in sample characterization, on ID15A. The results are published this week in Science.

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The asteroid Ryugu, located at 200 million kilometres from the Earth, is one of the most primitive objects of the solar system. The Japanese spacecraft Hayabusa2 explored it from 2018 until it came back to Earth two years later with minuscule multiple samples from the asteroid.

Two years later, and thanks to the international collaboration of institutes led by the Japan Aerospace Exploration Agency (JAXA), the first results on the analysis of the samples shed light on the history of Ryugu, from its formation to its collisional destruction.

Researchers used cosmochemical and physical methods at universities and institutes, including the ESRF and four other synchrotron radiation facilities in Japan, United States, and Europe.

The results combined with computer simulation have allowed scientists to picture the origins of Ryugu:  the Ryugu parent body accumulated about 2 million years after the formation of the solar system, and then heated up to about 50°C over the next 3 million years, resulting in chemical reactions between water and rock. The size of the impactor that destroyed the Ryugu parent body, which is about 100 km in diameter, is at most 10 km in diameter, and that the present-day Ryugu is composed of material from a region far from the impact point.

What the data explain

In particular, the seventeen Ryugu samples analysed contain particles (such as Ca- and Al-rich inclusions) that were formed in high-temperature environments (>1000°C). These high-temperature particles are thought to have formed near the Sun and then migrated to the outer solar system, where Ryugu was formed. This indicates that large-scale mixing of materials occurred between the inner and outer solar system at the time of its birth.

Based on the detection of the magnetic field left in the Ryugu samples, it is highly likely that the original asteroid from which the current Ryugu descended (Ryugu’s parent body) was born in the darkness of nebular gas, far from the Sun, where sunlight cannot reach.

The scientists also discovered liquid water trapped in a crystal in a sample. This water was carbonated water containing salts and organic matter, which was once present in the Ryugu parent body. Crystals shaped as coral reefs grew from the liquid water that existed inside Ryugu's parent body. Rocks that were deeper underground contained more water than those in the surface.

The hardness, heat transfer, and magnetic properties of the samples were measured. The results showed that the Ryugu sample was soft enough to be cut with a knife. The sample also contained many small magnets, behaving like a natural hard disk, recording the magnetic field of the past.

With all this information, the scientists carried out a computer simulation of the process from the birth of the Ryugu parent body to its destruction by a catastrophic impact. This is the first time that measurements of the hardness and thermal diffusivity of actual asteroid samples have been incorporated into a simulation of the formation and evolution of asteroids.

The role of the ESRF

Scientists from the Ghent University and Goethe University Frankfurt, together with Marco di Michiel at the ESRF, carried out X-ray fluorescence experiments at ESRF ID15A. “We combined very high incident energies of 90 keV with a 300 nm resolution scanning capability and a new high-count rate high-efficiency fluorescence detector”, explains Laszlo Vincze, researcher at Ghent University.

“Looking deep inside the mm-sized Ryugu asteroid rock fragments non-destructively by performing high resolution 2D/3D elemental mapping of heavy and rare earth elements (REE) has been made possible by the unique combination of beam properties enabled by ESRF EBS and new detection system”, Vincze says. “Access to such high energy submicron excitation yielded up-to-now unattainable information on the high atomic number and REE trace element patterns within these unique samples”, he adds. 

The study of the returned Ryugu rocks at the ESRF takes place in the framework of a long-term project at ID15A that aims to obtain quantitative chemical information to unravel the secrets of this ancient solar system material.

Vincze and his colleagues are no strangers to the ESRF: “These investigations follow a series of studies by our team at the ESRF spanning over more than a decade now, investigating cometary and interstellar material returned by NASA’s Stardust mission. This experience has prepared us to analyse samples of future return material, such as that of the OSIRIS-REx mission from asteroid Bennu, expected next year”, he concludes.

Reference:

Nakamura, T. et al., Science, 22 September 2022. DOI: 10.1126/science.abn8671

 

Top image: A coloured view of the C-type asteroid 162173 Ryugu, seen by the ONC-T camera on board of Hayabusa2. Credit: JAXA Hayabusa 2