Asteroid: Ryugu samples confronted with data from Hayabusa 2

The probe Hayabusa 2 made two sample collections of Ryugu in 2019, which allowed him to bring back to earth 5.4 grams of matter of the surface and sub-surface of theasteroid. During these two samples, many pictures were taken, which made it possible to evaluate the properties of the surface and the material ejected. These properties could now be compared to those of the material brought back to Earth.

The images show that the ejected rock fragments have morphological variations ranging from rough to smooth, similar to the rocks of Ryugu, and shapes ranging from near-spherical to flattened. Similar variations were observed more generally by the Franco-German (Cnes-DLR) Mascot probe and lander among the rocks on the surface of Ryugu. The material ejected during the collections is therefore representative of the surface material in general.

Porous and intact matter

The analysis of a sequence ofpictures of these harvests shows that one of ejected fragmented by colliding with the probe during its ejection. According to the researchers, this collision occurred at very low speed (about 0.1 m/s), while the meteorites Of type chondrite carbonaceous material, used as analogues of the Ryugu material, require an impact velocity greater than 1 m/s to be fragmented. This result shows that the surface of Ryugu consists partly of a very porous material and weak resistance mechanical, which agrees with measured spectral data in situ by the probe.

More than 200 fragments collected, measuring between 1 and 10 millimeters, have also been observed. The analysis of the structure, the morphology surface, form and some color of these fragments shows that they are similar to the surface material of Ryugu observed by the probe. These samples were therefore not affected by the process of harvesting and returning to Earth and thus do not suffer from any bias ofsampling. The samples have now moved on to the second, more detailed phase of physico-chemical analyses, which will allow us to learn more about thegeological history of the asteroid.

Asteroid: Ryugu samples reveal their first secrets

Article by Adrien Coffinet published on 03/01/2022

Just over a year ago, the probe Hayabusa-2 brought us samples from the asteroid Ryugu. The first analyzes of this material reveal unexpected information, questioning our knowledge of the history of the Solar System.

The Hayabusa-2 probe, in orbit around (162173) Ryugu from June 2018 to November 2019, reported to us on December 5, 2020 samples of this asteroid. This week the of them first articles about theanalysis of this material were published in Nature Astronomy.

Ryugu is a type C asteroid, i.e. a rocky and dark body high in carbon and in water. Carbonaceous chondrites, meteorites which would come from such asteroids, show many features indicating that they have been altered by fluids. These bodies, which would have formed at the edges of the asteroid belt, contained ice mixed with rock. Part of the ice cream meltedwhich produced minerals clay and carbonates (salts).

One of the objectives of the mission Hayabusa-2 was to study the link between C-type asteroids and chondrites carbonated. This is important because carbonaceous chondrites are probably the kind of objects that have brought water and organic compounds on Earth, allowing the life to emerge there.

Dust less dense than expected

Hayabusa-2 collected 5.4 grams of dust, the equivalent of a teaspoon. It may not seem like much, but there are still several thousand grains that can be analyzed individually. As a reminder, the initial objective was to collect 0.1 grams. As these samples are irreplaceable, the analyzes started with non-invasive and non-destructive observations and are currently followed by complex measurements that require the manipulation and preparation of the specimens.

Ryugu’s bulk density had been measured by Hayabusa-2 at about 1,190 kg/m³. This low value was expected since the asteroid is assumed to be a pile of debris gathered after the collision of a larger body and therefore have a porosity high. On the other hand, what was the surprise when the density of the material collected turned out to be almost as low, around 1,300 kg/m³ ! The collecting process should have compacted the grains, as well as the shaking during the transit and entry into theatmosphere. This density is also much lower than that of similar presumed meteorites at Ryugu.

The authors put forward two complementary reasons for this. Meteorites that ended up on Earth were previously ejected by collision from their parent asteroid. Unlike Ryugu’s material, which was protected by its capsule, their plunge into the earth’s atmosphere caused their fragmentation. Thus, meteorites that arrive on Earth have experienced at least one additional event that could decrease their porosity. Ryugu could also contain more low-density materials, such as organic molecules, than such meteorites. This is important because it implies that the Ryugu material has retained carbonaceous material that we have not been able to study before. This should allow us to learn more about the basic building blocks of life.

Preliminary grain analysis

The composition of many Ryugu grains has also been assessed by spectroscopy. It is not the most common technique for preliminary compositional analysis, but it has been used because it is non-destructive, requires no sample preparation, and allows direct comparison of grains with measurements taken at the surface of Ryugu by Hayabusa-2.

Not surprisingly, the grains and the surface of the asteroid have some spectra very similar, both with the same water footprint (as hydroxide, OH). The more detailed analysis in the laboratory, at wavelengths higher than those measured on the asteroid, revealed additional features, including one identified as coming from a nitrogen component which, according to the authors, could come from clay minerals containing ammonium or from an organic material rich in nitrogen. However, a different analytical technique will be required to determine the abundance of organic compounds in the samples.

The authors also found a huge carbonate grain about half a millimeter long that could be rich in ironvery characteristic of this type of meteorites.

Although a preliminary analysis, these papers show that the Ryugu material is primitive and sufficiently different from known meteorites to question the extent to which meteorites are representative of asteroids. This could change some aspects of our view of thesolar system history.