Beyond Earth: What samples of rocks and dust tell us about the Ryugu asteroid.

June 2022

JAXA Hayabusa 2

Ryugu is one of that category of near-Earth asteroids which belongs to the Apollo group. There are some 1400 Apollo asteroids, and these are both sufficiently large and come sufficiently close to Earth to be considered potentially hazardous.

For example Ryugu measures approximately 1 km in diameter and from earlier observations was classified as both a C-type (carbonaceous) and a B-type asteroid. B-type asteroids are generally similar to the C-type objects, but differ in that the ultraviolet absorption below 0.5 µm is small or absent, and the spectrum is slightly bluish rather than reddish. To investigate Ryugu in more detail, a Japanese spacecraft (The Hayabusa 2) was launched on 3rd December 2014. It landed on the asteroid 3.5 years later and after taking measurements and taking soil samples from above and below the surface of the Ryugu returned the sample capsule to Earth in December 2020. The returned samples are rock fragments ranging in size up to ~10 mm, with a total mass of 5.4 g.

This was the second Japanese mission to the asteroid belt. The target for the first mission, using the first of the Hayabusa spacecraft, was Itokawa - an asteroid with diameter of less than one kilometer, but also in the Apollo group. In 2005 the Hayabusa successfully collected samples of Itokawa dust and in December 2010 returned the capsule containing this asteroid material to Earth. Analysis of the Itokawa samples showed that this was an S-type or stony asteroid which was part of a much larger asteroid in the past. Its composition matched the common type of meteorites known as low-total-iron, low metal ordinary chondrites (non-metallic meteorite that has not been modified by either melting or differentiation of the parent body). After two different groups of researchers reported finding water in the asteroid particles (in the form of H₂O and OH), Itokawa was classified as a 'water-rich asteroid'.

A recent publication in the journal Science contains the first results from the analysis of the samples from Ryugu. The Ryugu samples were similar to those of a class of meteorites known as "Ivuna-type carbonaceous chondrites." The Ivuna meteorite landed near Ivuna in Tanzania on December 16 1938. It is one of only 9 known meteorites that are classified as type CI1 carbonaceous chondrites. These meteorites are of special interest because their compositions are essentially unaltered since they were formed at about the same time as our solar system, some 4.6 billion years ago. However these meteorites had been on Earth for quite some time before being discovered and it is likely that their chemical composition was altered by crashing through the atmosphere and lying exposed to the elements on Earth.

So having pristine samples from Ryugu is very special. Scientists were hoping Ryugu would help them to determine the relationship between C-type asteroids and the carbonaceous chondrite meteorites. Below is a summary of the key findings.

The Ryugu samples are mixtures of mechanical fragments - composed of fine-grained materials of phyllosilicate minerals and coarser grain materials. Phyllosilicates were identified as predominantly serpentine (hydrous magnesium-rich silicate minerals) and saponite (a clay mineral). Phyllosilicates are a group of minerals that are mainly composed of extended flat sheets of linked silicon-oxygen tetrahedra (pyramid-like in structure). Members of this family are micas and clays. The name is derived from the Greek word phyllos, meaning leaf.

Coarser-grained material was dominated by carbonates (dolomite (CaMg(CO₃)₂), breunnerite [(Mg, Fe, Mn)CO₃], and calcite (CaCO₃), magnetite (Fe₃O₄) , and sulfides eg .copper iron sulfide - Cubanite (CuFe₂S₃).

Ca-Al-rich inclusions (CAIs) or chondrules, which are characteristic constituents of most chondrite meteorites, were also in the Ryugu samples. No metal grains were identified. Anhydrous silicates, such as olivine and pyroxene, are common in chondrites, but are very rare in our Ryugu samples. Overall, the petrology and mineralogy of the Ryugu samples indeed most closely resembles that of the CI (Ivuna-like) group of chondrite meteorites. However, sulfates and ferrihydrite, which are commonly observed in CI chondrites, were not found in the Ryugu samples.

Although the samples brought to Earth appear relatively dry, the fragments show signs of having been soaked in water at some point in the past. The researchers measured gas release curves for both Ryugu samples and Ivuna. Simply put, in this experiment the samples are heated and the mass loss is measured. The mass loss is due to evaporation of water and the release of carbon dioxide. Water was found in both the Ivuna and Ryugu samples but Ivuna contained more H₂O than Ryugu.

So how did Ryugu form? The scientists put forward a plausible scenario of events. They postulate that Ryugu was created by the re-accumulation of material ejected from a parent body after an impact. Because of similarities between Ryugu and CI chondrites, the parent body was probably a CI chondrite. At some point the parent body was soaked in water:

"One must picture an aggregate of ice and dust floating in space that turned into a giant mudball when ice was melted by nuclear energy from the decay of radioactive elements that were present in the asteroid when it formed," said one of the authors of a recent publication.

The researchers believe that the 'aqueous event' took place approximately 5 million years after the Solar System was formed (based on the results from radioisotope dating). This extensive aqueous alteration caused the precipitation of dolomite and magnetite. The saponite produced by this fluid-assisted alteration likely contained large amounts of interlayer water in its crystals. The relatively small amount of water measured in the Ryugu samples suggests that much of this water later escaped into space probably after the parent body was destroyed and before Ryugu formed. The loss of water resulted from a combination of factors such as impact heating, solar heating, space weathering, and long-term exposure of the asteroid surface to the ultra-high vacuum of space. Temperatures probably did not need to be high as well, since laboratory experiments showed that heating the samples to 170^oC was sufficient to fully dehydrate them.

The researchers hope that this is just the beginning of solar exploration with many more spacecraft bringing samples from different asteroids, from Mars and possibly other planets in the coming decades.

Journal Reference:
1. Tetsuya Yokoyama et al. Samples returned from the asteroid Ryugu are similar to Ivuna-type carbonaceous meteorites. Science (2022); https://www.science.org/doi/10.1126/science.abn7850
2. Tomoki Nakamura et al. Itokawa Dust Particles: A Direct Link Between S-Type Asteroids and Ordinary Chondrites. Science (2011), vol 333, pp. 1113-1116.