Analysis of the sample returned from the C-type asteroid Ryugu reveals amino acids and the historic presence of water. The discovery strengthens the suspicion the building blocks for life were present in the material from which the Solar System formed and these were delivered to Earth on this type of asteroid.
In 2005 the mission Hayabusa collected the first asteroid sample to be returned from Earth. The target in that case, Itokawa, was known as an S-type for its high silicon abundance. S-types formed in the inner Solar System and are by far the most common there, making for an easy target.
The follow-up mission, Hayabusa2 was more ambitious, going after an asteroid thought to have migrated in from the outer Solar System, and preserve a more intact record of the nebula from which the Sun and planets formed. A paper in Proceedings of the Japan Academy published today reveals some of what was found, although arguably the most important part of the announcement – the presence of 20 amino acids – was leaked a few days earlier to the Japan Times. A second paper, published just three hours later in Science reaches similar conclusions.
Even before chemical analysis began, cutting open the Hayabusa2 revealed textures indicative of a freeze-thaw cycle. The samples are mostly fine-grained hydrous silicates, which on Earth we would call clay, with coarser-grained deposits sprinkled through them. Hydrous silicate forms through reactions between dry silicates and liquid water.
The water was present within a few million years of the Solar System's formation, subsequent evidence revealed. Ryugu's isotopes are in keeping with what would be expected of an object that formed from the protosolar nebula early in the lifetime of the Solar System and far out from the Sun.
Since Ryugu was too far from the Sun to have melted from external heat, the authors conclude, radioactive elements within it must have provided the energy to melt the ice. Within a few million years most of its more radioactive isotopes had decayed, reducing the heat production and causing the ice to refreeze.
Ryugu is only 435 meters (1,400 feet) across, however, which makes it too small for radioactive decay to have ever melted liquid water. Heat generated on an object that size would quickly escape to space. Consequently, the authors think Ryugu must have once been part of a more substantial object, several tens of kilometers in diameter at least. At some point, a collision broke this predecessor apart and Ryugu is one of the pieces.
Initially, Ryugu would have been more of a comet than an asteroid. Gravitational interactions with one or more planets drew it into the inner Solar System where exposure to sunlight turned its ice to water vapor and leaving it dry and porous. Previous modeling suggests jets of gas coming off during this process both increased Ryugu's rate of spin, and produced its distinctive shape. In the process it's likely material from deep inside was brought to the surface where Hayabusa2 could collect it.
Hayabusa2 collected samples at two different sites on Ryugu, which turned out to be a wise decision, since differences in their composition tell a story. It's likely the material from touchdown site 1 (TD1) is a mixture brought to the surface from different parts of the interior by a water vapor jet and redeposited there. TD2 is more uniform, probably because it was not stirred up in the same way, instead only being exposed during an impact.
Amino acids have been found in carbonaceous chondrite meteorites, leading to a strong suspicion they would be present on C-type asteroids, from which these meteorites come. However, since by definition meteorites have been exposed to the Earth's atmosphere, only a sample collected directly from an object like Ryugu could rule out the possibility a meteorite's organic material was the result of Earthly contamination.
Hayabusa2 has settled that question, making it more likely asteroids provided the materials needed to make DNA and RNA, which recent evidence shows can be catalyzed from nucleotides formed from amino acids. The fact it was once so rich in water strengthens the case that asteroids like this restocked the Earth's supplies after initial water boiled away during the Earth's immensely hot phase.
The Science paper noted the similarity between the Ryugu samples and the Ivuna meteorite, which fell over Tanzania. Ivuna is an example of the CI group of meteorites, one of the rarest types.
My first reaction was "holy sh**! This is truly amazing," said Dr Gretchen Benedix of Curtin University responding to the Science paper. "We've had other samples come back from planetary bodies before, but never the most primitive material in the Solar System."
Dr Alice Gorman of Flinders University, who like Benedix was not an author on either paper added."Out of over 50,000 known meteorites on Earth, there’s less than a dozen like Ivuna and Ryugu. They’re rich in water, and impacts from asteroids and meteorites like them may have been responsible for delivering water to Earth and the Moon early in the solar system’s history. The Earth examples, though, are fragile. They’ve been contaminated by weathering, and have absorbed moisture and other chemicals. The Ryugu sample is pure, so it can tell us so much more."
"Bizarrely, the composition of Ryugu is very like the outer layer of the Sun," Gorman continued "This indicates that its parent body was formed at around the same time at the beginnings of the solar system, about 4.6 billion years ago. It’s a fascinating window into a time when the planets were coming into being and the Sun was still young."