Asteroid Ryugu definitely from the outer Solar System, says new analysis

Scientists across the world are excited about findings from the 5.4 gram sample of rock from the asteroid known as “Ryugu”. It’s absolutely no ordinary dirt.

The dirt was brought back from the asteroid on the spacecraft Hayabusa2 and landed in the sands of South Australia almost two year ago. It has allowed researchers unprecedented insights into the history of our Solar System.

The sample of space dust is the culmination of a six-year, 5-billion-kilometre journey, and has now been analysed by an international team of more than 200 researchers. They used ultrabright X-ray beams, finding inside the rock, tiny water ‘inclusions’ with carbon dioxide inside.

The researchers say this is more evidence that Ryugu’s parent body formed in the outer Solar System, just  2 million years after the Solar System started forming.

“There is enough evidence that Ryugu started in the outer Solar System,” Argonne National Laboratory physicist Esen Alp says.

“Asteroids found in the outer reaches of the Solar System would have different characteristics than those found closer to the Sun.”

“For planetary scientists, this is first-degree information coming directly from the Solar System, and hence it is invaluable.”

At its closest orbit, Ryugu is only a quarter of the distance to Earth of the Moon, which might suggest that the asteroid would have been formed in the inner Solar System.

However, this research, and a study from earlier this year which backs up this finding, seems to suggest otherwise.

The team explain that the grains that make up the asteroid are much finer than you would expect if it was formed at higher temperatures found closer to the sun.

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Read more: Ryugu asteroid samples drop-shipped into South Australia contain super-rare space dirt

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Earlier this year, researchers determined that the structure was incredibly similar to a rare type of asteroid from the outer Solar System called CI chondrites.

“We’ve had other samples come back from other planetary bodies before, but never the most primitive material in the Solar System,” Curtin University astrogeologist Prof Gretchen Benedix explained at the time.

“On earth we have 70,000 meteorites (that we know of) – of these, only nine are classified as CI.”

These asteroids are assumed to form in the outer asteroid belt, more than four times the distance to Earth. This is because ‘4 AU’ is past the ‘snow line’ where the temperature is so low that all water will automatically freeze, but it’s also cold enough for volatile components like CO2 to condense into these grains of ice.

These asteroids also are more abundant in evidence of organic molecules and water in those little inclusions. Think of inclusions like the holes inside a sponge, rather then actual ‘drops’ of water.  

“Take the hydrogen and helium out of the sun and what you have is a CI chondrite,” said Phil Bland, director of the Space Science and Technology Centre at Curtin University.

“Because most of the mass of the Solar System is in the Sun, if you want to pick a composition for average Solar System stuff, it’s CI chondrite. It’s what everything was made from.”

With the finely tuned spectroscopy capabilities of a machine called the Advanced Photon Source, the new team was able to measure the amount of oxidation that the samples had undergone. This was especially interesting since the fragments themselves had never been exposed to oxygen — they were delivered in vacuum-sealed containers, in pristine condition from their trip across space.

The team also discovered something that set the Ryugu fragments apart from other CI chondrites – a large amount of an iron sulphide called pyrrhotite. This result also helps scientists put a limit on the temperature and location of Ryugu’s parent asteroid at the time it was formed.

“Our results and those from other teams show that these asteroid samples are different from meteorites, particularly because meteorites have been through fiery atmosphere entry, weatherization and in particular oxidation on Earth,” said Argonne National Laboratory physicist Michael Hu. ​

“This is exciting because it’s a completely different kind of sample, from way out in the Solar System.”

The research has been published in Science.