One such analysis was conducted by an international team of scientists, including Dauphas and Timo Hopp, then a postdoctoral researcher at the University of Chicago, who analyzed the isotopic “fingerprint” of the asteroid.
Elements come in different flavors with different masses, called isotopes. Any given rock will contain different amounts of different types of these isotopes. Because they vary in the source of their parent material and the conditions around them, they can tell scientists a lot about the origins of the rock.
In particular, matching isotopic fingerprints can indicate rocks that formed in the same place and at the same time. The team found that Ryugu resembled part of a group called Ivuna-like carbonaceous chondrites. Based on these and other measurements made on the Ryugu samples, scientists have proposed a new theory for the evolution of the solar system’s asteroid population.
Born with the Ice Giants
In the early days of the solar system, the young sun was surrounded by a disk of gas and dust called the protoplanetary disk. Over time, the material clumped together, forming the planets we know and love today, along with smaller pieces that became asteroids. Many of them ended up in the “main belt” of asteroids between Mars and Jupiter. Previously, these asteroids were thought not to have moved much from where they formed.
But scientists have suggested that some of these asteroids came from much further out in the solar system. A cluster has formed near the gas giants Jupiter and Saturn. Another group formed even further away. “This reservoir was located in the outer regions of the protoplanetary disk, possibly near the birthplaces of the ice giants Uranus and Neptune,” Hopp said. This group is the one that Ryugu matches.
Some of these distant asteroids flew off planets as comets. Eventually, the sun’s gravity captured them, sucked them in, and scorched their water and tails. This is what scientists think happened to Ryugu.
Because these asteroids formed so far from the sun, they could contain ice that then melted. This matches other analyzes by Ryugu showing that the rock has been weathered by water in its past.
The theory also fits with the fact that scientists have listed three different types of meteorites that have fallen to Earth. They are known as carbonaceous chondrites (CC); non-carbonaceous chondrites (NC); and the Ivuna-like (CI) carbonaceous chondrites, a strange group that resembles the carbonaceous chondrites but with a different iron isotopic composition. In this theory, the NC group formed in the inner solar system and remained there; the CC group formed near the gas giants; and the CI group formed near the ice giants.
“The new discovery completely overturns our previous idea that Ryugu and CI were just a part of CC meteorites born in a similar location in the early solar system,” said co-author Professor Tetsuya Yokoyama of the Tokyo Institute of Technology, whose lab processed and prepared Ryugu’s samples for testing. “I am happy to have been involved in decoding a solar system secret in this way.”
“There is still work to be done to confirm this theory, but if true, it would be very exciting because it is very difficult to bring back samples from these regions of the solar system,” Dauphas explained. “But in this case, we could look at these asteroids near us and learn things much further. We’re probably sampling some of the material that was in the outer disk when the sun was born.
Over the next few decades, scientists expect the arrival of several more pristine samples recovered by spacecraft; each will reveal more about the history of our solar system.
“It’s the excitement of exploring the unknown – you never know what you’re going to find,” Dauphas said.