Asteroids are many things: dinosaur killers, archives from the early days of the solar system, objectives for planetary defense – but they are not supposed to be water worlds. Correct?
Well, at least not these days. But in the early days of the Solar System’s formation, Ryugu, the diamond-shaped target of the Japan Aerospace Exploration Agency (JAXA) hayabusa2 mission – had a small ocean inside it.
Before it was the asteroid it is today, high-precision isotope analysis shows it was part of a larger, older parent before it was destroyed in a collision. But even more surprising is that within that small ocean, some dry silicates from the original parent asteroid managed to survive undisturbed. A new article from one of Hayabusa’s curator teams published this month in nature astronomy gets to what they show about the composition of Ryugu’s father and the very early Solar System asteroids.
WHAT’S NEW – In December 2020, Hayabusa2 returned just over five grams of Ryugu after a six-year mission. Because the samples are a relatively limited number of small beans, each one was labeled with its own name and number. In this case, the team’s analysis was based on just one of these particles, C0009.
Talking with reversecosmochemical isotope Ming Chang Liu at UCLA explains that C0009 was particularly interesting because “it was distinguished by having a small amount of anhydrous silicates,” that is, it contains oxygen-enriched minerals that are unaffected by water in the midst of a strongly H2O-altered sample.
Ryugu’s composition was significantly altered by the liquid water inside it. Despite forming deep in the cold outer Solar System, water and carbon dioxide ice accumulated in the protolith that made up Ryugu’s father along with short-lived radioactive isotopes. As those radioactive rocks heated the ice around them, Liu notes, they “began to float within the main body” and, over time, transformed the silicates and pyroxine that made up Ryugu’s predecessor into water-bearing phyllosilicates.
The remaining anhydrous silicates, then, give the team a clue as to what other materials in the early Solar System might have looked like before crashing into Ryugu’s tiny ocean. And the materials resemble the first materials formed in the Sun’s photosphere. Oxygen isotopes in the sample the team worked with show that the asteroid contains amoeboid olivine and magnesium-rich chrondrules that were incorporated directly from the solar nebula.
Motoo Ito, a cosmochemist at the Japan Agency for Earth and Marine Science Technology and a member of the larger Phase 2 team, was the lead author, along with Liu and others, of a study of the pristine particles of Ryuguwhich demonstrate the ways in which CI meteorites on Earth have been altered by our own much more volatile environment.
Talking with reverseIto points out that even if knowing the chemical composition “doesn’t tell us where the main body formed”, it still “allows us to construct some kind of history of Ryugu, how it formed in the outer solar system”.
BECAUSE IT IS IMPORTANT? This work grows out of the efforts of the larger Phase 2 healing team. After Hayabusa2 passed by Earth to drop off its cargo, the five grams of samples it brought back were divided among eight teams: six performed specific initial analysis: of chemical composition, stony and sandy materials, volatile organic materials, solid and soluble. on the materials, and two larger international teams working to clarify the potential scientific impact of the samples.
In June, the larger Liu and Ito team, based at Okayama University in western Japan, published their interpretation of the samples. They found that Ryugu phyllosilicates are like those found in CI chondrites, a rare and very primitive type of meteorite that has been collected primarily in Antarctica.
But because “they could have been sitting there for decades, years, ages before we pick them up,” Liu notes, “Earth has a very reactive atmosphere, so CI’s chondrite materials will interact with the atmosphere.” By comparison, the Hayabusa2 samples “are probably the most pristine chondrite materials obtainable.”
The survival of these elements from the Ryugu protolith is perhaps even more surprising in light of the work of some of the other teams. The Stony Analysis Team published their initial results this month in Sciences, which included liquid water from Ryugu trapped inside a crystal. Because Ryugu collected frozen carbon dioxide and water ice as it was forming, the liquid water found in the sample was carbonated.
WHAT’S NEXT – Part of Ryugu’s background is already on its way to Earth. Last May, NASA OSIRIS-REx The spacecraft left asteroid Bennu after picking up perhaps half a pound of rocks to begin its journey back to Earth. This was after OSIRIS-REx unexpectedly hit a 20 foot wide crater in the side of Bennu – as a result of it being held together much less firmly than anyone expected.
Like Ryugu, Bennu is a relatively pristine carbonaceous asteroid, though it’s a different type: B-type asteroids like Bennu look a little bluer than Ryugu and their C-type companions, which look reddish. But regardless of its color, according to cosmochemist Ito, finding equally complex carbonaceous components in the sample “will tell us about the distribution of organic components in the solar system.”
Although it answers questions about Ryugu’s composition, this work also raises questions about how Ryugu fits into the scheme of the earliest asteroids and meteorites. According to Liu, the team believes that despite the different categories that have emerged to cover all the different chondrites found on Earth over the years, “those starting materials could have been very similar.” “We just want to be a little bit provocative, stir the pot a little bit, try to change the paradigm,” he added.
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