TEXAS — Almost four years have passed since NASA’s OSIRIS-Rex spacecraft grabbed a piece of an asteroid. Now, scientists are excited to share what they’ve found in this chunk of space rock.
The asteroid, known as Bennu, is a near-Earth object with a surprising secret. It holds a significant amount of magnesium phosphate, a mineral. This discovery is unusual because these bright-white particles stand out against the dark rocks of Bennu, making it a rare find in space materials, according to experts.
Jessica Barnes, an assistant professor at the Lunar and Planetary Laboratory (LPL), is leading the study of this mineral in the asteroid sample. She mentioned, “We initially thought this might be a contaminant.” This comment was made during her presentation at the Lunar and Planetary Science Conference (LPSC) in Texas and online. She explained that finding a similar chemical on Earth is challenging because it’s either too fragile or disappears quickly after arriving on our planet. The presence of this mineral in the Bennu sample suggests various geological activities on the asteroid’s parent body.
The analysis of the samples also revealed glycine, the simplest amino acid and a key component of proteins. Additionally, scientists found water-bearing minerals like carbonates, sulfites, olivine, and magnetite. These discoveries provide solid evidence that Bennu’s parent body experienced several water-related events before its fragments formed Bennu.
Other researchers found plenty of water-altered compounds known as phyllosilicates, along with a rich array of other organic and hydrated minerals. Phyllosilicates, which bind structurally to water in meteorites, might have been crucial in delivering organics and water to Earth in its early history.
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‘A beautiful sample’
The sample, collected from Bennu in 2020 by the OSIRIS-REx mission, made its way back to Earth inside a secure capsule on September 24, 2023. The very next day, it arrived at NASA’s Johnson Space Center (JSC) in Houston for analysis. A team of scientists, known as the “tiger team,” began examining the material that had leaked from the spacecraft’s sample collector. Due to two troublesome screws on the container lid, accessing the majority of the sample was delayed until January. By then, scientists had cataloged 4.29 ounces (121.6 grams) of material, which was more than double the initial estimate.
During the LPSC, the mission team reported that the stones cataloged so far have various textures, hydrated minerals, and signs of space weathering, as expected from an ancient, airless rock.
“It’s a beautiful sample,” said Sara Russell, a planetary scientist at the Natural History Museum in London. She analyzed a small fragment of the sample and noted, “It’s not quite like any meteorite in our collection.”
The most pristine asteroid sample ever
Unlike most meteorites, which are altered by years of exposure to Earth’s atmosphere by the time they are found, pieces of Bennu are the most pristine space rocks scientists have ever studied.
“It’s so refreshing to see samples that aren’t altered to sulfates and all kinds of muck,” remarked Tim McCoy, a curator of meteorites at the Smithsonian’s National Museum of Natural History in Washington, D.C. The museum received a sample of the asteroid for analysis last November. “You’re seeing it literally the day it fell — it is remarkable to see something that fresh.”
Many of the cataloged rocks from Bennu are “hummocky boulders” with a rough, “sandpaper-like texture,” according to team member Andrew Ryan, a research scientist at LPL at the University of Arizona. He highlighted a 1.4-inch-wide (3.5 centimeters), 0.23-ounce (6.6 grams) rock as “our largest booty from the surface of Bennu,” showing off fresh 3D scans of the rock taken at the JSC curation lab.
Most measured minerals matched predictions made from remote sensing data collected by OSIRIS-REx as it approached Bennu. “We got it right with the remote sensing,” said Harold Connolly, a geologist at Rowan University in New Jersey. “And that directly influences how we analyze the sample and test our hypotheses.”
So far, the analysis aligns with the theory that Bennu broke off from a much larger asteroid between 2 billion and 700 million years ago. The latest findings show Bennu’s rocks filled with impact-related breccias, which are rock fragments loosely bound together like pebbles in concrete.
“These breccias probably didn’t form on Bennu,” McCoy, who’s leading the research on these features, explained. “They formed on the parent asteroid and then themselves became boulders incorporated into Bennu.”
The exact formation time of these breccias is still a mystery.
“We’re still in the very early days of this meticulous work,” McCoy stated. “There’s a lot we don’t know.”