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The fragile stones of the asteroid Bennu can facilitate the capture of a sample

When NASA's OSIRIS-REx reached the asteroid near Earth Bennu, scientists were dismayed to find a surface covered with dangerous-looking boulders.

But new research suggests those rocks are surprisingly brittle. This is potentially good news for the spacecraft, which is tasked with taking a piece of Bennu on October 20 and returning it to Earth in 2023 (SN: 1/15/19). If the rocks are crumbling, this could reduce the risk of damaging the ship’s equipment.

This type of rock can also be too brittle to survive the journey through the Earth’s atmosphere without burning. If so, scientists may be close to making use of a type of space rock never seen before, researchers report in a collection of articles published Oct. 8 in Science and Science Advances.

Data taken from Earth before the release of OSIRIS-REx suggested that the surface of Bennu would be sand. So it was a shock to find a rough landscape dotted with pebbles when the spacecraft arrived in 2018 (SN: 12/3/18).

“We were really convinced that Bennu was a smooth object,” says Daniella DellaGiustina, a planetary scientist at the University of Arizona in Tucson and a member of the OSIRIS-REx team. "As everyone saw in the first photos, that wasn't the case."

The team found a relatively clear crater, nicknamed Nightingale, from which to recover a sample of the space rock (SN: 12/12/19). Still, it remains concerned that pebbles could pose a safety hazard to the sampling system, which was designed to handle pebbles just a few inches in diameter.

From late April to early June 2019, planetary scientist Ben Rozitis of the Open University in Milton Keynes, England, and colleagues mapped how Bennu boulders retain heat, a clue to the structure of rocks. Dense materials withstand heat better than finer-grade ones, such as how a sandy beach cools rapidly after the sun, but large rocks remain warm.

This map shows where the carbon minerals (represented by redder colors) are found on the surface of Bennu. The opportunity to analyze these minerals could help scientists figure out how carbon came to Earth early.A. Simon et al / Science 2020

Based on these maps and maps of other surface properties, described in the series of works published on October 8, Bennu stones appear to present in two flavors: darker colored rocks that are weaker and porous, and lighter colored and denser rocks . stronger and less porous. Even the densest rocks are much more porous and brittle than similar asteroid meteorites found on Earth. Less dense meteorites are about 15 percent porous; Bennu rocks appear to be between 30 and 50 percent porous, Rozitis and colleagues found.

“This is exciting,” says DellaGiustina, co-author of the new works. The spacecraft and its instruments may "find some boulders at the sample site that might otherwise be difficult to ingest," she says, but "if they are porous and weak, then they could only decompose," so they will be easier to collect.

The lighter, denser rocks also appear to have been shot with carbonate veins, suggesting that they were in the presence of water flowing at some point in their past (SN: 12/10/18). NASA chose Bennu as an asteroid to visit in part because it resembles carbonaceous chondrite meteorites, which scientists think are time capsules of the first solar system. Similar space rocks could have delivered water and organic materials to Earth billions of years ago.

But Bennu’s more porous rocks appear to be nothing in the scientists ’current variety of meteorites, Rozitis says. “This is one of the interesting things about OSIRIS-REx: it’s very likely that it picks up new material that isn’t in our meteorite collection,” he says.

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That’s believable, says meteorologist Bill Cooke of NASA’s Marshall Space Flight Center in Huntsville, Alabama. Meteor observations have shown that low-density rocks and dust burn more in the Earth's atmosphere than higher-density rocks.

“The old conventional wisdom was that low-density material was comets and high-density material was asteroids,” he says. But recent observations show that some of the low-density rocks come from the orbits of asteroids. "So it's very plausible that Bennu's low-density things … burn higher in the atmosphere and don't have a chance to create meteorites."

If Bennu represents a missing piece in our understanding of the history of the solar system, studying that material in Earth labs will “help us cover an additional piece of the puzzle,” says Rozitis.

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