From telescopes on Earth, Bennu’s surface appears smooth. That’s one of the reasons why NASA picked the asteroid as a destination for its OSIRIS-REx spacecraft. But in 2018, when OSIRIS-REx approached the asteroid, scientists discovered that Bennu’s surface was covered with massive boulders. It turns out those boulders moved a lot over the last few hundred thousand years, according to recent research.
“When you think of small asteroids, you’d think they aren't very dynamic because they have no atmosphere or volcanic activity. But Bennu is so small and its gravity is so weak that material can move around much more easily than on a planet,” said Dr. Erica Jawin, a postdoctoral fellow in the Department of Mineral Sciences at the Smithsonian's National Museum of Natural History and the study’s lead author.
Bennu spun out of the asteroid belt millions of years ago and now circles the sun between Earth and Mars, much closer than its original location in the asteroid belt. Because the asteroid currently has an orbit near Earth’s, it is easier to sample it than any asteroid in the main belt. By modeling how Bennu’s boulders moved in the past, Jawin can predict where rocks in OSIRIS-REx's sample might have come from on the asteroid’s surface. Knowing those rocks’ origins will help scientists learn more about the composition of objects in the solar system and asteroid belt.
“Asteroids are always gravitationally interacting and essentially sharing material. Earth gets meteorites from asteroids and asteroids also get meteorites from other asteroids,” said Dr. Tim McCoy, Curator of Meteorites at the museum and a co-author on the study.
A moving history
Rocks on Bennu’s surface move so easily because the asteroid’s gravity is very weak. Because of the weak gravity, rotational forces can move the rocks. This is what causes boulders and rocks to move about or potentially fly into space.
“As Bennu rotates, its surface absorbs thermal energy from the Sun. It then radiates that heat back into space as the asteroid rotates. This provides a torque on the asteroid, which affects how quickly the asteroid rotates and over time can change the orbit of the asteroid. This effect also may have caused Bennu to leave the asteroid belt and come closer to Earth,” said Jawin.
Studying Bennu’s pristine rocks could reveal what material exists in the outer solar system. And that material could yield information about the composition of primordial Earth.
“On Earth, we’ve had life for potentially billions of years. Everything has been processed so much. In order to really understand how life started, you really need to go somewhere where there’s no life yet,” said Jawin.
Since Earth has an atmosphere and active plate tectonics, its oldest rocks are weathered or have been pushed deep into the mantle. So, researchers often use meteorites to learn more about both ancient Earth’s and the solar system’s composition.
“Meteorites have been described as the poor man’s space probe, because they are constantly coming to Earth. Just picking them up, we can learn about our solar system and its history,” said McCoy. “But at the same time, we’re trying to figure out what the entire asteroid belt and early solar system looked like from these bits and pieces.”
Examining Bennu’s rocks will give McCoy and his colleagues more tools, helping them trace meteorites in the museum’s collection back to the asteroid belt.
What happens next
Once the rock sample from Bennu finally reaches Earth in three years on September 24, 2023, part of it will be loaned to McCoy’s Smithsonian team. There, McCoy and Jawin will analyze it to see if any meteorites currently in the Smithsonian’s National Meteorite Collection have similar compositions. If there's a match, it could suggest that the object is related to Bennu or it was part of another asteroid in the region where Bennu came from.
“Most meteorites in our collection came from asteroids at some point, but we’ve only been able to link a very small fraction of the meteorites in our collection to their parent asteroids. If you just pick up a meteorite on the ground, you don’t know how long it’s been sitting there. So, it’s likely not in pristine condition,” said Jawin. “The OSIRIS-REx mission will give us pristine samples to compare to our collection and bridge that gap.”
McCoy also suspects the Bennu sample could yield rocks unlike anything on Earth, complicating what scientists know about the geology of the solar system.
“Every few years, we find a new kind of meteorite so it’s very possible that Bennu also has new kinds of rocks we don’t have in our collection. It’s possible we’ll get something entirely new,” said McCoy. These new rocks could maybe decode some of the collection’s more enigmatic meteorites.
The meteorite collection exists not only for scientists currently seeking to understand the solar system, but also for future scientists conducting experiments yet to be invented. Part of the Bennu sample will immediately be sealed for the foreseeable future, saved for the future as technology advances.
“We will be able to use tools and equipment that haven’t been invented yet to ask questions we haven’t even thought of yet. But because we have those samples, we'll be able to answer those questions,” said McCoy. “Think of it as the gift that keeps on giving.”