June 25, 2020
Asteroids Ryugu and Bennu, two top-shaped asteroids with orbits around the Sun similar to Earth’s, may stem from the rupture of a single, common parent asteroid, according to a study published May 27 in Nature Communications.
The study, conducted by an international research team that included planetary scientist Olivier Barnouin from the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, as well as astrophysicist and Queen lead guitarist Brian May, reveals a new way to explain how these asteroids ended up with the same peculiar shape but different amounts of water.
NASA’s Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) spacecraft has returned data and images of Bennu, where the spacecraft is currently orbiting, and it’s hard to miss how similar Bennu is to Ryugu, which Japan’s Hayabusa2 spacecraft imaged and sampled between 2018 and 2019. Both asteroids are small (between 0.5 and 1 kilometer, or more than a quarter- to a half-mile, in diameter) and have a similar density and, yes, an uncanny resemblance to a spinning top.
“The shapes of asteroids and their hydration level can serve as real tracers of their origin and history,” explained Patrick Michel, a planetologist at the Observatoire de la Côte d’Azur in France and the study’s lead author.
Small, top-shaped asteroids can occur through the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect, a process in which light reflected and emitted by an asteroid gradually spins it faster and faster. That rotation forces rocks and other materials toward the equator, creating a bulge that, after millions of years, makes the asteroid look like a top.
But Ryugu’s and Bennu’s cratered ridges at their equators aren’t the youngest parts of the asteroids, as expected from YORP — they’re the oldest.
How, then, did the asteroids end up looking so alike?
The team considered a process that affects all asteroids: collisions.
“Most asteroids less than a few kilometers in diameter are just fragments of larger, destroyed asteroids,” Barnouin said.
Using computers, the researchers simulated what happens when space rocks smash together, calculating the shapes and hydration levels of the rocks in the remaining debris field.
“We found that the gravity of ejected fragments sometimes pulled the pieces back together into little, squashed spheres, many of which closely resembled a top,” said Ron Ballouz, an astronomer and co-lead author from the University of Arizona in Tucson.
Barnouin had suspected even before Hayabusa2 and OSIRIS-REx arrived at their asteroid targets that asteroid collisions could produce top-shaped asteroids.
“I was pleased to see confirmation of this scenario,” he said. “There are so many fragments produced during catastrophic disruptions that it really isn’t a stretch that some would end up as Bennu- or Ryugu-shaped objects.”
Shape wasn’t the only thing the team’s simulations explained. Bennu has more water within its minerals than Ryugu, a detail that scientists have puzzled over, considering the asteroids’ many other similarities. But the team’s simulations showed this difference could arise from a collision as well.
During the simulated collisions, particles in the starting asteroid weren’t heated to the same extent. In fact, their temperatures covered a range that stretched well above and below the temperature at which minerals lose water. When the pieces later coalesced into new, top-shaped asteroids, their rocks didn’t have the same amount of water.
“That means Ryugu and Bennu don’t have to come from different asteroids — they could come from the same asteroid and still have different hydration levels. They could be siblings,” Barnouin said.
The simulations also showed that small top-shaped asteroids would likely have a uniform hydration level through this process, a finding that matches observational data for both asteroids.
Nailing down whether Ryugu and Bennu were birthed from the same asteroid will have to wait until Hayabusa2 returns a sample of Ryugu later this year and OSIRIS-REx returns a sample of Bennu in 2023, the team says.
Like using DNA to determine family relations, those samples will allow precise measurements of the age and composition of each asteroid, revealing whether these two asteroids are products of convergent evolution or just relatives from the same family.
Media contact: Jeremy Rehm, 240-592-3997, Jeremy.Rehm@jhuapl.edu
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