Planetary Sciences News

All About Bennu: A Rubble Pile with a Lot of Surprises

Asteroid Bennu has been under close scrutiny since December. Here are six key results from the first few months of data from OSIRIS-REx.

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The Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) arrived at a nearby asteroid called 101955 Bennu on 3 December 2018. The NASA mission aims to study the composition and physics of this near-Earth asteroid, collect a sample from its surface, and return that sample to Earth.

On 10 March 2019, mission scientists published a collection of papers that share some of the first results from the mission and discussed those results in a press conference. The first looks at the data show that Bennu is both reassuringly familiar and startlingly strange for an asteroid of its kind.

Despite Bennu’s surprises, the team has “great confidence that we’re going to be able to target a Bullseye TAG [touch-and-go] event for summer of 2020 and get that precious sample for stow and return back to Earth,” said Dante Lauretta, OSIRIS-REx principal investigator from the University of Arizona in Tucson.

For more details, here are six key results from the first few months of observations from OSIRIS-REx.

1. Unexpected Particle Ejections

Perhaps the most surprising discovery about Bennu so far is the detection of several particle ejection events from the surface of the asteroid using data from OSIRIS-REx’s navigational camera.

“Just 6 days after we got into orbit, we observed our first particle event,” said OSIRIS-REx flight navigator Coralie Adam.

The science and engineering groups quickly worked together to create tools to detect and analyze the ejections. The team detected 11 particle ejection events between 11 January and 18 February and continues to monitor for more. Some events ejected more than 100 particles.

“Bennu is part of a very small group of small bodies in the solar system called active asteroids,” Lauretta said. “We certainly did not expect to see this activity, because there are so few asteroids that are known to be active. There’s about a dozen of them of the 800,000 asteroids that we know of in the solar system.”

“This is definitely not what we expected to see,” he said.

The ejecta initially caused some worries for the team. “We were modestly concerned about particles being ejected from the surface of the asteroid unexpectedly,” said Rich Burns, OSIRIS-REx project manager at NASA Goddard Space Flight Center in Greenbelt, Md.

A quick assessment showed that “the spacecraft was safe in orbit” and the likelihood of a collision was very low, Burns said. The researchers have not yet assessed whether there will be an added risk during the sample collection maneuver. They expect that more data will reveal where the ejections come from, what mechanism may cause them, or how frequently they may occur.

The team is also working with observatories to identify a hypothetical “Bennid” meteor shower resulting from Bennu’s particle ejections, which could recur in September in Southern Hemisphere skies.

2. A Very Rugged Surface

A candidate sample selection site in Bennu’s northern hemisphere
A candidate sample selection site in Bennu’s northern hemisphere. OSIRIS-REx imaged this site on 7 March from about 5 kilometers away. Credit: NASA/Goddard/University of Arizona

“We had expected from ground-based infrared and radio astronomy that the ground was relatively smooth and dominated by centimeter-scale particles, which are optimal for our touch-and-go sample acquisition mechanism,” Lauretta said.

Now that the spacecraft is closer to Bennu, however, “we see a wide range of very large boulders on the surface. Our latest count is over 200 boulders larger than 10 meters in diameter.”

The unexpectedly rough surface caused some initial concern about the sample collection maneuver. “The asteroid is so rugged that there is unlikely to be any 25-meter-radius hazard-free zone,” Burns said. OSIRIS-REx was optimized to target an area of that size for its touch-and-go sample collection. The team has identified smaller areas with finer-grained regolith that it says are perfect for the sample collection and should not pose navigational difficulty.

“We have demonstrated meter-level navigation accuracy, which is even better than the Earth-based GPS on your cell phone,” Adam said. “Our predicted navigational accuracy is 10 times better than what we expected before we arrived at Bennu,” she said, and the craft should be able to do a so-called Bullseye TAG maneuver in the summer of 2020.

“Imagine that you’re shooting at a dartboard, just trying to hit the board,” Burns said. “Well, now we’re going to try to hit the center of the bull’s-eye.”

3. A Pile of Rubble

“Bennu has an expected spinning-top shape,” Lauretta said. “It’s roughly spherical with an equatorial ridge that seems to be dominated by its rotation.” But Bennu’s bulk density is much lower than would be expected for a solid object.

“It’s a pile of rubble,” he said, basically “gravel and boulders loosely bound together by their own small gravity field.” In this respect, Bennu is similar to some other asteroids, like Ryugu and Itokawa, visited by spacecraft.

As much as 60% of Bennu might be empty space.

A high-resolution video of Bennu’s rotation
This compilation of 40 high-resolution images shows a full rotation of Bennu from a distance of about 24 kilometers. OSIRIS-REx captured these images during a 4-hour period on 2 December 2018 just before its official arrival at Bennu. Credit: NASA/Goddard/University of Arizona

4. Abundant with Hydrated Minerals

“Bennu has turned out to be an ideal target from a compositional perspective,” Lauretta explained. Scientists detected phyllosilicates, which are clay minerals, and also magnetite, an iron oxide, in the spectra of surface rocks. These are indicative of “intense aqueous activity in the past history of this asteroid.”

In terms of spectra and minerology, Bennu’s rocks “look a lot like the rarest, most fragile meteorites in our collection,” Lauretta said, referring to the CM carbonaceous chondrites collected in 1969 in Murchison, Australia. “Those are the minerals that we were hoping to see.”

However, Bennu’s boulders physically look a lot different than hand samples of CM meteorites. “The nature of the boulders [is] suggesting that there’s more to the story than we know from our meteorite samples so far,” Lauretta said.

5. Spinning Faster and Faster

Bennu rotates once every 4.3 hours, but that rotation is slowly speeding up. Every day, the asteroid rotates about 4 millionths of a degree more than it did the day before.

“Because of the latest observations,” Lauretta said, “we can attribute this to a known phenomenon called the YORP [Yarkovsky-O’Keefe-Radzievskii-Paddack] effect, which is basically how interaction with sunlight causes the asteroid rotation rate to increase.”

A key goal of OSIRIS-REx is to study the YORP effect up close and better understand how it can change the orbits of near-Earth asteroids.

“In about 1.5 million years, we predict that Bennu will be spinning at twice its current rate,” Lauretta said.

Backtracking this acceleration would suggest that Bennu wasn’t rotating at all just 8 million years ago—which is pretty unlikely, Lauretta said, so the acceleration must be a relatively new phenomenon.

You can learn more about the YORP effect in the video below.

6. An Old yet Changing Surface

Bennu’s surface is between 100 million and 1 billion years old, which likely dates back before it shifted from the main asteroid belt to near-Earth space. Many boulders on the surface are cracked, which tells a tale of thermal stress and impacts.

Piles of boulders at low elevation points, a lack of small craters, and partially filled-in large craters suggest that loose surface material has shifted from the poles to the equator during the past million years in landslide-like events, Lauretta said. This might be due to the rotational acceleration, the particle rain, or a combination of effects.

The rotation “can trigger what we would call a landslide or a mass wasting event, moving material downslope and filling in low-lying regions of the asteroid,” Lauretta said. “We have active geology going on on the surface.”

Journey Just Beginning

OSIRIS-REx has about 2 years left at Bennu. During that time, the craft will continue to study the asteroid’s surface and near-surface environments and select a good touchdown site. The mission is expected to return in 2023.

Bennu provides “an excellent opportunity for us to understand active asteroids, volatile ejection, and loss from these bodies,” said Lauretta, as well as “the distribution of this kind of material in the solar system and, ultimately, its pathway to delivery to the surface of the Earth.”

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer

Citation: Cartier, K. M. S. (2019), All about Bennu: A rubble pile with a lot of surprises, Eos, 100, https://doi.org/10.1029/2019EO118773. Published on 21 March 2019.
Text © 2019. AGU. CC BY-NC-ND 3.0
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