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Bennu is Active Asteroid, OSIRIS-REx Team Says

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Bennu is Active Asteroid, OSIRIS-REx Team Says

 Dec 11, 2019

NASA’s OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) spacecraft recently observed several particle ejection events on the surface of the near-Earth asteroid Bennu . OSIRIS-REx principal investigator Dante Lauretta and colleagues have analyzed the properties and behavior of particles ejected from Bennu to determine the possible mechanisms of ejection and provide understanding of the broader population of active asteroids. Their results appear in the journal Science.

Bennu is Active Asteroid, OSIRIS-REx Team Says Image_7014-Bennu-Plumes
This view of asteroid Bennu ejecting particles from its surface on January 19 was created by combining two images taken on board NASA’s OSIRIS-REx spacecraft. Image credit: NASA / Goddard / University of Arizona / Lockheed Martin.
“Among Bennu’s many surprises, the particle ejections sparked our curiosity, and we’ve spent the last several months investigating this mystery. This is a great opportunity to expand our knowledge of how asteroids behave,” said Dr. Lauretta, a researcher at the University of Arizona.
Dr. Lauretta and colleagues observed three large particle ejection events on January 6 and 19, and February 11, 2019, and concluded that the events originated from different locations on Bennu’s surface.
The first event originated in the southern hemisphere, and the second and third events occurred near the equator. All three events took place in the late afternoon on Bennu.
The team found that, after ejection from the asteroid’s surface, the particles either briefly orbited Bennu and fell back to its surface or escaped from Bennu into space.
The observed particles traveled up to 10 feet per second (3 m/sec), and measured from smaller than an inch up to 4 inches (10 cm) in size. Approximately 200 particles were observed during the largest event, which took place on January 6.

Bennu is Active Asteroid, OSIRIS-REx Team Says Image_7893-Bennu-Plumes
Schematic diagram of orbit determination model output for the 19 January 2019 particle ejection event from Bennu observed by the OSIRIS-REx spacecraft: Bennu is depicted in gray and has a diameter of 1,640 feet (500 m). OSIRIS-REx is indicated with the brown dot, 1.2 miles (2 km) from Bennu’s center of mass; the cone represents the viewing angle; blue arcs are particle trajectories, ending or with gaps where the trajectories pass into shadow; the Sun-angular momentum frame coordinates are shown at bottom right: x – solar vector, y – Bennu orbital direction, z – Bennu north. Image credit: Lauretta et al, doi: 10.1126/science.aay3544.
The scientists investigated a wide variety of possible mechanisms that may have caused the ejection events, and narrowed the list to three candidates: meteoroid impacts, thermal stress fracturing, and released of water vapor.
Meteoroid impacts are common in the deep space neighborhood of Bennu, and it is possible that these small fragments of space rock could be hitting Bennu where OSIRIS-REx is not observing it, shaking loose particles with the momentum of their impact.
The team also determined that thermal fracturing is another reasonable explanation.
Bennu’s surface temperatures vary drastically over its 4.3-hour rotation period. Although it is extremely cold during the night hours, the asteroid’s surface warms significantly in the mid-afternoon, which is when the three major events occurred.
As a result of this temperature change, rocks may begin to crack and break down, and eventually particles could be ejected from the surface. This cycle is known as thermal stress fracturing.
Water release may also explain the asteroid’s activity. When Bennu’s water-locked clays are heated, the water could begin to release and create pressure.
It is possible that as pressure builds in cracks and pores in boulders where absorbed water is released, the surface could become agitated, causing particles to erupt.
But nature does not always allow for simple explanations.
“It could be that more than one of these possible mechanisms are at play. For example, thermal fracturing could be chopping the surface material into small pieces, making it far easier for meteoroid impacts to launch pebbles into space,” said Dr. Steve Chesley, senior research scientist at NASA’s Jet Propulsion Laboratory.
If thermal fracturing, meteoroid impacts, or both, are in fact the causes of these ejection events, then this phenomenon is likely happening on all small asteroids, as they all experience these mechanisms.
However, if water release is the cause of these ejection events, then this phenomenon would be specific to asteroids that contain water-bearing minerals, like Bennu.
D.S. Lauretta et al. 2019. Episodes of particle ejection from the surface of the active asteroid (101955) Bennu. Science 366 (6470): eaay3544; doi: 10.1126/science.aay3544

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