Meteoroid Impacts as a Source of Bennu's Particle Ejection Events

Description

Asteroid (101955) Bennu, a near-Earth object with a primitive carbonaceous chondrite-like composition, was observed by the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft to undergo multiple particle ejection events near perihelion between December 2018 and February 2019. The three largest events observed during this period, which all occurred 3.5 to 6 hr after local noon, placed numerous particles <10 cm on temporary orbits around Bennu. Here we examine whether these events could have been produced by sporadic meteoroid impacts using the National Aeronautics and Space Administration's (NASA) Meteoroid Engineering Model 3.0. Most projectiles that impact Bennu come from nearly isotropic or Jupiter-family comets and have evolved toward the Sun by Poynting-Robertson drag. We find that 7,000-J impacts on Bennu occur with a biweekly cadence near perihelion, with a preference to strike in the late afternoon (~6 pm local time). This timing matches observations. Crater scaling laws also indicate that these impact energies can reproduce the sizes and masses of the largest observed particles, provided the surface has the cohesive properties of weak, porous materials. Bennu's ejection events could be caused by the same kinds of meteoroid impacts that created the Moon's asymmetric debris cloud observed by the Lunar Atmosphere and Dust Environment Explorer (LADEE). Our findings also suggest that fewer ejection events should take place as Bennu moves further away from the Sun, a result that can be tested with future observations. Plain Language Summary The asteroid Bennu, the target of the OSIRIS-REx sample return mission, was observed to be ejecting tiny rocks shortly after the spacecraft entered orbit. The three largest ejection events took place in the late afternoon local time, with an average interval of 2 weeks. Each event launched multicentimeter-sized and smaller rocks into temporary orbits, where some escaped and others reimpacted Bennu. Given that all inner solar system objects are bombarded by cometary dust particles, we used a NASA model constructed to evaluate spacecraft impact risk to explore whether impacts could be the source of these events. We found that millimeter-sized cometary dust particles not only strike Bennu in the late afternoon, matching observations, but also produce enough ejected debris to explain the orbiting particles, provided that the material being pummeled is weak.

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