Shotgun-like meteoroid impacts and thermal stress trigger asteroid ejections
After NASA’s OSIRIS-REx spacecraft began orbiting the asteroid Bennu in 2019, its navigation camera witnessed multiple episodes where small rocks were being ejected from the surface at speeds up to 10 feet per second (3 metres per second). More than 300 particle ejection events have been seen to date.
Surprised by the observations, the science team came up with three possible explanations: meteoroid impacts, thermal stress and the release of water vapour. In research papers published 9 September in the Journal of Geophysical Research: Planets, the team has concluded the most likely explanation is a combination of thermal stress and meteoroid impacts.
“We thought that Bennu’s boulder-covered surface was the wild-card discovery at the asteroid, but these particle events definitely surprised us,” said Dante Lauretta, OSIRIS-REx principal investigator. “We’ve spent the last year investigating Bennu’s active surface, and it’s provided us with a remarkable opportunity to expand our knowledge of how active asteroids behave.”
Bennu rotates every 4.3 hours. As the temperature rises and falls, rocks can crack and break down, possibly ejecting smaller fragments from the surface in the extreme low-gravity environment. The observed particle showers occurred more often in the late afternoon on Bennu, when surface rocks heat up.
The timing of the events also matches up well with head-on meteoroid impacts in the inner solar system thanks to an unseen population of small fragments released from comets as they near the Sun and heat up or even break apart.
“One of the most significant things we’ve noticed is that the asteroid is frequently ejecting materials into space,” said Southwest Research Institute scientist William Bottke, lead author of a paper in JGR: Planets. “Tiny rocks are just flying off its surface, yet there is no evidence that they are propelled by sublimating ice, as one might expect from a comet.”
Using a computer model developed by NASA’s Meteoroid Environment Office at the Marshall Space Flight Center, Bottke found that sand-size meteoroids, hitting with the force of a shotgun blast, could explain many of the ejection events seen on Bennu. The model works best for asteroids with weak, porous surfaces, which pebble-strewn Bennu resembles
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A new study published this month in JGR Planets posits that the major particle ejections off the near-Earth asteroid Bennu may be the consequence of impacts by small, sand-sized particles called meteoroids onto its surface as the object nears the Sun. The study's primary author is Southwest Research Institute scientist Dr. William Bottke, who used data from NASA's OSIRIS-REx mission.
Launched in 2016, NASA's OSIRIS-REx spacecraft is currently orbiting Bennu with the aim of briefly touching on the surface and obtaining a sample from the asteroid in October 2020, and then returning to Earth.
"While in orbit, the spacecraft has been sending images of Bennu back to Earth," Bottke said. "One of the most significant things we've noticed is that the asteroid is frequently ejecting materials into space. Tiny rocks are just flying off its surface, yet there is no evidence that they are propelled by sublimating ice, as one might expect from a comet. The biggest events launch rocks as large as a few centimeters."
Even more curious is the fact that the observed major ejection events tend to occur in the late afternoon on Bennu. Determined to get to the bottom of these events, Bottke reached out to Althea Moorhead at NASA's Marshall Space Flight Center. Moorhead is a member of NASA's Meteoroid Environment Office, a group that monitors and models meteoroids that may be hazardous to spacecraft.
"Over the years, Althea and her team have built a computer model that determines the number of tiny particles impacting spacecraft," Bottke explained. "We used this software to calculate the number of meteoroid impacts Bennu would face in its current orbit."
Many meteoroids originated on comets. As comets approach the Sun, pieces break off as a consequence of solar heating. Some comets even break apart, producing far more small particles than asteroid collisions in the asteroid belt. For this reason, comet fragments are thought to be the major source of meteoroids that fill the inner solar system.
Interpreting their modeling results, Bottke's study suggests that as Bennu draws closer to the Sun in its orbit, it experiences a higher number of meteoroid impacts. Moreover, sand-sized meteoroids are predicted to hit Bennu with the force of a shotgun blast about once every two weeks, with most striking in the head-on direction. Their impact location on Bennu corresponds to late afternoon and early evening.
Furthermore, Bottke's study points out that the Lunar Atmosphere and Dust Environment Explorer (LADEE) previously made similar observations about impacts on the Moon. As with Bennu, most meteoroids hit the Moon head-on (with head-on defined with respect to the motion of the Earth-Moon system around the Sun). The key difference between Bennu and the Moon is how they rotate around their spin axes. The Moon spins west to east, so head-on impacts correspond to sunrise. Bennu spins in the opposite direction, so head-on impacts hit near dusk.
At first, Bottke's modeling work seem to predict that meteoroids would eject too little material from Bennu to explain the OSIRIS-REx observations. However, a better match could be obtained if Bennu has a weak porous surface. The possibility that Bennu has this property was recently strengthened by studies of the Bennu-like asteroid Ryugu, the target of Japan's Hayabusa2 sample return mission. Using explosives to launch a small projectile into Ryugu, the Hayabusa2 team produced a crater that was larger than expected by most impact experts. If Bennu's surface is indeed similar to Ryugu's, meteoroid impacts should be capable of ejecting relatively large amounts of debris.
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