Scientists capture shots of material ejected from asteroid Bennu's surface, only 'active' one to be seen up close
A near-Earth asteroid Bennu is throwing up unexpected findings and pushing the boundaries of scientific exploration.
Scientists have found centimeter-sized debris being ejected from its surface — a discovery made possible by close-up shots taken by cameras aboard NASA's OSIRIS-REx spacecraft. The finding puts Bennu (image by NASA) in a class of active asteroids — those that show evidence of ongoing mass loss — and marks the first time a spacecraft has seen the phenomenon first-hand and up close.
"The images offer a detailed look at small-scale mass loss events on an active asteroid, whereas before, observations have been limited to only the largest phenomena," say researchers.
The observations were made after the spacecraft entered orbit in January 2019. The cause of this particle ejection remains somewhat puzzling, say scientists, but the observations show that asteroids are far from being inert bodies.
"We initially detected this phenomenon in navigational images from January 6, 2019, one week after the spacecraft entered orbit and four days before Bennu perihelion. We subsequently detected multiple particle ejection events between December 2018 and February 2019. The largest observed events each released dozens of observed particles," says the researchers in their findings published in Science.
They say: "Our observations classify Bennu as an active asteroid. Active asteroids are commonly identified by major mass-loss events observable with telescopes, on scales much greater than we observed at Bennu. Our findings indicate that there is a continuum of mass loss event magnitudes among active asteroids."
According to Jessica Agarwal from the Max Planck Institute for Solar System Research in Göttingen, Germany, the discovery of particle ejections is surprising and may suggest that all asteroids are active at some level.
"However, previous spacecraft that have visited asteroids have not documented this type of event. Key answers may come from the Hayabusa-2 mission by the Japan Aerospace Exploration Agency to the asteroid Ryugu, which resembles Bennu in terms of shape, density and orbit but it is likely composed of a different material. Like OSIRIS-REx, Hayabusa-2 is characterizing the asteroid in great detail in order to bring back a sample of it in 2020. The presence or absence of activity will provide some additional constraints on the low-level activity of small asteroids," says Agarwal in a related perspective, also published in Science.
Bennu's a roughly spherical near-Earth asteroid
Asteroid Bennu has a diameter of approximately 500 m and the shape of a "spinning top" — roughly spherical, but somewhat pointed near the poles and bulgy in the equatorial zone. Bennu is categorized as a near-Earth asteroid because it is located between 0.9 and 1.4 times the distance between Earth and the sun.
Like most asteroids of its size, Bennu is likely a "rubble pile," a collection of boulders held together mainly by their own gravity and with half of the volume empty.
NASA's Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer or OSIRIS-REx mission was launched in 2016 to explore Bennu, which is the smallest body ever orbited by spacecraft. The primary purpose of the OSIRIS-REx mission is to obtain a sample from the surface of Bennu in 2020 and return it to Earth for analysis in 2023.
“We have observed centimeter-scale particles frequently being ejected and reimpacting the asteroid surface. It is possible that the collected sample will contain some particles that were ejected and returned to Bennu’s surface,” say researchers.
Studying Bennu will allow researchers to learn more about the origins of our solar system, the sources of water and organic molecules on Earth, the resources in near-Earth space, as well as improve our understanding of asteroids that could impact Earth.
According to NASA, Bennu is the target of NASA's OSIRIS-REx mission, which began orbiting the asteroid on December 31. Only slightly wider than the height of the Empire State Building, Bennu may contain unaltered material from the very beginning of our solar system, say, scientists.
"The OSIRIS-REx spacecraft has rendezvoused with the near-Earth asteroid (101955) Bennu. The selection of Bennu as the OSIRIS-REx target was partially based on its spectral similarity to some active asteroids," says Dante Lauretta, OSIRIS-REx principal investigator from the University of Arizona, Tucson, in the study.
More than 20,000 known near-Earth asteroids are traversing our solar system. From Earth, the vast majority appear inert with no evidence of smaller bits breaking free from their surface. Although rare, a small number of asteroids have been observed actively ejecting dust and particles in quantities large enough to create temporary clouds or comet-like tails viewable from Earth-based telescopes. Bennu is the only active asteroid to be observed up close.
"Active asteroids are small bodies in the solar system that show ongoing mass loss, such as the ejection of dust, which may be caused by large impacts, volatile release, or rotational acceleration. Studying them informs our understanding of the evolution and destruction of asteroids and the origin of volatile materials such as water on Earth," says Lauretta.
The findings
Since the beginning of 2019, the spacecraft has been thoroughly investigating the asteroid to pick and characterize the sampling site. And it finally made the first-ever close-up observations of particle plumes erupting from an asteroid's surface. The authors identified three major ejection events in early 2019.
"The discovery of plumes is one of the biggest surprises of my scientific career. And the rugged terrain went against all of our predictions. Bennu is already surprising us, and our exciting journey there is just getting started," says Lauretta.
Using the images, researchers describe close-up observations of mass loss from Bennu's surface. Appearing as bright points of light in the images captured, they have been identified as small, centimeter-scale objects traveling just above the surface of Bennu. Further analysis of the trajectory of the material revealed three discrete events during which material was ejected from the surface.
The scientists say the three source locations do not seem to have much in common or to be geologically different from the rest of the surface. But all three events took place in the local afternoon of Bennu.
According to the analysis, some of the shed objects remained in Bennu's orbit for several days before re-impacting with the surface, while others escaped into interplanetary space.
"Particles with diameters from (less than) <1 to about 10 cm were ejected from Bennu at speeds ranging from about 0.05 to (greater than) >3 m s–1. Estimated energies ranged from 270 mJ for the 6 January event to 8 mJ for the 11 February event. The three events arose from widely separated sites, which do not show any obvious geological distinction from the rest of Bennu's surface. However, these events all occurred in the late afternoon, between about 15:00 and 18:00 local solar time," says the study.
In addition to discrete ejection events, scientists detected a "persistent background of particles" in the Bennu environment. Some of these background particles have been observed to persist on temporary orbits that last several days, say experts.
The scientists suggest that micro-meteorite impacts, as well as dehydration and thermal stress fracturing, are the possible causes of the particle ejection.
"Plausible mechanisms for the large ejection events include thermal fracturing, volatile release through dehydration of phyllosilicates, and meteoroid impacts. The late-afternoon timing of the events is consistent with any of these mechanisms. Bennu's boulder geology indicates that thermal fracturing, perhaps enhanced by volatile release, could occur on the asteroid surface," the findings state.
Agarwal says the Bennu observations leave open a question of how to predict such low-level activity of millions of asteroids not targeted for missions. Estimating the amount of debris that is released into interplanetary space is important but challenging, she says.
"The activity on Bennu shows that an apparently inactive asteroid can harbor a complex dynamic of debris re-impacting the surface or feeding the interplanetary dust cloud. This process may have implications for Bennu's evolution," says Agarwal.