Craters, Boulders, and Regolith of (101955) Bennu Indicative of an Old and Dynamic Surface
by: K. J. Walsh, E. R. Jawin, R.-L. Ballouz, O. S. Barnouin, E. B. Bierhaus, H. C. Connolly Jr., J. L. Molaro, T. J. McCoy, M. Delbo’, C. M. Hartzell, M. Pajola, S. R. Schwartz, D. Trang, E. Asphaug, K. J. Becker, C. B. Beddingfield, C. A. Bennett, W. F. Bottke, K. N. Burke, B. C. Clark, M. G. Daly, D. N. DellaGiustina, J. P. Dworkin, C. M. Elder, D. R. Golish, A. R. Hildebrand, R. Malhotra, J. Marshall, P. Michel, M. C. Nolan, M. E. Perry, B. Rizk, A. Ryan, S. A. Sandford, D. J. Scheeres, H. C. M. Susorney, F. Thuillet, D. S. Lauretta and the OSIRIS-REx Team
Summarized by: Lisette Melendez
What data were used? Unlike geologic sites on Earth, scientists aren’t able to use field work to determine the geologic history of celestial objects like asteroids, planets, and distant moons. Instead, planetary geologists rely on data collected by scientific instruments on spacecraft, like cameras and spectrometers, to study these unreachable geologic features.
The data for this study was gathered from images taken by NASA’s ORISIS-Rex spacecraft, whose mission is to travel to a near-Earth asteroid named Bennu. Asteroids are the remains of the building blocks of our solar system that enabled the rise of planets and life, and most of them reside in the Main Asteroid Belt. However, sometimes asteroids are ejected and enter the inner solar system (i.e. the rocky planets: Mercury, Venus, Earth, and Mars), becoming near-Earth asteroids. This asteroid, Bennu, was chosen for the sample collection mission because of its proximity to Earth, large size (almost 500 meters long!), and carbonaceous (i.e., carbon-rich) composition. The carbon-rich part is important because these asteroids contain chemical compounds and amino acids that would have been present at the beginning of our Solar System. Even though the asteroid is relatively long compared to other asteroids, it’s only about as wide as the length of the Empire State Building!
The spacecraft is set to bring back a sample of this asteroid to Earth by 2023 for scientists to analyze. In late 2018, the spacecraft began the approach phase of the mission and used its cameras to take high-quality pictures of Bennu’s surface, as shown in Figure 1. These images are not only used to determine a good sample collection site, but scientists also use them to learn more about the geologic processes on Bennu’s surface. By weaving the images together, the team was able to produce a three-dimensional model of the asteroid and determine the location of boulders on the surface of Bennu.
Methods: The surface of Bennu was mapped out by visually analyzing images taken by cameras on OSIRIS-Rex. Scientists combined image and radar data to measure the size and distribution of boulders on Bennu’s surface. By applying the same foundational geologic concepts observed here on Earth, scientists can draw conclusions about the geologic features on asteroids and what forces potentially formed them.
Results: The orbit of a near-Earth asteroid is tumultuous, due to the possibility of collision with other asteroids and the forces exerted by Earth’s gravity, making a usual lifespan of a near-Earth asteroid only last around tens of millions of years. Usually, this would mean a young, consistently refreshed surface for these near-Earth asteroids. However, a detailed study of Bennu’s surface shows evidence of rocks that are hundreds of millions of years old – long before Bennu ever left the Main Asteroid Belt.
Boulders are the most prominent geologic feature on Bennu’s surface. As shown in Figure 2, they can be found all around the asteroid. Scientists noted that the size of various boulders are simply too large for them to have been formed in Bennu’s current orbit, pointing towards the possibility they were created during larger asteroid collisions in the main asteroid belt. This indicates that studying the boulders further may aid in the understanding of Bennu’s parent body (i.e., where the rocks were originally created) and conditions in the main asteroid belt.
Another interesting result from the study is that even though the resolution of the images was not clear enough to depict fine-grained particles, the scientists measured thermal inertia (tendency to resist changes in temperature) and found that the results were consistent with the existence of fine-grained particles on Bennu’s surface. Come the end of 2020, the spacecraft will start up the TAGSAM (Touch-and-Go-Sample-Acquisition-Mechanism) instrument, blow nitrogen gas onto the surface to stir up dust, and collect the sample – leading to even more scientific discoveries on the asteroid front.
Why is this study important? This study is a reminder of how fascinating geology is: scientists were able to predict the history of the asteroid solely by measuring the size and distribution of boulders on its surface. This group was able to differentiate between events that occurred while Bennu was in the Main Asteroid Belt versus a near-Earth orbit, which helps us understand the environment right outside of Earth and beyond.
The big picture: By looking into the early Solar System, the data gathered in this study will help scientists understand the processes behind the formation of planets, as well as the origins of life. Additionally, the study will enhance our understanding of the evolution of near-Earth asteroids as well as the possibility of the asteroids impacting Earth.
Citation: Walsh, K.J., Jawin, E.R., Ballouz, R. et al. Craters, boulders and regolith of (101955) Bennu indicative of an old and dynamic surface. Nat. Geosci. 12, 242–246 (2019). https://doi.org/10.1038/s41561-019-0326-6