Understanding the geologic history of a near-Earth Asteroid through the lens of NASA’s OSIRIS-Rex mission

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. 

Figure 1: Shows the size of various boulders on Bennu’s surface. The arrows point towards identified fractures, which may be indicative of large impact events or stress caused by rapid temperature changes.

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.  

Figure 2: Maps the abundance of boulders on Bennu’s surface, where red marks areas that are densely populated by boulders and blue marks areas where there are relatively less boulders.

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

Fossil hunting—On Mars!

Did you see this in the news? NASA is starting a new Mars mission, and this one has a very exciting goal: to find evidence of past life! And to study the habitability of Mars for past life and for humans in the future. 

A new rover, called Mars 2020 until a name is selected (update: Mars Perseverance Rover), will be sent to Mars this summer, with an arrival on Mars February 18, 2021. The rover will explore the Jezero crater for about one Martian year, equal to 687 Earth days. Jezero crater was chosen for study because there is evidence that this crater once contained a lake. 

Elevation map of Jezero Crater. Dark blue and purple are deeper areas, yellow is the highest. The circled area is the area of the mission. NASA/JPL-Caltech/MSSS/JHU-APL/ESA. https://photojournal.jpl.nasa.gov/catalog/PIA23511

Two rivers, on the left side of the picture, flowed into a crater. A flood like broke through the crater wall and allowed water to drain out of the crater (upper right). Inside of the crater is a former delta formed as sediments were deposited as the rivers entered the lakes and deposited sediment.  

Artist’s concept of the delta formed within the ancient lake. NASA/JPL-Caltech/University of Arizona, https://photojournal.jpl.nasa.gov/catalog/PIA22907

Spectral analyses of the deltas and fans have revealed the presence of carbonates and hydrated silicas.

Spectral analyses of the detlas and fans have revealed the presence of carbonates and hydrated silicas.

Carbonate is a chemical composed of carbon, oxygen, and a metal or hydrogen. For example, chalk, seashells, and egg shells are all made of calcium carbonate crystals (CaCO3). Carbonates need liquid water and an atmosphere with carbon dioxide to form. On Earth, carbonate rocks may be formed by the accumulation of tiny fossil shells, but carbonates can form abiotically (without life). Limestone, a carbonate rock, is a good preserver of body fossils and trace fossils. Silica, a combination of silicon and oxygen, forms in water. Chert and flint are examples of silica rocks. Chert is also formed by the accumulation of tiny shells, but these are made from silica, not carbonate.

Any fossils that are left on Mars from its warmer, wetter periods would likely be found in carbonate and silica deposits. Scientists expect that these fossils would be microorganisms (single celled organisms). 

In addition to searching for fossils, Mars 2020, Perseverance will also: 

  • Determine past climates that may have allowed ancient life to exist
  • Study the geology of Mars, including the processes that affected and altered Mars’s surface, as well as looking for rocks that formed in water and what they might reveal about past life
  • Help prepare for human explorers by studying radiation levels on Mars’s surface and chemicals common in martian soil that are known to be harmful to humans. 

This may be a very exciting mission, but the wait will be long! The search for fossils will be the last part of the mission. But we’ll keep you posted!

For more information, visit NASA’s about this mission: Mars 2020 Mission and Mars Perseverance Rover.