One hypothesis suggests that when the parent asteroid was disrupted, Bennu formed from material closer to the original surface, while Ryugu contained more material from near the parent body’s original center.Īnother possible explanation for the difference in hydration levels is that the fragments experienced different levels of heating during the parent asteroid’s disruption. Ryugu’s surface material is less water-rich than Bennu’s, which implies that Ryugu’s material experienced more heating at some point.Īssuming Bennu and Ryugu formed simultaneously, the paper explores two possible explanations for the different hydration levels of the two bodies based on the team’s computer simulations. In addition to their shapes, Bennu and Ryugu also both contain water-bearing surface material in the form of clay minerals. We would expect these craters to have disappeared with a recent YORP-induced reshaping of the asteroid.” “The presence of the large equatorial craters on these asteroids, as seen in images returned by the spacecraft, rules out that the asteroids experienced a recent reshaping due to the YORP effect. “Using computer simulations that model the impact that broke up Bennu’s parent body, we show that these asteroids either formed directly as top-shapes, or achieved the shape early after their formation in the main asteroid belt,” says co-lead author Ronald Ballouz, OSIRIS-REx postdoctoral research associate at the University of Arizona. Since the craters cover the equatorial ridges, their spinning-top shapes must also have been formed much earlier. Both asteroids have large impact craters on their equators, and their size suggests that these craters are some of Bennu’s oldest surface features. However, in a new paper in Nature Communications, scientists from the OSIRIS-REx and Hayabusa2 teams argue that the YORP effect may not explain the shape of either Bennu or Ryugu. The YORP effect increases the speed of the asteroid’s spin, and over millions of years, material near the poles could have migrated to accumulate on the equator, eventually forming a spinning-top shape-meaning that the shape would have formed relatively recently. Until now, scientists thought that this shape formed as the result of thermal forces, called the YORP effect. Arizona)īennu and Ryugu are both “spinning-top” asteroids, which means they have a pronounced equatorial ridge. From the spacecraft’s vantage point, half of Bennu is sunlit and half is in shadow. This image, showing asteroid Bennu’s spinning-top shape, was taken by the MapCam camera on NASA’s OSIRIS-REx spacecraft on April 29, from a distance of 5 miles. Now, scientists are looking to discover what processes led to specific characteristics of these asteroids, such as their shape and mineralogy. The small fragments reaccumulated to form an aggregate body.īennu and Ryugu may actually have formed in this way from the same original shattered parent body. Researchers with NASA’s first asteroid sample return mission, OSIRIS-REx, are gaining a new understanding of asteroid Bennu’s carbon-rich material and signature “spinning-top” shape.īennu, the target asteroid for the OSIRIS-REx mission, and Ryugu, the target of the Japan Aerospace Exploration Agency’s Hayabusa2 asteroid sample return mission, are composed of fragments of larger bodies that shattered upon colliding with other objects.
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