Both the asteroids have water-bearing surface material as clay minerals.

Origin of asteroids Bennu and Ryugu might have been found! A team of scientists, led by the University of Arizona (UA) and working with NASA on its first asteroid sample return mission, now have a better understanding of the carbon-rich material of Bennu and its “spinning top” shape, the UA said in a statement. They have found that the hydration pattern and the shape of the asteroid gives clues regarding its origin as well as that of asteroid Ryugu, the target asteroid for the sample return mission of Japan Aerospace Exploration Agency. The scientists have explained their findings in a paper published by the scientists in Nature Communications.

Origin of asteroids: How did Bennu and Ryugu form?

The statement stated that Bennu and Ryugu are both made of fragments of larger space objects which had shattered after colliding with other objects, and these small fragments rejoined to form these asteroids. Scientists now believe that the two asteroid bodies may have formed from the same original parent body that had shattered. Now, the team is focusing on finding out about the processes leading to the specific characteristics of the asteroids.

Asteroids Bennu and Ryugu: What we knew earlier

Bennu and Ryugu are both classified as asteroids with “spinning top”, which means that both the asteroids have strong equatorial ridges. Scientists, till now, believed that the shape was a result of thermal forces called the YORP effect, which increases the speed with which the asteroid spins. As a result of this, over millions of years, the mass around the poles migrates towards the equator, giving a spinning top shape to the asteroids. This would mean that the shape of these asteroids would have formed relatively recently.

Formation of asteroids: Arguments in the new paper

In the new paper, the scientists argue that the shape of these asteroids might not be explained through the YORP effect. They said that both Bennu and Ryugu have large impact craters on their equators and, as per their size, it seems like these craters are some of the oldest surface features on Bennu. This means that their equatorial ridge would have been unchanged for a long time, indicating that their spinning top shape formed much earlier.

The UA statement quotes lead author Ronald Ballouz as saying that they have used computer simulation to show that these asteroids were either formed with these top shapes or they acquired the shapes soon after their formation in the main asteroid belt.

Moreover, both the asteroids have water-bearing surface material as clay minerals. As compared to Bennu, Ryugu’s surface material is less rich, implying that it experienced more heating at some point.

On the assumption that both formed simultaneously, the scientists have explored two possible explanations for the difference in hydration levels. One suggests when the parent body disrupted, Bennu was formed from the material which was closer to the original surface, while Ryugu’s material was at the centre of the parent asteroid.

The second hypothesis is that the fragments that later formed the asteroids experienced different levels of heating when the parent asteroid disrupted. In this case, it would mean that Ryugu’s source material was near the impact point, since the collision would have caused the temperatures in the region to rise, while Bennu’s material would be from an area that didn’t undergo such heating and was farther from the impact point.

However, scientists would be able to provide a clearer idea on this only once the samples return and observational analysis of the surface of these asteroids is undertaken. While samples from Ryugu are expected to reach Earth later this year, the mission to collect samples from Bennu will make its first attempt on October 20 and return the samples in September 2023.