Why are Rubble Pile Asteroids Shaped Like Diamonds?

Scientists have the opportunity to see close-up images of Ryugu and Bennu, two near-Earth asteroids. For some reason, both asteroids are shaped like diamonds. Why is this? It was a puzzle up until now.
A team of scientists is now trying to answer the question.

Scientists continue to study asteroids. These are remnants of the primordial Solar System material, which was not swept up during planet formation. They are difficult to find because they are located far out in the asteroid Belt.

Some of them, however, have managed to escape the belt and are now close to Earth. Scientists have the best chance to study these asteroids near Earth. Both Ryugu, Bennu were visited by spacecraft that collected samples and returned them to Earth. Both the Japanese spacecraft Hayabusa 2 (which visited Ryugu) and NASA's OSIRIS REx (which visited Bennu), spent time studying their asteroid targets, taking detailed images, and both visited Ryugu.

Both are rubble pile asteroids. This means they are made up of small pieces of smaller material that are bound together by gravity. Both rotate quickly.

Near-Earth asteroids Ryugu and Bennu (l). Both are rubble-pile asteroids, and both are diamond-shaped. Image Credit Left: NASA/Goddard/University of Arizona Public Domain. Image Credit: By ISAS/JAXA. CC BY 4.0

Researchers from the Okinawa Institute of Science and Technology and Rutgers University published a paper that explained the shape of the asteroids' diamonds. The paper is called Bennu and Ryugu, diamonds in the sky. Tapan Sabuwala, OIST, is the lead author.

They are loosely held together by smaller pieces of material, which makes them more like granular matter than large, monolithic rocks. The team of scientists used granular Physics modelling to examine them and determine their diamond shape. A model of how sand grains flow was used to explain the diamond shape.

This is not the first time models have been used to explain the shapes of asteroids. The models used rotation to explain the shape but they didn't stand up to testing.

Tapan Sabuwala, the paper's lead author, stated that previous models had attributed the diamond-like shapes to the forces of rotation. This resulted material being driven from poles to equator. These models were used to simulate the asteroids. However, the shape of the asteroids was not flattened or symmetrical. This meant that something was wrong, Dr. Sabuwala stated in a press release.

In the press release that was released with the study, sand was shown flowing through a funnel. The conical shape of the sand forms when it is poured into it. There is more to an asteroid than just a pile of sand. Different gravitational forces exist. This was something we had to account for, along with the fact that asteroids rotation also plays an important role, Dr. Sabuwala stated.

Below is an image showing the asteroid 101955 Bennu. The right-hand side shows a simulation of the study. This model matches the asteroid's diamond shape. Image Credit: Sabuwala et al 2021.

The paper reveals that centrifugal force played an important role in Ryugus and Bennus shapes. Because it is weaker close to the poles of asteroids, more material accumulates there. This is what gives asteroids their distinctive appearance. This accumulation leads to higher elevations at poles and centrifugal movement increases elevations at equator. The authors state in their paper that poles would not have the elevated poles seen on Ryugu or Bennu without this accumulation.

This work is different from the previous models in a significant way. This shows that asteroids did not form in a different way and then slowly become diamond-shaped. The asteroid formed the diamond shape and material accumulated. Over time, the shape of asteroids has remained stable.

The model is based on the principle that falling grains on spinning, rubble pile asteroids will create a critical angle for repose. Our results show that the model is driven primarily by deposition. They first suggest that Bennu, Ryugu have acquired their distinctive shapes early on. Second, that any subsequent shaping by other effects may not be as significant, they write in the papers summary.

The researchers next want to investigate other rubble-pile asteroids which rotate slower. Because the centrifugal force in these asteroids is less, they should have a more spheroidal form. This is exactly what they did when they decreased the rotation rate in their modeling.

The researchers decreased the rotation rate of the models to reduce the centrifugal force. This resulted in a more spheroidal form. Image Credit: Sabuwala et al 2021.

Professor Chakraborty, OIST co-author, stated that we used simple concepts about how grains flow to explain the strange shapes of these asteroids. This work is perhaps most enjoyable because simple ideas can help solve complex problems.

Continue reading: