New Delhi: ‘Near Earth’ asteroids Bennu and Ryugu have a fascinating shape. They are literally like ‘diamonds in the sky’.
Scientists have now studied the two asteroids, using a granular physics model, and explained the reason why they are shaped like a diamond.
The study, published in the Granular Physics journal, was conducted by scientists from the Okinawa Institute of Science and Technology Graduate University (OIST) and Rutgers University. They used a novel model to explain the diamond shaped profiles.
Asteroids which spin rapidly, and have loosely aggregated materials, appear to take the shape of a diamond, with elevations occurring at the poles and the equator, the study found.
What Are Asteroids?
Rocky celestial bodies, orbiting the Sun, which were formed from leftover materials after the Big Bang, are called asteroids. These leftover materials refer to the matter that didn’t get absorbed into the large planets during the formation of the Solar System, around 4.6 billion years ago. Thus, asteroids serve as an integral tool to study how the Solar System and its planets were, during their initial days of formation.
The region between Jupiter and Mars, where asteroids are trapped, is known as the asteroid belt. Since asteroids are located at large distances from Earth, it could be difficult to study them. However, during times when asteroids come closer to Earth, their pictures could be captured with the help of unmanned spacecrafts.
NASA's OSIRIS-REx spacecraft had captured images of materials ejected from Asteroid Bennu in 2019. The Japan Aerospace Exploration Agency (JAXA) spacecraft Hayabusa2 had captured images of Asteroid Ryugu in 2018.
What Kind Of Asteroids Are Bennu And Ryugu?
Bennu and Ryugu are rubble-pile asteroids — they are weak aggregates of small and large components held together under the influence of gravity.
How Were Previous Models Different From One Used In This Study?
‘Deposition of material’ was the key ingredient missing in previous models, Dr. Tapan Sabuwala, lead author of the study, said in a statement issued by the Okinawa Institute of Science and Technology Graduate University (OIST).
The simulations performed using these models result in flattened and sometimes asymmetric shapes that do not conform to the observed shapes of Ryugu and Bennu, the study states. When other studies, which explain that violent fragmentation and coalescence of the fragments of the body resulted in diamond shapes, are simulated, then other shapes such as spheroids are observed. This is indicative of the fact that other processes also are responsible for the diamond shape of the asteroids, which previous models did not take into account, the study explains. The previous models did not suggest the fact that the asteroids could have gained their shapes during the early stages of formation.
The Shape Of Diamond
The models are based on the principle of debris accretion, where the rubble is deposited for rapidly spinning and loosely aggregated asteroids at a certain angle to the horizontal plane, causing them to attain the shape of diamonds. This angle is known as the critical angle of repose, and when the materials are deposited in that orientation, the poles and equator become elevated regions.
Using simulations of debris deposition, the scientists showed the consistency between the simulated results and analytical results. The study also states that non-uniform accumulation of debris, which has not been considered much in this research, may be an important determining factor in the formation of diamond-shaped asteroids.
The scientists have explained in the study that materials are accumulated at the poles because there, the centrifugal forces (outward force on a rotating body) are decreased. The elevations at the equator could be attributed to centrifugal migration of material, the study stated. If such accumulations do not occur, the poles would not be elevated.
Sabuwala and co-author Professor Pinaki Chakraborty applied the idea of sand or sugar being poured through a funnel, and the use of granular physics for the subsequent prediction of the shape attained by them, depending upon the forces acting on them, in these models.
Sabuwala explained that the orientation of gravity on these asteroids was different, compared to that experienced by a sand pile. He said this was factored into the model, along with the asteroids’ rotation.
The study states that the forces acting upon the asteroids are what resulted in them becoming diamond-shaped, instead of attaining a conical shape.
The models are therefore helpful in predicting that uniform deposition of material on a rotating object will cause accumulation at the poles and the equator due to the centrifugal force decreasing with latitude. The scientists have confirmed this prediction with recent simulations. Since deposition governs the model, the scientists deduced that Bennu and Ryugu could have attained their diamond-shape during their early stages of formation, and that the effect of other physical processes, which could change their shape, is negligible. Based on the size and distribution of craters on Bennu, the scientists analysed that the shapes could have been acquired in the early days of the asteroid’s birth, and the results obtained from the models are compatible with the analyses.
“We have used simple concepts of how grains flow to explain how these asteroids assumed their curious shapes,” Professor Chakraborty was quoted as saying in the OSIT statement.