Sabuwala and two co-authors reported in 2021 that rapidly spinning and loosely aggregated asteroids appear to take on diamond-shaped profiles, with elevated poles as well as equators. The evolutionary processes that form these characteristic shapes remain a matter of debate. In this paper, we propose a novel model, based on debris accretion, to explain these diamond-shaped profiles.
The authors derived an analytic expression for the shapes of such rapidly spinning rubble piles based on the principle that as rubble is deposited it assumes a critical angle of repose. We show that this expression correctly reproduces diamond shaped profiles. We also conduct granular simulations of debris deposition and show that simulated shapes are in striking accord with both observations and analytical results. Our results suggest that non-uniform debris accumulation, which is overlooked in current models, may play a cardinal role in the formation of diamond-shaped asteroids.
In summary, the authors used a simplified model to predict that material deposited uniformly onto a rotating body will tend to accumulate near both the poles and the equator due to a reduction in centrifugal force with latitude. We have confirmed this prediction in simulations, and we have found that both prediction and simulation agree with recent profiles of diamond-shaped asteroids. The model relies on the principle of a critical angle of repose assumed by falling grains deposited on spinning, rubble-pile asteroids.
Since the model is driven by deposition, their results suggest first that Bennu and Ryugu acquired their characteristic shapes at early times and second that subsequent reshaping by other effects, such as YORP-induced regolith migration, may be minimal. Their results were claimed to be in accord with recent analyses that also suggest an early-time formation of the characteristic asteroid shape based on the size and distribution of craters on Bennu [9, 10].
For Phys.org reporting on this paper, see: https://phys.org/news/2021-09-physics-rubble-pile-asteroids.html
[Also see: https://www.youtube.com/watch?v=-ndCrqjk7U8]
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