Possible particle ejection contributions to the shape and spin stability of small near-Earth asteroids

Top Shaped Asteroids (TSAs) have proven to be common amongst the near-Earth rubble pile population, with multiple examples confirmed via groundbased radar and spaceborne optical sensors through the past 20 years. A substantial body of literature has developed, exploring the formation of these unique shapes either through rotation-induced landslides and creep, or collisional reaccumulation. Models of such mass movements can provide good explanations for mid and low latitude material redistribution, but Bennu also shows a significant increase in radius in the high polar regions, which is harder to explain with these processes. The discovery of repeated and probably ongoing particle ejections around the 500 m diameter asteroid Bennu by the OSIRIS-REx mission suggests that we need to consider an alternate or additional mechanism which, we show, can anticipate the detailed variety of TSA shapes. This paper explores asteroid shape evolution as the result of particle ejections, modeled as being simply correlated with latitude via diurnal heating (or meteorite impacts), and re-accumulation using simulations including gravity and solar radiation pressure. Asteroid outlines are evolved with time as a function of particle ejection velocities and asteroid rotation rates. Bennu's shape can be anticipated by our model with RMS surface errors of less than 1.1% (2.7 m) although some southern latitudes have errors up to 10 m. Straightforward variation in conditions can produce shapes matching other TSAs. However, the observed particle fluxes on Bennu are approximately 3 orders of magnitude too low for this to be the only shaping mechanism on Bennu. The time necessary to form these shapes by our mechanism alone is far longer than the lifetimes of near-Earth asteroids, unless fluxes were once much greater, or there was an underlying oblate shape.