Surface slopes of asteroid pairs as indicators of mechanical properties and cohesion

Abstract

Asteroid pairs are those found to share similar heliocentric orbits but not positions. The leading theory suggests that each had a single progenitor that split due to rotational-fission of a weak, rubble-pile structured body. By constructing shape models of asteroid pairs from multiple-apparition observations and using a lightcurve inversion technique, we mapped the gravitational and rotational accelerations on the surfaces of these asteroids. This allows us to construct a map of local slopes on the asteroids' surfaces. In order to test for frictional failure, we determine the maximum rotation rate at which an area larger than half the surface area of the secondary member (assumed to be the ejected component) has a slope value >40 degrees, the angle of friction of lunar regolith, where loose material will begin sliding. We use this criterion to constrain the failure stress operating on the body, just before disruption at the commonly observed spin barrier of 2.2 h. Our current sample includes shape models of eleven primary members of asteroid pairs, observed from the Wise Observatory in the last decade. In the studied parameter space we find that the shape models only reach the spin barrier when their bulk density is larger than the ~2 g cm−3 measured for the rubble pile structured 25143 Itokawa, and better matches 433 Eros' value of 2.7 g cm−3, suggesting that km-sized asteroid pairs are dense compared to sub-km bodies. Assuming ejection of secondary components that are larger than those observed (up to the maximal size allowing separation), can also increase the spin barrier of the asteroids, thus supporting the previously suggested scenario of continuous disruption of the secondary. In addition, cohesion levels of hundreds of Pascals are also required to prevent these shape models from disrupting at spin rates slower than the usual spin barrier.