Rejuvenating Asteroids during Planetary Flybys: Applications to (99942) Apophis and Other Near-earth Asteroids

Introduction: Laboratory measurements of meteorite reflectance spectra rarely match telescopic observations of asteroid reflectance spectra due to the process (es) of space weathering, which rapidly darken and redden asteroid surface on million-year timescales [e.g. 1-2]. While most main belt asteroids appear weathered, a fraction of near-Earth asteroids (the Q-types) appear unweathered – possibly the result of a previously unrecognized resurfacing mechanism associated with planetary encounters [3-7]. If the number of " fresh, " unweathered asteroids is assumed to be in steady state, this resurfacing mechanism must operate large planetary flyby distances, between 5~20 planetary radii. These large flyby distances are well beyond typical tidal disruption distance of 2~5 planetary radii [8-10]. In this paper, we propose that the weaker tidal perturbations during these distant flybys are still large enough to trigger avalanching on the surface of these asteroids. While not as dramatic as tidal disruption, this process may still be able to resurface the uppermost layers of the asteroid, uncovering unweathered regolith material, and changing the asteroid's spectral properties. Constraining the efficiency of this process will provide insight into the widely debated space weathering timescale. Asteroid Resurfacing Model: Hill slope stability can be analyzed using classic Mohr-Coulomb failure criterion [e.g. 11]. A block of regolith, characterized by some angle of internal friction (ϕ) and cohesion (c) will remain stable if the shear stress at the failure plane remains below the critical shear stress (σ S): σ S = c + σ N tan(ϕ) where σ N is the normal stress. In terrestrial applications , the shear and normal stresses can be easily calculated by projecting the weight of the block due to gravity into the down-slope (shear) and normal directions. This threshold shear stress can be related to a maximum hill slope angle, or angle of repose. (It is important to note: for the case of an asteroid, the angle of repose is dependent on both the cohesion and absolute size and gravity of the asteroid.) For an asteroid during a planetary flyby, the total force acting on the regolith test block is non-trivial: it is the sum of gravi-tational force, centrifugal force, and tidal force (Fig 1). The gravitational force across an asteroid's surface can vary dramatically [e.g. 12-14], due to the irregular shapes of most asteroids. For simplicity (and to enable