Resurfacing asteroids from thermally induced surface degradation

Abstract

The spectral slopes of S and Q-type asteroids are altered by the weathering of their surfaces due to solar wind interactions and micrometeorite impacts, as well as any processes that work to remove that weathered material. These processes of space weathering and asteroid resurfacing compete with each other to determine the spectral slope of each asteroid, with space weathering raising the spectral slope and resurfacing lowering it. By considering the distribution of spectral slopes with respect to orbital location and size, we can determine which potential resurfacing processes are the most dominant. In this study, we show that the spectral slopes of S and Q-type Near-Earth Asteroids (NEAs) decrease with decreasing perihelion, but only for perihelia q≲0.9 AU. We then use this distribution of spectral slopes vs. perihelion as a constraint for modeling two proposed resurfacing mechanisms. For both models, we numerically integrate the orbits of NEAs over their lifetimes and track their spectral slopes. First, we model the resurfacing of asteroids due to close encounters with the terrestrial planets and found that no combination of parameters could explain the spectral slope vs. perihelion trend at q≲0.9 AU. We also model the resurfacing of asteroids due to thermally induced surface degradation, by assuming a power law relationship between the resurfacing timescale and the solar distance. For a space weathering timescale of an asteroid on a circular orbit at 1 AU of τSW0≲5 Myr, we find a range of resurfacing timescales and power law exponents that generate a spectral slope vs. perihelion distribution consistent with observations. We discuss that previous studies supporting the resurfacing of asteroids due to close encounters could be a result of confounding variables, as the correlation of Q-type asteroids' past orbits with the terrestrial planets are confounded by the higher fraction of Q-type asteroids at low perihelia, which could be created by another mechanism. We also discuss previous studies that qualitatively support the process of thermally induced surface degradation at low perihelia, and conclude that thermal processes are the most consistent explanation for resurfacing of asteroids with low perihelia q≲0.9 AU.