A spectral slope versus perihelion distance correlation for planet-crossing asteroids

The spectral slope of sunlight reflected by an asteroid surface may change with time as a result of several physical processes acting on the surface layer known as 'space weathering'. The effects of space weathering are important for asteroid studies because they may help us to explain the paucity of spectroscopic analogues of ordinary chondrites (OCs) among the main belt asteroids. In this Letter we report a new result that may be used to understand better how asteroid surfaces evolve with time. Using a large data set of spectroscopic observations of planet-crossing asteroids (near-Earth objects and Mars-crossers), we have found a statistically significant correlation between their spectral slope and the perihelion distance. We have also determined that the Q- and Sq-type planet-crossing asteroids are more abundant (relative to more space-weathered S-type asteroids) at small perihelion distances that characterize dynamically evolved orbits. We take these results as evidence for some (so far neglected) process that modifies surfaces of planet-crossing asteroids. As originally proposed by Nesvorny et al., the close encounters of these objects with the terrestrial planets may produce tidal perturbations to their surfaces that may partially, or even completely, remove the old and weathered layers and expose non-weathered OC-like material. Our findings may provide new evidence for this process because standard models for the dynamical evolution of asteroids show that planet-crossing asteroids with small perihelion distances have undergone more frequent planetary encounters than those residing in more distant orbits. Therefore, if planetary encounters are important, the distribution of Q-type asteroids in the planet-crossing space should show a correlation with the perihelion distance, exactly as we have found here.