Fresh Surfaces on Q-Type Asteroids: Close Planetary Encounters Unlikely to be the Source

The S-type Conundrum refers to a problem in planetary science in which Q-type asteroids, which match the spectra of the most common meteorite found on Earth, the ordinary chondrite (OC), are significantly outnumbered by S-type asteroids, whose color is weathered and reddened by comparison. The accepted solution holds that Q-type and S-type asteroids both have ordinary chondritic composition, but they have experienced a space weathering process that reddened their surfaces. This space weathering process changes the reflectance spectra of OC-composition asteroids from Q-type to S-type over an estimated timescale of about 1 million years. Collisional processes that might resurface asteroids have timescales that are much longer than this space weathering time scale, and therefore all asteroids with ordinary chondrite composition should have experienced extensive space weathering, and only weathered S-type asteroids should be observed at the present time. To explain the currently observed Q-type population, a refreshing mechanism must exist to disturb surface grains and reveal fresh, unweathered material. It has been proposed that tidal forces due to close planetary encounters may be the dominant refreshing mechanism, causing landslides and disturbing the surfaces of asteroids. The hypothesis predicts that most, if not all, Q-type asteroids have recently experienced close planetary encounters within the estimated space weathering timescale. We report that a significant fraction of Mars-crossing Q-type asteroids have not experienced close planetary encounters in the past 1 Myr, implying that other refreshing mechanisms must be active in the Mars-crossing region.