Do planetary encounters reset surfaces of near Earth asteroids?

Processes such as the solar wind sputtering and micrometeorite impacts can modify optical properties of surfaces of airless bodies. This explains why spectra of the main belt asteroids, exposed to these ‘space weathering’ processes over eons, do not match the laboratory spectra of ordinary chondrite (OC) meteorites. In contrast, an important fraction of Near Earth Asteroids (NEAs), defined as Q-types in the asteroid taxonomy, display spectral attributes that are a good match to OCs. Here we study the possibility that the Q-type NEAs underwent recent encounters with the terrestrial planets and that the tidal gravity (or other effects) during these encounters exposed fresh OC material on the surface (thus giving it the Q-type spectral properties). We used numerical integrations to determine the statistics of encounters of NEAs to planets. The results were used to calculate the fraction and orbital distribution of Q-type asteroids expected in the model as a function of the space weathering timescale, tsw (see main text for definition), and maximum distance, r∗, at which planetary encounters can reset the surface. We found that tsw ∼ 106 yr (at 1 AU) and r∗ ∼ 5Rpl, where Rpl is the planetary radius, best fit the data. Values tsw < 105 yr would require that r∗ > 20Rpl, which is probably implausible because these very distant encounters should be irrelevant. Also, the fraction of Q-type NEAs would be probably much larger than the one observed if tsw > 107 yr. We found that tsw ∝ q2, where q is the perihelion distance, expected if the solar wind sputtering controls tsw, provides a better match to the orbital distribution of Q-type NEAs than models with fixed tsw. We also discuss how the Earth magnetosphere and radiation effects such as YORP can influence the spectral properties of NEAs.