title: Earth encounters as the origin of fresh surfaces on near-Earth asteroids.
abstract: Telescopic measurements of asteroids' colours rarely match laboratory reflectance spectra of meteorites owing to a 'space weathering' process that rapidly reddens asteroid surfaces in less than 10(6) years. 'Unweathered' asteroids (those having spectra matching the most commonly falling ordinary chondrite meteorites), however, are seen among small bodies the orbits of which cross inside Mars and the Earth. Various explanations have been proposed for the origin of these fresh surface colours, ranging from collisions to planetary encounters. Less reddened asteroids seem to cross most deeply into the terrestrial planet region, strengthening the evidence for the planetary-encounter theory, but encounter details within 10(6) years remain to be shown. Here we report that asteroids displaying unweathered spectra (so-called 'Q-types') have experienced orbital intersections closer than the Earth-Moon distance within the past 5 x 10(5) years. These Q-type asteroids are not currently found among asteroids showing no evidence of recent close planetary encounters. Our results substantiate previous work: tidal stress, strong enough to disturb and expose unweathered surface grains, is the most likely dominant short-term asteroid resurfacing process. Although the seismology details are yet to be worked out, the identification of rapid physical processes that can produce both fresh and weathered asteroid surfaces resolves the decades-long puzzle of the difference in colour of asteroids and meteorites.
authors.affiliation: Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. rpb@mit.edu
authors.forename: Richard P
authors.initials: RP
authors.lastname: Binzel
authors.forename: Alessandro
authors.initials: A
authors.lastname: Morbidelli
authors.forename: Sihane
authors.initials: S
authors.lastname: Merouane
authors.forename: Francesca E
authors.initials: FE
authors.lastname: Demeo
authors.forename: Mirel
authors.initials: M
authors.lastname: Birlan
authors.forename: Pierre
authors.initials: P
authors.lastname: Vernazza
authors.forename: Cristina A
authors.initials: CA
authors.lastname: Thomas
authors.forename: Andrew S
authors.initials: AS
authors.lastname: Rivkin
authors.forename: Schelte J
authors.initials: SJ
authors.lastname: Bus
authors.forename: Alan T
authors.initials: AT
authors.lastname: Tokunaga