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.

Spectral properties of eight near-Earth asteroids

“…This agrees with the small bodies having relatively young surfaces, and Earth encounters being one of the origins for rejuvenating surfaces on near-Earth asteroids (Binzel et al 2010). Removing the exponential continuum and comparing again with spectra from the Relab database, we found a good fit to the spectrum with those of ordinary chondrite meteorites with high level of Fe, from petrologic class 4 (H4 -olivine-bronzite).…”

“…The MOID values are larger than those predicted by Binzel et al (2010), whose findings,however we cannot exclude. Our deterministic clones approach can reliably obtain NEAs positions only for a couple of thousands years backward in time (Nedelcu 2010).…”

“…It can be speculated that this young surface is due to a relatively recent close encounter with a planet (Binzel et al 2010). …”

Re-assessing the ordinary chondrites paradox

“…This theory seemed consistent with a trend of spectral inclinations and perihelion distances seen among NEAs (Marchi et al 2006). Recently, Binzel et al (2010) has published a paper that also proposes tidal effects to explain the presence of Q-type asteroids among NEAS.…”

Comet 322p/Soho 1: An Asteroid With the Smallest Perihelion Distance?*

“…It appears inactive, has a rotation period that is faster than any known comet and implies a density >1000 kg m −3 , has a higher albedo than has ever been measured for a comet nucleus, and has colors rather atypical for a comet and most consistent with V-and Q-type asteroids. The unweathered spectra of Q-types seem to correlate with recent close approaches to planets (Binzel et al 2010); given 322P's small-q, similar processing may be occurring. However, no comet nucleus has ever been studied following such a close perihelion passage, and we cannot exclude the possibility that 322P's unusual properties are caused by its extreme orbit, where equilibrium temperatures exceed 1000 K at perihelion.…”