Observations of “fresh” and Weathered Surfaces on Asteroid Pairs and Their Implications on the Rotational-fission Mechanism

The rotational-fission of a “rubble-pile” structured asteroid can result in an “asteroid pair” two un-bound asteroids sharing nearly identical heliocentric orbits. Models suggest that this mechanism exposes material from below the progenitor surface that previously had never have been exposed to the weathering conditions of space. Therefore, the surfaces of asteroid pairs offer the opportunity to observe non-weathered “fresh” spectra. Here we report near-infrared spectroscopic observations of 31 asteroids in pairs. In order to search for spectral indications of fresh surfaces we analyze their spectral slopes, parameters of their 1μm absorption band and taxonomic classification. Additionally, through backward dynamical integration we estimate the time elapsed since the disintegration of the pairs progenitors. Analyzing the 19 ordinary chondrite-like (S-complex) objects in our sample, we find two Q-type asteroids (19289 and 54827) that are the first of their kind to be observed in the main-belt of asteroids over the full visible and near-infrared range. This solidly demonstrates that the Q-type taxonomy is not limited to the NEA population. The pairs in our sample present a range of fresh and weathered surfaces with no clear evidence for a correlation with the ages of the pairs. However, our sample includes “old” pairs (2x10 ≥ age ≥ 1x10 years) that present relatively low, meteoritic-like spectral slopes (< 0.2% per 1μm). This illustrates a timescale of at least ∼ 2 million years before an object develops high spectral slope that is typical for S-type asteroids. We discuss three mechanisms that explain the existence of weathered pairs with young dynamical ages and find that the “secondary fission” model (Jacobson & Scheeres 2011) is the most robust with our observations. In this mechanism an additional and subsequent fission of the secondary component contributes the lion share of fresh material that re-settles on the primary’s surface and recoats it with fresh material. If the secondary breaks loose from the vicinity of the primary before its “secondary fission”, this main source of fresh dust is avoided. We prefer this secondary fission model since i) the secondary members in our sample present “fresh” parameters that tend to be “fresher” than their weathered primaries; ii) most of the fresh pairs in our sample have low size ratios between the secondary and the primary; iii) 33% of the primaries in our sample are fresh, similar to the prediction set by the secondary fission model (Jacobson & Scheeres 2011); iv) known satellites orbit two of the pairs in our sample with low size ratio (D2/D1) and fresh surface; v) there is no correlation between the weathering state and the primary shape as predicted by other models. Subject headings: Asteroids; Asteroids, rotation; Asteroids, surfaces; Rotational dynamics; Spectroscopy