Simultaneous Spectroscopic and Photometric Observations of Binary Asteroids

We present results of visible wavelengths spectroscopic measurements (0.45 to 0.72 microns) of two binary asteroids, obtained with the 1-m telescope at the Wise Observatory on January 2008. The asteroids (90) Antiope and (1509) Esclangona were observed to search for spectroscopic variations correlated with their rotation while presenting different regions of their surface to the viewer. Simultaneous photometric observations were performed with the Wise Observatory's 0.46-m telescope, to investigate the rotational phase behavior and possible eclipse events. (90) Antiope displayed an eclipse event during our observations. We could not measure any slope change of the spectroscopic albedo within the error range of 3%, except for a steady decrease in the total light flux while the eclipse took place. We conclude that the surface compositions of the two components do not differ dramatically, implying a common origin and history. (1509) Esclangona did not show an eclipse, but rather a unique lightcurve with three peaks and a wide and flat minimum, repeating with a period of 3.2524 hours. Careful measurements of the spectral albedo slopes reveal a color variation of 7 to 10 percent on the surface of (1509) Esclangona, which correlates with a specific region in the photometric lightcurve. This result suggests that the different features on the lightcurve are at least partially produced by color variations and could perhaps be explained by the existence of an exposed fresh surface on (1509) Esclangona.Comment: 21 pages, 14 figures, 1 table, accepted for publication in Meteoritics & Planetary Science (MAPS

Rotationally Resolved Spectroscopy of Asteroid Pairs: No Spectral Variation Suggests Fission is followed by Settling of Dust

We examine the spectral properties of asteroid pairs that were disrupted in the last 2 Myrs to examine whether the site of the fission can be revealed. We studied the possibility that the sub-surface material, perhaps on one hemisphere, has spectral characteristics differing from the original weathered surface, by performing rotationally-resolved spectroscopic observations to look for local variations as the asteroid rotates. We observed 11 asteroids in pairs in the near-IR and visible range. Photometry was also conducted to determine the rotational phases of a spectrum on the asteroid lightcurves. We do not detect any rotational spectral variations within the signal-to-noise, which allow us to constrain the extent of any existing surface heterogeneity. For each observed spectrum of a longitudinal segment of an asteroid, we estimate the maximal size of an un-detected "spot" with a spectral signature different than the average. For 5 asteroids the maximal diameter of such a spot is smaller by a factor of two than the diameter of the secondary member. Therefore, the site of the fission is larger than any area with a unique spectral parameters and the site of the fission does not have a unique spectrum. In the case of an S-complex asteroid, where the site of fission is expected to present non-weathered spectra, a lack of a fission spot can be explained if the rotational-fission process is followed by the spread of dust that re-accumulates on the primary asteroid and covers it homogeneously. This is demonstrated for the young asteroid 6070 that presents an Sq-type spectrum while its inner material, that is presumably revealed on the surface of its secondary member, 54827, has a fresher, Q-type spectrum. The spread of dust observed in the disruption event of asteroid P/2013 R3, might be an example of such a process and an indication that it was indeed formed in a rotational-fission event.Comment: 16 pages, 15 figures, 6 Tables. Accepted for publication in Icaru

Unexpected D-type Interlopers in the Inner Main Belt

Very red featureless asteroids (spectroscopic D-types) are expected to have formed in the outer solar system far from the sun. They comprise the majority of asteroids in the Jupiter Trojan population, and are also commonly found in the outer main belt and among Hildas. The first evidence for D-types in the inner and middle parts of the main belt was seen in the Sloan Digital Sky Survey (SDSS). Here we report follow-up observations of SDSS D-type candidates in the near-infrared. Based on follow up observations of 13 SDSS D-type candidates, we find a ~20% positive confirmation rate. Known inner belt D-types range in diameter from roughly 7 to 30 kilometers. Based on these detections we estimate there are ~100 inner belt D-types with diameters between 2.5 and 20km. The lower and upper limits for total mass of inner belt D-types is 2x$10^{16}$ kg to 2x$10^{17}$ kg which represents 0.01% to 0.1% of the mass of the inner belt. The inner belt D-types have albedos at or above the upper end typical for D-types which raises the question as to whether these inner belt bodies represent only a subset of D-types, they have been altered by external factors such as weathering processes, or if they are compositionally distinct from other D-types. All D-types and candidates have diameters less than 30km, yet there is no obvious parent body in the inner belt. Dynamical models have yet to show how D-types originating from the outer solar system could penetrate into the inner reaches of the Main Belt under current scenarios of planet formation and subsequent Yarkovsky drift.Comment: 16 pages, 3 figures, 4 tables -- accepted for publication in Icaru

Observations of "Fresh" and Weathered Surfaces on Asteroid Pairs and Their Implications on the Rotational-Fission Mechanism

The rotational-fission of a rubble-pile asteroid can result in an "asteroid pair", two un-bound asteroids sharing similar orbits. This mechanism might exposes material 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. We report near-IR spectroscopic observations of 31 asteroids in pairs. We analyze their spectral slopes, 1 {\mu}m absorption band, taxonomy, and estimate the time elapsed since their separation. Analyzing the 19 S-complex objects in our sample, we find two fresh Q-type asteroids that are the first of their kind to be observed in the main-belt over the full visible and near-IR range. This solidly demonstrates that Q-type objects are 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 (1 to 2 My) that present low spectral slopes. This illustrates a timescale of at least ~2 My before an object develops high spectral slope that is typical for S-type asteroids. We discuss 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 since: 1) the secondary members in our sample present fresh parameters that tend to be fresher than their weathered primaries; 2) most of the fresh pairs in our sample have low size ratios between the secondary and the primary; 3) 33% of the primaries in our sample are fresh, similar to the prediction set by this model; 4) known satellites orbit two of the pairs in our sample with low size ratio and fresh surface; 5) there is no correlation between the weathering state and the primary shape as predicted by other models.Comment: 19 pages, 17 figures, 4 tables. Accepted to Icaru