Searching for color variation on fast rotating asteroids with simultaneous V-J observations

Motivation: Boulders, rocks and regolith on fast rotating asteroids (~2.5 hours) might slide towards the equator due to a strong centrifugal force and a low cohesion force, as described by models (Walsh et al. 2008, Sánchez & Scheeres 2014). As a result, a fresh material might be exposed, if the surface consists of weathered ordinary chondrite (S-complex). Detecting color variation, due to the exposure of fresh material, will allow us to model the mass shedding process, its extent and age, and thus support or reject hypotheses of rotational-fission.Method: Detecting color variation on small and fast rotating asteroids is difficult with spectroscopy since color differences are mild while the exposure time must be short to measure a narrow rotational phase. Broadband photometry is also problematic since it introduces large systematic errors when images in different filters are calibrated with standard stars. We describe a novel technique in which the asteroid is simultaneously observed in the visible and near-IR wavelength ranges. This technique is possible if a dichroic split the light into two beams that hit two detectors. In this technique atmospheric interference are self-calibrated between the visible and the near-IR image. We use a V and a J filters since the distinction between fresh and weathered surfaces are most prominent in these wavelengths and range between 10-20%.Observations: We observed 3 asteroids with CTIO’s 1.3m telescope and ANDICAM detector. The asteroids were observed during 2 rotational cycles to confirm features on the color-curve. There is ~5% variation of the mean color. There are a few measurements with a larger/smaller color in the range of ~10%, but these do not repeat in a second rotation cycle and we cannot confirm them as real. Therefore, we cannot detect fresh colors (as seen on Q-type asteroids) on the surface. This suggests one of the following statements: 1. No landslides occurred within the timescale of space weathering. 2. Landslides occurred but the exposed patches are too small for the measurements’ uncertainty. 3. Slides occurred but did not expose fresh material.