Doublet craters on Charon and implications for km-sized binaries in the outer solar system

The trans-Neptunian region is observed to host a large proportion and a wide variety of binary systems (components with > a few tens of km in size) [1, 2], offering unique opportunities for studying planetesimal formation from the protoplanetary disk and subsequent evolution [3]. Since trans-Neptunian objects (TNOs) reside far from the Earth, observing TNOs smaller than km-scale remains challenging [4] and consequently, the binarity in this size range is unknown [5].Doublet craters are generally defined as a pair of adjacent, similarly-sized craters, and are hypothesized to form through simultaneous impacts of widely-separated binaries. The derivable impactor population for Pluto and Charon consists of TNOs smaller than km-scale based upon standard crater scaling laws [6, 7]. Hence, cratering records on these bodies likely contain valuable information about < km-sized widely-separated binaries.We will present results from our study of doublet craters on Vulcan Planum, Charon, which is the most suitable region owing to its relative low density of craters [6, 7]. We define a potential doublet as a pair of craters with a separation smaller than 1.4x the diameter of the larger crater, and with a ratio of the two diameters greater than 0.4. Through visual inspection, potential doublets are categorized as "unlikely" based on geomorphology such as superposition and/or different degree of degradation, and the rest as "possible". We obtained 39 possible doublets which yields 8% (39 out of 483 craters). Assuming that all possible doublets are true doublets and adopting 15% as the likely fraction of binary impacts resulting in doublets [8], approximately 54% of < km-scale TNOs may be widely-separated binaries. Moreover, we will discuss implications of spatial analyses and the inferred binary population among km-scale TNOs. Reference [1] Brunini, A. (2020) in "The Trans-Neptunian Solar System" Eds., D., Prialnik, M.A. Burucci, and L.A. Young [2] Noll, K.S., et al. (2020) in "The Trans-Neptunian Solar System" Eds., D., Prialnik, M.A. Burucci, and L.A. Young [3] Fraser, W., et al. (2017) Nature Astronomy, 1, 0088. [4] Arimatsu, K., et al. (2019) Nature Astronomy, 3, 301-306. [5] Thirouin, A., and S.S. Sheppard (2019) The Astronomical Journal, 157, 228. [6] Singer, K.N., et al. (2019) Science, 363, 955-959. [7] Robbins, S.J., et al. (2017) Icarus, 287, 187-206. [8] Miljkovic, K., et al. (2013) Earth and Planetary Science Letters, 363, 121-132.