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Impact craters on 2014 MU69: Implications for Kuiper belt object size-frequency distributions and planetesimal formation
  • Kelsi N. Singer
Kelsi N. Singer
Southwest Research Institute

Corresponding Author:[email protected]

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Impact craters in the Pluto system as observed by New Horizons in July of 2015 revealed a previously-unknown deficit of small Kuiper belt objects (KBOs) less than ~1–2 km in diameter (Singer et al., 2019, Science) . New Horizons was poised to test this observation 3.5 years later with its next KBO close flyby of the cold classical KBO (486958) 2014 MU69 (nicknamed Ultima Thule). The images returned by New Horizons in early 2019 show MU69 is only modestly cratered, and potential craters on the surface show a shallow size-frequency distribution (SFD) similar to that of craters on Pluto and Charon (Stern et al. 2019, Science; Singer et al., 2019, EPSC-DPS; Spencer et al. 2020, Science). Some features on MU69 do have multiple morphologic indicators suggesting they are impact craters. There are regions on MU69 with few-to-no craters, even where the lighting is favorable for recognizing topographic features. The ~7-km-diameter depression (informally called Maryland) on the small lobe of MU69 is the largest easily observed feature that is likely to be an impact crater. There are also craters under 1 km in diameter, but no obvious craters between 1 and 7 km in diameter. The possible impact craters < ~1 km in diameter were divided into subgroups based on morphology, likelihood of being a crater , lighting geometry, and also into one possible geologic unit. Measurements/ratings were provided by several team members. All subgroups produced shallow-sloped SFDs (differential power-law slope less than approximately −2) similar to those seen on Pluto and Charon for the same size craters (when scaled for gravity and impact velocity). Whole-sale resurfacing through geologic processes is not expected for MU69. Thus many more craters and at least some heavily cratered terrains would be predicted if the impactor SFD slope were steeper (an average differential slope closer to −3; Greenstreet et al. 2019, ApJ). Both the apparent lack of craters overall, and the shallow SFD slopes, are consistent with a relatively benign collisional environment for MU69 (McKinnon et al., 2020, Science).