Impact craters on 2014 MU69: Implications for Kuiper belt object
size-frequency distributions and planetesimal formation
Abstract
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).