Arrokoth’s necklace

Abstract

Flyby images of (486958) Arrokoth (Ultima Thule, 2014 MU69) show a comparatively bright “necklace” in the neck, or cleft, between its two lobes, in contrast to its generally low albedo. We suggest that the necklace may be the result of thermally controlled ice deposition. The necklace is found in the most (orbitally averaged) shaded part of the surface. It may consist of clean, high albedo, ice condensed from vapor sublimed by dirty, low albedo, ice elsewhere; ice accumulates where the maximum temperatures are the lowest. Ammonia and propane have the necessary mesovolatile vapor pressure. Surrounding gas in the proto-Solar System would facilitate redeposition of molecules sublimed by warmer parts of the surface into the cleft, as well as smoothing the surface and explaining, by hydrodynamic drag, Arrokoth’s slow (compared to its breakup rate) rotation. Alternatively, a layer of hoarfrost thick enough (≳ 0.1 μ) to have a high albedo could have formed more recently.

The Geophysical Environment of (486958) Arrokoth—A Small Kuiper Belt Object Explored by New Horizons

“…Thermal processes, including sublimation, may also play a role in shaping Arrokoth's geology (Grundy et al, 2020;Katz & Wang, 2021;Lisse et al, 2021;Spencer et al, 2020;Steckloff et al, 2021;Stern et al, 2019;Umurhan et al, 2022;Zhao et al, 2021). Since thermal processes also depend sensitively on shape and spin, there is some ambiguity between whether geophysical or thermophysical processes control Arrokoth's geology.…”