Arrokoth's necklace

Fly-by 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 vapour sublimed by dirty, low albedo, ice elsewhere; ice accumulates where the maximum temperatures are the lowest. Ammonia and propane have the necessary mesovolatile vapour 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 break-up rate) rotation. Alternatively, a layer of hoarfrost thick enough (≳ 0.1, μ) to have a high albedo could have formed more recently.