Surface Irradiation of Cold Classical Kuiper Belt Objects

The recent flyby of Arrokoth (2014 MU69) by the New Horizons spacecraft provided our first close view of a so-called cold classical Kuiper Belt Object (CCKBO) among those with near circular orbits at 40 47 AU and low inclinations < 5 degrees. Classical means that such objects of moderate inclination are not in resonance with Neptune and are remnants of the original KBO population, while cold refers to a history of minimal dynamical interactions and collisions with other objects. All CCKBOs have red colors as compared to increasingly bluer colors of bodies with larger inclinations > 12 degrees. The red colors may have arisen from differences in primordial composition and from subsequent space weathering of otherwise-undisturbed astrophysical ices over billions of years, while bluer colors are suggestive of resurfacing with less processed ices by collisional or cryovolcanic excavation of deeper layers. Detection of methanol and other unidentified complex hydrocarbon ices on Arrokoth may be indicative of hydrocarbon irradiation in water ice. We envisage a CCKBO surface as originally composed of three distinct layers: (1) highly irradiated crust at nanometer to sub-millimeter depth, (2) moderately irradiated mantle ice at millimeter to meter depth, and (3) unprocessed primordial mantle ice at greater depth. Solar wind plasma sputtering and micrometeoroid impacts from KBO dust (M. Horanyi, private communication, 2021), erode away the crust layer at millimeters per billion years, while reddish materials (e.g., tholins) accumulate in the increasingly exposed radiation mantle. Red color is indicative that the crust is eroded way faster than it is created, and that there has been minimal resurfacing from the primordial mantle. We compile surface irradiation spectra from Pioneer, Voyager, and New Horizons energetic particle data in the NASA Virtual Energetic Particle Observatory (VEPO), now an element of the OMNIWeb services for the NASA Space Physics Data Facility, to model irradiation rates in the three layers for the CCKBO region of the heliosphere. Application of this model to Centaurs and to hotter KBOs with red colors follows from assumption of minimal resurfacing by impacts and cryovolcanism among redder subgroups of those populations.