Retention of CO Ice and Gas Within 486958 Arrokoth

TL;DR

This paper presents a theoretical model showing that small, cold Kuiper Belt Objects like Arrokoth can retain their primordial CO ice over billions of years, aligning with recent observational constraints.

Contribution

It introduces an analytical framework assuming extreme cold conditions to assess volatile retention, providing a computationally efficient alternative to simulations.

Findings

Arrokoth can retain its original CO ice for billions of years.

The sublimation process creates a vapor-pressure equilibrium limiting CO loss.

Predicted CO expulsion rates agree with New Horizons observations.

Abstract

Kuiper Belt Objects (KBOs) represent some of the most ancient remnants of our solar system, having evaded significant thermal or evolutionary processing. This makes them important targets for exploration as they offer a unique opportunity to scrutinize materials that are remnants of the epoch of planet formation. Moreover, with recent and upcoming observations of KBOs, there is a growing interest in understanding the extent to which these objects can preserve their most primitive, hypervolatile ices. Here, we present a theoretical framework that revisits this issue for small, cold classical KBOs like Arrokoth. Our analytical approach is consistent with prior studies but assumes an extreme cold end-member thermophysical regime for Arrokoth, enabling us to capture the essential physics without computationally expensive simulations. Under reasonable assumptions for interior temperatures, thermal conductivities, and permeabilities, we demonstrate that Arrokoth can retain its original CO stock for Gyrs if it was assembled long after the decay of radionuclides. The sublimation of CO ice generates an effective CO `atmosphere' within Arrokoth's porous matrix, which remains in near vapor-pressure equilibrium with the ice layer just below, thereby limiting CO loss. According to our findings, Arrokoth expels no more than $\approx 10^{22}$ particles s$^{-1}$, in agreement with upper limits inferred from \textit{New Horizons}' 2019 flyby observations. While our framework challenges recent predictions, it can serve as a benchmark for existing numerical models and be applied to future KBO observations from next-generation telescopes.