On the origin & thermal stability of Arrokoth's and Pluto's ices

In this paper we discuss in a thermodynamic, geologically empirical way the long-term nature of the stable majority ices that could be present in Kuiper Belt object (KBO) 2014 MU₆₉ (also called Arrokoth; hereafter "MU₆₉") after its 4.6 Gyr residence in the Edgeworth-Kuiper belt (EKB) as a cold classical object. We compare the upper bounds for the gas production rate (~10²⁴ molecules/s) measured by the New Horizons (NH) spacecraft flyby on 01 Jan 2019 to estimates for the outgassing flux rates from a suite of common cometary and KBO ices at the average ~ 40 K sunlit surface temperature of MU₆₉, but do not find the upper limit very constraining except for the most volatile of species (e.g. CO, N₂, CH₄). More constraining is the stability versus sublimation into vacuum requirement over Myr to Gyr, and from this we find only 3 common ices that are truly refractory: HCN, CH₃OH, and H₂O (in order of increasing stability), while NH₃ and H₂CO ices are marginally stable and may be removed by any positive temperature excursions in the EKB, as produced every 10⁸-10⁹ years by nearby supernovae and passing O/B stars. To date the NH team has reported the presence of abundant CH₃OH and H₂O on MU₆₉'s surface (Stern et al., 2019; Grundy et al., 2020). NH₃ has been searched for, but not found. We predict that future absorption feature detections, if any are ever derived from higher signal-to-noise ratio spectra, will be due to an HCN or poly-H₂CO based species. Consideration of the conditions present in the EKB region during the formation era of MU₆₉ lead us to state that it is highly likely that it "formed in the dark", in an optically thick mid-plane, unable to see the nascent, variable, highly luminous Young Stellar Object (YSO)/TTauri Sun, and that KBOs contain HCN and CH₃OH ice phases in addition to the H₂O ice phases found in their short period (SP) comet descendants. Finally, when we apply our ice thermal stability analysis to bodies/populations related to MU₆₉, we find that methanol ice is likely ubiquitous in the outer solar system; that if Pluto isn't a fully differentiated body, then it must have gained its hypervolatile ices from proto-planetary disk (PPD) sources in the first few Myr of the solar system's existence; and that hypervolatile rich, highly primordial comet C/2016 R2 was placed onto an Oort Cloud orbit on a similar few Myr timescale.