Ices in KBO MU69 and Pluto — Implications for Their Formation & Evolution

The New Horizons (NH) mission flyby of 14 July 2015 verified the presence of an extensive surface ice sheet consisting of CO + N₂ ice in Sputnik Planitia, and a near-global covering of layered and structured CH₄ ice around the planet. Assuming Pluto was aggregated out of billions of icy planetesimals, the prominence of large amounts of N₂ ice is in tension with its ~0.2% vs water abundance found in inner system comets. A similar tension results from the ~1.0 % CH₄ vs water in comets. Using the results of the 01 Jan 2019 NH flyby of KBO MU69, we infer new constraints on the icy makeup of the smaller KBOs, which differ substantially from the icy makeup of inner system comets in having abundant amorphous hydrogen-bonded ices like H₂O, CH₃OH, and (maybe) HCN/H₂CO. Here we use this new information and new modeling of the thermodynamic properties of MU69's ices to argue that due to the action of solar insolation, short-lived radioactive isotope decay, micrometeorite bombardment, galactic cosmic rays, passing O/B stars, and nearby supernovae, hypervolatile ices like N₂, CO, and CH₄ exist today in small icy solar system bodies only as minority species in water ice phases. Only refractory hydrogen-bonded ices should remain after 4.56 Gyrs. Any pure hypervolatile ices that originally condensed "in the dark", while the solar system's midplane was optically thick, were lost within 1 Myr of the time of disk clearing. This implies that Pluto either formed very fast, before the time of disk clearing + 1 Myr, or is completely melted and differentiated through and through, allowing the release of all its minority hypervolatiles from trapped water ice phases and their rise to its surface and atmosphere.