  Few laboratory studies have investigated the vapor pressures of the volatiles
that may be present as ices in the outer solar system; even fewer studies have
investigated these species at the temperatures and pressures suitable to the
surfaces of icy bodies in the Saturnian and Uranian systems ($\lt$100 K,
$\lt10^{-9}$ bar). This study adds to the work of Grundy et al. (2024) in
extending the known equilibrium vapor pressures of outer solar system ices
through laboratory investigations at very low temperatures. Our experiments
with ammonia and oxygen ices provide new thermodynamic models for these
species' respective enthalpies of sublimation. We find that ammonia ice, and to
a lesser degree oxygen ice, are stable at higher temperatures than
extrapolations in previous literature have predicted. Our results show that
these ices should be retained over longer periods of time than previous
extrapolations would predict, and a greater amount of these solids is required
to support observation in exospheres of airless bodies in the outer solar
system.
