Implications of Evaporative Cooling by H2 for 1I/‘Oumuamua

Abstract

Abstract The first interstellar object observed in our solar system, 1I/‘Oumuamua, exhibited several peculiar properties, including extreme elongation and nongravitational acceleration. Bergner & Seligman (hereafter BS23) proposed that evaporation of trapped H2 created by cosmic rays can explain the nongravitational acceleration. However, their modeling of the thermal structure of 1I/‘Oumuamua ignored the crucial cooling effect of evaporating H2. By taking into account the cooling by H2 evaporation, we show that the surface temperature of H2-water ice is a factor of 9 lower than the case without evaporative cooling. As a result, the thermal speed of outgassing H2 is decreased by a factor of 3. Our one-dimensional thermal modeling that takes into account evaporative cooling for two chosen values of thermal conductivity of κ = 0.01 and 0.1 WK−1 m−1 shows that the water ice volume available for H2 sublimation at T > 30 K would be reduced by a factor of 9 and 5 compared to the results of BS23, not enabling enough hydrogen to propel 1I/‘Oumuamua.