Ortho-to-Para Ratios of Water and Ammonia in Comet C/2001 Q4 (NEAT): Comparison of Nuclear Spin Temperatures of Water, Ammonia, and Methane

Cold nuclear spin temperatures found in cometary molecules have been simply interpreted as the physical temperature when the population distribution among different nuclear spin isomers was determined through thermal equilibrium processes. However, the real meaning of cold nuclear spin temperatures is unclear due to a lack of experimental studies about the ortho-to-para ratios (OPRs) of molecules in cometary ice analogs. Here we report the nuclear spin temperatures (T_spin) of water and ammonia in comet C/2001 Q4 (NEAT). Measurements of the nuclear spin temperatures of these species and methane from previous work are all consistent with ~30 K. Consistency of the nuclear spin temperatures among different molecular species may suggest that OPRs (or abundance ratios of different nuclear spin isomers) of these molecules were last determined in thermal equilibrium. The obtained nuclear spin temperature of cometary ices is not consistent with molecular formation by hydrogen-atom addition reactions on cold grains, where the H atoms accreted from the gas phase onto grains. The condensation process on the grains might control the ortho-to-para ratios of the precometary ices, or conversion of OPRs within the ices might occur. The small diversity of the nuclear spin temperatures and lack of clear correlation between T_spin and chemical composition in several comets are consistent with the hypothesis that T_spin reflects the temperatures in the presolar nebula.