Direct Simulation Monte Carlo Modeling of Ammonia in Comet C/2014 Q2 (Lovejoy)

Abstract

Ammonia (NH3), likely the most abundant nitrogen-bearing molecule in cometary ices followed by hydrogen cyanide, is believed to be stored in the nucleus predominantly as a parent ice. However, spatial profiles of NH3 observed in comet C/2014 Q2 (Lovejoy) in the near-infrared region are consistent with a distributed source contribution (Dello Russo et al. 2022). We developed the direct simulation Monte Carlo model of ammonia in cometary coma and applied it to comet C/2014 Q2 (Lovejoy). Results suggest that NH3 molecules in the coma of C/2014 Q2 (Lovejoy) can plausibly originate from a combination of parent molecules of NH3 in the coma and a NH3 nucleus source. We demonstrate that the parents of NH3 having photodissociation lifetimes of several hundreds of seconds or longer (at 1 au from the Sun) can explain the observed spatial profile of NH3 in comet C/2014 Q2 (Lovejoy). Even though ammonia salts are possible candidates for parents of NH3, some simple ammonium salts such as NH4CN or NH4Cl may dissociate thermally within very short lifetimes after sublimation from the nucleus, so the contribution from those ammonium salts may be indistinguishable from the nucleus source. The lack of experimental data on photoprocesses for potential NH3 parent molecules prevent us from identifying the origin of NH3 in comets. Experimental and theoretical studies of photodissociation/ionization reactions of potential NH3 parent molecules by the solar UV radiation field are encouraged for the future identification of NH3 parents in comets.