Detection of an NH$_3$ absorption band at 2.2 $\mu$m on Europa

Abstract

The presence of NH$_3$-bearing components on icy planetary bodies has important implications for their geology and potential habitability. Here, I report the detection of a characteristic NH$_3$ absorption feature at 2.20 $\pm$ 0.02 $\mu$m on Europa, identified in an observation from the Galileo Near Infrared Mapping Spectrometer. Spectral modeling and band position indicate that NH$_3$-hydrate and NH$_4$-chloride are the most plausible candidates. Spatial correlation between detected ammonia signatures and Europa's microchaos, linear, and band geologic units suggests emplacement from the underground or shallow subsurface. I posit that NH$_3$-bearing materials were transported to the surface via effusive cryovolcanism or similar mechanisms during Europa's recent geological past. The presence of ammoniated compounds implies a thinner ice shell (Spohn & Schubert, 2003) and a thicker, chemically reduced, high-pH subsurface ocean on Europa (Hand et al. 2009). With the detection of NH$_3$-bearing components, this study presents the first evidence of a nitrogen-bearing species on Europa -- an observation of astrobiological significance given nitrogen's essential role in the chemistry of life.