The Origin of Nitrogen on Jupiter and Saturn from the ${}^{15}$N/${}^{14}$N Ratio

Abstract

The Texas Echelon cross Echelle Spectrograph (TEXES), mounted on NASA's Infrared Telescope Facility (IRTF), was used to map mid-infrared ammonia absorption features on both Jupiter and Saturn in February 2013. Ammonia is the principle reservoir of nitrogen on the giant planets, and the ratio of isotopologues (${}^{15}$N/${}^{14}$N) can reveal insights into the molecular carrier (e.g., as N${}_2$ or NH${}_3$) of nitrogen to the forming protoplanets, and hence the source reservoirs from which these worlds accreted. We targeted two spectral intervals (900 and 960 cm${}^{-1}$) that were relatively clear of terrestrial atmospheric contamination and contained close features of ${}^{14}$NH${}_3$ and ${}^{15}$NH${}_3$, allowing us to derive the ratio from a single spectrum without ambiguity due to radiometric calibration (the primary source of uncertainty in this study). We present the first ground-based determination of Jupiter's ${}^{15}$N/${}^{14}$N ratio (in the range from $1.4\times 10^{-3}$ to $2.5\times 10^{-3}$), which is consistent with both previous space-based studies and with the primordial value of the protosolar nebula. On Saturn, we present the first upper limit on the ${}^{15}$N/${}^{14}$N ratio of no larger than $2.0\times 10^{-3}$ for the 900-cm${}^{-1}$ channel and a less stringent requirement that the ratio be no larger than $2.8\times 10^{-3}$ for the 960-cm${}^{-1}$ channel ($1\sigma$ confidence). Specifically, the data rule out strong ${}^{15}$N-enrichments such as those observed in Titan's atmosphere and in cometary nitrogen compounds. To the extent possible with ground-based radiometric uncertainties, the saturnian and jovian ${}^{15}$N/${}^{14}$N ratios appear indistinguishable, implying that ${}^{15}$N-enriched ammonia ices could not have been a substantial contributor to the bulk nitrogen inventory of either planet, favouring the accretion of primordial N${}_2$ from the gas phase or as low-temperature ices.

arXiv comment33 pages, 19 figures, manuscript accepted for publication in Icarus

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