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 (15N/14N) can reveal insights into the molecular
carrier (e.g., as N2​ or NH3​) 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
14NH3​ and 15NH3​, 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 15N/14N ratio (in the range from 1.4×10−3 to
2.5×10−3), 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 15N/14N ratio of no larger than
2.0×10−3 for the 900-cm−1 channel and a less stringent
requirement that the ratio be no larger than 2.8×10−3 for the
960-cm−1 channel (1σ confidence). Specifically, the data rule out
strong 15N-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 15N/14N ratios
appear indistinguishable, implying that 15N-enriched ammonia ices could
not have been a substantial contributor to the bulk nitrogen inventory of
either planet, favouring the accretion of primordial N2​ from the gas phase
or as low-temperature ices.Comment: 33 pages, 19 figures, manuscript accepted for publication in Icaru