  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 $1.4\times10^{-3}$ to
$2.5\times10^{-3}$), 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
$2.0\times10^{-3}$ for the 900-cm$^{-1}$ channel and a less stringent
requirement that the ratio be no larger than $2.8\times10^{-3}$ 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.
