  Noble gases provide tracers of cosmic provenance that are accessible to a
future Uranus Atmospheric Probe. Argon and krypton are expected to be
well-mixed on Uranus with respect to H$_2$ and He, although condensation at the
winter pole may be possible. The Ar/H$_2$ and Ar/Kr ratios address whether the
materials accreted by Uranus resembled the extremely cold materials accreted by
Jupiter's atmosphere, or whether they were warmer like comet
67P/Churyumov-Gerasimenko, or whether Uranus is like neither. Xenon condenses
as an ice, probably on methane ice, in Uranus's upper troposphere. Condensation
may complicate interpretation of Xe/H$_2$, but it also presents an opportunity
to collect concentrated xenon samples suitable for measuring isotopes. Solar
System Xe tracks three distinct nucleosynthetic xenon reservoirs, one evident
in the Sun and in chondritic meteorites, a second in refractory presolar
grains, and a third evident in comet 67P/C-G and in Earth's air. The first and
third reservoirs appear to have been captured from different clouds of gas. The
two gases do not appear to have been well-mixed; moreover, the high
$^{129}$Xe/$^{132}$Xe ratio in 67P/C-G implies that the gas was captured before
the initial nucleosynthetic complement of $^{129}$I (15.7 Myr half-life) had
decayed. Xenon's isotopic peculiarities, if seen in Uranus, could usefully
upset our understanding of planetary origins. Krypton's isotopic anomalies are
more subtle and may prove hard to measure. There is a slight chance that neon
and helium fractionations can be used to constrain how Uranus acquired its
nebular envelope.
