  We explore the feasibility of using the atmosphere of Jupiter to detect
Ultra-High-Energy Cosmic Rays (UHECR's). The large surface area of Jupiter
allows us to probe cosmic rays of higher energies than previously accessible.
Cosmic ray extensive air showers in Jupiter's atmosphere could in principle be
detected by the Large Area Telescope (LAT) on the Fermi observatory. In order
to be observed, these air showers would need to be oriented toward the Earth,
and would need to occur sufficiently high in the atmosphere that the gamma rays
can penetrate. We demonstrate that, under these assumptions, Jupiter provides
an effective cosmic ray "detector" area of $3.3 \times 10^7$ km$^2$. We predict
that Fermi-LAT should be able to detect events of energy $>10^{21}$ eV with
fluence $10^{-7}$ erg cm$^{-2}$ at a rate of about one per month. The observed
number of air showers may provide an indirect measure of the flux of cosmic
rays $\gtrsim 10^{20}$ eV. Extensive air showers also produce a synchrotron
signature that may be measurable by ALMA. Simultaneous observations of Jupiter
with ALMA and Fermi-LAT could be used to provide broad constraints on the
energies of the initiating cosmic rays.
