  The ultra-faint dwarf galaxy Reticulum II was enriched by a rare and prolific
r-process event, such as a neutron star merger. To investigate the nature of
this event, we present high-resolution Magellan/MIKE spectroscopy of the
brightest star in this galaxy. The high signal-to-noise allows us to determine
the abundances of 41 elements, including the radioactive actinide element Th
and first ever detections of third r-process peak elements (Os and Ir) in a
star outside the Milky Way. The observed neutron-capture element abundances
closely match the solar r-process component, except for the first r-process
peak which is significantly lower than solar but matches other r-process
enhanced stars. The ratio of first peak to heavier r-process elements implies
the r-process site produces roughly equal masses of high and low electron
fraction ejecta, within a factor of 2. We compare the detailed abundance
pattern to predictions from nucleosynthesis calculations of neutron star
mergers and magneto-rotationally driven jet supernovae, finding that nuclear
physics uncertainties dominate over astrophysical uncertainties. We measure
$\log\mbox{Th/Eu} = -0.84 \pm 0.06\,\text{(stat)} \pm 0.22\,\text{(sys)}$,
somewhat lower than all previous Th/Eu observations. The youngest age we derive
from this ratio is $21.7 \pm 2.8\,\text{(stat)} \pm 10.3\,\text{(sys)}$ Gyr,
indicating that current initial production ratios do not well describe the
r-process event in Reticulum II. The abundance of light elements up to Zn are
consistent with extremely metal-poor Milky Way halo stars. They may eventually
provide a way to distinguish between neutron star mergers and
magneto-rotationally driven jet supernovae, but this would require more
detailed knowledge of the chemical evolution of Reticulum II.
