  Material ejected during (or immediately following) the merger of two neutron
stars may assemble into heavy elements by the r-process. The subsequent
radioactive decay of the nuclei can power electromagnetic emission similar to,
but significantly dimmer than, an ordinary supernova. Identifying such events
is an important goal of future transient surveys, offering new perspectives on
the origin of r-process nuclei and the astrophysical sources of gravitational
waves. Predictions of the transient light curves and spectra, however, have
suffered from the uncertain optical properties of heavy ions. Here we consider
the opacity of expanding r-process material and argue that it is dominated by
line transitions from those ions with the most complex valence electron
structure, namely the lanthanides. For a few representative ions, we run atomic
structure models to calculate radiative data for tens of millions of lines. We
find that the resulting r-process opacities are orders of magnitude larger than
that of ordinary (e.g., iron-rich) supernova ejecta. Radiative transport
calculations using these new opacities indicate that the transient emission
should be dimmer and redder than previously thought. The spectra appear
pseudo-blackbody, with broad absorption features, and peak in the infrared (~1
micron). We discuss uncertainties in the opacities and attempt to quantify
their impact on the spectral predictions. The results have important
implications for observational strategies to find and study the radioactively
powered electromagnetic counterparts to compact object mergers.
