  We present near-UV transmission spectroscopy of the highly irradiated
transiting exoplanet WASP-12b, obtained with the Cosmic Origins Spectrograph on
the Hubble Space Telescope. The spectra cover three distinct wavelength ranges:
NUVA (2539-2580 {\AA}), NUVB (2655-2696 {\AA}), and NUVC (2770-2811 {\AA}).
Three independent methods all reveal enhanced transit depths attributable to
absorption by resonance lines of metals in the exosphere of WASP-12b. Light
curves of total counts in the NUVA and NUVC wavelength ranges show a detection
at a 2.5{\sigma} level. We detect extra absorption in the Mg II
{\lambda}{\lambda}2800 resonance line cores at the 2.8{\sigma} level. The NUVA,
NUVB, and NUVC light curves imply effective radii of 2.69+/-0.24 RJ,
2.18+/-0.18 RJ, and 2.66+/-0.22 RJ respectively, suggesting the planet is
surrounded by an absorbing cloud which overfills the Roche lobe. We detect
enhanced transit depths at the wavelengths of resonance lines of neutral
sodium, tin, and manganese, and at singly ionized ytterbium, scandium,
manganese, aluminum, vanadium, and magnesium. We also find the statistically
expected number of anomalous transit depths at wavelengths not associated with
any known resonance line. Our data are limited by photon noise, but taken as a
whole the results are strong evidence for an extended absorbing exosphere
surrounding the planet. The NUVA data exhibit an early ingress, contrary to
model expectations; we speculate this could be due to the presence of a disk of
previously stripped material.
