  We present a primary transit observation for the ultra hot (Teq~2400K) gas
giant expolanet WASP-121b, made using the Hubble Space Telescope Wide Field
Camera 3 in spectroscopic mode across the 1.12-1.64 micron wavelength range.
The 1.4 micron water absorption band is detected at high confidence (5.4 sigma)
in the planetary atmosphere. We also reanalyze ground-based photometric
lightcurves taken in the B, r', and z' filters. Significantly deeper transits
are measured in these optical bandpasses relative to the near-infrared
wavelengths. We conclude that scattering by high-altitude haze alone is
unlikely to account for this difference, and instead interpret it as evidence
for titanium oxide and vanadium oxide absorption. Enhanced opacity is also
inferred across the 1.12-1.3 micron wavelength range, possibly due to iron
hydride absorption. If confirmed, WASP-121b will be the first exoplanet with
titanium oxide, vanadium oxide, and iron hydride detected in transmission. The
latter are important species in M/L dwarfs, and their presence is likely to
have a significant effect on the overall physics and chemistry of the
atmosphere, including the production of a strong thermal inversion.
