  We present HST optical transmission spectra of the transiting hot Jupiter
WASP-12b, taken with the STIS instrument. From the transmission spectra, we are
able to decisively rule out prominent absorption by TiO in the exoplanet's
atmosphere. Strong pressure-broadened Na and K absorption signatures are also
excluded, as are significant metal-hydride features. We compare our combined
broadband spectrum to a wide variety of existing aerosol-free atmospheric
models, though none are satisfactory fits. However, we do find that the full
transmission spectrum can be described by models which include significant
opacity from aerosols: including Rayleigh scattering, Mie scattering, tholin
haze, and settling dust profiles. The transmission spectrum follows an
effective extinction cross section with a power-law of index alpha, with the
slope of the transmission spectrum constraining the quantity alphaT =
-3528+/-660 K, where T is the atmospheric temperature. Rayleigh scattering
(alpha=-4) is among the best fitting models, though requires low terminator
temperatures near 900 K. Sub-micron size aerosol particles can provide equally
good fits to the entire transmission spectrum for a wide range of temperatures,
and we explore corundum as a plausible dust aerosol. The presence of
atmospheric aerosols also helps to explain the modestly bright albedo implied
by Spitzer observations, as well as the near black body nature of the emission
spectrum. Ti-bearing condensates on the cooler night-side is the most natural
explanation for the overall lack of TiO signatures in WASP-12b, indicating the
day/night cold-trap is an important effect for very hot Jupiters. These finding
indicate that aerosols can play a significant atmospheric role for the entire
wide range of hot-Jupiter atmospheres, potentially affecting their overall
spectrum and energy balance.(abridged)
