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We present HST optical transmission spectra of the transiting hot JupiterWASP-12b, taken with the STIS instrument. From the transmission spectra, we areable to decisively rule out prominent absorption by TiO in the exoplanet'satmosphere. Strong pressure-broadened Na and K absorption signatures are alsoexcluded, as are significant metal-hydride features. We compare our combinedbroadband spectrum to a wide variety of existing aerosol-free atmosphericmodels, though none are satisfactory fits. However, we do find that the fulltransmission spectrum can be described by models which include significantopacity from aerosols: including Rayleigh scattering, Mie scattering, tholinhaze, and settling dust profiles. The transmission spectrum follows aneffective extinction cross section with a power-law of index alpha, with theslope 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 terminatortemperatures near 900 K. Sub-micron size aerosol particles can provide equallygood fits to the entire transmission spectrum for a wide range of temperatures,and we explore corundum as a plausible dust aerosol. The presence ofatmospheric aerosols also helps to explain the modestly bright albedo impliedby Spitzer observations, as well as the near black body nature of the emissionspectrum. Ti-bearing condensates on the cooler night-side is the most naturalexplanation for the overall lack of TiO signatures in WASP-12b, indicating theday/night cold-trap is an important effect for very hot Jupiters. These findingindicate that aerosols can play a significant atmospheric role for the entirewide range of hot-Jupiter atmospheres, potentially affecting their overallspectrum and energy balance.(abridged)

HST hot Jupiter transmission spectral survey: evidence for aerosols and lack of TiO in the atmosphere of WASP-12b