Constraints on Uranus's haze structure, formation and transport

Microphysical simulations have been performed to constrain the formation and structure of haze in Uranus's atmosphere. These simulations were coupled to a radiative-transfer code to fit observations performed by the SINFONI Integral Field Unit Spectrometer on the Very Large Telescope (VLT) and by the Wide Field Camera 3 (WFC3) of the Hubble Space Telescope (HST) in 2014. Our simulations yield an effective radius of similar to 0.2 mu m for the haze particles in the tropopause and a density of similar to 2.9 particles per cm(3). Our simulations also provide an estimate for the haze production rate in the stratosphere of between similar to 3.10(-16) and 3.10(-15) kg m(-2 )s(-1), about 100 times smaller than that found in Titan's atmosphere (e.g. Rannou et al., 2004). This range of values is very similar to that derived by Pollack et al. (1987) from Voyager-2 observations in 1986, suggesting microphysical timescales greater than the elapsed time between these observations (28 years, or 1/3 of a Uranian year). This result is in agreement with analyses performed with our microphysical model that show timescales for haze particles to grow and settle out to be > similar to 30 years at pressure levels > 0.1 bar. However, these timescales are too big to explain the observed variations in the haze structure over Uranus's northern hemisphere after 2007 equinox (e.g. de Pater et al., 2015). This indicates that dynamics may be the main factor controlling the spatial and temporal distribution of the haze over the poles. A meridional stratospheric transport of haze particles with winds velocities > similar to 0.025 m s(-1) would result in dynamics timescales shorter than 30 years and thus may explain the observed variations in the haze structure.