Constraints on Uranus's haze structure, formation and transport

Daniel Toledo,Patrick G.J Irwin,Pascal Rannou,Nicholas A Teanby,Amy A Simon,Michael H Wong,Glenn S Orton

doi:10.1016/j.icarus.2019.05.018

Abstract

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 ∼0.2 μm for the haze particles in the tropopause and a density of ∼2.9 particles per cm3. Our simulations also provide an estimate for the haze production rate in the stratosphere of between ∼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 >∼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 >∼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.

Highlights

Voyager-2 observations revealed the presence of hazes in the atmosphere of Uranus (Pollack et al, 1987; Rages et al, 1991)
Fall velocity (m/sec) winter hemisphere where they sink (Rannou et al, 2004). If these seasonal changes in the Uranus haze structure over the north pole are due to this possible meridional circulation, the timescales for stratospheric meridional transport should be smaller than those given for microphysics
From Eq (4) and the timescales shown in Fig. 8, we can compute for each altitude the equivalent wind velocity (V∗) that represents the wind velocity for which the timescale for haze meridional transport equals the microphysics timescale

Summary

Introduction

Voyager-2 observations revealed the presence of hazes in the atmosphere of Uranus (Pollack et al, 1987; Rages et al, 1991). In addition to condensing ices of acetylene, ethane, and diacetylene that were considered by all prior photochemical models (Orton et al, 2014b; Pollack et al, 1987), Rages et al (1991) discussed condensation of C6H2, while Moses et al (2005) discussed condensation of C3H8, C4H10, and C6H6. These species have condensation levels in the 0.1 to 20-mbar region

Methods

Results

Conclusion