Nature of the stratospheric haze on Uranus: Evidence for condensed hydrocarbons

We have used a number of models to analyze Voyager images of Uranus obtained at several high phase angles to derive physical and chemical properties of particulate matter present in the planet's lower stratosphere. These models include a multiple-scattering algorithm for plane parallel atmospheres, a spherical atmosphere code for performing limb inversions, a microphysical model of aerosol formation, growth, and sedimentation, and a photochemical model of methane photolysis. We obtain definitive evidence for the presence of aerosols at pressure levels ranging from a few millibars to about 100 mbar. There are two possible sets of particle properties that can fit radiances observed close to but somewhat interior to the limb at several phase angles in four visible wavelength bands. The low-density solution is characterized by particles having modal radius and number density equal to 0.13+/-0.02 μm and 2+/-1 particles/cm³, respectively, at a pressure level of 44 mbar. The alternative, high-density solution is characterized by particles having a modal radius that is 0.6-0.7 times that of the low-density solution at the reference level and a density that is 2 orders of magnitude larger. Since the high-density solution implies a mass production rate for the stratospheric aerosols that is much larger than those that can plausibly be supplied by photochemically produced gases that condense, whereas the low-density solution does not, we favor the low-density solution. Inversion of narrow-angle, high-resolution images of the limb provides a definition of a variable that provides a measure of the amount of the aerosol scattering at high phase angles. The vertical profile of this variable shows a decrease of several orders of magnitude from pressure levels of tens to a few millibars.

This decrease is due chiefly to the particle size of the aerosols becoming small compared to a wavelength. Above the base of the stratosphere the aerosol optical depth is approximately 0.01 in the mid-visible. A major source for the stratospheric aerosols is the condensation of ethane, acetylene, and diacetylene gas species at pressure levels of approximately 14, 2.5, and 0.1 mbar, respectively. These gases are produced at much higher altitudes by solar UV photolysis of methane and diffuse to the lower stratosphere, where they condense. In addition, diacetylene is also produced photochemically within its condensation region. Condensation of locally produced diacetylene may represent a significant fraction of the total hydrocarbon condensation. Such a local source of condensation may be required by the inversions to the limb profiles, which indicate that at least half of the ice condensation occurs at altitudes above the 5-mbar level. The hypothesis that the stratospheric particles are made of hydrocarbon ices is supported by the approximate agreement between the total ice condensation rate predicted by the methane photochemical model and the aerosol mass production rate derived for the low-density solution of the Voyager data.

The aerosol mass production rate derived from the Voyager data is equal to 2-15×10^-17 g/cm²/s. Additional but weaker support for this hypothesis is provided by the Voyager radio occultation temperature profiles. It is suggested that solar UV radiation promotes solid state chemistry within the lower order hydrocarbon ices, resulting in the production of polymers capable of absorbing at visible wavelengths. Thus this altered material could play a key role in the planet's heat budget. Ethane, acetylene, and diacetylene ices evaporate at approximaely the 600-, 900-, and 3000-mbar levels of the upper troposphere. The polymeric material is expected to evaporate at pressures in excess of the evaporation level for diacetylene.