Aerosol properties in the upper haze of Venus from SPICAV IR data

SPICAV IR, a channel of the SPICAV/SOIR suite of instruments onboard Venus Express spacecraft measured spectra in nadir and solar occultation modes in the range of 0.65-1.7 μm. We report results from 222 solar occultations observed from May 2006 to November 2014. The vertical resolution of measurements varies from 1 to 25 km depending on the distance of the spacecraft to the limb of Venus. The vertical profiles of atmospheric extinction were obtained at 10 near-IR wavelengths in the altitude range from 70 to 95 km. This allowed us to derive microphysical properties of the mesospheric haze. The aerosol haze top is higher near the equator than near the pole. In the upper haze, the aerosol scale height is found to be 3.3 ± 0.7 km. Detached haze layers were detected at altitudes from 70 to 90 km. Particle size and number density profiles are retrieved from extinction coefficients using Mie scattering theory adopting H2SO4 refractive indices. Bimodal distribution of particles is consistent with data for some orbits with mean radius for mode 1 reff1 = 0.12 ± 0.03 μm and reff2 = 0.84 ± 0.16 μm for mode 2. Particle radii tend to cluster within occultation campaign and vary on the time scale of several months. The radius for the single mode case equals Reff = 0.54 ± 0.25 μm, and they are also 1.5-2 times smaller in the polar region (60°N-90°N) than in nonpolar regions (60°S-60°N). In bimodal case the number density profiles decrease smoothly for both modes, from ∼500 cm−3 at 75 km to ∼50 cm−3 at 90 km for mode 1, and from ∼1 cm−3 at 75 km to ∼0.1 cm−3 at 90 km for mode 2.