Jordan Pollack

Probing Venus's cloud structure with Galileo NIMS

The spectral image cubes obtained by the Near-Infrared Mapping Spectrometer (NIMS) on Galileo as it flew by Venus have been analyzed to constrain the vertical structure of the clouds, the nature of the aerosol particles, and the location and particle properties of the opacity variations responsible for high contrast features observed in the near-infrared windows at 1.7 and 2.3 μm. A radiative transfer program was used to simulate mid-latitude curves of limb darkening at 3.7 μm. Best-fit models to these curves demonstrate that the upper clouds are dominated by mode 2 particles (r = 1.0 μm), with a contribution of ≈15% of opacity from mode 1 particles (r = 0.3 μm). The low-latitude upper cloud is well represented by a dual scale-height model, with a particle scale height of ≈1 km from an altitude of 61–63 km, and a scale height of ≈ 6 km above this, up to the level where τ = 1 at approximately 71 km. This model also successfully simulates limb-darkening curves at 11.5 μm from the Pioneer Venus Orbiter Infrared Radiometer. Successful simulations of correlation plots of 1.7 vs 2.3 μm intensities reveal that mode 3 particles (r = 3.65 μm) represent the dominant source of opacity in the lower and middle clouds, and that variation in total cloud opacity reflects chiefly the addition and removal of mode 3 particles near the cloud base. We find that the full spectrum of brightnesses at 1.7 and 2.3 μm implies that the total cloud optical depth varies from ≈ 25 to ≈ 40.

WATER IN THE DEEP ATMOSPHERE OF VENUS FROM HIGH-RESOLUTION SPECTRA OF THE NIGHT SIDE

High-resolution, near-infrared (1.09 to 2.5 μm) spectra of the night side of Venus have been obtained in 1990 and 1991 using the Fourier Transform Spectrometer at the 3.6-m Canada-France-Hawaii telescope. Absorptions due to H2O were detected in spectral windows near 2.3, 1.74, and 1.18 μm. Our analysis of these absorptions constrains the abundance of water vapor in three different altitude ranges located between the clouds and the surface: 30–40 km, 15–25 km and 0–15 km. A constant water vapor mixing ratio of 30±15 ppm below the clouds can fit the observations. These values are consistent with recent near-infrared studies of the night side of Venus at lower spectral resolution. The atmosphere of Venus appears to be dryer than originally suggested by the in-situ measurements made by the Pioneer Venus and Venera mass-spectrometers and gas-chromatographs.

VARIATIONS IN VENUS CLOUD-PARTICLE PROPERTIES: A NEW VIEW OF VENUS'S CLOUD MORPHOLOGY AS OBSERVED BY THE GALILEO NEAR INFRARED MAPPING SPECTROMETER

Using Venus nightside data obtained by the Galileo Near-Infrared Mapping Spectrometer, we have studied the correlation of 1.74 and 2.30 μm radiation which is transmitted through the clouds. Since the scattering and absorption properties of the cloud particles are different at these two wavelengths, one can distinguish between abundance variations and variations in the properties of the cloud particles themselves. The correlation of intensities shows a clustering of data into five distinct branches. Using radiative transfer calculations, we interpret these branches as regions of distinct but different mixes of Mode 2′ and 3 particles. The data and calculations indicate large differences in these modal ratios, the active cloud regions varying in content from nearly pure Mode 2′ particles to almost wholly Mode 3. The spatial distribution of these branches shows large scale sizes and both hemispheric symmetries and asymmetries. High-latitude concentrations of large particles are seen in both hemispheres and there is banded structure of small particles seen in both the North and South which may be related. The mean particle size in the Northern Hemisphere is greater than found in the South. If these different branch regions are due to mixing of vertically stratified source regions (e.g. photochemical and condensation source mechanisms), then the mixing must be coherent over very large spatial scales.

Search for spatial variations of the H2O abundance in the lower atmosphere of Venus from NIMS-Galileo

Abstract The spectroscopic data of the Near-Infrared Mapping Spectrometer (NIMS), recorded during the Galileo flyby of Venus, are analysed to retrieve the water vapour abundance variations in the lower atmosphere of Venus at night. The 1.18 μm spectral window, which probes altitude levels below 20 km, is used for this purpose. Constraints on the CO2 continuum and far-wing opacity from existing ground-based high-resolution observations are included in the modelling of the NIMS spectra. The NIMS measurements can be fitted with a water vapour mixing ratio of 30 ± 15 ppm, in agreement with analyses of ground-based nightside observations. The water vapour abundance shows no horizontal variations exceeding 20% over a wide latitude range (40°S, 50°N) on the nightside of Venus. Within the same selection of NIMS spectra, a large enhancement in the O2 fluorescence emission at 1.27 μm is observed at a latitude of 40°S, over a spatial area about 100 km wide.

The upper clouds of Venus: determination of the scale height from NIMS-Galileo infrared data

Abstract The 3–5 μm thermal emission of the nightside of Venus, recorded by the NIMS instrument at the time of the Galileo flyby of Venus, is analysed to infer the properties of the upper cloud boundary. From the global maps of Venus at fixed wavelengths, the limb darkening of the flux is measured at several latitudes, within each infrared channel. By using the nominal Pioneer Venus thermal profile, these data give access to two parameters: the cloud deck temperature and the cloud scale height. It is verified independently, from the NIMS spectra, that this thermal profile is consistent with all the NIMS observations, and that the thermal structure does not vary significantly in the latitude range (25°S, 30°N). Within this range, the cloud scale height is found to be constant with latitude, and is H = 5.2 km, with an accuracy of about 15%, taking into account the various sources of theoretical and observational uncertainties. At higher latitudes, the temperature profile becomes more isothermal and the presented method to retrieve H is no longer valid.