Planetary surfaces

Abstract:

Venus as a planet resembles the Earth, but has a much hotter and denser atmosphere due to an extreme case of the greenhouse effect, caused by compositional differences and tile thick cloud cover. Studies of the lower atmosphere are inhibited by the cloud opacity, which makes remote measurements at most frequencies short. of the radio range quite difficult. Progress in understanding of the com position and thermal structure below the clouds has been made by the Pioneer and Venera entry probes of the 1970s, and more recently with results from the Galileo fly-by in 1990. The latter exploited the newly discovered near-infrared 'windows' to achieve measurements of carbon monoxide and water vapour abundances in the deep atmosphere, and provided the first detailed view of the global cloud structure. The morphology and spatial variations seen in the main mass of clouds are remarkable, and suggest a powerful and diverse meteorology dominated by convection. Carbon monoxide is significantly more abundant at high northern latitudes than at low latitudes in either hemisphere.

Abstract:

A radiometric experiment was conducted to investigate the thermodynamic behaviour of the H2O and CO2 pressure modulated cells of the Pressure Modulator Infrared Radiometer (PMIRR) which flew on "Mars Observer" in 1992. The wave forms and phases of the transmission, emission, pressure and piston position cycles, together with the mean absorption were, for the first time, all measured during the same experiment. The measured transmission and emission cycles were used to calculate the phase and amplitude of the temperature cycles and were found to be super-adiabatic for both cells, in reasonable agreement with a thermal diffusion model. Anomalies were seen in the H2O modulator driving frequency, as has been observed by other authors, but none were evident in the measured cell wave forms suggesting that the water vapour behaves as an ideal gas in the cell. © 1994.

Abstract:

We describe the design, the development, and the calibration of a radiometer to measure water vapor abundance in the middle atmosphere, using the solar occultation technique from an orbiting platform. The use of gas cells containing water vapor and carbon dioxide leads to a relatively simple, but sensitive, design. This radiometer is shown to be capable of detecting water vapor in the mesosphere, where, because of dissociation by the Sun, the amounts are very small, of the order of 1 part per 10(6) or less. A flight version of the instrument will be launched in 1992 on board the European Retrievable Carrier space platform, and the data will be employed for a better understanding of the Earth's water budget.