Radiometry of the atmosphere of Mars
This thesis is concerned with a project to apply the method of pressure modulation radiometry to measurements of the temperature and water vapour distributions in the atmosphere of Mars. The technique has already been used successfully to measure temperature (and occasionally composition) in the atmospheres of Earth and Venus. Two pressure modulators, containing carbon dioxide and water vapour respectively, are used in the Pressure Modulator Infrared Radiometer (PMIRR), on the payload of Mars Observer, scheduled for launch in September 1992. The composition of the Martian atmosphere is almost pure CO2, so the emission lines are unusually broad, so that the mean pressure in the CO2 modulator has to be much larger than for previous modulators, thus increasing the power consumption. The limited power available in a Martian spacecraft requires that the power consumption of the pressure modulator and drive be minimized. As a result a highefficiency drive circuit was designed and developed. Water vapour is of major importance to the Martian environment and there are several outstanding questions about its behaviour. Measurements using the pressure modulator technique for terrestrial water vapour measurements have had limited success, raising questions about its suitability for condensible, affinitive molecules. A new model of the thermodynamics of pressure modulators was developed to predict their mechanical and spectroscopic behaviour, and spectroscopic measurements of the pressure modulator cell transmission, with high spectral and temporal resolution, using a tunable diode laser spectrometer (TDLS), were performed. The measured transmission agrees well with the predictions of the model, indicating that pressure modulation radiometry can be applied to measurements of water vapour, giving confidence in the successful study of Martian climatology by the PMIRR instrument.