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Title: Hunting for water with MCS
Author: Lolachi, Ramin
ISNI:       0000 0004 8503 0494
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2019
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Mars Climate Sounder (MCS) is an infrared radiometer aboard NASA's Mars Reconnaissance Orbiter (MRO) launched in August 2005 and designed to characterize the temporal and spatial variation of temperature, dust and water profiles. MCS has nine spectral channels in the range 0.3-50 μm. An original goal of the mission was retrieval of vertical water profiles. This was to be done using two spectrally co-located channels (centred at 42 μm)-a narrow channel (B3), which was mainly sensitive to dust, and a broader overlapping channel (B2) sensitive to both dust and water. This has been severely compromised by the fact that the B2 and B3 filters did not meet their specification, with a much wider than expected spectral response. This thesis is split into three parts attempting to assess the current ability of MCS to retrieve water vapour. In the first part, work was done assessing the ramifications on the water vapour sensitivity of the B3-B2 pair and the B2 channel alone through extensive simulation work. The B3-B2 pair were found to have a drastically reduced sensitivity to water vapour that was further reduced by the presence of dust and low temperatures. Radiance residuals of B2 from MCS observations showed that a water vapour signal is present in the channel. Simulations of MCS radiances and retrieval of synthetic observations revealed that the accuracy of water vapour retrievals is sensitive to the choice of aerosol particle size and that in the presence of moderate aerosol opacity (dust and water ice), aerosol radiance contribution in B2 is falsely misinterpreted as water vapour. In the second part, work was done measuring the spectral response of the B3 filter in order to understand the process behind the spectral broadening in the B-channels. It was thought that random forward scattering of out-of-bandpass radiation by the mesh filter may be the cause of the widening. Using a custom-made jig and FTIR spectrometer, measurements were taken at several filter-detector separation distances. No change in the B3 response FWHM was observed as the distance was varied. In the final part, a two-stage retrieval method was applied to MCS observations for MY28 northern hemisphere summer (Ls=111-173°). Nightside vertical profiles of water vapour were retrieved for the first time showing interaction with the aphelion cloud belt directly. Possible interaction between dust and water cycles was seen. Retrieval results compared favourably with observations by others for both column abundances and vertical profiles. Of particular note was good agreement with results from Clancy et al. (2017) using vertical water profiles derived from CRISM retrievals of 1.27 μm O2 (1g) dayglow. A number of spurious retrievals were seen and these are attributed to the aerosol model choice problem seen in the simulations.
Supervisor: Irwin, Pat Sponsor: Science and Technology Facilities Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Astronomy ; Atmospheric Physics