Development of high sensitivity spectroscopic techniques for remote sensing of trace gases in the Matian atmosphere.

Measurement of methane gas in the atmosphere of Mars is very important to the search for extant life. Even at a very low level, methane's presence implies either a geo-chemical or a biological production mechanism, since it is easily photolysed with a lifetime in the atmosphere of approximately 300 years. To measure methane by remote sensing requires very sensitive detection techniques at high (<0.01cm-1) resolution to remove contaminating spectral features from dust, water vapour and carbon dioxide. The tentative detections of methane by ground-based observations (Krasnopolsky et al. 2004) and the Planetary Fourier Transform spectrometer on Mars Express (Formisano et al. 2004) have remained controversial. Both measured very low-levels, approximately 10ppb, close to the instruments' lower limits of detection. New, more accurate measurements are clearly required. In Earth observing applications, trace gas-measurements are typically made by high-resolution (<0.001cm-1) Fourier Transform Spectrometers. However, the high mass (e.g. 320kg for MIPAS) of these instruments eliminates them as candidates for planetary missions. Fortunately newly developed compact instruments based on high resolution, high sensitivity techniques are becoming available. One such technique is infrared laser heterodyne radiometry (LHR). This project will take the LHR system currently under development at the CASE partner, the Molecular Spectroscopy Facility (MSF), Rutherford Appleton Laboratory (RAL), and adapt it for use with methane gas measurements. The LHR technique (Weidmann et al. 2007) mixes the radiation from the atmosphere with a tuneable laser source to pick-out spectral features. This allows high spectral resolution (<0.001cm-1) measurements over a narrow spectral range (e.g. 10cm-1), targeting specific gas species in a compact instrument with far fewer moving parts and lower mass than traditional high-resolution techniques. The development of such species-specific instruments is highly complimentary to broader spectral range compact instruments already in a more advanced state of development to flight readiness. Instrument design, modelling and sensitivity studies will be carried out at the sub-department of Atmospheric, Oceanic and Planetary Physics, department of Physics, University of Oxford. The experimental work will be carried out at the MSF using the existing quantum cascade laser (QCL) LHR setup to develop a prototype instrument. A large part of the experimental work will be testing the MSF breadboard LHR using the gas cell facilities at RAL to reproduce representative Martian atmospheric paths, including typical dust and condensate loadings using the specialised aerosol cells. This will test the instrument's discrimination of overlapping spectral features and sensitivity to contamination from the dust present in the Martian atmosphere. The project will further consider the requirements for miniaturisation and steps needed to bring the instrument to a level so that it can be considered for proposal to future Martian orbiter and lander opportunities. References: Formisano et al., Science, No. 306, p. 1758, 2004 Krasnopolsky et al. Icarus, No. 172, p. 537, 2004 D. Weidmann, W. J. Reburn, and K. M. Smith, Rev. Sci. Instrum. 78, 073107, (2007)