Remote Sensing of Methane in the Martian Atmosphere using Infrared Laser Heterodyne Radiometry

Details Remote Sensing of Methane in the Martian Atmosphere using Infrared Laser Heterodyne Radiometry Remote Sensing of Methane in the Martian Atmosphere using Infrared Laser Heterodyne Radiometry

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p21a1642p&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P21A-1642

Mathematics

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[0343] Atmospheric Composition And Structure / Planetary Atmospheres, [0394] Atmospheric Composition And Structure / Instruments And Techniques, [6225] Planetary Sciences: Solar System Objects / Mars

Scientific paper

In the last few years, several research teams have reported the detection of methane in the atmosphere of Mars, measuring 10 ppb on average [1][2][3]. The source of the methane is still unknown, but its identification is important as its presence could imply a biological origin. However, the detection limits of current instruments lie below the requirements for an unambiguous determination of concentration mapping and distribution. We investigate the viability of detecting methane in the Martian atmosphere via a high sensitivity remote sensing technique known as passive mid-infrared laser heterodyne radiometry. Although heterodyne spectroscopy is not a new idea, recent advancements in local oscillator technology [4] offer the possibility of significant instrument miniaturisation relevant to space deployment. We present our current work on a laser heterodyne radiometer (LHR) which involves adapting an existing 10 μm laser breadboard design, which was used with much success to study stratospheric ozone [5], to operate at 7.7 μm in order to target the ν4 fundamental band of methane. The core of the LHR consists of a distributed-feedback quantum cascade laser (QCL) operating in continuous-wave mode, which acts as the local oscillator. QCLs are ideal local oscillators for this type of instrument as they emit with high spectral purity and the necessary optical power in the mid-infrared region where characteristic spectral lines of interest lie. Atmospheric modelling of the Martian atmosphere and instrument sensitivity studies enabled simulated methane spectral features to be studied in detail, which subsequently determined the focus for experimental efforts in the laboratory. Testing of the LHR was initially carried out on small gas cells containing pure methane gas, but in order to test the instrument more rigorously for atmospheric studies a larger gas cell was constructed that approximates the Martian atmosphere in the laboratory. Trace quantities of methane were introduced into this representative Martian atmospheric path and here we present an assessment of the LHR's ability to discern spectral features of methane from the spectral features of the other 'background' gases such as carbon dioxide and water vapour. The advantages of the laser heterodyne radiometer over conventional remote sensing spectral instruments, including its compact lightweight design, are also reviewed.

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