Absolute Measurements of Methane on Mars: The Current Status

Details Absolute Measurements of Methane on Mars: The Current Status Absolute Measurements of Methane on Mars: The Current Status

Oct 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007dps....39.3102m&link_type=abstract

American Astronomical Society, DPS meeting #39, #31.02; Bulletin of the American Astronomical Society, Vol. 39, p.471

Astronomy and Astrophysics

Astronomy

Scientific paper

Our study of methane on Mars now extends over three Mars years, sampling a wide range of seasons (Ls) with significant spatial coverage. By orienting the spectrometer slit North-South on the planet, we obtain simultaneous spectra at latitudes along the central meridian. Successive longitudes are presented as the planet rotates, and the combination then permits partial maps of the planet. We earlier reported differential detections of methane. Here, we present absolute extractions of methane for our entire database.
The new results are based on improved analytical procedures developed since 2005. We now identify and correct instrumental effects such as variations in resolving power along the slit, second-order optical fringe removal, correction of (minor) internal scattered light, and highly precise spectral re-sampling and wavelength calibration. With a multi-layer atmospheric model, we synthesize terrestrial transmittance spectra at 100 m/sec resolution using mixed line-shape functions for H2O, CH4, and O3, specifically including pressure shift effects for CH4 and H2O. Synthesizing the fully-resolved transmittance spectrum is critical for accurately correcting Martian spectral lines - and abundances - for telluric extinction. We convolve the fully-resolved transmittance spectrum to the instrumental resolution, and subtract it from the combined spectrum. Solar Fraunhofer lines are removed from the residual Mars spectra along with ro-vibronic bands of water and carbon dioxide isotopes. The residuals are then inspected for signatures of methane and other possible trace constituents.
On certain dates, the residual spectra display spectral lines at the positions expected for Doppler shifted CH4. Mixing ratios derived from those residuals exceed (by a factor of ten) upper limits obtained on other dates. The positive indications favor certain seasons and locations; these variations could be consistent with episodic release. Details will be presented.
Supported by NASA's Planetary Astronomy Program (344-32-51-96) and Astrobiology Program (344-53-51), and NSF's RUI Program (AST-0505765).

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