Absolute Abundance of Methane and Water on Mars: Spatial Maps

Details Absolute Abundance of Methane and Water on Mars: Spatial Maps Absolute Abundance of Methane and Water on Mars: Spatial Maps

Aug 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005dps....37.2704m&link_type=abstract

American Astronomical Society, DPS meeting #37, #27.04; Bulletin of the American Astronomical Society, Vol. 37, p.669

Astronomy and Astrophysics

Astronomy

4

Scientific paper

We earlier reported ground-based detections of methane on Mars, and presented evidence for local enhancements in its mixing ratio consistent with local release. Our long-slit infrared observations (now extending over two Mars years) sample a wide range of seasons and areocentric longitudes with significant spatial coverage. By orienting the slit North-South on the planet, we obtain simultaneous spectra at many latitudes on Mars.
Heretofore, we presented differential abundances using one spectrum as a standard of comparison for spectra taken at other latitudes; this approach provides excellent removal of telluric features at the expense of reducing the intensity of residual spectral lines from Mars. Our differential extractions for water agreed with simultaneous measurements acquired with TES. Our differential extractions for methane implied strong latitudinal gradients - these are contrary to predictions if photochemistry limits the lifetime of methane on Mars, but instead require local release and a much shorter lifetime. The lifetime against destruction cannot be much longer than equator-to-pole transport times imposed by the Hadley circulation (weeks). A shorter lifetime requires that estimated production rates be revised upwards commensurately.
The extraction of absolute spectral signatures for Mars requires both special instrumental factors (high sensitivity, high resolving power) and an appropriate spectral synthesis technique. The telluric transmittance spectrum must be synthesized at very high resolution (approaching 100 m/sec), as otherwise the subtraction of synthetic from measured spectra produces unacceptably large residuals. This requires inclusion of mixed line-shape functions for H2O, CH4, and O3, and of pressure shift effects for CH4 and H2O. Here, we present absolute extractions of methane and water, based on such multi-layer atmospheric models for Mars and Earth. We will compare and contrast the new results with our earlier findings.
Supported by Grants to MJM from NASA's Planetary Astronomy Program (344-32-51-96) and Astrobiology Program (344-53-51), and NSF RUI Grant AST-0205397 to REN. GLV was supported by the NAS-NRC under the NASA Resident Research Associateship Program.

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