ABSTRACT

Hydrogen gas dominates the atmospheres of giant planets, ensuring that most atmospheric carbon is fully reduced and is present as methane. On the terrestrial planets, the severe depletion of hydrogen causes most carbon to be chemically bound with oxygen, and so atmospheric carbon is found mainly in the form of carbon dioxide. Indeed, carbon dioxide is the dominant atmospheric gas on Mars and Venus and it is the principal form of atmospheric carbon on Earth. A tiny fraction of terrestrial carbon is found as atmospheric methane, produced almost entirely biologically with only a very small contribution from abiotic (geothermal) processes.

On Mars, the photochemical lifetime of methane is very short (~300 years), and any methane now in its atmosphere must have been released recently. The methane release rate can be inferred from its atmospheric abundance, and provides an important quantitative constraint for assessing biogenic vs. primordial or geothermal origins. For this reason, methane on Mars has been sought for decades using increasingly sensitive instruments, but has eluded detection. Recently, three groups have reported independent detections of methane on Mars.

I will review the current status of these searches and will present evidence for the detection of strong latitudinal gradients by our team. Such gradients require both intense local sources and also require that a rapid destruction mechanism be operating. The lifetime against destruction cannot be much longer than equator-to-pole transport times imposed by the Hadley circulation (weeks), and must certainly be far shorter than the photochemical lifetime. Heterogenous reactions with oxidants adsorbed on airborne aerosol grains are a possible explanation. Additional chemical tests can help to constrain biogenic vs. abiotic production, but measurement of isotopic variations with sufficient accuracy to test origins will likely require investigations from space or landers.