METHANE ON MARS: THERMODYNAMIC EQUILIBRIUM AND PHOTOCHEMICAL CALCULATIONS. J. S. Levine', M. E. Summers2 and M. Ewell2, 'NASA Langley Research Center, Hamp- ton, VA 2368' (joel.s.levine@nasa.gov ), 2Geroge Mason University, Fairfax, VA 22030 ([email protected]) .

Introduction: The detection of methane (CH4) in Table 2. Assumed Composition of the atmosphere of Mars by Mars Express [1] and the Atmosphere of Mars [ 5] Earth-based spectroscopy [2,3] is very surprising, very puzzling, and very intriguing. On Earth, about 90% of • Carbon dioxide (CO 2): 95.32% atmospheric ozone is produced by living systems [4]. • Nitrogen (N 2): 2.7% A major question concerning methane on Mars is its • Argon (Ar): 1.6% origin —biological or geological. • Oxygen (O 2): 0.13% Thermodynamic equilibrium caculations indicated • Carbon monoxide (CO): 0.07% that methane cannot be produced by atmospheric • Water vapor (H 2O): 10 ppmv at surface • Mean atmospheric surface pressure: 6.4 mb chemical/photochemical reactions [5]. Thermodynamic • Surface temperature range from equilibrium calculations for three gases, methane, am- • 148K (polar winter) to 290K (southern summer) monia (NH3) and nitrous oxide (N 2 O) in the Earth’s atmosphere are summarized in Table 1. The calcula- Table 3. Calculated Trace Gases in tions indicate that these three gses should not exist in the Atmosphere of Mars Based on Thermodynamic the Earth’s atmosphere. Yet they do, with methane, Equilibrium Calculations [5] ammonia and nitrous oxide enhanced 139, 50 and 12 orders of magnitude above their calculated thermody- Gas T=100KT= 200K T = 300K namic equilibrium concentration due to the impact of life! Thermodynamic equilibrium calculations have -100 -100 -100 Methane (CH4) <10 <10 <10 been performed for the same three gases in the atmos- phere of Mars based on the assumed composition of -100 -89 -62 the Mars atmosphere shown in Table 2. The calculated Ammonia (NH 3 ) <10 2 x 10 4 x 10 thermodynamc equilibrium concentrations of the same Nitrous oxide -23 three gases in the amosphere of Mars is shown in Ta- 6 x 10-54 4 x 10-30 5 x 10 (N O) ble 3. Clearly, based on thermodynamic equilibrium 2 calculations, methane should not be present in the at- mosphere of Mars, but it is in concentrations approach- In the atmospheres of Earth and Mars, methane is ing 30 ppbv from three distinct regions on Mars [3]. destroyed by photolysis by sola r ultraviolet radiation and by reaction with the hydtoxyl radical (OH)., ac- Table 1. Calculated Trace Gases in the Atmosphere cording to the following reactions [6,7]: of Earth Based on Thermodynamic Equilibrium CH4 + hv 4 CH2 + H2 , and, Calculations [5] CH4 + OH 4 CH 3 + H2O The hydroxyl raical forms from the following two

Thermo- reactions involving water vapor (H 2 O);

dynamic Measured H2 O+ hv 4 OH+H Atmospheric 1 Equilibrium Concentration H2O + O( D) 4 2 OH, Gas Enhancement Concentration (Mole where O(1D) is excited atomic oxygen. (Mole Fraction) The methane measurements are very intriguing Fraction) since it appears that methane emanates and forms large

Methane -145 -6 139 seasonal plumes from three distinct areas of Mars— 10 1.7 x 10 10 (CH4) Terra Sabae, Nili Fossae and Syrtis Major [3]. In addi- tion, the measurements suggest that methane has an Ammonia -10 1050 2 x 10-60 10 atmospheric lifetime considerably less than the ap- (NH3) proximately 300 years lifetime calculated using meth- Nitrous ane loss from solar photolysis and reaction with the -19 -7 12 oxide 2 x 10 3 x 10 10 hydroxyl radical. This suggests that a major methane (N2O) sink other than solar photolysis and reaction with the hydroxyl radical may exist. Other potential methane sumed eddy diffusion profiles, etc. The sensitivity of sinks, including surface sinks will be assessed. methane to these parameters will be discussed. Photochemical calculations to determine concentra- tion, the vertical distribution and atmospheric lifetime References: [1] Formisano et al. (2004) Science of methane on Mars for a wide range of atmospheric 306, 1758-1761. [2] Krasnopolsky et al. (2004) Icarus conditions were performed using a one-dimensional 172, 537-547. [3] Mumma et al. (2009) Science 323, photochemical model of the atmosphere of Mars [8]. 1041-1045. [4] Reeburgh (2004) Treatise on Geochem- The vertical distribution of species in the atmosphere istry, Vol. 4, The Atmosphere, Elsevier Pergamon, 65- of Mars using this model is shown in Figure 1. To 89. [5] Levine et al. (1989) The Case for Mars III: assess the sensitivity of the methane concentration, its Strategies for Exploration-Technical, American Astro- vertical distribution and atmospheric lifetime to a nautical Society277-282. [6] Levine (1985) The Photo- whole range of atmospheric parameters, sensitivity chemistry of Atmospheres, Academic Press, Inc., 518 studies have been conducted for a wide range of at- pages. [7] Levine et al. (1985) Nature, 318, 254-257. mospheic parameters, including assumed lower bound- [8] Summers et al. (2002) Geophysical Research Let- ary flux conditions, assumed water vapor profiles, as- ters, 10.1029/2002GL015377.

Figure 1. Standard Model of the Atmosphere of Mars (CO 2 = 6 mbar) [8].