The Methane Diurnal Variation and Microseepage Flux at Gale Crater, Mars as Constrained by the ExoMars Trace Gas Orbiter and Curiosity Observations

• Published in: Geophysical research letters Vol. 46; no. 16; pp. 9430 - 9438
• Main Authors: Moores, John E., King, Penelope L., Smith, Christina L., Martinez, German M., Newman, Claire E., Guzewich, Scott D., Meslin, Pierre‐Yves, Webster, Christopher R., Mahaffy, Paul R., Atreya, Sushil K., Schuerger, Andrew C.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28.08.2019; American Geophysical Union
• Subjects: Astrophysics; Atmosphere; Atmospheric convection; Atmospheric methane; boundary layer; Constraints; Convection; Curiosity (Mars rover); Detection; Detection limits; Dilution; Diurnal; Diurnal variations; dust; Earth and Planetary Astrophysics; Earth Sciences; Fluxes; Gas seepage; Mars; Mars craters; Mars rovers; Mars surface; Measuring instruments; Methane; Night; Planetology; Sciences of the Universe; Seepage; Seepages; Spacecraft; Trace gases; Upper bounds
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2019GL083800

The upper bound of 50 parts per trillion by volume for Mars methane above 5 km established by the ExoMars Trace Gas Orbiter, substantially lower than the 410 parts per trillion by volume average measured overnight by the Curiosity Rover, places a strong constraint on the daytime methane flux at the Gale crater. We propose that these measurements may be largely reconciled by the inhibition of mixing near the surface overnight, whereby methane emitted from the subsurface accumulates within meters of the surface before being mixed below detection limits at dawn. A model of this scenario allows the first precise calculation of microseepage fluxes at Gale to be derived, consistent with a constant 1.5 × 10−10 kg·m−2·sol−1 (5.4 × 10−5 tonnes·km−2·year−1) source at depth. Under this scenario, only 2.7 × 104 km2 of Mars's surface may be emitting methane, unless a fast destruction mechanism exists. Plain Language Summary The ExoMars Trace Gas Orbiter and the Curiosity Rover have recorded different amounts of methane in the atmosphere on Mars. The Trace Gas Orbiter measured very little methane (<50 parts per trillion by volume) above 5 km in the sunlit atmosphere, while Curiosity measured substantially more (410 parts per trillion by volume) near the surface at night. In this paper we describe a framework which explains both measurements by suggesting that a small amount of methane seeps out of the ground constantly. During the day, this small amount of methane is rapidly mixed and diluted by vigorous convection, leading to low overall levels within the atmosphere. During the night, convection lessens, allowing methane to build up near the surface. At dawn, convection intensifies and the near‐surface methane is mixed and diluted with much more atmosphere. Using this model and methane concentrations from both approaches, we are able—for the first time—to place a single number on the rate of seepage of methane at Gale crater which we find equivalent to 2.8 kg per Martian day. Future spacecraft measuring methane near the surface of Mars could determine how much methane seeps out of the ground in different locations, providing insight into what processes create that methane in the subsurface. Key Points Nighttime SAM‐TLS seasonal cycle enrichment measurements and TGO sunset/sunrise measurements are not in opposition Microseepage fluxes must be local to Gale, range from 0.82 to 4.6 kg/sol, and are consistent with a constant source at depth Little of Mars experiences microseepage unless a fast destruction mechanism exists or Gale is very unusual