Table 1.

Summary of Potential Methane Seepage Sites on Mars (References within Main Text)

Type of Site	General areas	Specific areas	Comments
Mud-volcano-like structures	Deep lowland basins	Acidalia and Utopia Planitiae Many thousands of mud-volcano-like structures	Locations in major depo-centers for fluid and fine-grained sediment. Thermogenesis potential by burial and impact heating. FTT reactions possible in Noachian buried crust. Proximity to volcanic edifices may enhance heat flow and thermogenesis potential as well as extent/duration of liquid water for serpentinization and H2 production for FTT reactions. Permafrost seals could trap ancient CH4. Clathrates and zeolites could also store CH4. Mud-volcano-like mounds and giant polygons provide conduits for seepage. Key sites for potential biotic or abiotic CH4 seepage
	Lowlands areas, Arabia Terra, Valles Marineris	Isidis, Scandia, Chryse, Crommelin and Firsoff craters, Candor Chaos, Candor and Coprates Chasmata	Various mud-volcano-like morphologies.
Ancient springs	Gusev crater Arabia Terra Valles Marineris	Columbia Hills, adjacent to “Home Plate” SW part of Vernal crater Various locations	Data from Spirit rover. Opaline silica with digitate textures; associated volcanics; possible hot spring deposit. Elliptical albedo features, circumferential faults, aligned knobs suggestive of fluid flow. ∼8 km trend. Possible spring deposit.
Flow structures at crater rims	Lowlands	Many examples, especially in ghost craters in N. Chryse and S. Acidalia	Location on crater rims suggestive of hydrothermal flow associated with impacts. Example in N. Chryse also has mud-volcano-like mounds and giant polygons within crater.
Geothermal-volcanic centers	Tharsis; Elysium Mons, Syrtis Major	Satellite volcanoes, vents, fissures	Potential for recurring gas exhalation. CH4 content would be quite low based on Earth analogs.
Microseepage zones	Faulted sedimentary and serpentinized areas	Nili Fossae (W. of Isidis) Dichotomy (e.g., Gusev and Gale craters, Nili Fossae) Cerberus Fossae Argyre	Relationship to Isidis impact and location on the dichotomy provide potential for deep fault conduits for seepage. Evidence for past serpentinization; Chassigny-like materials with FTT catalysts possible. Key site for potential abiotic CH4 seepage Deep fault potential, possibility to tap CH4 generated at depth in lowlands basins. Long fractures; recent fluid injection. Deep source for liquid water. Possible site for serpentinization but FTT catalysts uncertain. Long-lived fluid flow and volcanic activity. Chassigny-like materials. Serpentinization, FTT reactions, organic thermogenesis possible.