Instruments on Mars orbiters and rovers have detected trace amounts of atmospheric methane () that exhibit seasonal fluctuations, peaking during the late summer. Three scientists propose different models to explain the source and behavior of this methane.
Scientist 1 (Biogenic Model)
Martian methane is produced by subsurface methanogenic archaea (microbes). These microbes inhabit deep hydrothermal aquifers where liquid water is stable. The archaea combine hydrogen () and carbon dioxide () from Martian rocks and fluid reservoirs to produce and water as metabolic byproducts. Because microbial metabolic rates are temperature-dependent, methane production increases during the warmer summer months, leading to the observed seasonal fluctuations in atmospheric methane levels.
Scientist 2 (Geochemical Model)
Martian methane is generated through serpentinization, an abiotic (non-biological) reaction that occurs when subsurface olivine-rich rocks react with liquid water in the presence of dissolved carbon dioxide (). This reaction releases gas, which is initially trapped in subsurface ice lattices (clathrates). During the Martian summer, warmer surface temperatures cause thermal expansion and micro-fracturing in the overlying permafrost, allowing the trapped geologic methane to escape into the atmosphere and producing the seasonal cycle.
Scientist 3 (Exogenous Model)
Martian methane is produced on the planet's surface via the ultraviolet (UV) photolysis of organic matter. Martian dust contains organic carbon compounds delivered by carbonaceous chondrite meteorites and micrometeorites that continuously bombard the planet. When exposed to solar UV radiation, these surface organic compounds degrade, releasing . The seasonal variation is driven directly by changes in solar UV flux, which peaks during the Martian summer due to the tilt of the planet's rotational axis. Liquid water is not involved in this surface reaction.
Based on the models provided, match each scientific claim on the left with the correct consensus status among the three scientists on the right.
- Methane concentrations in the Martian atmosphere vary periodically according to the time of year.Agreed upon by all three scientists.
- The presence of subsurface liquid water is a necessary condition for the production of Martian methane.Agreed upon by Scientist 1 and Scientist 2, but not Scientist 3.
- The carbon source for Martian methane is exogenous, arriving via meteorites and cosmic dust.Supported only by Scientist 3, and not proposed by Scientist 1 or Scientist 2.