Question

Martian Methane Plumes

In 2018, planetary scientists confirmed seasonal fluctuations in the concentration of atmospheric methane ($CH_4$) on Mars, peaking during the late northern summer. Three hypotheses have been proposed to explain the origin and release mechanism of this methane.

*Hypothesis 1*
Methane is produced continuously by active methanogenic microbes residing in deep, warm subsurface aquifers where liquid water is stable. This biologically produced gas migrates upward and becomes trapped in subsurface clathrates (crystalline water-based solids physically caging gas molecules). During the warmer summer months, the thermal gradient in the upper regolith shifts, destabilizing the shallowest clathrates. This physical destabilization releases pulsed streams of methane gas through micro-fractures into the atmosphere.

*Hypothesis 2*
Methane is generated abiotically through serpentinization—a reaction in which liquid water chemically alters olivine-rich rocks within the Martian crust, producing hydrogen gas ($H_2$) which then reacts with carbon dioxide ($CO_2$) to form $CH_4$. This process occurs continuously at depth. The resulting methane migrates upward and is weakly bound (adsorbed) to the surfaces of clay minerals in the cold, dry shallow regolith. During summer, increased solar ultraviolet (UV) radiation heats the shallow regolith, supplying the thermal energy required to desorb the methane from the clay surfaces, releasing it into the atmosphere.

*Hypothesis 3*
Methane is produced entirely at the surface from exogenous (external) sources. Carbonaceous meteorites and interplanetary dust particles continuously deposit organic macromolecular material onto the Martian surface. This accumulated organic matter, when exposed to the high intensity of solar UV radiation during the summer peak, undergoes photolysis (light-activated chemical breakdown), directly releasing $CH_4$ gas into the thin atmosphere. In this view, no subsurface reservoirs or internal geological/biological processes are involved in generating the methane.

Based on the descriptions of the three hypotheses, which of the following statements best identifies the core claim of Hypothesis 2 regarding the generation and release of Martian methane?