Question

### Martian Methane Sources

In 2004, sensors detected trace amounts of methane ($CH_4$) in the atmosphere of Mars. Because methane is rapidly destroyed by solar ultraviolet radiation, its presence indicates an active source on or beneath the Martian surface. Three scientists present competing hypotheses regarding the primary source of Martian methane.

Scientist 1
Martian methane is produced by methanotrophic-like microbial life (methanogens) residing in deep, liquid-water aquifers beneath the cryosphere. These microbes utilize carbon dioxide ($CO_2$) and subsurface hydrogen ($H_2$) to produce energy, releasing $CH_4$ as a metabolic byproduct. The hydrogen is generated by radiolysis of water in deep crustal rocks. The seasonal variation in atmospheric methane levels is due to temperature-controlled changes in the permeability of the overlying permafrost, which traps the methane in winter and cracks in summer, releasing the gas.

Scientist 2
Martian methane is abiotic in origin, produced by a geochemical process called serpentinization. Deep underground, olivine-rich rocks react with liquid water and dissolved $CO_2$ at high temperatures (above $150^\circ\text{C}$) to form serpentine minerals, magnetite, and hydrogen gas. In the presence of metal catalysts, this hydrogen reacts with carbon dioxide via the Sabatier reaction to yield $CH_4$. The methane is stored in subterranean clathrate hydrates (icy cages). Micro-fractures caused by tidal stresses from Mars’s moons, Phobos and Deimos, periodically rupture these clathrates, releasing methane plumes into the atmosphere.

Scientist 3
Martian methane is delivered exogenously by carbonaceous chondrite meteorites and interplanetary dust particles (IDPs) falling onto the Martian surface. These materials contain organic macromolecular carbon (carbon polymers). Solar UV radiation photochemically breaks down these organic polymers on the surface of Mars, generating methane gas directly in the atmosphere. The seasonal peak in atmospheric methane coincides with the annual increase in solar UV irradiance during the Martian perihelion (the point in orbit closest to the Sun), which accelerates the photochemical reaction rate.

Based on the hypotheses presented, which of the following statements best describes how Scientist 1 and Scientist 2 differ regarding the role of hydrogen in the production of Martian methane?