Martian Atmospheric Methane Instruments on Mars have detected seasonal fluctuations in atm… — ACT

Martian Atmospheric Methane

Instruments on Mars have detected seasonal fluctuations in atmospheric methane ( $CH_4$ ), peaking during the Martian summer. Two scientists propose differing explanations for the source and release mechanism of this methane.

Scientist 1
The seasonal methane spikes are caused by the destabilization of subsurface methane clathrates (crystalline water-based solids physically trapping methane gas). These clathrates were formed billions of years ago when Mars possessed abundant surface water. Under current Martian conditions, clathrates are only thermodynamically stable at depths of $10\text{ meters}$ or more. During the Martian summer, solar heating warms the upper regolith, sending a thermal wave downward that destabilizes the uppermost clathrates, releasing the trapped $CH_4$ , which diffuses through the porous soil into the atmosphere.

Scientist 2
The methane is produced abiotically by modern serpentinization—a geochemical reaction between liquid water and olivine-rich rock. This reaction occurs in the deep crust ( $>1\text{ km}$ depth) where geothermal heat keeps water liquid. The generated $CH_4$ gas accumulates in deep geologic traps. During the Martian summer, the peak gravitational tidal forces exerted by Mars's moons deform the crust, reopening micro-fractures and allowing the pressurized methane to rapidly escape to the surface.

Which of the following is an underlying assumption of Scientist 1's explanation but NOT of Scientist 2's explanation?

The seasonal temperature variation on Mars penetrates the regolith to a depth of at least $10\text{ meters}$ with sufficient intensity to destabilize clathrates.Answer
B
Methane clathrates are abiotically replenished by active serpentinization occurring at depths shallower than $10\text{ meters}$ .
C
The thermal stability of methane clathrates is completely independent of the ambient temperature of the surrounding regolith.
D
Crustal deformation caused by the tidal forces of Mars's moons is strongest during the Martian winter.

Answer

The seasonal temperature variation on Mars penetrates the regolith to a depth of at least

10\text{ meters}

with sufficient intensity to destabilize clathrates.

The correct option correctly identifies the physical prerequisite for Scientist 1's proposed release mechanism. Scientist 1 specifies that clathrates are only stable at depths of

10\text{ meters}

or deeper. Therefore, to destabilize these clathrates via seasonal solar warming, the thermal wave must be able to penetrate the regolith to a depth of at least

10\text{ meters}

with enough energy to disrupt their stability. Without this assumption, the solar heating mechanism would be unable to reach and release the trapped methane.

Step-by-Step Solution

Identify the core mechanism proposed by Scientist 1.

Scientist 1 proposes that summer solar heating warms the regolith, sending a thermal wave downward that destabilizes clathrates at their minimum stable depth of

10\text{ meters}

or more.

This establishes the physical process that must take place for the hypothesis to be valid.

Determine the physical constraints mentioned in Scientist 1's viewpoint.

Clathrates are only thermodynamically stable at depths of

10\text{ meters}

or more under current conditions.

This sets a spatial boundary: any clathrates that can be destabilized must reside at or below

10\text{ meters}

Formulate the implicit physical requirement linking the mechanism and the constraints.

For solar heating to destabilize clathrates starting at

10\text{ meters}

, the heat wave must propagate down to at least

10\text{ meters}

with enough energy to exceed the clathrates' stability threshold.

If the thermal wave did not reach this depth, the clathrates would remain unaffected, rendering the proposed seasonal release mechanism impossible.

Key Concept

Identifying implicit physical assumptions behind a proposed mechanism in conflicting viewpoints.

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