### Titan's Atmospheric Methane Titan, Saturn's largest moon, has a thick atmosphere rich … — ACT

### Titan's Atmospheric Methane

Titan, Saturn's largest moon, has a thick atmosphere rich in methane ( $CH_4$ ). Because solar ultraviolet radiation continuously breaks down atmospheric $CH_4$ through photochemical reactions, Titan's atmospheric $CH_4$ must be replenished from an internal reservoir to maintain its observed levels. Two scientists propose differing mechanisms for this replenishment.

Scientist 1
Titan's $CH_4$ is stored as methane clathrate hydrates—compounds in which $CH_4$ molecules are trapped inside cages of water ice—within its outer icy crust. These clathrates were incorporated into Titan during its accretion from the cold solar nebula. Periodically, thermal plumes rising from Titan's rocky core warm the base of the crust, causing the clathrates to dissociate (break apart) and release gaseous $CH_4$ . This gas then migrates upward through fractures in the ice shell and enters the atmosphere.

Scientist 2
Titan's $CH_4$ is continuously produced by serpentinization within its rocky core. Liquid water from Titan's subsurface ocean circulates through the olivine-rich rocky core at high temperatures (exceeding $150^\circ\text{C}$ ). The chemical reaction between water and olivine produces hydrogen gas ( $H_2$ ), which then reacts with carbon dioxide ( $CO_2$ ) via the Sabatier reaction to synthesize $CH_4$ . This newly formed $CH_4$ rises through the subsurface ocean and the overlying ice shell to replenish the atmosphere.

Scientist 2's hypothesis relies on which of the following underlying assumptions regarding Titan's internal structure?

Titan's subsurface liquid water ocean is in direct contact with its rocky core.Answer
B
Methane clathrate hydrates are entirely absent from Titan's outer icy crust.
C
The temperature of Titan's rocky core remains below $150^\circ\text{C}$ .
D
Photochemical reactions in Titan's atmosphere do not destroy methane molecules.

Answer

Titan's subsurface liquid water ocean is in direct contact with its rocky core.

Scientist 2 proposes that liquid water from the subsurface ocean circulates through the rocky core to react with olivine. For this chemical process to occur, the liquid water must be in direct contact with the rocky core. If a barrier, such as a layer of high-pressure ice, separated the ocean from the core, the water could not circulate through the core, and no serpentinization or methane synthesis could take place.

Step-by-Step Solution

Analyze Scientist 2's proposed mechanism for methane production.

Scientist 2 claims that methane is synthesized when liquid water from Titan's subsurface ocean circulates through the rocky core to undergo chemical reactions (serpentinization).

Understanding the physical route required for the proposed chemical process.

Identify the physical requirements of this process.

For liquid water from the ocean to circulate through the rocky core, the ocean and the core must be in direct contact without any solid barriers separating them.

Determining the implicit physical condition needed for the proposed mechanism to occur.

Evaluate the options to find the statement that represents this required condition.

The statement regarding direct contact between the subsurface ocean and the rocky core is the only option that describes this necessary underlying assumption.

Selecting the correct assumption.

Key Concept

Identifying Underlying Assumptions and Premises

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