### Early Martian Clays Phyllosilicates (clay minerals) are widely distributed across the … — ACT

### Early Martian Clays

Phyllosilicates (clay minerals) are widely distributed across the oldest terrains of Mars, dating back to the Noachian epoch (approximately 4 billion years ago). Because clays require water to form, their presence indicates that liquid water was active on early Mars. However, scientists disagree on the environmental conditions under which these clays formed.

Hypothesis 1 (Surface Weathering Model)
Phyllosilicates formed primarily at the Martian surface through the chemical weathering of basaltic rocks by ambient liquid water. During the Noachian epoch, Mars had a dense greenhouse atmosphere composed mainly of carbon dioxide ( $CO_2$ ) and water vapor ( $H_2O$ ). This atmosphere maintained a warm and wet climate, allowing for rainfall, surface runoff, and stable open bodies of water. Basaltic rocks exposed to this precipitation slowly weathered into clays over millions of years.

Hypothesis 2 (Subsurface Hydrothermal Model)
Phyllosilicates formed primarily in the Martian subsurface through hydrothermal alteration. During the Noachian epoch, Mars had a cold, thin atmosphere, and surface water was frozen as ice. However, heat from geothermal processes and frequent meteoroid impacts warmed groundwater, circulating it through fractures in the basaltic crust. This warm subsurface water chemically altered the surrounding rock into clays. The clays were later exposed at the surface by impact excavation and wind erosion.

Based on Hypothesis 1, which of the following atmospheric and environmental conditions was required for the formation of Martian phyllosilicates?

A warm and wet climate maintained by a dense greenhouse atmosphere rich in $CO_2$ and $H_2O$ Answer
B
A cold, thin atmosphere that caused surface water to remain frozen as ice sheets
C
Subsurface geothermal activity that circulated heated groundwater through fractured crust
D
Frequent meteoroid impacts that generated transient heat to melt local surface ice

Answer

A warm and wet climate maintained by a dense greenhouse atmosphere rich in

CO_2

and

H_2O

According to Hypothesis 1, phyllosilicates formed at the surface due to chemical weathering by liquid water, which required a warm and wet climate sustained by a dense greenhouse atmosphere composed of

CO_2

and

H_2O

Step-by-Step Solution

Identify the target model in the question prompt.

The question asks about Hypothesis 1 (Surface Weathering Model).

This focuses the search to the first hypothesis in the passage.

Locate the description of the atmospheric and environmental conditions required by Hypothesis 1.

Hypothesis 1 states that a dense greenhouse atmosphere composed of

CO_2

and

H_2O

maintained a warm and wet climate, allowing rainfall and surface runoff to weather basaltic rocks.

This directly matches the core claim of Hypothesis 1 regarding how clays formed.

Compare the located information to the given options.

The option describing a warm and wet climate maintained by a dense greenhouse atmosphere matches the text exactly.

This identifies the correct claim, while the other options describe the mechanisms or conditions of Hypothesis 2.

Key Concept

Identifying Core Claims and Hypotheses

Estimated Time:1m 0s

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