### Models of the Early Martian Atmosphere Two models were proposed to explain the presenc… — ACT

### Models of the Early Martian Atmosphere

Two models were proposed to explain the presence of liquid water features on early Mars, despite the young Sun being 30% fainter than it is today.

Model 1 (Warm and Wet Greenhouse Model)
Early Mars possessed a thick, stable atmosphere composed primarily of $CO_2$ and $H_2O$ gas, with a surface pressure of $1.5\text{ to }2.0\text{ bar}$ . This thick greenhouse gas envelope was maintained by continuous, global volcanic outgassing. The high surface pressure and potent greenhouse effect raised the average surface temperature above $273\text{ K}$ ( $0^\circ\text{C}$ ), allowing for long-term liquid water oceans and a persistent hydrologic cycle. This model assumes that Mars’s magnetic field was strong enough to protect the thick atmosphere from solar wind stripping during its first 500 million years.

Model 2 (Cold and Icy Impact Model)
Early Mars had a thin, dry $CO_2$ atmosphere with a surface pressure of less than $0.1\text{ bar}$ . The average surface temperature was well below $220\text{ K}$ , and the surface water was frozen as planet-wide ice sheets. Large meteoroid impacts, which occurred frequently during the Late Heavy Bombardment, delivered transient heat and vast quantities of water vapor. Each major impact event vaporized local ice sheets and injected $H_2O$ and $CO_2$ into the atmosphere, creating a temporary, warm greenhouse effect. Surface temperatures rose above $273\text{ K}$ for periods of only tens to hundreds of years, causing localized, rapid melting and catastrophic flash floods that carved the valley networks before the atmosphere cooled and froze again.

According to the descriptions of the two models, which of the following statements best contrasts the atmospheric pressures and surface temperature dynamics required by Model 1 and Model 2 to explain the presence of liquid water features on early Mars?

Model 1 requires a stable surface pressure of $1.5\text{ to }2.0\text{ bar}$ with a constant surface temperature above $273\text{ K}$ , whereas Model 2 requires a thin atmosphere where surface temperatures only rise above $273\text{ K}$ in short, transient intervals.Answer
B
Model 1 requires a thin atmosphere where surface temperatures only rise above $273\text{ K}$ in short, transient intervals, whereas Model 2 requires a stable surface pressure of $1.5\text{ to }2.0\text{ bar}$ with a constant surface temperature above $273\text{ K}$ .
C
Both models require a stable surface pressure of $1.5\text{ to }2.0\text{ bar}$ with surface temperatures constantly maintained above $273\text{ K}$ , but Model 1 attributes this to meteoroid impacts and Model 2 to volcanic outgassing.
D
Both models require a thin atmosphere with surface temperatures below $220\text{ K}$ , but Model 1 attributes liquid water features to continuous volcanic outgassing and Model 2 to stable solar radiation.

Answer

Model 1 requires a stable surface pressure of

1.5\text{ to }2.0\text{ bar}

with a constant surface temperature above

273\text{ K}

, whereas Model 2 requires a thin atmosphere where surface temperatures only rise above

273\text{ K}

in short, transient intervals.

The correct answer accurately contrasts the two models: Model 1 describes a thick atmosphere with stable, warm conditions above freezing (

273\text{ K}

) to maintain liquid oceans, while Model 2 describes a thin atmosphere with cold conditions where temperatures rise above freezing only during transient periods following meteoroid impacts.

Step-by-Step Solution

Analyze Model 1 to determine its required atmospheric pressure and temperature conditions.

Model 1 requires a thick atmosphere (

1.5\text{ to }2.0\text{ bar}

) and surface temperatures constantly above freezing (

273\text{ K}

This establishes the physical parameters proposed by the first model.

Analyze Model 2 to determine its required atmospheric pressure and temperature conditions.

Model 2 requires a thin atmosphere (

< 0.1\text{ bar}

) with a cold baseline temperature (

< 220\text{ K}

) that only rises above freezing (

273\text{ K}

) temporarily.

This establishes the physical parameters proposed by the second model.

Compare the findings from the two models to identify the option that accurately contrasts these conditions.

The correct option must state that Model 1 requires stable high pressure and constant warm temperatures, while Model 2 requires thin pressure and transient warm temperatures.

Comparing the core mechanisms and assumptions allows us to identify the correct contrast statement.

Key Concept

Contrasting competing scientific models based on their distinct physical parameters and atmospheric assumptions.

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