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Difficulty: MediumIdentifying Underlying Assumptions and Premises

### Formation of Martian Gullies

Martian gullies are geologically young, sharp-edged channel systems found on steep slopes on Mars. Scientists debate the mechanism responsible for carving these features.

Scientist 1
Martian gullies are formed by the flow of liquid water. Although the Martian surface is cold and has low atmospheric pressure, subsurface liquid water can be released to the surface during warm seasons. When liquid water contains dissolved salts, it forms a brine that has a significantly lower freezing point and a slower evaporation rate than pure water. This allows the salty liquid water to remain stable on the surface long enough to flow downslope, carving the alcoves, channels, and depositional aprons characteristic of water-carved gullies on Earth.

Scientist 2
Martian gullies are formed by dry mass-wasting processes triggered by the seasonal sublimation of carbon dioxide (CO2CO_2) frost. Under current Martian atmospheric conditions, liquid water is highly unstable and would rapidly freeze or evaporate, preventing it from flowing in quantities sufficient to carve gullies. Instead, winter temperatures allow CO2CO_2 frost to condense in gully alcoves. In spring, solar heating causes the bottom of the frost layer to sublimate directly into gas. The pressure of this escaping gas fluidizes the overlying dry sand and dust, causing it to flow downslope and erode the gullies without liquid water.

Based on the passage, Scientist 2's explanation of gully formation relies on which of the following assumptions?

  1. A
    Liquid water is stable on the Martian surface under current temperature and atmospheric conditions.
  2. The sublimation of CO2CO_2 frost can generate sufficient gas pressure to fluidize and transport dry sediments downslope.Answer
  3. C
    Martian subsurface water contains a high concentration of dissolved salts that lowers its freezing point.
  4. D
    CO2CO_2 frost accumulation only occurs in regions of Mars that have localized high-pressure atmospheric conditions.

Answer

The sublimation of CO2CO_2 frost can generate sufficient gas pressure to fluidize and transport dry sediments downslope.
The correct answer is that the sublimation of carbon dioxide frost can generate sufficient gas pressure to fluidize and transport dry sediments. Scientist 2's explanation hinges on the physical mechanism of gas pressure from sublimating CO2CO_2 frost fluidizing dry sediment. If this sublimation process cannot generate enough pressure to move sand and dust, the proposed mechanism would be impossible. Thus, this is a required underlying assumption of Scientist 2's model.

Step-by-Step Solution

1
Identify the core claim of Scientist 2's model.
Scientist 2 proposes that Martian gullies are formed by dry flows of sand and dust fluidized by the gas escaping from sublimating carbon dioxide (CO2CO_2) frost.
This establishes the physical mechanism proposed by Scientist 2.
2
Determine what must be true for Scientist 2's proposed mechanism to function.
For sublimating CO2CO_2 frost to carve gullies without water, the escaping gas must physically be able to exert enough pressure to fluidize and transport the overlying soil.
An underlying assumption is a premise that must be true for the argument or model to hold.
3
Evaluate the choices to find this necessary premise.
The premise that CO2CO_2 sublimation can generate enough gas pressure to fluidize and transport dry sediments downslope is identified as the correct assumption.
This choice directly supports the viability of Scientist 2's model.

Key Concept

Identifying Underlying Assumptions and Premises
Estimated Time:1m 30s
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Identifying Underlying Assumptions and Premises

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