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Difficulty: MediumLogical Inferences

Planetary scientists studying Gale Crater on Mars have identified deposits of manganese oxides, minerals that on Earth form only under highly oxidizing conditions, often facilitated by atmospheric oxygen or microbial activity. Since Mars lacks a modern oxygen-rich atmosphere, some researchers inferred that the Martian atmosphere was once highly oxygenated. However, geologist Chloe Vance demonstrated in laboratory simulations that manganese can be oxidized by perchlorates—which are highly abundant in Martian soil—when exposed to ultraviolet radiation, even under anoxic conditions. Which choice most logically completes the text?

  1. A
    microbial life must have played a role in the formation of Martian manganese oxides before the atmosphere lost its oxygen.
  2. the presence of manganese oxides on Mars does not necessarily indicate that the planet once had an oxygen-rich atmosphere.Answer
  3. C
    perchlorates are more efficient oxidizing agents than atmospheric oxygen under typical planetary conditions.
  4. D
    ancient Mars lacked ultraviolet radiation at the time the manganese oxide deposits in Gale Crater were formed.

Answer

the presence of manganese oxides on Mars does not necessarily indicate that the planet once had an oxygen-rich atmosphere
The correct answer correctly identifies that since manganese can be oxidized by perchlorates under ultraviolet radiation without oxygen, the presence of manganese oxides does not guarantee that ancient Mars had an oxygenated atmosphere.

Step-by-Step Solution

1
Identify the primary hypothesis discussed in the text.
Some researchers hypothesized that ancient Mars had a highly oxygenated atmosphere because manganese oxides on Earth typically require oxygen or microbial activity to form.
This establishes the initial assumption that the new findings will challenge.
2
Examine the experimental findings presented as counterevidence.
Chloe Vance's simulations showed that manganese can be oxidized by perchlorates (abundant in Martian soil) under ultraviolet radiation, even in anoxic (oxygen-free) environments.
This introduces an alternative, oxygen-free mechanism for the formation of the observed minerals.
3
Determine the logical implication of the alternative mechanism.
Because an alternative chemical pathway exists that does not require oxygen, the presence of manganese oxides is not conclusive proof that Mars once had an oxygenated atmosphere.
This matches the logical flow of the passage and completes the argument.

Key Concept

Logical Inferences
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Logical Inferences

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