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Difficulty: Very hardLogical Inferences

The detection of phosphine (PH3PH_3) in the clouds of Venus is a potential biosignature because abiotic pathways on temperate planets produce it in negligible amounts. On Venus, thermodynamic models show that atmospheric conditions rapidly oxidize phosphine into phosphorus oxides, requiring a continuous source of replenishment. Some researchers propose that active volcanism releases sufficient phosphorus to explain the observed levels. However, Venusian volcanic gases contain too little water to reduce volcanic phosphorus into phosphine, and the direct photolysis of phosphorus oxides by ultraviolet light requires a hydrogen donor that is absent in the upper atmosphere. Which choice most logically completes the text?

  1. A
    the presence of phosphine constitutes definitive proof of biological activity in the Venusian clouds.
  2. B
    volcanic activity on Venus must be far less widespread than previously estimated by atmospheric models.
  3. any abiotic mechanism replenishing Venusian phosphine must rely on pathways other than the proposed volcanic emissions or upper-atmosphere photolysis.Answer
  4. D
    phosphine is capable of resisting oxidation under the high pressure and temperature of the lower Venusian atmosphere.

Answer

The correct answer states that any abiotic mechanism replenishing Venusian phosphine must rely on pathways other than the proposed volcanic emissions or upper-atmosphere photolysis.
The text states that phosphine on Venus is rapidly oxidized and requires continuous replenishment. It evaluates two proposed abiotic mechanisms for this replenishment: volcanic activity (ruled out because the gases contain too little water to reduce phosphorus) and ultraviolet photolysis (ruled out because the upper atmosphere lacks a hydrogen donor). Therefore, if phosphine is indeed present and being replenished, any abiotic explanation must involve processes other than these two refuted pathways.

Step-by-Step Solution

1
Identify the primary premise regarding phosphine stability on Venus.
Phosphine is unstable on Venus and is rapidly oxidized, meaning its presence requires a continuous source of replenishment.
This establishes that some active replenishment mechanism must exist if phosphine is present.
2
Analyze the text's evaluation of the first proposed replenishment pathway (volcanism).
Volcanism is ruled out as the source because Venusian volcanic gases lack sufficient water to chemically reduce phosphorus into phosphine.
This eliminates volcanic emissions as a viable mechanism for phosphine generation under the stated constraints.
3
Analyze the text's evaluation of the second proposed replenishment pathway (photolysis).
Upper-atmosphere photolysis by ultraviolet light is ruled out because it requires a hydrogen donor that is absent in that region.
This eliminates ultraviolet photolysis as a viable mechanism for phosphine generation under the stated constraints.
4
Synthesize the findings to draw a logical conclusion.
Since both proposed abiotic mechanisms are inviable, any valid abiotic explanation for phosphine replenishment must operate through alternative pathways.
This directly connects the elimination of the two proposed pathways to the logical necessity of a different mechanism, without introducing outside assumptions.

Key Concept

Drawing logical conclusions based solely on the elimination of proposed pathways described in the text.
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Logical Inferences

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