Astronomers detected a spectral line at 1.12\text{ mm} in the atmosphere of Venus, which t… — ACT

Astronomers detected a spectral line at $1.12\text{ mm}$ in the atmosphere of Venus, which they attribute to phosphine ( $PH_3$ ) at a concentration of approximately $20\text{ ppb}$ (parts per billion). Three hypotheses were proposed to explain this observation.

*Hypothesis 1*
Phosphine is produced by biological activity. Anaerobic microorganisms living in the Venusian cloud decks generate $PH_3$ as a metabolic byproduct, similar to certain microbial life forms on Earth.

*Hypothesis 2*
Phosphine is produced by abiotic volcanism. Active volcanoes on Venus eject phosphide minerals from the mantle. These minerals are carried into the atmosphere, where they react with sulfuric acid droplets to produce $PH_3$ gas.

*Hypothesis 3*
The detection is a data processing artifact. The spectral line at $1.12\text{ mm}$ was actually caused by sulfur dioxide ( $SO_2$ ), which is highly abundant in the Venusian atmosphere. The line was mistaken for $PH_3$ due to noise-reduction filtering of the radio telescope data.

Suppose a new study finds that $PH_3$ is rapidly destroyed in the Venusian atmosphere by ultraviolet radiation and acid reactions, requiring a continuous replenishment rate of $1.5 \\times 10^7\text{ kg/year}$ to maintain a concentration of $20\text{ ppb}$ . The study also calculates that Venusian volcanic activity can release at most $2.0 \\times 10^3\text{ kg/year}$ of phosphorus-containing gases. Based on the hypotheses presented, how does this new evidence affect the validity of Hypothesis 2?

It weakens Hypothesis 2, because the maximum rate of volcanic phosphorus release is far lower than the rate required to maintain the observed phosphine concentration.Answer
B
It supports Hypothesis 2, because it confirms that volcanic eruptions actively release phosphorus-containing gases that can react to form phosphine.
C
It weakens Hypothesis 2, because the rapid destruction of phosphine in the clouds prevents volcanic gases from rising high enough to be detected.
D
It has no effect on Hypothesis 2, because the rate of phosphine destruction in the clouds is independent of the rate at which volcanic gases are released from the surface.

Answer

The correct option weakens Hypothesis 2 because the maximum volcanic release of phosphorus is insufficient to sustain the observed concentration of phosphine.

The correct option weakens Hypothesis 2 because it shows a severe quantitative deficit in the volcanic supply. To maintain the observed concentration of phosphine, a source must replenish it at a rate of

1.5 \\times 10^7\text{ kg/year}

due to rapid destruction. Because Venusian volcanism can supply at most

2.0 \\times 10^3\text{ kg/year}

, it cannot be the primary source, undermining Hypothesis 2.

Step-by-Step Solution

Identify the replenishment rate of phosphine (

PH_3

) required to offset its atmospheric destruction as described in the new evidence.

The required replenishment rate is

1.5 \\times 10^7\text{ kg/year}

This establishes the minimum source rate needed to sustain the observed concentration of

20\text{ ppb}

PH_3

against photolysis and acid reactions.

Identify the maximum rate of volcanic phosphorus-containing gas release on Venus from the new evidence.

The maximum volcanic release rate is

2.0 \\times 10^3\text{ kg/year}

This represents the maximum rate at which the abiotic source proposed in Hypothesis 2 can supply phosphorus to form phosphine.

Compare the required replenishment rate with the maximum volcanic release rate to determine the impact on Hypothesis 2.

The volcanic release rate (

2.0 \\times 10^3\text{ kg/year}

) is four orders of magnitude (10,000 times) lower than the required replenishment rate (

1.5 \\times 10^7\text{ kg/year}

Because the volcanic source cannot supply phosphorus fast enough to balance the rapid destruction of phosphine, the evidence strongly undermines the claim that volcanism is the source of the detected gas, thereby weakening Hypothesis 2.

Key Concept

Evaluating the Impact of New Evidence

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