### The Xenon Paradox Compared to chondritic meteorites, Earth’s atmosphere contains only … — ACT

### The Xenon Paradox

Compared to chondritic meteorites, Earth’s atmosphere contains only about $10\%$ of the xenon ( $Xe$ ) expected relative to other noble gases, such as krypton ( $Kr$ ) and argon ( $Ar$ ). Two scientists discuss competing hypotheses for this "missing xenon."

Scientist 1
The missing xenon is sequestered in Earth’s deep interior. At pressures exceeding $100\text{ GPa}$ and temperatures above $3000\text{ K}$ , which are characteristic of the core-mantle boundary, xenon ceases to be chemically inert. Under these extreme conditions, xenon reacts with iron ( $Fe$ ) and nickel ( $Ni$ ) to form stable intermetallic compounds that sink into the core. In contrast, krypton and argon do not form stable compounds with iron or nickel under core conditions, allowing them to remain in the atmosphere.

Scientist 2
The missing xenon escaped into space early in Earth's history. During the Hadean eon, solar extreme ultraviolet (EUV) radiation was much stronger than it is today. Xenon has a lower first ionization energy ( $12.1\text{ eV}$ ) than krypton ( $14.0\text{ eV}$ ) and argon ( $15.8\text{ eV}$ ). Consequently, xenon was selectively ionized by EUV radiation. The resulting $Xe^+$ ions were dragged out of the atmosphere along with escaping hydrogen ( $H^+$ ) ions driven by hydrodynamic escape. Because krypton and argon remained mostly neutral, they were unaffected by the electromagnetic drag and remained bound to Earth.

Suppose new laboratory experiments demonstrate that at pressures of $120\text{ GPa}$ and temperatures of $3200\text{ K}$ , krypton and argon form stable intermetallic compounds with iron and nickel that are just as stable and dense as those formed by xenon. This finding would most directly support or weaken which of the scientists' hypotheses?

A
It supports Scientist 1's hypothesis, because it confirms that noble gases can react with core metals at high pressures and temperatures.
B
It weakens Scientist 2's hypothesis, because it demonstrates that chemical sequestration in the core is a more thermodynamically favorable process than hydrodynamic escape.
It weakens Scientist 1's hypothesis, because it suggests that krypton and argon would also have been sequestered in Earth's core, leaving the atmosphere depleted of these gases as well.Answer
D
It supports Scientist 2's hypothesis, because it provides an alternative mechanism for atmospheric noble gas retention that does not rely on ionization potential.

Answer

The finding weakens Scientist 1's hypothesis, because it suggests that krypton and argon would also have been sequestered in Earth's core, leaving the atmosphere depleted of these gases as well.

The correct option is the one stating that the finding weakens Scientist 1's hypothesis. Scientist 1 argues that xenon is depleted in the atmosphere because it selectively reacts with core metals under high temperature and pressure, whereas krypton and argon do not react and thus remain in the atmosphere. If new evidence shows that krypton and argon also react with core metals under these conditions, it implies that they should also be depleted in the atmosphere. Since they are not depleted, this contradicts Scientist 1's model, thereby weakening it.

Step-by-Step Solution

Identify the core mechanism and assumptions of Scientist 1's hypothesis.

Scientist 1 argues that xenon is depleted because it selectively reacts with iron and nickel in the core, while krypton and argon do not react and thus remain in the atmosphere.

This establishes the baseline assumption that must be tested against the new evidence.

Analyze the implications of the new experimental evidence.

The new evidence shows that krypton and argon also form stable, dense compounds with iron and nickel under core-like conditions, behaving identically to xenon.

This step determines what physical outcomes are predicted by the new evidence.

Compare the implications of the evidence with the observed atmospheric composition to evaluate Scientist 1's model.

If krypton and argon also react and sink into the core, they should be depleted in the atmosphere. Since they are not depleted, Scientist 1's selective-sequestration model is contradicted, meaning the new evidence weakens the hypothesis.

This step logically connects the experimental results to the validity of the hypothesis.

Key Concept

Evaluating how new experimental evidence that contradicts a hypothesis's key assumptions weakens that hypothesis.

Estimated Time:2m 30s

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