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A 2025 University of Copenhagen project proposes testing whether sulfur could trap xenon as it crystallizes. It is a new candidate explanation, not a confirmed solution: the project description outlines planned experiments and calculations but reports no result showing that sulfur holds xenon under geological conditions. The broader question—why planetary atmospheres contain far less xenon than expected—remains unresolved.
What is the missing xenon paradox?
Xenon is a noble gas, a group whose members are often compared when scientists reconstruct how planets formed and evolved. The paradox is that xenon is unusually scarce in planetary atmospheres relative to expectations from cosmochemical models and relative to other noble gases. The 2026 review by Avinash Kumar Both, Avradip Ghosh and Chin Li Cheung reports atmospheric xenon depletion on Earth, Venus and Mars.
For Earth, that review cites a comparison from Dauphas (2003): atmospheric xenon is depleted by a factor of 4.8 × 104 relative to solar composition. It gives a corresponding krypton depletion factor of 3.3 × 104. These are figures reported in the 2026 review from the cited earlier work, not new measurements made in 2026. The comparison helps frame the puzzle: xenon is scarce, but krypton is also depleted, and the pattern is not simply that all noble gases are missing to the same degree.
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The sulfur proposal, described by Villum Fonden in its 2025 project announcement, is that molten sulfur might provide a host for xenon as sulfur crystallizes. Xenon is highly polarizable, a property that can allow physical trapping in structures such as clathrates. The project asks whether sulfur could offer another route.
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Why molten sulfur is the focus
The project description says stable S8 rings are too small to encapsulate xenon. Molten sulfur, however, can contain changing chains and rings. The researchers suggest that this evolving mix could act as a “dynamic combinatorial library,” with xenon potentially templating sulfur structures during crystallization. In other words, the proposal is not simply that a fixed sulfur ring has an empty space large enough for xenon; it is that sulfur’s changing structures might form a xenon-bearing arrangement under suitable conditions.
What the project plans to test
The announced work includes crystallizing sulfur under pressure, characterizing the resulting structures, and computationally screening sulfur structures for favorable xenon binding. Those steps could test whether xenon-bearing sulfur arrangements are possible and stable in the studied conditions. The announcement does not establish that such structures have been found, that they occur in Earth’s crust or interior, or that they could account for the planet’s missing xenon.
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How does the sulfur idea compare with other explanations?
The 2026 review treats the paradox as a problem with multiple candidate mechanisms, rather than one settled cause. Explanations differ in where xenon might be stored or lost, when the process would operate, and what evidence bears on it. More than one process may be needed to explain both the elemental depletion and xenon’s isotope patterns.
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|---|---|---|
| Atmospheric escape or interaction with the solar wind | Atmosphere to space; during atmospheric evolution | Included among explanations surveyed by the 2026 review; the review does not identify it as a complete, settled solution. |
| Capture in sulfur structures | Potentially in sulfur as it crystallizes | A 2025 project hypothesis with planned high-pressure, structural and computational tests; the project page reports no confirming result. |
| Partitioning into the core or loss during magma-ocean outgassing | Planetary interior; during early differentiation | Listed as possible contributors to the deep-mantle xenon deficit in a 2022 study, not as proof of a single global explanation. |
| Xenon-poor material building Earth | Earth’s parent bodies; during accretion | Also listed as a possible contributor in the 2022 study. |
| Biological processes | Potentially during later planetary evolution | Included among possibilities surveyed by the 2026 review; this does not establish that biology explains the atmospheric deficit. |
The table reflects candidate processes discussed in the 2026 review and the 2022 deep-mantle study; it is not a ranking of their likelihood. The 2022 study reports that the pre-subduction deep mantle is depleted in xenon relative to krypton and chondrites. It identifies core partitioning, magma-ocean outgassing and a xenon deficit in Earth’s parent bodies as explanations that could have acted together. That finding concerns the deep-mantle inventory; it does not test or validate sulfur capture.
Do high-pressure xenon compounds solve the puzzle?
High pressure can make xenon compounds theoretically stable, but predicted stability alone does not show that a compound is a natural reservoir for Earth’s xenon. A 2013 Nature Chemistry study predicted stability thresholds for three xenon oxides:
| Predicted compound | Stability threshold reported by the 2013 study | Important constraint |
|---|---|---|
| XeO | Above 83 GPa | The study concluded that xenon oxides are unstable in equilibrium with metallic iron in the lower mantle. |
| XeO2 | Above 102 GPa | The threshold is a theoretical prediction, not a measured concentration in a natural Earth reservoir. |
| XeO3 | Above 114 GPa | The study discussed possible retention at defects in mantle silicates and oxides, but did not establish this as the missing-xenon solution. |
The thresholds are the study’s predicted stability pressures, not observations that these compounds store Earth’s xenon. Its stated limitation—instability in equilibrium with metallic iron in the lower mantle—matters when assessing whether the compounds could persist in a proposed natural setting.
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What evidence would make sulfur a credible explanation?
There are two separate questions: can xenon bind to sulfur in the laboratory or in calculations, and could that interaction store a meaningful share of planetary xenon in a real geological setting? Evidence for the first would not by itself answer the second.
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- Geological relevance: those conditions would need a defensible connection to where sulfur crystallized in Earth, rather than simply being achievable in a laboratory.
- Inventory and isotope fit: a proposed reservoir would need to help account for the scale and pattern of xenon depletion, including how it relates to other noble gases and xenon isotopes.
Until such evidence is reported, sulfur capture is best understood as an experimentally testable possibility. The project description’s conditional statement is that “Validating xenon’s interaction with sulfur could inform new extraction and recovery strategies.” It is a prospective implication, not evidence of a validated capture method.
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What is the status of the proposed solution?
The missing xenon paradox remains open. The 2025 sulfur project offers a specific way to test whether molten sulfur could host xenon during crystallization, while the 2026 review and earlier work describe other possible losses and reservoirs. No result in the project announcement shows that sulfur accounts for Earth’s missing xenon. The proposal will become an explanation only if xenon-bearing sulfur structures are demonstrated and their geological importance can be established.
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