Strange messenger: a new history of hydrogen on Earth, as told by xenon
Open accessAbstract
Atmospheric xenon is strongly mass fractionated, the result of a process that apparently continued through the Archean and perhaps beyond. Previous models that explain Xe fractionation by hydrodynamic hydrogen escape cannot gracefully explain how Xe escaped when Ar and Kr did not, nor allow Xe to escape in the Archean. Here we show that Xe is the only noble gas that can escape as an ion in a photo-ionized hydrogen wind, possible in the absence of a geomagnetic field or along polar magnetic field lines that open into interplanetary space. To quantify the hypothesis we construct new 1-D models of hydrodynamic diffusion-limited hydrogen escape from highly-irradiated CO2-H2-H atmospheres. The models reveal three minimum requirements for Xe escape: solar EUV irradiation needs to exceed 10× that of the modern Sun; the total hydrogen mixing ratio in the atmosphere needs to exceed 1% (equiv. to 0.5% CH4); and transport amongst the ions in the lower ionosphere needs to lift the Xe ions to the base of the outflowing hydrogen corona. The long duration of Xe escape implies that, if a constant process, Earth lost the hydrogen from at least one ocean of water, roughly evenly split between the Hadean and the Archean. However, to account for both Xe’s fractionation and also its depletion with respect to Kr and primordial 244Pu, Xe escape must have been limited to small apertures or short episodes, which suggests that Xe escape was restricted to polar windows by a geomagnetic field, or dominated by outbursts of high solar activity, or limited to transient episodes of abundant hydrogen, or a combination of these. Xenon escape stopped when the hydrogen (or methane) mixing ratio became too small, or EUV radiation from the aging Sun became too weak, or charge exchange between Xe+ and O2 rendered Xe neutral. In our model, Xe fractionation attests to an extended history of hydrogen escape and Earth oxidation preceding and ending with the Great Oxidation Event (GOE).
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Above the cold trap, mainly the light dissociation products H2 and H will be abundant.
“Above the cold trap, mainly the light dissociation products H 2 , and H will be abundant, as discussed in detail by Zahnle et al. 2019.”
From The young Sun's XUV-activity as a constraint for lower CO2-limits in the Earth's Archean atmosphere (2021) · cites this paper as Zahnle et al. 2019
The Introduction (p. 2) says H2 or CH4 can be abundant above Earth's water-vapor cold trap, and that CH4 decomposes at still higher altitudes, making H2 more abundant near the top of the atmosphere. The paper also models an upper atmosphere containing atomic H (Section 2, p. 7), but it does not establish that H2 and H are the main abundant species throughout the region above the cold trap; CH4 may also be abundant there. (LLM, conf=0.84)
Experimental — accuracy not yet measuredEvidence points to high levels of Archean CH4.
“Evidence points to high levels of Archean CH4 (Fig. 3B).”
From The Archean atmosphere (2020) · cites this paper as [187]
The Introduction (p. 2) reports circumstantial evidence for methane in the late Archean. The Conclusions (p. 28) infer that H2 or CH4 reached an atmospheric mixing ratio of at least 1% at times during the Archean, but do not establish high CH4 specifically rather than H2. (LLM, conf=0.85)
Experimental — accuracy not yet measuredFractionation of xenon isotopes in the Archean suggests that ~3.5 Ga ago CH4 levels were >0.5%, i.e., >5000 ppmv.
“Later, we discuss how fractionation of xenon isotopes in the Archean suggests that, ~3.5-Ga ago, CH4 levels were >0.5%, i.e., >5000 ppmv (187).”
From The Archean atmosphere (2020) · cites this paper as [187]
The numerical results (p. 19) say Archean Xe escape is consistent with an atmosphere containing >1% hydrogen, or an equivalent >0.5% methane. The Conclusions (p. 28) specify that H2 or CH4 reached the required level at times; they do not establish that CH4 itself exceeded 0.5% around 3.5 Ga. (LLM, conf=0.88)
Experimental — accuracy not yet measuredThe total oxidation during the Archean is equivalent to oxygen from a tenth or more of an ocean.
“The total oxidation during the Archean is equivalent to oxygen from a tenth of more of an ocean (187).”
From The Archean atmosphere (2020) · cites this paper as [187]
Example 1 (p. 24) models Archean hydrogen losses of 0.35 and 0.86 oceans of water, consistent with oxidation equivalent to oxygen from at least a tenth of an ocean in those scenarios. But the Conclusions (p. 28) say the amount lost is not a robust result of the model; the paper does not establish a tenth of an ocean as an unconditional minimum for total Archean oxidation. (LLM, conf=0.84)
Experimental — accuracy not yet measuredWhen Zahnle et al. (2019) neglect N2 as a second main atmospheric species and neglect dissociation of CO2 molecules and escape of its dissociation products, their simplified model will not yield accurate results.
“However, when they neglect N 2 as a second main atmospheric species and neglect dissociation of CO 2 molecules and escape of its dissociation products, then their simplified model will not yield accurate results.”
From The young Sun's XUV-activity as a constraint for lower CO2-limits in the Earth's Archean atmosphere (2021) · cites this paper as Zahnle et al. (2019)
Section 2 (p. 7) says the model omits N2, and Appendix A (pp. 31, 39) says it overlooks CO2 dissociation and treats CO2 as non-escaping. Appendix A.1 (p. 31) also says pathways producing escaping oxygen ions are omitted. The paper does not conclude that these omissions make its results inaccurate; it presents the model as a first approximation. (LLM, conf=0.88)
Experimental — accuracy not yet measured
Zahnle et al. (2019) found that under their assumptions in the homosphere where CO2 is abundant, radiative heating and radiative cooling are in balance, the temperature of the gas is determined by the bulk gas and very little energy is channeled into hydrogen escape.
“Zahnle et al. (2019) also found that under their assumptions in the homosphere (lower thermosphere) where CO 2 is abundant, radiative heating and radiative cooling are in balance, the temperature of the gas is determined by the bulk gas and very little energy is channeled into hydrogen escape.”
From The young Sun's XUV-activity as a constraint for lower CO2-limits in the Earth's Archean atmosphere (2021) · cites this paper as Zahnle et al. (2019)
Split into 2 atoms — 1 confirmed, 1 partially supported, 0 disputed, 0 misattributed, 0 unverifiable. Expand the claim to see per-atom verdicts.
Experimental — accuracy not yet measuredThe differences attributable to CO2-related photochemistry included in Zahnle et al. (2019) but neglected by Tian et al. (2005) and Kuramoto et al. (2013) were more or less negligible.
“The differences that are attributable to the CO 2 -related photochemistry that was included in the model of Zahnle et al. (2019) but neglected by Tian et al. (2005) and Kuramoto et al. (2013) were more or less negligible.”
From The young Sun's XUV-activity as a constraint for lower CO2-limits in the Earth's Archean atmosphere (2021) · cites this paper as Zahnle et al. (2019)
In §2.4 (p. 12), the paper says its results are similar to Kuramoto et al. (2013), with minor differences attributable to CO2 absorption of solar XUV, which Kuramoto's model omits. It also says Tian et al. (2005) predicts significantly less hydrogen escape and does not establish that CO2-related photochemistry accounts for only negligible differences with that model. (LLM, conf=0.84)
Experimental — accuracy not yet measuredZahnle et al. (2019) could reproduce Earth's present Xe isotope paradox if the assumed CO2-dominated atmospheres had mixing ratios of at least 1% hydrogen or 0.5% CH4 above the H2O cold trap if exposed to solar EUV irradiation at least 10 times higher than today's solar value.
“Zahnle et al. 2019 could reproduce Earth's present Xe isotope paradox if the assumed CO 2 -dominated atmospheres had mixing ratios of at least 1% hydrogen or 0.5% CH 4 above the H 2 O cold trap if they are exposed to a solar EUV irradiation that was ≥ 10 times higher than today's solar value.”
From The young Sun's XUV-activity as a constraint for lower CO2-limits in the Earth's Archean atmosphere (2021) · cites this paper as Zahnle et al. 2019
Split into 2 atoms — 1 confirmed, 1 partially supported, 0 disputed, 0 misattributed, 0 unverifiable. Expand the claim to see per-atom verdicts.
Experimental — accuracy not yet measuredZahnle et al. (2019) neglected dissociation of CO2 molecules and escape of its dissociation products.
“Although Zahnle et al. (2019) included radiative cooling of CO 2 in their study of hydrogen escape from a CO 2 -dominated atmosphere, they neglected dissociation of CO 2 molecules and escape of its dissociation products.”
From The young Sun's XUV-activity as a constraint for lower CO2-limits in the Earth's Archean atmosphere (2021) · cites this paper as Zahnle et al. 2019
Xenon is easily ionized by solar UV or charge exchange with H+ ions, so Xe+ can be dragged out to space by escaping H+ ions.
“Unlike lighter krypton, xenon is easily ionized by solar UV or charge exchange with H+ ions, so Xe+ can be dragged out to space by escaping H+ ions (187).”
From The Archean atmosphere (2020) · cites this paper as [187]
Xenon dragged out into space by escaping hydrogen during the Archean and Hadean best explains the progressive mass fractionation.
“Xenon dragged out into space by escaping hydrogen during the Archean and Hadean best explains the progressive mass fractionation (49, 187).”
From The Archean atmosphere (2020) · cites this paper as [187]
Our preliminary model finds that Xe can escape when the total hydrogen mixing ratio exceeds ~1% for the solar extreme UV flux expected around ~3.5 Ga ago.
“Our preliminary model finds that Xe can escape when the total hydrogen mixing ratio exceeds ~1% for the solar extreme UV flux expected around ~3.5 Ga ago (187).”
From The Archean atmosphere (2020) · cites this paper as [187]
Xe isotopes potentially tell us about O2, CH4, and H2 levels.
“Changes in atmospheric Xe isotopes through the Archean stop after the GOE (analogous to S-MIF) (49) and potentially tell us about O2, CH4, and H2 levels (187), including in the otherwise hidden Hadean, as discussed below.”
From The Archean atmosphere (2020) · cites this paper as [187]
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References 136 · Cited by 175
Research works printed but not linked: 4
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- Hayes J. M. (1994). Global methanotrophy at the Archaean-Proterozoic transition. In Early Life on Earth, Nobel Symposium 84, S. Bengtson (Ed), pp. 220-236.
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- Authors
- Zahnle, K. J., Gacesa, M., & Catling, D. C.
- Affiliations
- Ames Research Center (US)
- Space Sciences Laboratory (US)
- University of Washington (US)
- Corresponding author
- Zahnle, Kevin J.
- Venue
- Geochimica et Cosmochimica Acta
- Publisher
- Elsevier BV
- Published
- 2019-01
- Volume
- 244
- Pages
- 56-85
- ISSN
- 0016-7037 (ISSN-L)
- OpenAlex
- W2891240414
- Cited by
- 175
- References
- 136
- Citations by year
- 2026: 9 · 2025: 23 · 2024: 23 · 2023: 16 · 2022: 30 · 2021: 14 · 2020: 40 · 2019: 18 · 2018: 1
- FWCI
- 9.79
- Type
- journal article
- Language
- en
- Open access
- hybrid
- doi.org
- Licence
- Locations
- Geochimica et Cosmochimica Acta (publishedVersion, cc-by-nc-nd)
- arXiv (Cornell University) (submittedVersion)
- arXiv (Cornell University)
- AGU Fall Meeting Abstracts
- Funders
- National Science Foundation (1338810)
- National Aeronautics and Space Administration
- Simons Foundation (511570)
- Topics
- Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Astro and Planetary Science
- Physical Sciences › Earth and Planetary Sciences › Paleontology › Paleontology and Stratigraphy of Fossils
- Physical Sciences › Earth and Planetary Sciences › Geophysics › Geological and Geochemical Analysis
- Keywords
- atmospheric xenon, noble gas fractionation, hydrogen escape, Archean atmosphere, EUV irradiation, Great Oxidation Event
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