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This figure from Wikipedia's Atmosphere of Earth shows a hydrogen fraction of 0.000055 percent by volume.

Question: Where does molecular hydrogen in the atmosphere come from?

Does this come directly from biological production like much of the CO2 and CH4, or is this coming from geological sources or breakdown of methane by atmospheric chemistry for example, or someplace else?

Related: What are the sources of molecular hydrogen in human breath?

Gas proportions in the Earth's lower atmosphere; "The proportion of water vapor (H₂O) is variable so it is not represented in this chart, however it averages about 1% in the troposphere."

Atmosphere gas proportions

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    $\begingroup$ I can't give a per-centage but there is some contribution from refineries and chemical plants. H is difficult to contain and small leaks at gaskets are common . These are generally small leaks that require test equipment to detect. Several different processes use high pressure H so a great number of flanges. It can also diffuse through steel at temperatures above 500F and process temperatures to 900 F are common. Also welding must release some H but I am unaware of any measurements; The only H from the welding that is of concern is the H that enters the steel an may embrittle it. $\endgroup$
    – blacksmith37
    Mar 25, 2019 at 21:24
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    $\begingroup$ Hydrogen gas is produced by some bacteria and algae and is a natural component of flatus, HCHO photolysis. $\endgroup$
    – Keith McClary
    Mar 25, 2019 at 22:26
  • $\begingroup$ @KeithMcClary do you think there is enough there for an answer? So far no other explanations have been proposed. $\endgroup$
    – uhoh
    Jun 20, 2019 at 7:37
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    $\begingroup$ I don't think I will write it (I would just copypaste from wiki). I see there is recent interest in geological sources: h2euro.org/latest-news/… , researchgate.net/publication/… , greeleytribune.com/news/… (the latter has some good info between the crackpotty bits). $\endgroup$
    – Keith McClary
    Jun 21, 2019 at 3:13

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I've found two sources for molecular hydrogen.

  1. The photolysis of formaldehyde (See chemical formula 10 and see Novelli et al. 1999)
  2. Anthropogenic sources.

While it isn't a source I think I should mention the exosphere. The exosphere is not very dense, so hydrogen (probably) doesn't react with anything. Since it is the lightest element, it probably doesn't go anywhere except out of the atmosphere.

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  • $\begingroup$ Excellent, thank you! $\endgroup$
    – uhoh
    Aug 30, 2020 at 0:18
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Besides man-made sources, hydrogen can be produced naturally through a process called serpentization. Mafic and ultramafic rocks, which are rich in magnesium and iron silicates, react with water to produce a variety of breakdown products summarized in the Wikipedia article on serpentine

Serpentinization is a geological low-temperature metamorphic process involving heat and water in which low-silica mafic and ultramafic rocks are oxidized (anaerobic oxidation of Fe2+ by the protons of water leading to the formation of H2) and hydrolyzed with water into serpentinite. Peridotite, including dunite, at and near the seafloor and in mountain belts is converted to serpentine, brucite, magnetite, and other minerals — some rare, such as awaruite (Ni3Fe), and even native iron. In the process large amounts of water are absorbed into the rock increasing the volume, reducing the density and destroying the structure.[1]

The density changes from 3.3 to 2.7 g/cm3 with a concurrent volume increase on the order of 30-40%. The reaction is highly exothermic and rock temperatures can be raised by about 260 °C (500 °F),[1] providing an energy source for formation of non-volcanic hydrothermal vents. The magnetite-forming chemical reactions produce hydrogen gas under anaerobic conditions prevailing deep in the mantle, far from the Earth's atmosphere. Carbonates and sulfates are subsequently reduced by hydrogen and form methane and hydrogen sulfide. The hydrogen, methane, and hydrogen sulfide provide energy sources for deep sea chemotroph microorganisms.[1]

Roughly speaking, the magnesium-bearing component of the mafic/ultramafic rock is basic and undergoes acid-base reactions with water, generating no hydrogen (but possibly producing magnesium hydroxide, which indicates said basic conditions). The iron-bearing components, by contrast, are reducing, especially under the basic conditions associated with the magnesium compounds, and they do generate hydrogen:

Serpentine is the product of the reaction between water and fayalite's ferrous (Fe2+) ions. The process is of interest because it generates hydrogen gas:[2][3]

$3 \text{Fe}_2\text{SiO}_4 + 2 \text{H}_2\text{O} → 2 \text{Fe}_3\text{O}_4 + 3 \text{SiO}_2 + 2 \text{H}_2$

The reaction can be viewed simplistically as follows:[4][5]

$6 \text{Fe(OH)}_2 → 2 \text{Fe}_3\text{O}_4 + 4 \text{H}_2\text{O} + 2 \text{H}_2$.

In addition, in the presence of carbonates and sulfates the reduction process can also produce methane and hydrogen sulfide. The references cited in this passage explore these processes as sources of raw material and chemically stored energy for undersea life both in Earth and in subterranean ocean worlds elsewhere in the Solar System.

Cited references:

1.
"Serpentinization: The Heat Engine at Lost City and Sponge of the Oceanic Crust". Link

2. "Methane and hydrogen formation from rocks – Energy sources for life". Retrieved 6 November 2011. Link

3. Sleep, N.H.; A. Meibom, Th. Fridriksson, R.G. Coleman, D.K. Bird (2004). "H2-rich fluids from serpentinization: Geochemical and biotic implications". Proceedings of the National Academy of Sciences of the United States of America. 101 (35): 12818–12823. Bibcode:2004PNAS..10112818S. doi:10.1073/pnas.0405289101. PMC 516479. PMID 15326313.

4. Russell, M. J.; Hall, A. J.; Martin, W. (2010). "Serpentinization as a source of energy at the origin of life". Geobiology. 8 (5): 355–371. doi:10.1111/j.1472-4669.2010.00249.x. PMID 20572872.

5. Schrenk, M. O.; Brazelton, W. J.; Lang, S. Q. (2013). "Serpentinization, Carbon, and Deep Life". Reviews in Mineralogy and Geochemistry. 75 (1): 575–606. Bibcode:2013RvMG...75..575S. doi:10.2138/rmg.2013.75.18.

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