The Source of Hydrogen in Earth’s Building Blocks

By Keith Cowing
Status Report
June 21, 2024
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The Source of Hydrogen in Earth’s Building Blocks
Optical image of the clastic matrix region in LAR 12252 that was studied using S-XANES. Notable phases include enstatite (example outlined in red), sulphides (example outlined in yellow), a small chondrule fragment (outlined in orange), and fine matrix (example highlighted in pink). b Corresponding map calculated from the SXANES spectra of the region shown in a, showing the amplitude of the H-S peak at each point normalised to the average amplitude of point spectra measured from chondrule mesostasis. Black pixels correspond to points where the S-XANES spectra that did not contain a resolvable H-S peak. White pixels correspond to points where the SXANES spectra could not be fitted due to their quality or noise in the spectra. — astro-ph.EP

Despite being pivotal to the habitability of our planet, the process by which Earth gained its present-day hydrogen budget is unclear. Due to their isotopic similarity to terrestrial rocks across a range of elements, enstatite chondrites (ECs) are thought to be the meteorites that best represent Earth’s building blocks.

Because of ECs’ nominally anhydrous mineralogy, these building blocks have long been presumed to have supplied negligible hydrogen to the proto-Earth. Instead, hydrogen has been proposed to have been delivered to our planet after its main stage of formation by impacts from hydrated asteroids. In this case, our planet’s habitability would have its origins in a stochastic process. However, ECs have recently been found to unexpectedly contain enough hydrogen to readily explain Earth’s present-day water budget.

Although this result would transform the processes we believe are required for rocky planets to be suitable to life, the mineralogical source of ~80% of hydrogen in these meteorites was previously unknown. As such, the reason ECs are seemingly rich in hydrogen was unclear. Here, we apply sulfur X-ray absorption near edge structure (S-XANES) spectroscopy to ECs, finding that most (~70%) of their hydrogen is bonded to sulfur.

Moreover, the concentration of the S-H bond is intimately linked to the abundance of micrometre-scale pyrrhotite (Fe1-xS, 0<x<0.125), suggesting most hydrogen in these meteorites is carried in this phase. These findings elucidate the presence of hydrogen in Earth’s building blocks, providing the key evidence that unlocks a systematic, rather than stochastic, origin of Earth’s hydrogen.

Thomas J Barrett (1), James F. J. Bryson (1), Kalotina Geraki (2) ((1) University of Oxford, Department of Earth Sciences, (2) Diamond Light Source)

Comments: 19 pages, 12 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2406.13637 [astro-ph.EP] (or arXiv:2406.13637v1 [astro-ph.EP] for this version)
Submission history
From: Thomas Barrett
[v1] Wed, 19 Jun 2024 15:32:02 UTC (4,138 KB)
Astrobiology, Astrochemistry, Astrogeology,

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻