  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.
