The origin of Earth's water is one of those foundational issues that explains our planet's habitability. The idea that water was delivered by comets and asteroids has been on the minds of planetary scientists for a long time. Both types of bodies, and especially comets, are known to have water, and both types are known to have crashed into Earth in large numbers during the planet's early ages.

But there's also a growing understanding that rocky planets form with a primordial envelope rich in hydrogen and helium that they acquire from their solar nebulae . Since water is—obviously—H2O, then Earth was born with substantial amounts of hydrogen, and all that's required was oxygen. Did that need to be delivered to Earth somehow?

Sort of, but not exactly, and new research shows why. It's titled " Dry pebble rain could have delivered water to Earth through hydrogen reduction of iron oxide ," and it's published in Nature Geoscience.…