Where did Earth’s water come from?

A fraction of the hydrogen in any water is deuterium, a heavier isotope. The ratio depends on the temperature and conditions where that water formed, which is what makes it useful: the same measurement made on meteorites, comets and moons can say where each of them formed.

So the question can be asked as a matching problem. Earth’s oceans have a particular ratio; any proposed source has to have that ratio or be mixed from things that average to it.

A 2012 study measured hydrogen isotopes in 86 primitive meteorites and found the water in carbonaceous chondrites distinct from the water in comets, with the hydrogen and nitrogen of the CI group suggesting it was the principal source of Earth’s volatiles. That implies their parent bodies formed in a different region from the comets, contrary to what recent dynamical models had predicted.

Comets have been measured one at a time, and the ratios differ between them by as much as a factor of four. The comet Rosetta reached in 2014 measured about three times Earth’s, and that brought its family into question as a source.

A 2024 reanalysis of the whole mission challenges that reading. Its authors find that ice on dust grains in the coma releases water far richer in deuterium than the nucleus, and that the ratio measured where the gas is well mixed is 1.2 to 1.6 times Earth’s, in line with the other Jupiter-family comets measured. A 2022 analysis of the same instrument’s data had found the ratio high and steady wherever the spacecraft was, and that is the result the reanalysis disputes.

Delivery by impacts after Earth had largely formed is one model: a small late addition of water-rich material from the outer solar system. Another has the water built in from the start. Enstatite chondrites, meteorites whose isotopes resemble Earth’s rocks, were presumed dry because they formed in the inner solar system; a 2020 study found enough hydrogen in them to have supplied at least three times the water in the oceans. And some may have come from the nebula itself, dissolved into an early magma ocean from a thick primordial atmosphere whose traces are still in the mantle, though one estimate puts that share at a tenth or less of the mantle’s volatiles.

These are not exclusive, and the mixture is what is being argued about. Much of the water may not be in the oceans at all: one mass balance puts up to nine tenths of it in the mantle, and work on core formation suggests the core took most of the planet’s hydrogen. The mantle’s hydrogen is also lighter than the oceans’, so matching the oceans tests only part of the inventory.

The ratio in Earth’s oceans today is not necessarily the ratio Earth started with. Hydrogen escapes to space preferentially over deuterium, and one model of an early hydrogen-rich atmosphere raises the ocean’s ratio by a factor of two to nine, which would mean the match with the meteorites does not by itself point to them. A 2026 calculation adds core formation, which it finds drew light hydrogen into the core and left the rest of the planet richer in deuterium.

Against that, a 2012 synthesis finds terrestrial hydrogen and nitrogen inside the meteorite range, and explains them by mixing two reservoirs without needing any escape fractionation. The matching exercise assumes a target that may have moved, and whether it moved is itself unsettled.