Where did Earth's water come from?

Earth assembled in the warm inner zone of the Sun’s planet-forming disk, where water ice could not condense; by the simplest reading of the rules, this planet should be bone dry. Instead its surface is two-thirds ocean. Every glass of water is imported goods from somewhere, and the question of where has been argued in earnest since geologists first tried to write the history of seawater in the early 1950s.

Why it matters

Water is the solvent of life, and Earth’s habitability hangs on having received the right amount at the right time: enough for oceans, not so much that the planet drowned under a global sea with no exposed rock to weather. Knowing how water reaches rocky planets tells us whether worlds like ours are routine or lucky, which quietly sets the odds inside every question about life elsewhere. It also anchors the history of our own solar system, since each proposed water source implies a different story of how the planets shuffled and scattered material in their first hundred million years.

What has been tried

The main tool is a fingerprint: the ratio of heavy hydrogen to ordinary hydrogen, which varies by where in the disk a water reservoir formed. Earth’s oceans have been compared against every candidate courier. Most comets measured so far carry water too heavy to be the main source, though at least one has matched. Water-rich carbonaceous asteroids, sampled through meteorites, match well and remain the leading suspects. A newer line of evidence complicated the picture: enstatite chondrites, dry-looking meteorites long assumed waterless that resemble Earth’s original building blocks, turn out to hold enough hydrogen to supply oceans several times over, suggesting Earth may have carried much of its water from birth rather than importing it late. The young Sun’s nebular hydrogen, reacting with molten rock, may have contributed too.

Where the edge is

The question has shifted from which source to in what proportions and when. Isotopes constrain but do not yet settle the mix of inherited water, asteroid delivery, and nebular gas, nor how much water hides in the mantle, nor how much the Moon-forming impact boiled away and what survived it.

What would count as an answer

A budget that closes: isotopic and chemical fingerprints tight enough to assign the ocean’s contents to specific sources in specific fractions, consistent with the timeline of Earth’s assembly. The water is here; the receipts are still being audited.