EARTH × ATMOSPHERE

Where Earth's Water Came From

~4.5 to 4.4 billion years ago (origin debated)

Hadean

Nobody knows for certain where the ocean came from. The meteorites that best match the Earth's own composition were long taken for bone-dry, born too close to the Sun to keep any water; measured finely, they turn out to hold enough hydrogen to fill the ocean at least three times over, in which case the water came with the planet's own building blocks, there from the start. But the grains brought back from the asteroid Ryugu tell another story: they resemble a rare and exceptionally water-rich family of meteorites, rare on the ground largely because material so fragile seldom survives the plunge through the atmosphere. Asteroids of that kind may be widespread, and could have delivered the ocean ready-made.

Why it matters

If planets are born with their water, oceans should be common across the galaxy; if impacts have to bring it afterwards, oceans are a lottery. The answer decides how many other worlds we should expect to find with seas.

Dating & uncertaintyDisputed

Each side rests on an isotopic fingerprint. The hydrogen locked in enstatite chondrites, the meteorites closest to Earth's make-up, matches the hydrogen of the Earth's mantle; the oxygen of the Ryugu grains comes closest to that of the CI chondrites, water-rich bodies born far from the Sun. Both prints are convincing, and that is the difficulty: the measurements alone no longer single out one source.

Sources

1. Piani et al. (2020), Science · Earth's water may have been inherited from material similar to enstatite chondrite meteorites
2. Greenwood et al. (2022), Nature Astronomy · Oxygen isotope evidence from Ryugu samples for early water delivery to Earth by CI chondrites
3. Piani et al. (2018), Nature Astronomy · A dual origin for water in carbonaceous asteroids revealed by CM chondrites

Earth history as one day

00:28:3223 h 31 min before midnight

if Earth’s 4.54 billion years were compressed into a single day