Where Did All of Earth’s Water Come From?
A few years ago, I came across a science illustration that stopped me in my tracks. It showed, in a single glance, how much water exists on Earth. We know that water covers 75% of our planet’s surface, so instinctively we might think it’s a massive quantity. But when gathered together, all that water (excluding water bound up in Earth’s mantle) would form a single sphere just 1,384 km in diameter—about the size of a small moon.
In the illustration, this “big” water sphere sits above Earth, which appears as if its oceans have been drained, exposing the sea floor. The overall shape of the planet remains essentially unchanged. Next to the large sphere is a much smaller one—this represents all the freshwater on Earth. Most of it is groundwater. Even smaller, barely more than a dot, is the volume of water in lakes and rivers combined.
Despite appearances, the total water sphere is still substantial—its diameter is nearly 1,400 km. Given Earth’s diameter of about 12,700 km, water accounts for roughly 0.15% of our planet’s volume, or about 0.03% of its mass. With an average ocean depth of 3.8 km, if Earth’s surface were perfectly smooth, the entire planet would be covered by almost 3 km of water. In that sense, Earth truly deserves the name “water world.”
A Clue from a Comet
In January 2015, Science published results from the European Space Agency’s Rosetta mission to comet 67P/Churyumov–Gerasimenko. After a decade-long journey, Rosetta deployed the lander Philae onto the comet’s surface and continues to orbit and study it. One highlight: Rosetta’s mass spectrometer measured the ratio of deuterium to hydrogen (D/H) in the comet’s water molecules.
Comet 67P’s D/H ratio was found to be 0.00053—meaning 53 deuterium atoms per 100,000 hydrogen atoms—more than three times higher than Earth’s ratio of 0.00015. This suggests that comets like 67P were unlikely to be the primary source of Earth’s water.
No Oceans on the Early Earth
When the solar system formed 4.6 billion years ago, the newborn Earth was hot. Any water molecules present would have existed as vapor in the atmosphere or trapped in rock. As the surface cooled below 100°C, liquid water could condense—but by then, much of it would have escaped into space. The inner planets, including Venus and Mars, shared this problem.
Where, then, did Earth’s oceans come from? Evidence points to delivery by water-rich bodies from farther out in the solar system—mainly asteroids, and to a lesser extent comets. Measurements show that the D/H ratio in certain asteroids closely matches that of Earth’s oceans, implying a shared origin.
Deuterium is a stable isotope, so a match in D/H ratio strongly suggests that two water sources are related. By contrast, the Sun’s D/H ratio (measured in the solar wind) is about 0.00002—seven times lower than Earth’s—making it impossible for our oceans to be primordial solar system water simply retained during Earth’s formation.
The Snow Line and the Great Delivery
In the early solar system, there was a “snow line” between the orbits of Mars and Jupiter. Inside this boundary, temperatures were too high for water to condense as ice; beyond it, ice could form and be incorporated into planetesimals. Asteroids that formed beyond the snow line contained abundant ice.
Early on, Jupiter’s orbit was unstable. Its gravitational disturbances scattered many of these icy asteroids inward, where some collided with the young Earth, delivering vast quantities of water.
Previously, scientists thought this bombardment happened hundreds of millions of years after Earth’s formation. But newer research suggests it occurred much earlier—within a few tens of millions of years—meaning that by 100 million years after its birth, Earth may already have looked much like the “blue planet” we know today. This earlier timeline could also shift back estimates for when life first appeared.
