Debate continues over how water came to be on Earth

It is a mystery whether Earth’s water formed along with Earth or if it came after Earth formed. There is good scientific evidence for either scenario.

Water is common in the universe. Telescopes have located spectral signatures of water in nebulae in distant reaches of the galaxy.

Earth formed from tiny bits of matter coalescing within a vast gaseous nebula consisting mostly of interstellar hydrogen and helium mixed in with tiny amounts of heavier atoms from supernova explosions.

Slowly, gravity began collapsing a section of the nebula to form a slowly rotating disk. Hydrogen and helium, compressed by gravitational attraction in the center of the disk, began nuclear fusion when the density reached a certain threshold, forming the sun.

Meanwhile, matter in the outer disk collected to form rings. Within the rings, countless particles of ice and dust made of heavier atoms such as silicon, oxygen, aluminum and iron collided and stuck together, forming small bodies called planetesimals. Countless collisions of planetesimals over million of years formed larger protoplanets.

The third protoplanet from the sun was proto-Earth. As it orbited the sun, gravity vacuumed in matter from its orbit. Energy from the collisions transformed into heat. Proto-Earth grew from each collision as it got hotter and hotter.

When it became large enough and soft enough, gravity shaped it into a spherical planet. Continuing impacts melted the planet, allowing the heavier iron to sink to the center, forming the core. Lighter silicon, oxygen, aluminum and others remained near the surface.

As the crust solidified, remaining chunks of matter struck Earth as meteorites, pulverizing and perhaps remelting it and hypothetically bringing water. Then a Mars-sized protoplanet collided with Earth, spraying debris into space. Gravity pulled this debris into a ring, which eventually coalesced to form the moon.

After 700 million years, Earth’s surface had cooled enough to solidify, but the interior remained molten with gases dissolved in it. Volcanic eruptions released gases to form the primeval atmosphere rich in water and carbon dioxide, the two main gases released in volcanoes.

Even at today’s relatively low rate of volcanic eruptions, enough water vapor is erupted into the atmosphere to fill the ocean basins in a little over a billion years.

Scientists in the recent past hypothesized that Earth’s water came from collisions with comets that formed in the remote Kuiper Belt. Hydrogen isotope ratios in water from comets Halley, Hyakutake and Hale-Bopp are different from Earth’s water, so Earth’s water is not likely from comets.

Recent studies of primitive meteorites called carbonaceous chondrites reveal that these meteorites represent protoplanets of rock and ice formed in the outer asteroid belt, not in the distant Kuiper Belt. Furthermore, asteroids contain the same ratio of hydrogen isotopes as Earth and its moon.

It would appear that Earth is made at least in part from carbonaceous chondrites.

The water is from asteroids, but the water itself is older than the sun. Last month, researchers at Carnegie-Mellon University reported that a significant fraction of the water in our solar system originated in interstellar space.

Richard Brill is a professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.