0.2%
Fraction of Earth's mass that is water
ON DECEMBER 6, 2020, dozens of scientists and technicians from the Japanese space agency, JAXA, roamed the South Australian desert, waiting for a sign from above. They were looking for a bright streak of light that would cross the sky, bringing with it new knowledge about the origins of our Solar System. It arrived just before dawn, accompanied by a radio signal that revealed the location of the treasure left behind: a capsule filled with about 5 grams of the near-Earth asteroid 162173 Ryugu, recovered by the Hayabusa2 robotic spacecraft more than one year earlier and dropped as the craft whizzed by Earth.
Ryugu itself might only be 9 million years old, but it's made of debris from a much older asteroid, a rocky remnant from just after the Solar System's formation about 4.6 billion years ago. Asteroids and their icy cousins, the comets, act as time capsules, preserving chemical details that can help us understand how the planets came to be the way they are. One of those details is where our planet's water comes from.
Scientists have debated the origin of Earth's water for decades. While some think that water was embedded in Earth's original building blocks, many more think it came from elsewhere — for instance, from comets and asteroids that collided with the young Earth when the Solar System was in its infancy. Others think the Sun itself might have played a role. So far, none of these ideas can provide an unequivocal answer.
The quest for water's origin is not just about hydrogen and oxygen. It's part of the larger mystery of how Earth got its volatile elements, from the nitrogen that makes up 78% of our atmosphere to the carbon that dominates life's chemistry. The same mechanism most likely furnished Earth with all of these elements, the very things that make our blue planet what it is.
Foreign water
Based on its ocean-covered surface, Earth looks like a planet overflowing with water. Even more hides
