HOW DID EARTH GET ITS WATER?

Moon rocks suggest that the water might have been here all along

- Charles Q. Choi

Earth's water may have been here since the planet formed, and not delivered later by collisions with icy comets. New research analysed Moon rocks brought to Earth by the Apollo program and sheds light on our planet's earliest days. Although more than 70 per cent of Earth's surface is covered in water, overall our planet is relatively poor in water and other volatile molecules compared with most other bodies in the Solar System, said Lars Borg, a planetary scientist at Lawrence Livermore National Laboratory in California.

Scientists have long debated how Earth came to possess water. Two major scenarios prevail, and both involve ancient cosmic impacts, the most notorious of which saw the proto-Earth collide with a Mars-size rock dubbed Theia that helped give birth to the Moon. In the first scenario, "Earth is born dry and inherits its water through the addition of material from water-rich bodies such as comets and primitive meteorites," Borg said. "In the second, Earth forms with volatile element abundances that are similar to the Solar System average, but loses most of them during the giant impact that formed the Moon."