NEW YORK (NYTIMES) - Water covers 70 per cent of the Earth's surface. It's the source of life and home to countless living things. But where did it come from?
Scientists have devised many ways to account for Earth's water. It could have been here from the start. It could have arrived with dirty ice balls called comets, or as part of interplanetary dust.
And then there are asteroids, rock objects of varying sizes that have come crashing to Earth at different times in its history, sometimes in great numbers.
More than four billion years ago, they pummelled the planet in a period called the late bombardment.
Even though samples of asteroids, in particular the kind called carbonaceous chondrites, show that they contained water, it seemed that the water would be lost in the intense heat of the impacts.
But Dr R. Terik Daly, now a researcher at Johns Hopkins, and Dr Peter Schultz, a planetary scientist at Brown University, conducted small-scale experiments that suggest the water in asteroids could have been captured in Earth's rock.
Dr Schultz said the results were surprising: "I've been doing this for 38 years, and what I've discovered is that every time we do an experiment, you think you know what's going to happen and it doesn't happen that way."
The results were published in Science Advances.
Dr Daly, who was working on a doctorate at Brown, and Dr Schultz, his adviser, used Nasa's vertical gun, a three-story apparatus designed to fire projectiles at a target at different angles.
They shot pea-size pebbles of a rock called antigorite, similar to the material of carbonaceous chondrites, at a small sample of pumice, meant to mimic a dry earth.
The gun only had to shoot the pebbles about 10 feet (3.04 metres), but at a speed of more than 17,700 kmh.
At that speed, the energy of the impact generated such heat that the water in the antigorite was vaporised, but it was absorbed by the melted pumice as it solidified and fused with bits of antigorite.
At an impact angle of 45 degrees, for instance, about 30 per cent of the water was captured by the mix of impact melt and rock fragments called breccia.
If that happened with real asteroids, that water might later be liberated, for instance as steam from lava flows, and contribute to the conditions that made life on Earth possible.
It wouldn't account for all of Earth's water, Dr Schultz said, but it's new information for a very important subject.