The water that makes Earth a majestic blue marble was here from the time of our planet's birth, according to a new study of ancient meteorites, scientists reported Thursday.
Where do the oceans come from? The study headed by Adam Sarafian of the Woods Hole Oceanographic Institution (WHOI) in Woods Hole, Massachusetts, found that our seas may have arrived much earlier on our planet than previously thought.
The study pushes back the clock on the origin of Earth's water by hundreds of millions of years, to around 4.6 billion years ago, when all the worlds of the inner solar system were still forming.
Scientists had suspected that our planet formed dry, with high-energy impacts creating a molten surface on the infant Earth. Water came much later, went the thinking, thanks to collisions with wet comets and asteroids.
"Some people have argued that any water molecules that were present as the planets were forming would have evaporated or been blown off into space," said study co-author Horst Marschall, a geologist at WHOI.
For that reason, he said, scientists thought that "surface water as it exists on our planet today must have come much, much later-hundreds of millions of years later."
But no one was certain. To pin down the exact time of the arrival of Earth's water, the study team turned to analyzing meteorites thought to have formed at different times in the history of the solar system.
First, they looked at carbonaceous chondrite meteorites that have been dated as the oldest ones known. They formed around the same time as the sun, before the first planets.
Next they examined meteorites that are thought to have originated from the large asteroid Vesta, which formed in the same region as Earth, some 14 million years after the solar system's birth.
"These primitive meteorites resemble the bulk solar system composition," said Sune Nielsen of the WHOI, a study co-author. "They have quite a lot of water in them, and have been thought of before as candidates for the origin of Earth's water."
The team's measurements show that meteorites from Vesta have the same chemistry as the carbonaceous chondrites and rocks found on Earth. This means that carbonaceous chondrites are the most likely common source of water.
"The study shows that Earth's water most likely accreted at the same time as the rock," said Marschall.
"The planet formed as a wet planet with water on the surface."
While the authors are not ruling out that some of the water that covers 70 per cent of Earth today may have arrived later, their findings suggest that there was enough already here for life to have begun earlier than thought.
"Knowing that water came early to the inner solar system also means that the other inner planets could have been wet early and evolved life before they became the harsh environments they are today," explained Nielsen.
Circling the sun between the orbits of Mars and Jupiter in the main asteroid belt, Vesta is the second largest asteroid known and has an ancient, battered surface.
For sky-watchers with binoculars, the magnitude 7.8 Vesta looks like a very faint, star like object in a sea of stars. It is visible low in the south-western sky after dusk, about 6 degrees above the bright orange star Antares, but only from a dark location. For those stuck under light-polluted city skies, I recommend looking at the asteroid with a pair of binoculars or small telescope.
Although you can easily see the asteroid with binoculars, a telescope will allow you to watch it move in front of a background of stars.
With a help of the star charts above, steadily held binoculars, and some patience, you should be able to find distant Vesta, some 267 million miles (430 million kilometres) away. Sky and Telescope's website has some printable sky charts for extra help.
is a science writer, broadcaster, and lecturer