Scientists say clues from meteorites suggests there was water on Earth more than a hundred million years earlier than had previously been thought -- and that perhaps life started earlier than had been believed.
Previous theories have held that the Earth and the other inner planet formed dry and that water arrived later courtesy of comets or asteroids containing ice and gases.
"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, and that surface water as it exists on our planet today, must have come much, much later -- hundreds of millions of years later," says Adam Sarafian of MIT and the Woods Hole Oceanographic Institution, who was the lead author of a study published in the journal Science.
However, the researchers suggest an earlier source of water could have been the most primitive form of meteorites, known as carbonaceous chondrites, which arose from the same disk of dust, ice and gas out of which the sun formed more than 4.5 billion years ago.
"These primitive meteorites resemble the bulk solar system composition," says WHOI geologist and study coauthor Sune Nielsen. "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 researchers used measurements of the ratio of the two hydrogen isotopes, deuterium and hydrogen, which vary considerably in different parts of our solar system and can be used to estimate when water formed in meteors, asteroids and planets.
They focused on an asteroid, 4-Vesta, and meteorite samples from it that have been found in Antarctica.
Those meteor fragments from the asteroid, which came together in the same part of the solar system as our planet, display a signature of one of the oldest reservoirs of hydrogen in the solar system, the researchers said.
Their age makes them perfect for estimating the amounts of water in the inner solar system during the period when Earth was in its major building phase, they said.
While the Earth grew and changed over the next 4 billion years or so, Vesta remained frozen in time, Sarafian says.
"Vesta gives us a snapshot of what Earth maybe looked like when it was first forming," he says.
The same hydrogen isotopic composition exists in Vesta as in carbonaceous chondrites, evidence that the chondrites could have been the likely common origin of water for both the asteroid and the early planets, the researchers said.
The study findings don't rule out a later accumulation of water on the Earth, they acknowledge, but water from chondrites was present early in significant amounts, they say.
"An implication of that is that life on our planet could have started to begin very early," Nielsen noted. "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."
