By Mark Waghorn via SWNS
Water on Earth came from the edge of the solar system, according to new research.
It was carried by meteorites that formed far out in space where it is cold enough for CO2 to freeze.
Other space rocks would have been too dry - ruling them out as the source of our lakes, rivers and oceans, say scientists.
The discovery came as a surprise - and could help identify worlds most like to host extra-terrestrials.
Lead author Dr. Megan Newcombe, of the University of Maryland, said: "We wanted to understand how our planet managed to get water because it's not completely obvious.
"Getting water and having surface oceans on a planet that is small and relatively near the sun is a challenge."
The study shows, contrary to popular belief, not all outer solar system objects are rich in water.
It means water was delivered to Earth via unmelted, or chondritic, meteorites.
The findings have applications beyond geology. Alien hunters are interested in the origin of Earth's water because of its deep connections with life.
Newcombe said: "Water is considered to be an ingredient for life to be able to flourish so as we're looking out into the universe and finding all of these exoplanets, we're starting to work out which of those planetary systems could be potential hosts for life.
"In order to be able to understand these other solar systems, we want to understand our own."
The finding is based on an analysis of seven melted space rocks.
They crashed into Earth billions of years after splintering from at least five "planetesimals" that collided to form Earth, Mercury, Venus and Mars.
The parent bodies were heated up by the decay of radioactive elements in the early solar system - causing them to separate into layers with a crust, mantle and core.
In the first analysis of its time the U.S. team used scanners to measure levels of volatiles such as magnesium, iron, calcium, silicon - and water.
Co-author Dr. Conel Alexander, of the Carnegie Institution for Science in Washington DC, said: "The challenge of analyzing water in extremely dry materials is that any terrestrial water on the sample's surface or inside the measuring instrument can easily be detected - tainting the results."
Samples were baked in a low-temperature vacuum oven to remove any surface water - and then dried out.
Newcombe said: "I had to leave the samples under a turbo pump—a really high-quality vacuum—for more than a month to draw down the terrestrial water enough."
Some came from the warm and dry inner solar system which is home to Earth and others from the colder, icier outer reaches.
Water is thought to have originated in the latter but the objects that carried it across remain a mystery.
Co-author Dr. Sune Nielsen, of Woods Hole Oceanographic Institution in Massachusetts, said: "We knew plenty of outer solar system objects were differentiated.
"But it was sort of implicitly assumed because they were from the outer solar system, they must also contain a lot of water.
"Our paper shows this is definitely not the case. As soon as meteorites melt, there is no remaining water."
It comprised less than two-millionths of the samples' mass. The wettest meteorites contain up to about a fifth of water by weight - or 100,000 times more.
Heating and melting leads to near-total loss - regardless of how much they started out with. The study is in the journal Nature.
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