Researchers studying a meteorite that fell to Earth in 2000 found that the water in its parent asteroid disappeared soon after the space rock formed, when its insides were still warm.
Asteroids that slammed into Earth several hundred million years after the solar system's birth were thus probably relatively dry, researchers said.
"So, our results suggest that the water [was] supplied to Earth in the period when planets formed rather than the period of late heavy bombardment from 4.1 billion years to 3.8 billion years ago," study lead author Yuki Kimura, of Tohoku University in Japan, told LiveScience.
Kimura and his colleagues analysed the Tagish Lake meteorite, which landed in Canada's Yukon Territory in January 2000.
The scientists used a transmission electron microscope to observe tiny particles of magnetite, which arranged themselves within the meteorite into three-dimensional "colloidal crystals."
These crystals can be formed during the sublimation of water - the transition of the material directly from ice to vapour - but not during freezing, Kimura said.
This implies that the parent asteroid's bulk water disappeared in the early stages of the solar system's formation, before the space rock's innards had a chance to cool down, he added.
In addition to water, impacts likely delivered to the young Earth organic molecules, the carbon-containing building blocks of life.
Indeed, the colloidal crystals in the Tagish Lake meteorite have an organic layer on their surface, Kimura said.


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