Asked by: Paul Leslie, Chelmsford
As Earth was intensely hot following its formation 4.6 billion years ago, little of today’s water is likely to date back that far. Instead, it’s thought to have arrived later, in collisions with objects from elsewhere in the Solar System.
Comets were long thought to be the most likely source, but data sent back from the recent Rosetta mission has confirmed suspicions that these ‘dirty snowballs’ contain water with a mix of isotopes different to water found on Earth.
So attention has now switched to so-called Kuiper Belt Objects (KBOs) orbiting far beyond Neptune. Studies of these asteroid-like objects have revealed the presence of water, and they are now suspected of having delivered it to Earth when swarms of them smashed into our planet around 3.8 billion years ago.