A vast cloud of vapour expelled from boiled asteroids may have lingered in the solar system for millions of years before raining down on Earth, according to a new idea for how our planet got its water.
The origin of Earth鈥檚 water has long puzzled scientists. It is hard to argue that our planet has always had the water we see today, because the young sun would have been so hot that it should have driven off any that existed at that time.
Alternatively, it could have come from asteroids that smashed into Earth over millions of years, but this requires a complex and specific dance of planets and asteroids for there to have been enough impacts.
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Now, at the Paris Observatory in France and his colleagues have suggested a different idea, which is that icy asteroids that existed early in the first few million years of the solar system were heated as the sun grew hotter, around 20 million years after it formed, producing a cloud of water vapour, which was then pulled towards Earth by gravity.
鈥淭his theory is more natural and simpler,鈥� says Kral. 鈥淵ou don鈥檛 need a complicated history for the Earth, where you have to play pool, pushing the asteroids or comets in the right direction.鈥�
Kral and his colleagues simulated a belt of icy asteroids, using what we know about their composition and the solar system鈥檚 early conditions, and worked out how long it would take for the ice to turn to water vapour and how large a disc it would form. They then calculated how much of this water would reach Earth and found it could match the amount seen on our planet.
This also showed that the process would result in the same ratio of hydrogen isotopes that we measure in Earth鈥檚 water today. This ratio can be used as a chemical fingerprint to trace water鈥檚 origin, and that in Earth鈥檚 water is very similar to what we see in some asteroids.
However, this is also the case for a scenario where our planet got its water from impacting asteroids, making it impossible to tell the two scenarios apart by just looking at our own solar system, says Kral. A piece of confirming evidence for the latest idea, however, would be to find the proposed process happening in asteroid belts around other young stars, he says.
Astronomy & Astrophysics
