Since water can obviously be found on our planet today, it means that the water we are now enjoying on Earth has been there since its formation, says a study which sheds new light into the ancient puzzle surrounding the origin of water on the planet.
For the study, published in the journal Nature Geoscience, the researchers looked far beyond Earth — up to Venus — to investigate the origin of terrestrial water.
While everybody agrees that our blue planet is rich in water, this observation is at odd, first, with the exploration of other rocky planets, genuinely lacking surface water, and second, with the idea of a giant impact between the proto-Earth and a planetary embryo the size of Mars that created the Moon.
Such a catastrophic event should have vaporized any pre-existing water, leaving behind a dry Earth.
After the giant impact catastrophe, we have thus two options to explain the presence of water on Earth — either water was brought back later, after the catastrophe, notably by icy or water-rich asteroids or the giant impact was not big enough to vaporise all the water on the planet.
Because of the importance of water to sustain life, the question of the origin of water on Earth is primordial. A major challenge in investigating this question is that Earth has lost all the traces of its formation since it is an active planet.
Therefore the team of numerical modelers and geochemists led by Cedric Gillmann from the Free University of Brussels (ULB ) in Belgium decided to look up to Venus to investigate the origin of terrestrial water.
While Earth and Venus could be considered as twin sisters, their respective geological and climatic evolutions diverged dramatically in the past.
But because of their proximity, the Earth and Venus should have received the same type of material during their history, the study said.
Using numerical simulations of impacts of different types of asteroids containing various amounts of water, the team discovered that water-rich asteroids colliding with Venus and releasing their water as vapour cannot explain the composition of Venus' atmosphere as scientists measure it today.
It means that the asteroidal material that came to Venus, and thus to Earth, after the giant impact must have been dry, therefore preventing the replenishment of the Earth in water.
The results suggest that the water we are now enjoying on Earth has been there since its formation, likely buried deep in the Earth so it could survive the giant impact.
The findings have very deep implications in terms of habitability of ancient Earth, Venus and Mars, as it suggests that planets likely formed with their near-full budget in water, and slowly lost it with time.
Because Mars is much smaller, it likely lost all its water while life developed on Earth.
For Venus, those results shine a complementary light on recent work advocating that water oceans existed at the surface of the planet, and help constrain the maximum amount of water that can be expected on Venus.
They will also help prepare the next generation of space missions to Venus..