Earth's earliest water uncovered

The early Earth was awash with water, new research has shown.

The planet we inhabit formed about 4.5 billion years ago. But what's unclear is whether the water that's here now arrived only much later, delivered by incoming comets and asteroids, which is the favoured theory, or whether the material that formed the Earth in the first place also contained abundant water.

Now Glasgow University scientist Lydia Hallis has uncovered signs of the earliest water on the Earth and shown that both proposals are correct.

The ratio of heavy and lighter forms - or isotopes - of hydrogen in water can be used like a chemical fingerprint to establish its origin. Water in the oceans, for example, bears a close isotopic similarity to the water on some asteroids.

But unlocking what water was present at the time the Earth first formed is challenging, since the oldest rocks on Earth's surface, that might contain water samples, go back only about 4.1 billion years because tectonic plate movements have consumed anything older.

Instead, Hallis and her colleagues have turned to the planet's interior for the answer. Magma erupted from volcanoes at various places, including Hawaii, is derived from the Earth's mantle, which sits beneath the crust.

Crystals in this material contain water samples, which bear a very different isotopic signature to the water in the Earth's oceans. "And this suggests that when the planet formed, there was a lot of water there already," says Hallis.

The findings are important because they provide an insight into the composition of the dust and gas from which the Earth assembled itself as the solar system formed initially. "And this is relevant to the other rocky planets too, which formed in the same way," she says. "Earth's pretty wet, and so, possibly was Mars, in its infancy."