Earth's first water found

Compared with our planetary neighbours, the Earth is a very wet place. But what scientists are unclear about is how much of its water the Earth was born with when it formed, and how much water arrived afterwards aboard incoming comets and asteroids.

Now, Lydia Hallis, from the University of Glasgow, has found some of the first water that formed with the Earth, four and a half billion years ago. She knows that's what it is because the chemical composition of the water is different from what's in the oceans today; it's made up of a lighter form of hydrogen, and it shows that when the Earth formed, it came with a lot of water built in. Lydia Hallis explained to Chris Smith...

Lydia -  Up until, I would say, about 15 years ago, maybe less than that, we were looking at Earth as water rich, but we thought that most of Earth's water was present in the oceans and in the rivers and in the atmosphere, and what we're finding more and more is that, actually, when you start to look at the interior of the Earth, in the mantle, there are really a lot of places that you can store water, that you would never think you can retain water, but it turns out that if you pressurise really common minerals and silicates in the mantle, you can actually retain a lot of water in what is essentially rock. It may be that there's actually more water in the interior of Earth than there is on the surface.

Chris - So there therefore could be two things going on here?  There could be water born with the Earth, which is inside, and then there could be this veneer of water on the outside, which is a mixture of what it was born with and what was delivered here afterward then?

Lydia - Yes, so what we were trying to figure out whether the two are completely separate. So whether we could actually find a sample of rock on the Earth that really did represent this deep mantle, and whether there was a reservoir of water down in the deep mantle that was completely separate from surface water that we could analyse and that would help us to understand where the Earth's original water came from.

Chris - How can you get at rocks that date from the time when the Earth was formed though?  Do they exist?

Lydia - We don't find rocks on the surface that are from the beginning of the Earth's formation, no, because we have plate tectonics on Earth, all of our crustal rocks eventually get recycled.  So we are looking for a reservoir deep in the Earth that does represent the Earth's primordial water, its original water.  The way that we get at that reservoir is to sample larvas that come from deep mantle plumes.  I suppose the most famous area would be Hawaii, and the reason there are volcanos there is because there's this huge plume of material that's thought to come from the deep mantle and erupts onto the surface.  Therefore, we get rocks on the surface that are sourced in the deep mantle.

Chris - When you do this, and you look at the flavour of the water that is coming out of the rock and compare it with the water on the surface of the Earth.  Are they the same, or are they different?

Lydia -   They're actually quite different and it shows that, in the deep mantle, there are isolated sources that have water that has been unaffected by any processing on the surface of Earth.  So, it's been there since the formation of the Earth.  It's the original water that Earth was formed with.

Chris -   Therefore, when the Earth formed, there must have been a lot of water in the material that gave rise to the Earth for that water to be inside the planet like that?  Is that the implication of this finding?

Lydia -   Yes, especially because if you imagine when Earth was accreting or any planetary body is accreting from small dust particles into a planet sized body, you get a lot of heat in there from radioactive elements and, also, just the heat from friction in accretion, it causes the whole body to melt. And originally, the Earth would have been essentially a big ball of lava in space, really, really hot at the surface and so any water that would have been accreting in there, that was able to escape as a gas onto the surface of Earth would have escaped.  We would have lost a lot of water into space, so it shows that whatever was accreting must have been really, really rich in water for us to be able to retain the amount of water we have today having, we assume, lost a lot, a big percentage of that water.

Chris - Is this important because it begins to give us an insight into what was in that disc of material from which the rocky worlds, that we are one of, formed.

Lydia - Yes.  I think it's really important, not only in terms of the Earth's formation, but also the other rocky bodies in the solar system, especially places like Mars where we are potentially interested in going to explore as humans.  It shows, if we assume that Mars and Earth formed in a similar type of mechanism, you would expect then that not only is Earth is quite rich in water, but Mars is also, in its interior, got quite a lot of water and it starts to pose the questions of what happened to Mars' water.  Is it still there, is it retained as subsurface ice, and it's things that we really don't know much about any of the other planets at all, really.  It just goes to show that we don't know that much about our own planet.