Image credit: Jack Cook

Over 70% of the Earth’s surface is glazed with glistening ocean. This, of course, is where the phrase ‘The Blue Marble’ originated from. All this water amounts to a total volume equal to 1.3 billion cubic kilometres – but did you know that according the model of the Sun’s proto-planetary disk, Earth actually has a deficit of this familiar and life sustaining substance?  Astronomers studying how the Earth initially formed have been stuck trying to explain why we lack so much of the seemingly abundant liquid. Why? In theory, Earth should have a similar composition to Neptune or Uranus—in essence, all signs point to Earth being a water world.

This almost paradoxical question stems from the ‘snow line’. It is the distance from the Sun that marks the boundary of where any ice in the vacuum of space should be melted. At the moment, our line lies within the asteroid belt (between the orbit of Mars and Jupiter). The conventional model of early solar system suggests that the disk of dust, gas and debris inside the snow line was fully ionised. As such, a substantial amount of material would have been funneled onto the hot, young star. This line is thought to have started out at least one billion miles from the sun, but as the temperatures dwindled, it retreated inwards past Earth’s orbit BEFORE the planet had time to form. But if this line hadn’t moved before Earth formed, we should have been built from ice, rather than the heavy metals and silica compounds known of today.

Image credit: NASA, ESA

So evidently we have a problem; Earth couldn’t have formed before the snow line moved inwards, as there was nothing there to build it from. Furthermore, if it formed outside it, we would have resembled Neptune or Uranus. What’s the answer then? Well a recent study is saying that the proto-planetary disk couldn’t have been fully ionised because main sequences stars - such as our own - simply don’t have the ‘energetic punch’ to do it. This means that the normal mechanisms going on can’t function as previously thought, and consequently; the matter, which would have flowed into the star in the old model, now sits in orbit.

This leads a ‘dead zone’ about 0.1 AU out. It acts much like a wall that prevents material from moving beyond it. The ice is still melted and vaporised in a region stretching out a few AUs, but the dry material left over in the inner solar system was enough to build the small planets we see today. So now that we have an accurate explanation of why Earth’s composition lacks so much water, where did our thin layer of water come from? Without water, life as we know it would be impossible. By proxy, Earth would be a very different place to what it is today.

Credit: Photo:Colourbox

Up until very recently the most prevalent theory for the origin of our water was that it has always been here. It was thought that during the molten stages of our planet's evolution, volcanic activity was incredibly high, and the water locked within the crust or mantle reached the surface and formed a part of a thick carbon dioxide rich atmosphere. How and when the Earth’s surface cooled isn’t clear, as the impact that created the moon would have melted huge portions of the crust – vaporising even heavy metals. Eventually though, it did cool; ultimately  allowing thick clouds of water vapour to condense to form the oceans. But we now think that this is wrong.

Water isn’t as uniform across the universe as you may have originally thought it was; there are actually different forms, out of the 2 most common, one is much denser than the other (tritium is very rare). Hydrogen exists as protium (1 proton, and generally the one we are all familiar with) and deuterium (1 proton and 1 neutron). Deuterium shares near enough the exact same chemical properties as protium, such that it too can bond with oxygen atoms to form a molecule that we call ‘heavy water’. This is rare, and accounts for about 0.016% of the ocean. Nevertheless, this ratio is important as it holds the key to the possible origin of our water. If Earth got its water right at the beginning of its life we’d expect a much higher ratio of deuterium to protium, and so looking elsewhere for the ratio we do see could hold the answer.

Artist rendering of the late heavy bombardment period (credit: NASA)

It has probably already occurred to you where this is going, but comets are the new theory. After the Earth formed as a dry ball of rock and the snow line retreated, comets bombarded the Earth for millions of years (we now call this the late heavy bombardment period). But how can we determine if they were our watery messengers? Well it isn’t easy, but is has been done. We have flown right through the tail of a comet and measured the ratios of deuterium to protium water molecules – and the results are promising, but not conclusive. The next big break through may come in the next few days and; as the much-publicised Comet ISON enters the inner solar system.

By carefully analysing what is thought to also be one of the best cosmic shows of a generation, we are hoping to use the data to further push forward the theory that comets delivered our life-giving liquid. Studying the event may also uncover many more mysteries about the early solar system - though for now, all we can do is wait and see.

In the meantime, you may enjoy a related article with bits of interesting information on Earth's water. Check it out here.

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