Friday, 17 October 2014

The origin of water on the Earth.

In GCSE chemistry it is taught that the Earth’s atmosphere is formed through the activity of volcanoes. We know that volcanoes produce huge amount of water vapour, estimates suggest up to 60% of the gaseous emissions from a volcano is water vapour. We know the early Earth had a high amount of volcanism and the theories follow that this produced the water vapour that would eventually cool down and condense to form the oceans.

However the issue with this theory is that it assumes that there was water present in the Earth already. That the water was heated due to pressure and the heat of the Earth’s core into steam in order to erupt out of volcanoes. This gives rise to the question of where water on the Earth came from.

A paper published today (17/10/14) shows that Mimas – one of Saturn’s moons – has an orbital wobble greater than first predicted. The authors suggest that this is due to either a ‘rugby ball’ shaped core or an ocean of subsurface water.

This is of interest because we are finding that ice or water is more common amongst moons and planets than first thought. For instance Jupiter’s moon Europa has a surface of solid ice and Enceladus has sheets of ice and ice volcanoes giving rise to a whole geology built upon water rather than rock. These ice eruptions have been shown to add to Saturn’s rings. Recent images from NASA’s MESSENGER have shown that Mercury has ice at its poles. We know that Mercury has extreme temperatures and very little atmosphere so the finding that there is ice present is surprising. However this leads to the conclusion that if ice can be found on a very extreme environment such as Mercury than it is possibly much more abundant than originally thought. As water is the solvent which all life on Earth requires then it stands this would increase the likelihood of life of a non-terrestrial origin.

The current Rosetta mission is aiming to drill a comet in order to see if amino acids (building blocks of proteins and as such all life) are present in comets. If this is shown to be the case then not only could it provide information on where life on this planet originated from but also possibly suggest that impacts on different bodies in the solar system could have led to amino acids being deposited on their surfaces. It has been shown that life on Earth can survive in very hostile conditions this means that if there is water present alongside amino acids then the probability of life of any form (most likely very simple bacteria) would increase.

 The issue arises though not with the presence of absence of amino acids – which have been shown to form given the right building blocks in the Miller/Urey experiment but in the formation of water, especially on the early Earth. Although it should be pointed out that there are issues with the Miller/Urey experiment and the likelihood of this happening on the Earth and thus create amino acids or indeed for those molecules to have stayed in the early Earth's atmosphere.

The Earth when it was formed was incredibly hot and without a dense atmosphere (which would not form for a long time). This means that any early water would not stay on the surface but be lost to space, the only difference would be ice which could survive on the surface (similar to Mercury and the Moon).  

There are many theories as to where the water on the Earth came from. We know that there was a spike in the oxygen levels of the early atmosphere caused by the increase in cyanobacteria, however this could not be the source of the Earths water only a way of increasing it because the cyanobacteria would need water in order to evolve in the first place. The idea of extra-terrestrial water from comets etc. while seductive is also not likely to have either produced the necessary volumes of water or indeed would the water be likely to make its way into volcanos or form the oceans by itself. The most likely origin of water is the theory is that redox reactions in the rocks (specifically with aluminium oxide) would create free oxygen to combine with hydrogen to create water. It has also been demonstrated that this could have been done with iron (which is plentiful in the core) to produce water. This water could easily have then found its way to the volcanoes in order to be pushed out into the atmosphere. We know that the early Earth will have had the raw materials to make the oceans and that events such as the impact that created the Moon will have sped up this process whilst also producing lots of carbon dioxide. The volume of carbon dioxide will have had an effect on the formation of liquid water as water can be formed at higher temperatures than 100oC if it is under pressure. The carbon dioxide would provide the pressure due to the nature of it being denser than the water vapour allowing for surface water to appear while the Earth was still young and hot. This would explain how water would be able to find its way into the mantle of the young Earth and exist in a liquid state ready to be boiled in a volcano. This would allow for the situation which is taught at C2 to arise and for the larger bodies of water on the Earth to be formed by volcanism.

Whilst it is difficult to accurately pinpoint the origin of water on the Earth it is also very interesting to consider that something which is so taken for granted might have a very interesting back story.  As with most of science the truth is probably not simply one or the other but a combination of many of them. There is probably water that has been formed through redox reactions in the mantle, there is almost certainly water formed from the actions of cyanobacteria and from the impacts on the early Earth. It is also most likely that there is water on the planet that has been formed from the impacts of comets. However while this may not have provided the bulk of the water that is found on the Earth there is a chance that something greater came from the solar system. If Rosetta can prove that comets contain amino acids then not only would the elements which make life have come from space (via supernovae) but there would be mounting evidence that living organisms themselves have direct an extra-terrestrial origin. Whilst water having a non-Earth origin is an exciting thought the fact that the building blocks of life are not formed on this planet but somewhere else in the cosmos is of infinitely greater excitement. 

*Edit - Just to add this review here has some interesting points on the origin of water looking at the deuterium ratios and concludes that we really don't yet have a solid idea where water at all comes from let alone terrestrial water. In fact it goes so far as to say that there is conflicting theories which could account for most water but not all - CG.* 

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