Water: Where did it come from?

I haven’t done a proper science post for a while and I’m sorry for that. I saw a news story pop up on Twitter from the ESA Science Team (@esascience) about the origin of Earth’s water. Just where did it come from?

This is an area that really interests me, in fact I get rather too excited about it. We had a long and detailed question on it pop up in S283 (an OU planetary science course) and I thoroughly enjoyed researching and developing my answer. I leapt at this chance to discuss it further.

The origin of Earth's water?

The origin of Earth’s water?

It’s pretty obvious surely? Comets right? They’re mainly composed of water ice, we know the planets were pummeled by them in the late heavy bombardment about 4 billion years ago, its got to be them hasn’t it?

There has been no way to test this hypothesis until very recently. You need to send a spacecraft to a comet to test it – a very expensive but totally worthwhile test.

Now we’ve finally managed to study 4 comets in detail and the results are interesting. What we need to study is what’s called the deuterium/hydrogen isotope ratio. Deuterium is just basically a slightly heavier version of hydrogen, it has an extra neutron (technically not an extra one because hydrogen doesn’t have any neutrons).

If comets are the origin of the Earth’s water we’d expect there to be a very similar ratio of hydrogen and deuterium to the ratio of these isotopes in ocean water. From the comets that have been studied it turns out that this probably isn’t the case. Comets appear to have twice as much deuterium than ocean water, meaning that comets are an unlikely cause for our waters origins. As we’ve said already though, only a few comets have been analysed in detail. They might not be representative of all comets.

Another theory states that water-bearing grains are responsible. The distance from the Sun at which the Earth formed though casts doubt on this. It would have been so warm that water couldn’t have existed here. Not if they were incorporated within hydrated minerals though. As the planet formed (and after) these hydrated minerals would, over time, degas out into the atmosphere via volcanic eruptions. Eventually, enough was degassed  to form today’s oceans. This has been held as the most plausible explanation.

A spanner seems to have been thrown in the works though, the debate has been reignited. The Herschel infrared space observatory has been looking at comet Hartley 2 and has found that its deuterium/hydrogen ratio is pretty much exactly the same as Earth’s oceans. This comet is suspected to originally have been a trans-Neptunian object flung into the inner solar system have a gravitational tug of war. These comets, forming under different conditions to those that formed between Jupiter and Saturn, probably have slightly different compositions, specifically the deuterium/hydrogen ratio.

A recent study shows that there was likely a 5th giant planet in the solar system, but after gravitational encounters with other planets was flung out of the solar system, stirring up all the trans-Neptunian comets on its way. Is this the reason for the late heavy bombardment? It lends weight to comets being the origin of Earth’s water.

I’m still sceptical though. This is only one comet. We’re going to need to study many, many more before we reach a definitive conclusion. From what I’ve studied, hydrated minerals seem to fit best with the available evidence, but as more comes in I’m willing to change my mind.

The report from the ESA science can be read here
The report on a possible 5th giant/ice giant can be read here 

New moon discovered around Pluto

Hubble’s still turning up surprises, even today. Whilst focusing her gaze at Pluto, doing some extra research in preparation for the New Horizons flyby in 2015, Hubble has found an extra blob in its vicinity. That’s right Pluto has a new moon.

New Moon 'P4' Discovered Around Pluto

Pluto already has 3 moons. Charon is the biggest at over 1000km across and is the largest moon in the solar system when compared to the size of its companion, Pluto. There’s also Nix, Hydra and now P4, although that isn’t it’s official name just a nickname for the time being.

The new moon could be between about 10-40km across, making it Pluto’s smallest moon. It’s difficult to judge its size initially. It’s probably a very icy body, like most things out this far – 5.9 billion km – meaning that it’d have a fairly icy surface. If this is the case the moon reflects a lot of the light from Sun and that’ll mean it’d be on the smaller side of things. If it’s reflecting a small amount of light though, meaning it’s a darker body (this could be due to radiation effects) then it’d be on the larger side of things.

The Pluto System

P4 fits nicely between the orbits of Nix and Hydra. It’s thought that the moons were created in a large impact event much like the theory behind our own moons formation where a large body hit the planet, flung material into space and then re-coalesced to form the moon.

We’ll be finding out more about this intriguing, unexplored part of the Solar System when New Horizons flies by in 4 years time. It’s a pretty exciting mission where we’ll be looking in detail at Kuiper Belt Objects for the first time.

More information on this discovery from NASA, the BBC and New Scientist