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 

Images a’ coming!

I had a bit of a lazy morning this morning and got up just before 10 (I had very good reason, I was on an early yesterday and needed some decent sleep). I went straight to my laptop to look at the latest images from Stardust but couldn’t find anything. I went to Ustream and caught the last 2 minutes of JPL’s broadcast (the American’s all went to bed I guess). What happened?

Well as it turns out everything pretty much went according to plan with the flyby. The spacecraft turned round to point its high-gain antenna at the Earth and hit the transmit button. Seems as if the pictures are coming in the wrong order though. It was programmed to send us the really exciting close-ups first but has instead decided to send them in the order they were photographed. This means we do actually have some pictures at the moment but were going to have to wait a few more hours to get the good’uns!

As good as it gets for the time being!

As you can see, a bit grainy and lacking in detail at the moment. The ones to come will be on orders of magnitude more impressive. I’m going to be at work when they come through though which is a bit of a shame. I’ll try and grab a sneaky peek when it quietens down a tad and blog about it tomorrow!

Return to Tempel 1

The American’s have hyped this up a bit, ‘Valentine encounter for Stardust and comet’ when strictly speaking (if you follow GMT/UTC) then it’s the day after valentines. But hey, I guess if it gets people interested, why not.

Impression of Stardust-NExT's encounter with Comet Tempel 1

The Deep Impact spacecraft originally flew past the comet back in July 2005 and launched a projectile at the comet. Continue reading