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Sunday, 22 July 2012

Dry planet

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Why is Earth so dry? OK, I realise that this is a bit of U turn when, not so long ago, I ran a story about how much water Earth has gained [1] from asteroids and comets. But here's the kicker - at least some large percentage of the water on Earth was imported by asteroids or comets. And, even with that, Earth is only about than a percent water by weight.

Image above: The huge carbonaceous asteroid Mathilde. Space rocks like these can hold a lot of water within themselves, and are thought to have been the source of much of the water on Earth. Image courtesy of Pearson Prentice Hall, Inc.

That's pointing to a big plot hole in the theories of our planets formation [2]. A plot hole that runs like this:

Earth formed from the protoplanetary disk, of dust and gas and debris around the growing Sun. Within this disk is a temperature boundary called 'the frost line', running around the Sun: If you're outside it, things are cold, water can condense to form solid ice, and massive amounts of water should get incorporated to any growing body. If you are inside it things are warm, water cannot condense, and anything that grows in that area is dry.

Video above: a very cool artist impression of the solar system forming out of the protoplanetary disk. Why is it in here? Why not? Video courtesy of University of Copenhagen/Lars Buchhave.

Yes, Earth was born from the collision between two protoplanets [3], which should have dried it out a bit. Temperatures of ten thousand degrees Celsius do that. But our models of planet growth predict that, at least for some of the time, Earth should have been growing beyond the frost line.

So Earth should not merely be wet, with oceans up to eight thousand metres deep. It should be at least ten percent water, potentially with oceans hundreds of kilometres deep.

So what gives?

Well, what gives - paradoxically - is that the young Sun didn't heat the protoplanetary disk as well as we thought. In the previous models of the accretion disk, the material near the inner edge of the disk is fully ionised [4] -  which allows the Suns magnetic field to gulp matter down [5] onto its surface. While this is going on the inner disk is hot. But this phase doesn't last long enough for a planet like Earth to grow, and when it ends the disk cools, the frost line migrates inwards, and the Earth should be growing in a region where water ice is abundant.

A Tale of Two Disk Models

Image above: The standard model of the protoplanetary disk, and the new model. The 'dead zone' around the Sun heats up by gravity, and actually warms more of ther disk than the Sun could. Image courtesy of [6]
In a new version of the model [7], the Sun doesn't have enough power to fully ionise the inner disk, so it can't scoop material off the inner edge. Dust and gas builds up on the inner edge, and it heats up of its own accord, by gravitational compression [8]. This heat source is steady, and warms the inner disk for long enough to keep the region Earth grows in dry.


*If simple means years of painstaking work, simulations run on a super computer, all checked against observations of young star systems by the Hubble Space Telescope [9].

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