2012: Advent Atmosphere: Volcanoes and Snowballs

by on December 4, 2012

So. A problem. How did Earth get its water?

We know it didn’t come from the primordial gas cloud, because it was too hot for that when the Earth was condensing out as a big ball of rock. So the water on our surface and in our atmosphere must have come from somewhere else.
There are two main theories: outgassing and cometary bombardment.

Outgassing says that the water used to be locked up in the rocks of the crust, and was processed into the atmosphere by volcanoes. The Earth did have some really enthusiastic volcanic activity way back when, and if you test the gases given out by volcanoes you find that 60% of it is water vapour.

Unfortunately for this theory, the rocks being processed by modern volcanoes are, on average, on their eighth round of the rock cycle and were mostly seabed on their most recent incarnation, which is why they’re wet. Oh dear. So we can’t tell anything about primordial volcanic outgassing by checking modern day volcanoes. We can’t disprove it, either, but there’s a more convincing theory: cometary bombardment.

Go back to a few million years after the Sun formed from the dust cloud, and the solar system is simply teeming with comets, which are mostly made of ice. All the planets are being rained down on by these lumps of ice, as an inevitable consequence of gravity. The heat of impact turns that ice to water vapour, and Earth doesn’t have much of an atmosphere yet. The pressure is low, so even at minus 18 Celcius, the water stays gaseous. And as more comets hit, more water builds up in the burgeoning atmosphere, and it acts as a greenhouse gas, warming the surface of the planet, until one day there’s enough water for it to condense out of the air, and Earth has its first rainfall.

Enough comets hit the Earth over the first billion years or so of its existence to create all the oceans of the world. Enough giant snowballs to fill the Pacific Ocean and all the rest. Imagine just the Antarctic ice sheet cut up into comet-sized lumps, and think how many times we got smacked by space rocks to make it.

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