Earth's Elements

30 January 2019

A planet-sized game of pool? It reminds me of Red Dwarf episode, but that's how Earth got a lot of its elements.

I was talking with a customer the other day about water on Earth. We both found it mind-boggling that most of our water came from space in the form of harmless comet collisions. The majority of comets are cricket-ball sized "dirty snowballs". When they hit our atmosphere they turn to steam, and fall as rain.

But what about the other stuff?

Over 4 billion years back, our newly-formed Sun was creating a giant disc of dust and garbage gathered from previous supernovas in the area. Astronomers call this a proto-planetary disc. They've seen it happening around other new stars as well well - this photo is from a system called HL Tauri.

In this disc, elements were gathering in bands at different distances from the centre. It's really an astronomical-sized centrifuge. As planets formed in the disc, they started off with just the elements they could vacuum from their local bands. You can actually see it happening in the photo. The lump that was to become the Earth got a lot of rock-forming elements such as iron and silicon, but not a lot of others.

So how did Earth end up with such a wide variety of elements?

Recently, scientists at Rice University in Texas came up with a theory. Most of our carbon and nitrogen (as well as some other stuff like sulphur) came from a stray planet that crashed into the lump of rock that was to become the Earth. The collision was so massive that it also formed our Moon, just through junk blasted away.

This rogue planet had been in a zone that was rich in these elements, and had been knocked out of its orbit (probably between Mars and Jupiter) in another planetary collision, like balls on a pool table.

So collisions with other bodies might be a threat to life on Earth, but it's how the Earth gets a lot of exotic elements. And without those, there probably wouldn't have been life at all.

(Image: NASA - APOD 10 Nov 2014)