The LHB, or the ‘Late Heavy Bombardment’ may be the earliest ‘late’ event you have ever heard of. It happened around 4 billion years before you were late for school, and made our planet a particularly unpleasant place to be.
Geologists have a nice name for the period between the formation of the planet and ~4Ga (geology shorthand for 4 billion years ago); we call it the Hadean. Those of you familiar with Greek mythology, or Disney’s Hercules, will know Hades was the god of the Underworld, the guardian of Hell. It is after hell that this time was named, and rightly so. For the first hundred million years or so the earth is not yet solid, it’s a big ball of mixed up melted rock and metal. Later the planet cools down a bit and things are looking up, until along comes the LHB; three hundred million years of continuous bombardment by meteorites.
Like most things in science, this is a theory. That means lots of people believe it happened, but some people disagree. If it weren’t for this disagreement, scientists wouldn’t have so many tea-drinking (research) jobs, so it’s completely healthy and necessary.
So what’s this got to do with us? One branch of the LHB theory which I particularly like involves elements like platinum and palladium, very expensive metals with uses in catalytic converters, aeroplanes, plasma displays and many other things from our day to day life.
Imagine the hot, early earth in the first hundred million years is liquid. It’s made of lots of different elements, but largely things like iron, nickel, magnesium, silicon, oxygen and aluminium. At first these are all mixed together nicely, but remember the earth is a big heavy ball that has a gravitational field, so all the heavy stuff starts to get pulled downwards. Iron and nickel fall to the centre of the new planet as they are heavy, and silicon, oxygen and friends float to the surface. Some elements are so light, like helium and hydrogen, that they leave the planet entirely and go to space. Platinum is what’s known as a siderophile element, that means that it does what iron does, if it can. Therefore, when the iron sunk to the core, it took the platinum with it, so we are left with an earth with no platinum on the surface.
So how do we make those catalytic converters without platinum? The answer is in the meteorites. The meteorites, like the early liquid earth also had some platinum in them, scattered evenly through the rock. So, take one Planet Earth which has lost all its platinum to the metal core, allow it to cool and solidify a bit, then add hundreds of thousands of meteorites all containing platinum over hundreds of millions of years. What do you know, the platinum is back!
I was wondering if there is a take-home message here. Maybe to summarise I should say: meteorites might be scary when they’re coming straight at you, but they also might be bringing useful gifts from space so they’re not all bad.