I write this column a few weeks before publication, and one of the news stories at the moment is about Martian rocks, well a single rock being sold at auction for a great deal of money. I’ve no intention of going into the politics of this sort of thing, but the sale itself does raise some interesting questions for the person in the street.
Firstly, how do we know that the rock is from Mars? This is an interesting question and is also very easy to answer properly. It all boils down to taking a sample of the rock and cutting or drilling into it in a vacuum chamber. This releases a tiny amount of the gases trapped in the rock, a sort of molecular fingerprint of the place where the rock came from. We then compare this sample with the results of the air samples from the Viking spacecraft that landed on Mars nearly 50 years ago and lo and behold, they match each other. Definitive proof of the origin of the rock.
Are there many Martian rocks on Earth? No there aren’t, not many at all. Somewhere around 400 out of the 72.000 meteorites that have been catalogued originated on Mars. The latest one is a whopping 25 kilograms in weight, it’s the biggest found so far which is why it’s worth millions.
Now for the biggest question, how do all these rocks get from Mars to Earth. There are two ways that this happened, either by an impact from something smacking into Mars or by volcanic eruption spitting rocks into space. Mars has a much weaker gravity field than we do on Earth so it’s relatively easy for this to occur. As you might imagine, the rock types are quite different so they get special names (shergottites, nakhalites etc if you want to look them up). Once the rocks have reached space they will orbit around randomly until one day they meet with the Earth and come plummeting down to the ground, awaiting someone to make a fortune from them.
Last bit, how old are these rocks? Well, the Earth is about 4.5 billion years old and the oldest Martian meteorite is about 4 billion years old. The youngest one is only a few million years old so these rocks cover quite a range in ages. A part of the work going on around the world is to understand just what the history of the planets actually is, and study of the rocks that come to us from space forms a very important part of this, it’s science in action.
That’s all for this month, enjoy the night sky.