Rocks In My Head

NWA 4522, full slice

I recently told a friend how I sometimes stand in my garden at night, gazing up at the moon while holding a piece of it in my hand. She looked at me sideways, probably wondering if I’d been enjoying a different kind of moonshine.

This scene is one of the entrancing aspects of meteorites. As a life-long amateur astronomer, I knew a little about them:

“A meteoroid is a rock floating in space. When it enters the earth’s atmosphere, sometimes at 20 miles per second, it heats up and becomes a meteor, giving off heat, light and mass as it plunges downwards. If anything survives to reach the ground, it’s called a meteorite.”

But that textbook definition doesn’t begin to describe their charm and mystery. How do those rocks get to be ‘floating in space’ in the first place? Many come from collisions between dwarf planets out in the asteroid belt. We know this because if you grind up samples and look at how they reflect different colours of light, you get a good match with telescopic observations of the light coming from their presumed parent asteroids.

The HED group of meteorites, for instance, seem to have been blasted from the surface of minor planet Vesta by a huge impact. We may even have the smoking gun, in the form of a 280 mile wide crater – a monster, when you consider the asteroid is only 330 miles across. Vesta was lucky to survive.

Or take bigger targets, like the Moon and Mars. If the impacts are powerful enough, tonnes of rock can be ejected into space. Very, very occasionally, after tens of millions of years wandering through the solar system, some of it punches through our atmosphere and crashes to earth, giving us the Lunar and Martian meteorites. We know they’re from the Moon, because analysis shows their similarity to the material brought back by the Apollo astronauts: from Mars, because tiny gas bubbles trapped in the rock match the atmosphere sampled by Martian landers.

 

Meteorites are old. The most primitive (and thus oldest) are the carbonaceous chondrites, which have been dated to 4,567 million years. They’re left over from the gas and dust which condensed to form our solar system. Hold a piece of Allende, which fell in Mexico a few months before the moon landings, and you’re holding something more ancient than the planet you’re standing on.

If that doesn’t wow you, how about this. Allende contains particles called ‘pre-solar grains’. These are tiny deposits of minerals, including nanometer-sized diamonds, which originated in other stars – pushed out from the atmosphere of supergiants, or hurled violently in our direction by nearby supernovae. It’s difficult to calculate the age of these grains, but they must be many millions of years older than the solar system.

That piece of Allende you’re holding contains – quite literally – the cores of long-dead stars, the wispy breath of distant red giants.

This was the point where I got hooked. I’d seen them in museums; it never occurred to me you could actually own a meteorite. Aren’t they rare and exotic?

Well, yes. In the whole of history, something like 30,000 tonnes of gold have been mined. Perhaps 1,000 tonnes of diamonds. The total weight of recovered meteorites on the planet is about 700 tonnes – and 60 of that still lies where it fell, in Hoba, Namibia.

Rarer than diamonds, and far more diverse: each has a name and a story.

In 1492, just before Columbus set off for the New World, the Ensisheim meteorite landed in a field in Alsace. On hearing this, King Maximillian I, demonstrating acute political opportunism, declared it a sign from God that he should wage war against the French. (He also decreed that the rock should be chained to the rafters of a local church, in case it ever decided to fly back heavenwards – clearly a better politician than scientist).

A shower of 3,000 stones which fell in L’Aigle, France, in 1803 was critical in persuading scientists that rocks really did fall from the heavens. So much material was recovered, and so many witnessed the event, that it was no longer possible to dismiss the stories as peasant hearsay.

There are many more. (1911, Egypt: 10kg space rock strikes and kills dog).

Collecting meteorites is the ultimate crossover hobby. The combination of science, aesthetics and history is unique; and to me, magical. There are the human stories, for sure. Some meteorites are very beautiful. Much of the science is understood, but like astronomy, there is still room for amateurs to make real contributions.

So go grab yourself a piece of the moon. As a gift to a partner, it’s a whole lot more romantic than roses.

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