When showing my collection to people, I’m often asked two questions. First, how do you know these are really meteorites? And second – what are meteorites?
Meteorites are pieces of solar system debris, some weighing many tonnes, which penetrate the earth’s atmosphere and come crashing to the surface. The very largest are the exceptions (which is just as well – ask the dinosaurs); the best-known meteorites tend to weigh in the few kilos to a few hundred kilos range.
There are three main kinds of meteorite, classed by their composition. Stones are mainly composed of stoney material, although they invariably contain some metal in one form or another. Irons are largely composed of iron/nickel alloy. Finally, stony-irons (mesosiderites and pallasites) sit part way between the two.
Their composition depends on the kind of parent body, and where within that body they formed (see diagram):
As well as the structural differences, a meteorite will either be a find (just what it sounds like – spotted, picked up and tested, and subsequently shown to be a meteorite) or a fall - witnessed falling to earth on the last stage of its journey through the solar system.
Falls are particularly important, as the meteoritical material is fresh and hasn’t had a chance to weather (degrade) due to being out in the elements. As well as that, in historical times falls were finally accepted as proof that rocks really do fall from the sky (proving that “educated” scientists could only deny for so long what “ordinary” commoners knew was happening from their own experience!).
I became interested in meteorites because of my background in astronomy. I find them to be endlessly varied, often very beautiful; I like the human stories that surround them; and I find it amazing that I can hold in my hand something which may have taken millions of years to reach me from the Moon, Mars or the asteroid belt, and which may contain material which is older than the earth, or even the solar system itself.
If you’re in the UK or Ireland (or even if you’re not!), consider contacting the BIMS for more information or membership.