What kinds of meteorites are there?

Unlike the shooting stars we see in meteor showers, which burn up entirely in our atmosphere, meteorites are the fragments we find that have made it all the way down to the ground. Like anything that enters our atmosphere, meteorites have also been burned by the friction of our air, which means that the pieces that we find are only a fraction of the original object; the rest of it was burnt up as a colourful vapor in our sky, like the pieces of grit that make up a meteor shower.

Meteorites are usually made of a combination of one of two substances: rock, or iron. With the infinite inventiveness of astronomers, we duly called them stony, iron, or stony-iron, if they’re a relatively even blend of the two. Stony meteorites can (and often do) contain some small fraction of iron, but only those meteorites which are almost entirely metal are classified as iron meteorites. The vast majority - over 90% - of meteorites that fall to earth are stony, leaving less than 10% of all meteorites as primarily iron. However, the iron ones are a little more well known, and that’s because for a long time, the iron ones were much easier to identify as meteorites.

Stony meteorites look like stones (see the picture just below), so unless you know exactly what you’re looking for, they can be hard to spot. In certain parts of the world, they can be a little easier to pick out - the deserts of Africa or the high deserts of Chile will let unusual-looking rocks stand out a little more. The best place to look for stony meteorites, however, is the glaciers of Antarctica. The ice there is so old and so slow-moving that meteorites that fall there will stay put for a long time. Meteorites are also quite easy to spot there, since anything dark will stand out for miles against the white ice.

That said, even though they look like one of many other stones on our planet, they are still fantastically interesting to scientists. Some of these stony meteorites are some of the oldest untouched rocks in the solar system, and they give us a sense of what the early solar system was made of and how it built itself up.

We also have some stony meteorites which have been blasted off of other worlds. In particular, we have a little collection of about 30 different meteorites which were originally part of Mars, one of which is shown just below. These are chunks of the surface of Mars which were flung so far away from the surface after another impact that they never fell back to the surface, and wandered the solar system until they encountered the Earth. These martian meteorites tend to be much younger than the rest, and are composed of a different set of elements and minerals, so we can pick them out without too much trouble.

Iron meteorites are much easier to spot than the stony ones. This is partially because it’s relatively unusual to find weathered chunks of iron sitting on the ground, so they are more easily recognized as out of place. As with the stony meteorites, they are most easily found in deserts and in the glaciers of Antarctica, where they are likely to stand out more starkly from their background, but they can also be relatively easily picked out in other places. Iron meteorites don’t wear down as much by erosion as the stony ones do, so they’re also more likely to stay around for a longer period of time. The iron meteorites are not actually completely made of iron, but contain a significant fraction of nickel, and this blend of metals is part of what makes them unique.

This one has been sliced in half and polished to show off the metal crystals inside it - but without this extra treatment, they look much less dramatic.

Opportunity, one of our Mars rovers, discovered a basketball-sized iron meteorite on Mars in 2005, which people got very excited about. The discovery of that meteorite was the first time we’d found a meteorite on a planet other than our own. This is much more what iron meteorites tend to look like before being sliced in half.

Iron meteorites and stony meteorites also tend to behave a little differently as they come through the atmosphere. Stony ones tend to be a little more fragile against the force of the atmosphere, and as a result, they can shatter more easily before they hit the ground. Iron meteorites are a little more resilient; heating up the outer layers of a metal lump doesn’t create enough stress to fracture them the way the stony ones would.

The Chelyabinsk meteor that exploded over Russia a little over a year ago also left behind some pieces for us to examine; the largest of the pieces were extracted out of a lake. One of the confirmed pieces is shown at the top. The meteorite pieces confirmed that the object that came through our atmosphere was a stony type meteor. It had already been suspected of being stony since it had exploded so brilliantly in the air.

Both iron and stony meteorites give us a fascinating look at the early solar system, and help us to understand how the planets formed and when, so finding more and more meteorites helps us to understand the variety that was present when the planets were first being formed.

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Thanks for your patience while my thesis was being completed, everyone!