Falls and finds

Every meteorite is either a fall or a find. Knowing the difference helps explain why museums value some meteorites so highly.

What counts as a fall?

A fall is a meteorite that was seen or detected coming down and was then recovered. The fall might have been witnessed by people, filmed by fireball cameras, picked up on radar, or heard as a sonic boom followed by the discovery of fresh stones.

Falls are scientifically precious for two main reasons. First, they are fresh. They have spent only a short time exposed to air, water and soil, so their minerals and delicate chemicals are almost untouched. Second, because falls are noticed regardless of what they look like, the mix of falls gives a fairer picture of what actually arrives from space.

Fewer than 1,500 falls have been recorded through history, which is a small fraction of all known meteorites.

What counts as a find?

A find is a meteorite discovered without anyone having seen it land. Finds are often picked up years, centuries or even hundreds of thousands of years after they arrived. Farmers ploughing fields, people walking in deserts, and expeditions on the Antarctic ice have all discovered finds.

Finds make up the vast majority of known meteorites. Among them are many rare types that have never been seen to fall, so finds are hugely important too.

However, finds have usually been changed by their time on Earth. Metal rusts, minerals take up water, and Earth chemicals can creep into cracks. Scientists describe this damage as terrestrial weathering, and they grade it carefully, because it can affect the results of chemical tests. The longer a find has been on Earth, the more careful scientists must be.

A biased sample

Finds do not represent what falls from space fairly, because some meteorites are much easier to notice and survive much longer than others.

Iron meteorites are a good example. Only about 4 or 5 per cent of falls are irons. But among finds outside deserts and Antarctica, irons are far more common, because they are tough, heavy, unusual-looking and slow to break down. A farmer is much more likely to notice a heavy lump of metal in a field than a weathered grey stone.

Stony meteorites, by contrast, crumble in wet climates and blend in with local rocks. So the mix of falls tells us what Earth receives, while the mix of finds also reflects what survives and what people notice. Scientists always keep this difference in mind when they compare numbers.

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