Antarctic meteorite concentration

Why are most of the world's recovered meteorites picked up from a few patches of ice on one continent?

Meteorites fall on the Earth without preference. Nothing about an incoming stone knows where the continents are, and over any long stretch of time the falls per square kilometre should be much the same in Kansas as in Kenya as on the polar plateau.

And yet the great majority of the world's classified meteorites have been picked up from Antarctica, and within Antarctica from a modest number of named patches of exposed ice. If the falls are uniform, the finds are wildly not. So the anomaly cannot be in the sky. It has to be something the ice does after the stone lands — which raises the useful question: what would you have to build, on purpose, to concentrate a uniform rain of small objects?

You would need three things, and it is worth separating them, because only one is about ice being cold.

First, a conveyor that gathers. Snow falls on the interior plateau, buries whatever landed on it, compacts to ice, and flows outward under its own weight. A meteorite landing in the interior does not stay there: it is packaged into the ice column and carried. Now suppose that somewhere downstream the ice cannot continue — a buried mountain range, or a subglacial ridge, forces it upward and blocks its escape to the sea. The ice arriving from upstream must go somewhere, and if it cannot flow away it can only be removed from the top.

Removed how? Not by melting — it is far too cold. By sublimation, ice passing straight to vapour, driven relentlessly by the dry katabatic winds pouring down off the plateau. Where that removal outpaces snowfall, the surface is stripped back to old, bubble-poor ice: the blue ice fields.

Now do the accounting, because it gives the whole answer in one line. Suppose the falls arrive at some rate per unit area, and every stone that landed on a catchment of area A is eventually delivered by the flow to a stranding surface of area S. Then the number per square metre on that surface exceeds the global average by roughly the ratio A / S. That is the mechanism, and notice what it does not require: no extra falls, no cosmic bias, no unusual sky. A large catchment draining onto a small ablation window is a concentrator, in exactly the arithmetic sense.

Second, a preservative. A meteorite in a temperate field is attacked by water, oxygen, soil acids and roots, and is unrecognisable within centuries or a few thousand years. In Antarctica it lies in a cold, arid, chemically quiet place. Terrestrial ages measured on Antarctic finds — from cosmogenic isotopes that stop being produced once the stone is shielded — commonly run into tens and hundreds of thousands of years. I am recalling that range rather than quoting a paper, and the maximum is argued over; but the direction is unambiguous, and the consequence is straightforward. If the accumulating surface holds stones a hundred times longer, it collects a hundred times as many before losses balance arrivals. That factor multiplies the first one.

Third, contrast. A dark stone on white ice, with no soil, no vegetation and no local outcrop to confuse it with, is found nearly every time you walk past it. In a ploughed field it is found almost never. Recovery efficiency is the factor most people leave out, and it may be the largest of the three.

Multiply the three and the anomaly dissolves: a geometric concentration, times a long residence time, times a near-perfect detection rate.

Is this testable? Sharply. It predicts that dense fields occur only where ablation exceeds accumulation and flow is obstructed — not on blue ice generally, and never on freely-draining coastal ice that calves into the sea, which delivers its cargo to the ocean floor. It predicts terrestrial ages spread over a very long span rather than clustered. And it predicts that the recovered population, corrected for weathering, should resemble falls elsewhere, since nothing selected them. The observation that would refute it: rich concentrations found on fast-flowing coastal ice with net snow accumulation, or terrestrial ages all near zero — either would mean the ice is not doing the collecting, and something about recent falls is.

One genuine open point: how much of the concentration comes from long englacial transport versus stones that simply fell directly onto a stranding surface already exposed for a very long time. Both operate, and their relative weight is still argued in the literature.

The tempting shortcut is that Antarctica "gets more meteorites". It does not. It gets the same rain and keeps the finds, in a place where they are also easy to see. Every one of the three factors is about what happens after arrival.

A second refinement connects the pieces: the catchment ratio and the long terrestrial age are not independent conveniences but the same slowness, and the cold that stops the ice melting is the cold that stops the stone weathering. One climate produces all three effects.

And a limit worth stating: the concentration is a geometric artefact, not a physical sorting. It does not favour any type of meteorite, except through weathering, which erodes fragile stones preferentially and can bias the counts of the smallest and most friable classes. Numbers of recovered specimens are quoted very differently by different collections depending on how paired fragments — multiple pieces of one original fall — are counted, so I am deliberately not stating a total.

• How cosmogenic isotopes are used to separate a meteorite's time in space from its time on the ground.
• How hot deserts became the second great collecting ground, and which of the three factors they share.

Every term the collection defines is gathered in the glossary.