How long do meteorites last on Earth's surface?
It depends mainly on climate: in wet places a meteorite may rust away within decades to thousands of years, while in hot deserts many survive for tens of thousands of years and in Antarctic ice some last more than a million.
As soon as a meteorite lands, Earth starts to break it down. Most meteorites contain grains of iron-nickel metal and iron sulfide, minerals that formed in space with no water or oxygen around. On Earth, rain, dew and oxygen turn that metal into rust. The rust swells, prises the stone apart along tiny cracks and stains the inside brown. Scientists rate this damage on a weathering grade scale, from fresh to heavily altered.
Climate decides how fast this happens. In a damp, temperate place such as Britain, a small stony meteorite lying in soil may crumble within decades to a few thousand years, which is one reason British finds are so rare. Iron meteorites are tougher but still rust. The great Willamette iron in Oregon has deep hollows that formed largely as it weathered in that wet climate.
Dry and cold places act as natural freezers. In hot deserts such as the Sahara, many meteorites survive for tens of thousands of years, and some for much longer. Antarctica is better still: meteorites buried in the ice are sealed away from liquid water, and some have lain there for more than a million years before ice flow brings them back to the surface. Scientists measure how long a meteorite has been on Earth, its terrestrial age, from radioactive atoms made by cosmic rays in space, which slowly decay once the stone is shielded by Earth's atmosphere.
There are even fossil meteorites. In limestone quarries in southern Sweden, geologists have found more than a hundred meteorites that fell into an ancient sea around 470 million years ago. The original minerals have been almost entirely replaced, but tough grains of the mineral chromite survive and reveal their space origin.