What happens to a meteorite as it weathers on Earth?
Water and air rust its metal, alter its minerals and fill its cracks with Earth minerals. Over thousands of years, it crumbles and eventually disappears.
As soon as a meteorite lands, it begins to change. Earth's surface is full of water and oxygen, which space rocks rarely encountered in such quantities.
Rusting metal. Iron-nickel metal and iron sulfide react with water and oxygen to form iron oxides and hydroxides, such as goethite and akaganeite. Brown stains spread through the rock from metal grains. In iron meteorites, rust forms a crust that may flake off. Chloride from soil water speeds up rusting and can cause specimens to crack even after collection.
Mineral changes. Over longer periods, silicate minerals, such as olivine, slowly alter to clays. Glass in fusion crust and chondrules is attacked.
Gains and losses. Weathering can add elements from the environment. Desert meteorites often gain barium, strontium and calcium from soils, and calcium carbonate or gypsum fills their cracks. Some elements are leached out.
Physical breakdown. As rust expands, it cracks the meteorite. Temperature changes and frost also break it apart. Eventually, it crumbles into soil.
Timescales. In humid climates such as Britain, a stony meteorite may last only decades to a few thousand years on the surface. In hot deserts, stony meteorites can survive tens of thousands of years, and in Antarctica, where they are frozen, some have lasted more than a million years. Iron meteorites survive longer.
Weathering affects science, altering isotopes and chemistry, so scientists prefer fresh samples. For collectors, it means meteorites must be stored dry.