Heat-affected zone
The heat-affected zone is a thin layer just beneath a meteorite's fusion crust whose structure was altered by the heat of atmospheric entry.
A meteorite's flight through the atmosphere lasts only seconds, and its surface melts and blows away while the interior stays cold. Heat still soaks a little way in, however, creating a band just below the fusion crust where the rock or metal has been changed. This band, usually only a few millimetres thick, is the heat-affected zone.
It is clearest in iron meteorites. On an etched slice, the Widmanstätten pattern or Neumann lines run right up to the zone and then fade into a fine-grained, featureless rim where the metal was briefly heated enough to transform. Specialists call this altered kamacite the α2 zone. Its width helps estimate how much surface was lost to ablation and reveals where the original surface lay. If an iron has no heat-affected zone along an edge, that edge was probably cut, broken or weathered away after the fall.
Well-preserved irons such as Sikhote-Alin, which fell in Russia in 1947, show heat-altered rims beneath their crust. In stony meteorites the effect is subtler, seen mostly with a microscope. Look along the edges of an etched iron slice for a thin band that does not share the pattern.
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