Thermal metamorphism
Thermal metamorphism is the slow heating of rock inside an asteroid, below its melting point, which recrystallises minerals and blurs original textures.
Not long after asteroids formed, they warmed from within, mostly through the decay of radioactive aluminium-26. In bodies that did not melt, this heat baked the rock for millions of years. Grains grew, minerals exchanged atoms until they reached equilibrium, glass crystallised into feldspar, and chondrule outlines softened. This process is thermal metamorphism.
In chondrites, the degree of metamorphism is recorded as petrologic types 3 to 6, with type 6 the most heated, probably reaching very roughly 800–950 °C. A popular model, the "onion shell" asteroid, places type 6 material deep in the hot centre of the parent body and lightly heated type 3 material near the cool surface. Radiometric ages and cooling rates measured in different meteorites help test this idea. Metamorphism also erases fragile primitive features, which is why type 3 meteorites are preferred for studying the early nebula.
H chondrites span the whole range, from type 3 to the strongly recrystallised H6 Kernouvé, which fell in France in 1869. On a cut face, increasing metamorphism shows as fewer, fainter chondrules and a more uniform, granular texture.
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