These posters provide additional information about chondrites. They cover non-mass-dependent fractionation of oxygen isotopes (particular changes in the proportions of different oxygen isotopes that predate the formation of the Solar System), changes in the texture of ordinary chondrites that resulted from reheating and chemical exchange between minerals (a process known as thermal metamorphism), and the cosmic-ray exposure (CRE) ages of meteorites (a measure of how long these rocks traveled in interplanetary space as meter-size objects). The heating responsible for the thermal metamorphism of chondrites can come from two sources � the decay of short-lived radioactive isotopes (mainly aluminum with 13 protons and 13 neutrons) and collisions of meteoroids with the asteroid surface.