On the cooling rate evolution of asteroid fragments

Abstract

Recent observations of rapid cooling rates at high temperatures in several ordinary chondrite and primitive achondrite meteorite groups suggest the fragmentation of their parent bodies near peak temperatures. We use analytic models for conductive cooling of collisional fragments to show that the actively cooling part of a fragment experiences similar cooling rates over a large range of temperatures. This characteristic cooling rate decays inversely with time and the larger a fragment the longer this decay continues. Measurements of high temperature cooling rates therefore place a lower bound on the cooling time and size of collisional fragments. Applying this model to the cooling rates in H chondrites, which vary from 101 to 10−2 °C/yr at temperatures between 700 to 850 °C, we estimate cooling times of at least 101 and 104 years in fragments at least 102 to 103 m in radius.