Planetesimals were processed by heat from the decay of 26Al (in some cases leading to differentiation) and by collisions as the solar system’s debris disk dissipated. These processes are recorded in meteorites and, in particular, their timing can be established through the study of extinct radioisotopes. Among these, 129I has a relatively long half life of 16Myr, meaning that it can provide useful chronological information across the first ~100Myr of solar system history. This is the basis of the I-Xe system.
Chondritic meteorites represent fragements of planetesimals that survived differentiation. A baseline model would predict that the peak temperature of such a body would anti-correlate with the subsequent cooling rate (higher peak temperatures, slower cooling), since both depend on burial depth. Data deviate from this simple relationship; in some cases minerals cool through closure temperatures later in less processed (lower peak temperature) material. This suggests variations with time in the thermal properties of the parent planetesimal, perhaps driven by collisions.
The range of late ages for material separated from the most primitive samples can also be attributed to collisional processing. Studies of individual chondrules separated from the meteorite Chainpur, so far the only sample studied in sufficient detail, suggest the impact rate declined by a factor of 5 to 50 over the first 50 Myr of solar system history.