The Leonard Award Address: Presented 2000 August 30, Chicago,Illinois, USA: Early solar system events and timescales

Abstract— Some recent information on the Mn‐Cr and Al‐Mg systems is reviewed. This information is used to derive constraints on the timing of processes and events, which took place in the early solar system. Using reasonable assumptions, a timeline is constructed where the estimated age of the solar system is ～4571 Ma. This age is taken to mark the time when most calcium‐aluminum‐rich inclusions (CAIs) were starting to form, a process that may have lasted for several 10⁵ years. Almost contemporaneously small planetesimals have accreted that served to store these CAIs for later dispersal among larger planetesimals. By the time large numbers of planetesimals of several tens of kilometers in size had formed, the interior of these objects started to melt through the decay of ²⁶Al. Collisional disruption of these planetesimals allowed gases, dust, and melt to escape into the surrounding space. The fine droplets of melt reacted with gas and dust to form chondrules, which, after rapid cooling, were partially re‐accreted onto the residual rubble pile. This process of primary chondrule formation, in most cases involving several generations of planetesimals, most plausibly lasted only for ～2 Ma. Towards the end of this period and during the following 3 to 4 Ma planetary objects of several hundred kilometers in size were formed. They still stored enough energy to continue melting from the inside to finally differentiate into chemically stratified layers, with basaltic volcanism occurring within a few million years.