Chondrites and the birth of the Solar System

Chondrites are the least changed rocks we have. Their components formed in the disc around the young Sun and date the start of the Solar System.

The ingredients

Chondrules are once-molten droplets, typically 0.1 to a few millimetres across, that formed by brief, intense heating events in the disc, then cooled in minutes to hours. What heated them is still debated. Calcium-aluminium-rich inclusions (CAIs) are irregular white grains made of minerals that condense at very high temperatures. They are rare in most chondrites but common in CV chondrites such as Allende. A fine-grained matrix of dust binds everything together.

The first solids

CAIs are the oldest dated solids, at about 4,567 million years by uranium–lead methods, and this is taken as the age of the Solar System. Chondrules formed over the next few million years. Because the timing is known so precisely, chondrites record the sequence of events in the first few million years in a way no other material can.

The Sun's recipe and older stars

The rare CI chondrites, such as Orgueil, have proportions of most elements almost identical to those measured in the Sun's outer layers from its spectrum, except for the most volatile elements such as hydrogen, helium, carbon and nitrogen. They are therefore used as the reference composition of the Solar System.

The matrix of the most primitive chondrites also contains tiny presolar grains, such as diamonds, silicon carbide and graphite, that formed around other stars before the Sun existed. Their unusual isotope ratios record the nuclear processes in those stars.

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