What is a calcium-aluminium-rich inclusion (CAI)?
A CAI is a pale inclusion in some chondrites made of high-temperature minerals. CAIs are the oldest solids formed in the Solar System, about 4,567 million years ago.
Calcium-aluminium-rich inclusions, or CAIs, are irregular white or pink objects found mainly in carbonaceous chondrites. They range from microscopic specks to several centimetres across. The Allende meteorite, which fell in Mexico in 1969, is famous for its large, abundant CAIs.
CAIs are made of minerals rich in calcium, aluminium and titanium, such as melilite, spinel, hibonite, perovskite and calcium-rich pyroxene. These minerals form at very high temperatures, above about 1,300 °C, and match the first minerals expected to condense from a cooling gas of solar composition.
Their age is their greatest significance. Lead–lead dating gives CAIs an age of about 4,567 million years, older than any other solid formed in the Solar System. Scientists use this as 'time zero', the conventional start of the Solar System, and date other events relative to it.
CAIs also preserve evidence of short-lived radioactive isotopes. Many contain excess magnesium-26 from the decay of aluminium-26, showing that this isotope was abundant when they formed. Their oxygen isotopes are rich in oxygen-16, like the Sun, unlike most other Solar System materials.
CAIs probably formed close to the young Sun and were later carried outwards, because they appear in chondrites that formed farther out, and even in comet dust returned by the Stardust mission. How they travelled so far is a key question about how material mixed in the young disc.