CAIs
This is a slice of a meteorite known as Allende, a carbonaceous chondrite that fell from the sky over Mexico in 1969. Allende is one of the most heavily researched meteorites in history because it is a large sample, with abundant material available to send to labs, because it fell in early 1969 just as labs were gearing up to process samples from the Apollo program, and because it preserves chemical evidence of the formation of the solar system.
There are so many papers out on samples like Allende that it’s hard to pick just one component to look at. The spherical objects are called chondrules; they’re the remnants of silicate minerals that melted and formed blobs out in space. The dark groundmass is carbon-rich, containing even complex organic molecules. But take a look at the irregular white patches; that’s the component I want to focus on today.
The irregular white patches are made of minerals such as anorthite, melilite, perovskite, aluminous spinel, and hibonite. Those minerals have a couple chemical components in common, including calcium and aluminum, as well as a bit of titanium.
These components tell a story of condensation in the early solar system. At some point, the materials in the inner solar system were heated so much that everything was vaporized. If you vaporize every element on the periodic table then slowly cool it, each element will come out of the gas and form a solid at a distinct temperature –called the condensation temperature of that element.
Although there are 90 elements found in nature, only a handful of these are abundant enough to dominate the chemistry of the rock. The first moderately abundant element to condense from a rock vapor is titanium; the next abundant elements to form are aluminum and calcium.
The Calcium and Aluminum Rich Inclusions (CAIs) found in a meteorite like Allende are therefore the remnants of the first solids to form in the inner solar system. The white bits of this rock formed solids when it was too hot for most other minerals to form, so they literally are the oldest part of our solar system. Some of these CAIs have a bit of uranium in them as well and therefore they can be dated; age dates on these grains push the origin of the solar system all the way back to 4.568 billion years ago.
-JBB
Image credit: http://bit.ly/1Y6Tbsu
References: http://s.si.edu/1TrwIqY http://bit.ly/1LScdQJ