CAIs: THE OLDEST ROCKS
CRYSTALS CONTAINING CALCIUM AND ALUMINUM DATE TO THE BEGINNING OF THE SOLAR SYSTEM.
A SOLAR SYSTEM SUCH AS OUR OWN BEGINS when a massive gas and dust cloud collapses on itself and starts to spin. At first the temperature increases dramatically near the center of the spinning disk, where a young star begins to radiate energy. But gradually, the disk cools. As the temperature drops, solid particles begin to condense in the central region. The first rocks to condense in our solar system are known as calcium-aluminum inclusions, or CAIs. These oldest rocks can still be found in certain "primitive" meteorites.
Watch an animation showing remaining particles within solar nebula colliding to form larger objects. Some of these objects later became asteroids, the most common source of meteorites. (Get RealPlayer.)
The solar nebula, the disk of gas and dust that formed our own solar system, cooled relatively rapidly. In less than one million years, the temperature was low enough for CAIs to form. Some CAIs later melted, but others have not changed significantly since their formation. Both types offer insights into the origins of the solar system.
HOW DO WE KNOW?
DETERMINING THE AGE OF CAIs
How do we know that calcium-aluminum inclusions, or CAIs, are the oldest rocks that formed in our solar system? In a technique somewhat like the radiocarbon dating method used on fossils, researchers measure the amounts of certain forms, or isotopes, of the elements uranium and lead present in CAIs. Isotopes of uranium undergo radioactive decay to form lead at known rates, providing a clock that can be used to date material from the early solar system. These studies indicate that the oldest CAIs formed 4.568 billion years ago—less than one million years after our Sun first ignited.
What is an isotope? Chemical elements come in different forms, known as isotopes. Isotopes of the same element have the same number of protons in the atomic nucleus but different numbers of neutrons.
HISTORY OF OUR SOLAR SYSTEM
The evolution of our solar system from the rotating disk of dust and gas into the Sun and planets took place relatively rapidly. After the formation of CAIs, the first rocks in the solar system, it took about 30 million years for the planets to take shape.
A TREASURE TROVE OF CAIs
This fragment of Allende contains two kinds of calcium-aluminum inclusions (CAIs): round and irregularly shaped. Round CAIs melted and resolidified at some point in their history. Irregularly shaped CAIs, however, never melted: they crystallized directly from the gas of the early solar nebula. The two tons of Allende recovered for study provided scientists with more and larger CAIs than all previously known meteorites combined.
VARIATIONS ON A THEME
Not every primitive meteorite contains calcium-aluminum inclusions; some meteorites, like Leoville, have abundant CAIs, whereas others, like Clovis have none. Variations in the abundance of CAIs as well as other meteorite components suggest that the chemical elements may not have been evenly distributed in the early solar nebula.
HIDDEN CAIs
The CAIs in Coolidge are no longer visible. As this meteorite sat in a field in Kansas, water rusted the iron in the rock, discoloring the CAIs.
LOOK CLOSELY
The tiny white calcium-aluminum inclusions, or CAIs, in Murchison are exceptionally small—much smaller than the CAIs found in meteorites like Allende. Scientists have not determined why the size of CAIs can vary so much from meteorite to meteorite.
COSMIC SNOWFLAKES
Snowflakes form high in Earth's atmosphere, where the temperature and air pressure are low. Under these conditions, water molecules that are present in clouds as a gas solidify, or condense, into ice crystals, bypassing a liquid water stage.
A similar process occurred in our solar nebula. At the very low pressures of the cooling gas cloud, certain elements present as gas molecules condensed directly into mineral crystals. Oxide minerals rich in calcium, aluminum and titanium—elements that are abundant in calcium-aluminum inclusions (CAIs)—were the first to condense, at around 1,500°C (2,700°F). At lower temperatures, crystals of minerals containing magnesium, silicon and iron began to condense. These minerals are found in chondrules, which began to form around one million years after CAIs.
MOON ROCKS AND METEORITES
The Allende meteorite fell over northern Mexico in early 1969, just as scientific laboratories were preparing to study moon rocks. The first moon rocks were brought back in July 1969 by Apollo astronauts like Buzz Aldrin, shown here collecting soil samples on the Moon. After pieces of Allende were collected, geologists and chemists also turned their attention to these abundant meteorite samples—and the CAIs contained inside.
