Ordinary (but also Extraordinary!): Chondrite Meteorites
Meteorites are very special objects that originate in outer space.
Some basics about meteorites:
- Meteorites originate from asteroids, comets, and meteoroids (small rocky or metallic body travelling through space, ranging in size from small grains to 1 m).
- As the object passes through the Earth’s atmosphere, friction and pressure causes it to heat up, appearing to us on Earth as a meteor or a shooting star. If the original object is large enough, it is referred to as a superbolide and appears as a very bright fireball.
- Many meteors completely burn up in the atmosphere. They only become meteorites when they survive their fall to Earth. Meteorites land all over the surface of the Earth, but one place where they are concentrated and relatively easy to spot is Antarctica. NASA and NSF fund annual expeditions to Antarctica (called ANSMET) to search for new meteorites. You can hear an interview of one of the 2015/2016 ANSMET team, Nina Lanza to learn about what it is like to live on the ice and search for space rocks in this Science Friday Podcast.
- During the passage through the atmosphere, the original object may break up into many pieces. When a bunch of meteorites land at the same time, in the same general vicinity, and are known to have come from the same parent object, they are called a meteorite fall. A recent, local fall event occurred in Grimsby, ON on September 25, 2009.
- When meteorites are large enough, or coherent enough, they may impact the Earth with such force that they create circular depressions in the surface of the planet or moon, called impact craters. (More on these features later…)
One more very amazing fact about meteorites: Meteorites include the oldest solid materials of our Solar System.
About 86% of all meteorites that land on Earth are a stony (non-metallic) type called chondrites. Chondrite meteorites are aggregates of the various types of dust and small grains that were present in the early solar system and that accreted to form primitive asteroids.- Chondrites are named for small, round particles called chondrules. Chondrules represent one of the oldest solid materials within the solar system, dated as 4.55 billion years, and are thought to have formed from molten droplets within the solar nebula. Chondrules are believed to be the building blocks of the planetary system.
- Some chondrites contain the oldest substances of the solar system: submillimeter to centimeter-sized calcium-aluminium-rich inclusions (CAIs). CAIs are light colored with blob-like shapes and are among the first solids condensed from the cooling proplanetary disk. Radiometric dating indicates that CAIs formed up to 3 million years before chondrules first appeared.
- Chondrites also contain small blebs of iron-nickel metal.
There are different kinds of chondrites. Two of these are worth describing.
- Ordinary chondrites: As their name implies, these are the most abundant type of chondrite meteorite. Over 90% of all chondrites are ordinary chondrites. Most, but not all have experienced metamorphism (solid-state modification at high temperatures, <500 degrees Kelvin) on their parent asteroid.
- Carbonaceous chondrites: Less than 5% of chondrites belong to a relatively unmetamorphosed, or primitive group that is notable for its abundance of carbon compounds, including amino acids, as well as volatiles, such as water. Study of carbonanaceous chondrites provides important clues for the origin of life. Chondrite meteorites retain features (chondrules and CAIs) originating from the earliest stages of our Solar System because they have not been modified by melting and planetary differentiation (separation into iron-rich core, mantle, and crust). The remainder of this module will discuss how planets are thought to form from cosmic dust.
