There are little surprises more treasured in the night sky than the sudden stream of light seen streaking across the darkness only to disappear as soon as it comes. This ball of rock that burns up in the Earth’s atmosphere to create that magic sight is a meteor. Any given meteor most likely comes from space debris (such as from an asteroid) that got caught in our planet’s gravity and plummeted. As there are different types of asteroids, there are subsequently different types of meteors. However, some do come from more interesting origins such as the explosions of stars or planets. The main ones that astronomers have observed are called stones, irons, and stony-irons.
Stony – Stony ‘meteorites’ (what meteors are called once they actually hit the ground) are the most common but unfortunately, also the easiest to confuse with typical Earth rock. They are believed to have originated from the outer crust of a planet or asteroid and are split up into two categories, chondrites and achondrites. Chondrites are characterized by pockets of silicates (compounds containing silicon) as well as the tiny embodiments on them called chondrules, from the Greek word for “sand grains.” Because the composition of chondrites is so similar to our Sun, scientists say that these came from the solar nebula, the cloud of gas and dust that formed our solar system. This means that they are the oldest known piece of matter that Earthlings have ever held in their hands and can tell us a lot about the start of our solar system. The most basic of all chondrites is the kind that is rich in water, sulfur, and organic material, which is why these ‘carbonaceous’ chondrites are thought to have brought the materials that made life on Earth possible. Achondrites, on the other hand, are stony meteorites that don’t have these chondrules because of a volatile parent body that erased them. Achondrites are volcanic rock, meaning that they first melted into magma, cooled, then crystallized to create a concentric layered structure inside. This process is how rocky planets are theorized to have formed and so achondrites are very useful in teaching us more about the formation of planets, especially since this is the category that Martian and lunar meteorites fit into. How lucky are we that these very important geologic processes are found in the most common type of meteorite.
Iron – These are the minority, only making up about 10% of all meteorites found on our planet. Iron meteors are special because scientists believe that they originate from the metallic core of asteroids in the Asteroid Belt between Mars and Jupiter. This is because iron is heavy and so when a celestial body goes through a melting process, the iron sinks to the center to form a metallic core, leaving a silicate mantle surrounding it. Since this is similar to the rocky planets in our solar system, iron meteorites can tell us more about the metallic cores of planets. Like stony meteorites, irons are split up into two categories themselves, based on their amount of iron-nickel alloy minerals, taenite and kamacite. Taenites are rich in nickel, containing around 30 – 70%, while kamacites are rich in iron, containing only about 5 – 10% nickel. When the meteorite is treated with acid, the combination of those minerals is what gives iron meteorites their distinct crystalline structure, known as the Widmanstatten pattern, that makes it real easy to distinguish them from terrestrial rocks.
Stony-Iron – While iron meteorites are pretty much resilient to anything and stony meteorites erode quite fast, stony-iron meteors are, of course, just right. They are arguably the most beautiful and exotic-looking, making up only 2% of all meteorites found on our planet, and are believed to have come from the boundary between the core and mantle of a planet body, a place where there is a mixture of metal and silicate. Like the other two types, stony-iron meteors are split into two categories: mesosiderites for those that are composed of stone and metal, and pallasites for those that are made of nickel-iron infused with olivine crystals, which actually form the August birthstone gem peridot. Although these are the two most common subcategories, the overall category of stony-iron meteors is mainly just a place to put all the oddball meteorites that don’t quite fit in with irons and stones.
Stones, irons, and stony-irons seem to fit the current limit of our knowledge. However, as future meteorites are discovered and scientists learn more about their compositions, the classification of meteorites is sure to change.