Meteorites, Tektites, and Shatter Cones page
This on-line exhibit is based on, and expands on, the meteorite exhibit at the Lafayette Science Museum in Lafayette, LA. You can visit the Museum and see these intriguing rocks from space, the actual meteorites, tektites, and other artifacts found in this on-line exhibit (hours, entry fees, and other important information are available through the Lafayette Science Museum link or Facebook page). There’s even a nickel-iron meteorite that you can actually hold at the end of planetarium programs!
Meteorites contain a record of our solar system's evolution and suggest how meteorite impacts could affect our future. The Lafayette Science Museum’s collection includes different types of meteorites found around the world, including two found in Louisiana, one from the Moon, and another that’s likely from Mars!
Meteoroids are bits of rock and metal orbiting the sun. They may be little more than specks, but some are larger. The American Meteor Society considers the dividing line between meteoroids and asteroids to be one meter (slightly larger than one yard)—smaller than that is a meteoroid, and larger than that is an asteroid.
Meteors refer to the streaks of light in the sky caused when meteoroids enter Earth’s atmosphere. When these objects enter the atmosphere, shock compression, pressure, and chemical interactions with atmospheric gases cause them to heat up and radiate energy. Because of the great heat of entry, much of a meteor’s streak is actually due to glowing atoms and molecules in Earth’s air. Most meteors are caused by tiny bits of space debris that are vaporized during entry through the atmosphere, but some are large enough to reach the ground.
Meteorites are pieces of rock and metal from outer space that fall to the ground through the atmosphere. Although many (possibly most) meteors are caused by debris left behind by comets, most meteorites seem to have their origins from asteroids, small rocky and metal bodies orbiting the sun mostly in the Asteroid Belt between Mars and Jupiter. Tiny meteorites smaller than a millimeter (less than 1/16 inch) are called micrometeorites, and they do include debris from comets as well as other sources.
Meteorites can be classified in several ways. In one widely used classification system,
stony meteorites are composed of rocky material with a small amount of iron;
iron meteorites are composed primarily of nickel and iron;
stony-iron meteorites are composed of similar amounts of stony material and nickel-iron (some scientists include certain stony-irons as sub-groups of either stony or nickel-iron meteorites).
All known meteorites are thought to have originated within the solar system, but a few types include extremely small amounts of material that appear to have come from beyond or before the solar system. The study of meteorites indicates that particles in space consist mostly of substances similar to those found on Earth.
Nickel-Iron Meteorites
Nickel-iron meteorites are denser than normal rocks and surprisingly heavy for their size. In fact, the largest one at the Lafayette Science Museum is about the size of bowling ball but weighs 78 pounds (35.6 kilograms)!
Nickel-iron meteorites can be classified as octahedrites, hexahedrites, or ataxites, depending on the types of nickel-iron mineral crystals they have and the percentage of nickel present (there is a separate type of classification based on chemistry).
When an octahedrite is sliced, polished, and etched with acid, a “criss-cross” pattern called a Widmanstätten pattern emerges. The pattern (named for Count Alois von Widmanstätten, who studied them in 1808) is related to the amount of nickel in the meteorite. Astronomers use these patterns to determine if a piece of iron comes from a meteorite since Widmanstätten patterns are not found in Earth rocks. As the percentage of nickel increases in an octahedrite, the Widmanstätten pattern changes from coarse to fine.
Hexahedrites don’t have Widmanstätten patterns because their nickel-iron crystal structures are different from those of octahedrites. Instead, a sliced, polished, and treated hexahedrite shows a set of very fine, parallel lines called a Neumann pattern (named for Franz Ernst Neumann who found them in 1848).
If the nickel content gets above about 16% by weight, the meteorite’s Widmanstätten pattern becomes microscopic and the meteorite is classified as an ataxite (from Greek roots suggesting no structure). Nickel tests and microscopic examination show their true meteoritic natures.
Nickel-iron meteorites can tend to rust because they are exposed to humidity and oxygen here on Earth, which is an alien environment for these bits of space debris. Chemical and vapor treatments help protect these meteorites from the elements, and prevent or slow the rusting process.
Stony Meteorites
As their name implies, stony meteorites are primarily rock, although they also contain iron and iron-based minerals. They can be divided into chondrites and achondrites.
Chondrites are the most common type of meteorite and are characterized by the presence chondrules, which as appear as round spots within the chondrite. Chondrules are the building blocks of chondrites, forming as molten or partially molten droplets in space that come together to form larger bodies. Members of a rare class of meteorite called carbonaceous chondrites have water molecules as part of their mineral structures and are richer in carbon than other meteorites. They may also include amino acids, the so-called building blocks of life, that are not found on Earth. The Lafayette Science Museum has several small carbonaceous chondrites in its collection.
Achondrites are stony meteorites with no chondrules. They are formed by lava flows at or near the surface of very large asteroids. The rare meteorites originating on the moon and Mars are also achondrites.
Stony-Iron Meteorites
As the name suggests, stony-iron meteorites have a mix of stony material and iron. There is some iron present in most stony meteorites, too, but stony-irons have a much larger amount—very roughly as much iron as stone. Stony-iron meteorites appear to come from the mantles of differentiated asteroids, meaning asteroids that are (or were!) big enough to develop a core, an outer crust, and a mantle in between the core and crust. By contrast, stony meteorites seem to match up better with the crusts of asteroids, and with smaller asteroids that did not differentiate. The two most common types of stony-irons are pallasites (where minerals dominated by olivine are held in an iron network), and mesosiderites (where the iron is mostly in chunks held by stony material). The beautiful appearance of pallasites is quite striking! The Lafayette Science Museum has examples of both kinds of stony-iron meteorites.
Special Meteorites at the Lafayette Science Museum
Louisiana Meteorites
Only three meteorites have ever been found in Louisiana, and the Lafayette Science Museum has slices of two of them. The Atlanta meteorite was named after the town of the same name in Winnfield Parish, and the Greenwell Springs meteorite was found near Baton Rouge. A third small meteorite hit a house in New Orleans shortly before Hurricane Katrina in 2005, but most of it was lost in the aftermath of the storm.
Martian Meteorites
The Lafayette Science Museum’s Zagami stony meteorite is an actual piece of the planet Mars! Out of tens of thousands of collected meteorites, as of 2021 fewer than 300 are thought to be Martian meteorites because they match well with Martian rocks studied with lander spacecraft. Martian rocks and meteorites are volcanic and significantly younger than most meteorites, and Mars is one of the few places in the solar system with volcanic activity in geologically recent times. Finally, suspected Martian meteorites have atoms matching the Martian atmosphere, as measured by the US Viking landers of the late 1970s, between their mineral crystals.
Lunar Meteorites
Some meteorites are rocks from the Moon, known to be lunar because their mineralogy matches well with known Moon rocks. Lunar meteorites were ejected from the Moon by impacts by other objects, but their pre-ejection locations are unknown. They are quite rare, but scientifically valuable because Earth labs have such a limited number of samples from the US Apollo program (and far fewer additional samples from Russian and Chinese robotic return missions). Lunar meteorites allow scientists to study random lunar locations in addition to the known sites of sample return missions.
Vesta Meteorites
Several meteorites in the Lafayette Science Museum’s collection appear to be from the asteroid Vesta, which has a giant crater at its south pole. These meteorites match well with the minerals in the surface of Vesta as confirmed by the American Dawn spacecraft, which orbited Vesta during 2011 and 2012, capturing images and taking other measurements.
Are There Impact Craters on Earth?
Yes!
Impact craters are caused by the collision of large asteroids with Earth’s surface. More than 150 impact craters are known on Earth, but they can be difficult or even impossible to see because they are buried by sediment or have eroded. Geologists can find them through different forms of aerial imaging, gravity studies, and drilled core samples of suspected craters. Impact craters are much larger than the objects that create them. Earth’s largest known impact crater has a diameter of over 180 miles (300 kilometers).
The impact crater shown to the left is near Winslow, Arizona. Although it is almost a mile (1.6 km) in diameter, it's considered small. If it were on the moon, a telescope about 18" in diameter would be needed to see it as only a dot!
What are Tektites?
Tektites are an unusual type of natural glass that forms when material from our planet’s surface is blasted into the upper atmosphere by asteroid impacts (although there is no conclusive evidence of comet impacts on Earth, that could be another source of tektite formation). The material melts, solidifies, and falls back to Earth. This process creates distinctive odd shapes, gouges, and bubbles in the glass. Some tektites entered the atmosphere while still nearly molten, stretching into “dog bone” shapes as they rotated. Teardrop shapes may be dog bones that rotated fast enough to split in half. Small pits and gouges in tektites were caused by entry into the atmosphere at very high speeds. Regions where tektites and meteorites are found are called strewn fields.
What Are Shatter Cones?
Shatter cones are Earth rocks found below impact craters. The cone-shaped grooves and cracks in them happen when shock waves from an impact go through the target rock. Shatter cones are key features in determining whether a crater is volcanic in origin or due to the impact of an asteroid because shatter cones are only found under impact craters and nuclear bomb test sites. In the Lafayette Science Museum’s collection, the feathery shatter cone shape is especially visible in the small shatter cone from Sierra Madera, Texas, seen in the picture to the left.
Look! Up in the Sky!
At 9:59 p.m. on Friday, March 15, 1957, a brilliant fireball streaked across the southern United States, turning a clear moonlit night into day. Its cosmic journey ended somewhere over southern Louisiana or the Gulf of Mexico. A rumbling sonic boom followed, shaking houses and windows throughout Acadiana. In the grip of the Cold War a half year before the first satellite was launched, many observers worried that it might be an atomic attack on Baton Rouge or New Orleans. Visible for less than 10 seconds, for some it would leave memories that would last a lifetime.
Have you just seen a very bright meteor? Report it on the American Meteor Society's Fireball Report Form!