Meteorites and Craters (2023)

1. Fusion crust

Meteorites that have fallen recently have a black, glassy or ashy crust on their surface. When a meteorite falls through the Earth's atmosphere a very thin layer on the outer surface melts. This thin crust is called a fusion crust. It is often black and looks like an eggshell coating the rock. However, this crust weathers to a rusty brown color after several years of exposure on the Earth's surface and will eventually disappear altogether. In the image below, the fusion crust is the thin, black coating on the outside of the meteorite.

2. Irregular Shape

The surface of a meteorite is generally very smooth and featureless, but often has shallow depressions and deep cavities resembling thumbprints in wet clay or Play-Doh. Most iron meteorites, like the example at right, have well-developed regmaglypts all over their surface. Ordinary chondrites and stony meteorites like the one at left have smooth surfaces or regmaglypts.

3. Density

Unusual density is one of meteorites' more characteristic features. It's not enough to say your rock is heavy. Density is how heavy a rock is for its size or compared with other rocks. Iron meteorites are 3.5 times as heavy as ordinary Earth rocks of the same size, while stony meteorites are about 1.5 times as heavy. Lumps or fragments of human-made materials, ore rocks, slag (the byproduct of industrial processes) and the iron oxides magnetite and hematite, are also common all throughout the world and are frequently dense and metallic. So this test is helpful but not definitive.

4. Magnetism

Most meteorites contain some iron-nickel metal and attract a magnet easily. You can use an ordinary refrigerator magnet to test this property. A magnet will stick to the meteorite if it contains much metal. Some meteorites, such as stony meteorites, contain only a small amount of metal, but will attract a magnet hanging on a string. Metal detectors can alert you to whether a rock contains metal, but not all metal is magnetic. For instance, aluminum sets off metal detectors but is not magnetic. So, if you find a rock with a metal detector, try the magnet test too. In addition to meteorites containing iron, there are human-made and naturally-occurring materials that are magnetic and are easily confused with meteorites. Magnetite and hematite are common iron-bearing minerals that are often mistaken for meteorites. Both minerals can occur as large masses with smooth surfaces that are heavier than typical rocks, but have some features which resemble meteorites. Magnetite is very magnetic (hence its name) and hematite is mildly magnetic. Use the streak test below to distinguish these minerals.

5. Iron-nickel metal

Most meteorites contain at least some iron metal (actually an alloy of iron and nickel). You can see the metal shining on a broken surface. Meteorites without metal in them are extremely rare and they need to have some of the other characteristics of meteorites to be able to identify them as meteorites. Iron meteorites have a dense, silvery appearing interior with no holes or crystals. Stony iron meteorites are about half metal, half crystals of green or orange olivine. Stony meteorites contain small flecks of metal that are evenly distributed throughout the meteorite. The metal in a meteorite has the unusual characteristic of containing up to 7% nickel. This is a definitive test of a meteorite, but requires a chemical analysis or acid etching to detect.

Meteorites & Craters of Alabama

Do you think you might have found a meteorite? We’ll be happy to help you - scroll down for more information and a worksheet before you contact us.

  1. How do we know it’s a meteorite from Mars anyway?

  2. How do we know it’s a meteorite from the Moon anyway?

  3. Where are impact craters on the Earth? Check out the Earth Impact Database.

Do you think you’ve found a meteorite?

Meteorites are pieces of asteroids and other bodies like the moon and Mars that travel through space and fall to the earth. They are rocks that are similar in many ways to Earth rocks, but it is exciting to find a piece of another planet here on Earth. Meteorites fall to Earth all the time and are distributed over the entire planet, so you could even find one in your own backyard!

Meteorites have some distinguishing characteristics to help you identify them. You can use this list to guide you through them. Sometimes detailed chemical analyses need to be done, but since this takes time and money, you should look for the easy characteristics first.

If you want your specimen professionally tested, there are many places that will help you with this pursuit - check this web page. While you’re there, there’s also an excellent list of meteor-wrongs that are actually common terrestrial rocks.

Characteristics of meteorites

1. Fusion crust

2. Irregular shape

3. Density

4. Magnetism

5. Iron-nickel metal

Characteristics of meteor-wrongs

1. Round shape

2. Holes

3. Crystals

4. Heat or Radioactivity

5. Streak

6. Other kinds of metal

1. Round (Spherical) Shape

Meteorites are almost never perfectly round or spherical and rarely are they aerodynamically shaped. They are usually very irregular in appearance and come in a variety of different shapes and sizes.

2. Bubbles or holes

Many people believe that meteorites have the appearance of being molten, perhaps having a frothy appearance or bubbles on their surfaces. However, this is not the case. The outer portion of a meteorite, the fusion crust, is either smooth or has the characteristic regmaglypts (thumb prints) described earlier. However, many terrestrial igneous rocks are porous and have holes in them. These holes or 'vesicles' were produced by bubbles of gas that formed in the magma as it was erupted. If you find a rock that is porous or contains vesicles it is a terrestrial rock.

3. Crystals

If there is quartz (a clear or milky white crystal) it is not a meteorite. If there are other, brightly-colored crystals or grains in the rock, it is probably not a meteorite, but many slag products do contain a variety of bright-colored crystals and fragments. If there is an easily visible crystal structure it might not be a meteorite. This is not conclusive because some of the rarer meteorites do have some crystal structure. However, most ordinary meteorites do not unless viewed under a microscope.

4. Hot or Radioactive

Most meteorites are cold when they hit the Earth's surface and do not start fires on the ground. Their trip through the atmosphere is extremely short and the friction heat that burns up the outside does not have a chance to heat up the inside of the meteorite. When you see a shooting star of fireball go down to the horizon, remember that the horizon is 3 miles away! It is rare to see a meteorite fall from a fireball. Meteorites are made of the same elements and minerals as terrestrial rocks and are not any more radioactive than terrestrial rocks, so you can't find them with a Geiger counter.

5. Streak

Streak is what the rock leaves behind, like a crayon. Common ceramic tile, such as a bathroom or kitchen tile, has a smooth glazed slide and an unfinished dull side which is stuck to the wall when installed. Take the sample that you think is a meteorite and scratch it vigorously on the unglazed side of the tile. If it leaves a black gray streak the sample is almost certainly magnetite, and if it leaves a red-brown streak it is almost certainly hematite. A meteorite, unless it is very heavily weathered, will not leave a streak on the tile. If you don't have a ceramic tile, you can also use the inside of your toilet tank cover (the heavy rectangular lid on top of the tank) - it is heavy, so be careful.

6. Other kinds of metal

Human activity has produced objects made from pure iron for centuries, so it is possible to confuse lumps of man-made iron with meteoritic materials. Objects such as iron grinding balls often have a smooth rounded appearance and may be thought be meteorites. Lumps of iron slag from smelting processes can also have some similarities to meteorites. The major difference between iron produced by human activity and meteoritic iron is the presence of the element nickel. Iron metal in all meteorites contains at least some nickel whereas man-made metal objects generally do not. In addition, the interior structure of iron meteorites is unique and unlike any man-made metal alloys. Special analysis and preparation techniques are required to examine the internal structure and composition of a suspect meteorite. The results of such tests are, however, completely definitive.

Last Modified: March 8, 2016

Web Services provided by Bob Dean (MITS)

NASA Official: Barbara Cohen


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