Classifying Meteorites
The in-person activity (attached below) requires a physical sample set of meteorites and related objects. We usually have a box with two or more of each meteorite class along with several impact-related objects like tektites, shatter cones, and impact breccia. We do run this lab online without physical samples which is appended below.
As you learned while studying the Moon in Lesson 2, samples are really important. This week you'll be learning about a new and very important set of samples we have: Meteorites! What we have learned from meteorites will be important throughout the rest of the quarter, so the more you discuss here, the more you will get out of the remainder of this course.
Start by reading the following before you post your answers.
Introduction
Meteorites are fragments of other worlds that have survived entry into the Earth’s atmosphere. Most meteorites originate in the asteroid belt from bodies that formed very early in the history of the Solar System. Almost all of the information we have learned about the Solar System, such as its age, history, and chemical composition is due to the detailed study of meteorites.
There are three basic types of meteorites we will learn about in this class: stony, stony-iron, and iron. Meteoriticists recognize many more types of meteorites and have reconstructed a marvelously detailed history of the Solar System from their subtle differences.
When a large meteorite strikes the Earth, the kinetic energy of the meteorite is converted to thermal, mechanical, and acoustic energy that creates a shockwave that passes through the ground and distorts, fractures, and ejects pieces of the surface. This modified surface material is often all that remains of a crater after millions of years of geologic activity. Therefore the recognition of this material plays an important role in understanding impact events. The most common types of impact-modified material we will investigate are: impact breccia, shatter cones, and tektites.
Basic Descriptions of Sample Types
Stony Meteorites
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Stony meteorites are a broad class of meteorites encompassing several subtypes you will study in this exercise. They are the most common meteorites that fall to Earth. Since they tend to have similar appearance and density to Earth rocks, stony meteorites are difficult to recognize in the field. Unless someone sees them fall, they usually go uncollected. Therefore although stony meteorites are the most common type out in space, they are rarer than iron meteorites in collections on Earth.
Stony meteorites show a wide variety of appearances: some light, some dark, some coarse grained, some fine grained, but almost all stony meteorites contain some metallic iron. Chemically they are also diverse, though they all have a telltale composition that tells us they are not from Earth. Most stony meteorites are from an asteroid that suffered destruction by collision. Some are pieces of lava flows from the surface (Achondrites), some are pieces of impact breccia (also Achondrites), and some are pieces of material that apparently never existed in a much larger body (Ordinary or Carbonaceous Chondrites). Meteorites that come from such small, undifferentiated bodies are called primitive meteorites.
Carbonaceous Chondrite Meteorites
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An especially important meteorite is the Carbonaceous Chondrite Meteorite, a specific type of stony meteorite that originates from primitive asteroids. In this context, primitive means that the asteroid has been altered very little over the age of the Solar System. More specifically, it means that the asteroid has not been heated to the point that would change the original material that makes up the asteroid. Carbonaceous Chondrite Meteorites are black to dark gray in color (see image to the left), are rich in carbon (thus their dark, black appearance), and contain small, spherical, droplet-like inclusions called chondrules.
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The image is a slice of a type of carbonaceous chondrite. Note the numerous chondrules (the small, light-colored spherical inclusions) in the image. Carbonaceous Chondrite Meteorites are among the most primitive objects in the Solar System, having survived almost unchanged for 4.6 billion years.
Carbonaceous chondrites were the first place amino acids were found outside of Earth, and it has been recently learned that some of the materials in these meteorites were formed outside of our Solar System before our Solar System was even formed so they are not only an important probe into our early Solar System history, but they may supply us with samples of materials from beyond our Solar System! Carbonaceous chondrites are rare among meteorites that fall to Earth (about 4%). Added to the fact that they look like Earth rocks, means that they are very rare in collections. They also weather very easily and do not survive very long on the surface of Earth.
Stony-Iron Meteorites
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Stony-Iron meteorites are the rarest class of meteorites, comprising only about 1% of meteorites that fall to Earth. There are two broad classes of stony-iron meteorites: Pallasites and Mesosiderites. Pallasites are composed primarily of iron with crystals of a rock mineral, called olivine, embedded in it. The image to the left shows a polished slice of a pallasite meteorite. The roundish chunks of embedded material are the rocky material, olivine, surrounded by metallic iron. Pallasites are thought to be a material from the boundary zone between the iron cores and stony outer mantels of destroyed asteroids. Mesosiderites are theorized to be formed when an impact on an asteroid mixes material from the rocky mantel with iron from the core (a type of Impact Breccia).
Iron Meteorites
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Iron meteorites are the most easily recognizable meteorites. Since even a casual examination shows that they are not ordinary rocks, they tend to be very common in collections although they are rare in space, comprising only about 5% of all meteorites. They are very dense and except for a thin crust made by the melting of the exterior during their passage through the atmosphere, they look and feel like metal. Chemically they are composed mostly of iron with a few percent nickel and a little cobalt. When sawed in half, polished, and etched with a mild acid, they display a geometrical pattern called a Widmanstätten pattern as in the figure. The pattern is actually crystals of iron and nickel that form as a result of the meteorite having cooled very slowly (about 1oC per 1 million years) under very high pressure. The existence of a Widmanstätten pattern is our best evidence that iron meteorites were once the cores of larger, differentiated bodies. Buried deep in a body, the mass of the overlying rocks provide the high pressure and insulation for slow cooling.
Impact Breccia
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Impact breccias form when a crater-forming meteorite shatters, pulverizes and melts the world’s surface material. They are composed of rock and mineral fragments embedded in a matrix of fine-grained material. The fragments are usually sharp and angular, and vary greatly in both size and shape. The composition of the fragments depends on the surface material. Impact breccias often have the appearance of poorly mixed concrete (see images above). On airless, impact-covered worlds like the Moon, impact breccias are a very common type of rock. The most common type of sample returned by the Apollo lunar missions was impact breccia. Unfortunately, rocks that look a lot like impact breccias can be formed by volcanic and tectonic processes, so finding a breccia is not always a clear indication of an impact event.
Shatter Cones
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Shatter cones form when the shockwave from a meteorite impact event passes through the surface rocks and modifies them. The resulting rocks have distinctive, curved, striated fractures that typically form partial to complete cones like the one in the image to the right. Shatter cones can form in all types of surface rocks. The better-looking shatter cones form in fine-grained rocks like sandstones. They can range in size from centimeters to many tens of meters. Shatter cones are now accepted as a unique identifier of a meteorite impact event. This means that if you find a shatter cone, you have found a place where a meteorite has hit. Since the Earth is such a dynamic world, it will erase impact craters over a short period of time. Often, shatter cones are all that is left to identify an impact crater. An interesting feature of shatter cones is that the tips point toward the origin of the shockwave. This means that you can use shatter cones to reconstruct the size and shape of ancient impact craters that have subsequently been modified by other processes.
Tektites
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Tektites have been controversial objects since their discovery, with both their origin and source being subject to hot debate for more than a century. Tektites are small, glassy objects with shapes like spheres, ellipsoids, dumbbells and other forms characteristic of isolated molten blobs (see figure). They are typically black, but can be brown, gray or even green. Tektites look a lot like volcanic glass (e.g., obsidian) but are chemically distinct. The most telling chemical difference is that unlike volcanic glasses, tektites contain virtually no water. Current scientific consensus are that tektites are terrestrial material that has been melted and ejected from an impact event. Their shape is derived from cooling aerodynamically, during flight from the impact. Tektites are fairly common all over the Earth. However, linking them with particular impact events has proven problematic. When exactly the tektites are formed during the impact event and why they are found at only a few craters are two of the more obvious problems that have yet to be satisfactorily solved.
Goals
Meteorites and impact-related objects are an important part of this course, therefore it helps to have some first-hand understanding of their physical properties. Samples are also an important component of this class. The meteorites, after all, are samples from worlds in the Solar System that we would otherwise not have access to. The goal of today’s experiment is to simply familiarize you with the look and feel of these objects and give you a chance to see the objects that result from the tremendous energy released during these impact events. I have actual samples of these in the department so if you'd like to get your hands on a real meteorite or related object, feel free to stop by and ask me to see them!
Procedure
Examine the images of each sample carefully, paying attention to color, chondrules, metallic characteristics, etc. You may see one of these on the final exam so pay particular attention to the details of each! Print out the data table (
Accessibility score: High Click to improveMeteoriteIDTable.pdfActions ) or record your observations on a separate sheet of paper. Record all information that you feel would help you characterize and identify each object - note that I gave you a description for example for in the 'stony-iron' meteorite (feel free to add other observations for that object there or on the back as well).
When you are finished with your observations, please address the following 5 questions in your discussion posts:
1. The classification of "stony" that we use includes all chondrite classes and achondrites. The achondrite meteorites you examined probably came from what part of a differentiated world? What does differentiated mean?
2. We discussed that achondrites, stony-iron, and iron meteorites all come from the same body. Give a possible scenario for what these meteorites have gone through over the past 4 billion years or so that explains this.
3. Is it possible that carbonaceous chondrites or ordinary chondrites were ever part of a world similar to the one described in question #2? Explain why or why not.
4. You can clearly see the Widmanstatten pattern in the iron meteorite sample but this pattern is not seen when a piece of iron ore from the Earth's crust is cut open and etched. In fact, the pattern can't even be replicated in a lab! What do you think is the likely cause of this difference? (Note: its okay to speculate here, I don't necessarily expect you to know the correct answer, but give it your best shot!)
5. We discussed that most achondrites found on Earth come predominantly from one particular source. What was it? Can you name at least two other sources for achondrite meteorites found on Earth?
A full credit response includes a clear and thoughtful response to or comments on each of the 5 questions.
Credits: Adapted from "Learning Astronomy by Doing Astronomy: Collaborative Lecture Activities" by Stacy Palen & Ana M. Larson