Investigating Meteorites: Bridging Earth Science to Space Science

From this activity students should:
-discover that some meteorites come from the cores of previously existing planets (parent bodies).
-discover that making measurements can be used to gain more understanding of a meteorite (specifically: measuring crystal width could correlate to cooling rate and then parent body size)
-discover that knowledge learned in one discipline (Geology) can be applied to another (Space Science) [Specifically the cooling/crystal size correlation seen in Earth Rocks like granite can be applied to space science].
-Practice both using observation and inference and distinguishing between the two.
-Communicate their observations and inferences to classmates
-Practice measuring with a ruler and practice recording measurements
-Practice formulating questions about observations of an iron-nickel meteorite
-Information discrimination: students should gain practice determining which types of information would be relevant to answer the questions they have developed about their observations.

Context for Use:

This activity is designed to be an in-class guided inquiry lab investigation with follow-up homework required by the student. It is designed for a class with a maximum of 20 students, because verbal individualized teacher guidance should be provided to each group of students. The lab component of the activity can be completed in a single 50-minute class period. One homework assignment and a 30 minute follow up discussion are also needed.

Background:

A meteorite is any rocky body that has fallen to Earth. These sometimes impact Earth with a range of effects related to the size of the meteorite- sometimes they impact with minimal effect and sometimes they impact with catastrophic effects, vaporizing the meteorite itself and causing mass extinctions. Evidence of this can be seen in Earth's history; for example, the chixuclub . Scientists and the general public did not widely accept that rocks on Earth could have fallen from space until the eighteenth century.

Differentiation of Planets- In order for a planet or asteroid to have layers of differing composition, the body needs to be large enough to differentiate, that is to form layers where the densest materials sink to the middle forming a dense core.

Meterorites are classified mostly based on composition into chondrites, achondrites, iron and stony-iron meteorites. There are further sub-categories of each of these classifications. These meteorites have been providing scientists with a large amount of useful information about the formation and composition of the solar systems, as well as useful information about the particular parent bodies from whence they came.

Chondrites: Some meteorites have come from parent bodies that are likely undifferentiated and exhibit metal mixed with silicate material. Current scientific

understanding considers chondrites to be composed of the most primitive solar system material and the other classes of meteorites to be composed from larger solar system bodies that formed more recently than the chondrite material. Chondrites are considered primitive because their composition is similar to the Sun's composition. This is based on the idea that the sun is a good measure of average solar system composition because it contains more than 99% of the material in the solar system. There are also a few other techniques, such as measuring decay rates of radioactive elements, that help to confirm their primitive status. Chondrites are named for chondrules; round grains containing crystal and glass- thought to be droplets that condensed from a liquid. Radiometric dating ages them at about 4.566 billion years old, with some tiny grains such as micro-diamonds even older than this date. These microdiamonds are thought to be likely survivors of a previous supernova.

Achondrites: Achondrites are igneous, having crysallized from a silicate melt and they do not contain chondrules. Because they are igneous, they are thought to be remnants of differentiated planetesimals (or dwarf planets). Some achondrites have been matched to their respective parent bodies, Mars, Earth's Moon and Vesta.

Iron Meteorites:

In order for a planet or asteroid to have layers of differing composition, the body needs to be large enough to differentiate, that is to form layers where the densest materials sink to the middle forming a dense core. So, meteorites composed of iron and nickel crystals (and almost no silicates) are thought to be from the cores of planetary bodies that have since been shattered. As in Earth geology, the size of the crystal is correlated to its cooling rate, which in turn, can be related to the size of the original body. This is based on the idea that a larger body would cool more slowly. Most of these iron meterorites, with the exception of those containing very small or very large amounts of nickel, have a Widmannstatten pattern which is a intergrowth of crystals (this pattern can only be observed if the meteorite is polished and etched) Scientists would measure the different alloys of iron and nickel to ascertain more precise cooling rates (this will be simplified for middle school students by having them measure crystals indiscriminately).

As with the other categories of meteorites, iron meteorites are also further classified. One of these classifications is based on cooling rate. For example, one class has been calculated to have cooled extremely quickly at a rate of 210⁰ F (100⁰C) per hour with the corresponding parent body having a size of 24 miles (40 km) in radius. Another class has been calculated to have cooled at 9⁰F (5⁰C) per year. Overall, the iron meteorites that have been studied seem to come from parent bodies ranging from 6-105 miles (10-170 km) in radius.

The parent bodies yielding iron meteorites are thought to have fully solidified by 4.6 billion years ago. Using cosmic ray techniques (discussed in next section), these meteorites are thought to have broken from their parent bodies and been in space for times ranging from 200 million years-1 billion years before landing on Earth.

Stony Iron meteorites- These meteorites are composed of almost equal parts silicate and metal. These meteorites have compositions in between iron meteorites and achondrites, so are thought to be from differentiated parent bodies with incorporated core and mantle- some of these mixtures are difficult for scientists to explain. These stony iron meteorites can also be broken down into finer classes.

Some additional interesting information that can be gathered from meteorites in general:

On Earth, the minerals olivine and pyroxene are usually found in igneous rocks- these minerals can also be seen in meteorites, indicating that the original parent body formed by crystallizing from a liquid.

Iron and nickel is also found in meteorites, but the alloys of these in meteorites (taenite and kamacite) are very rare on the surface of Earth. These are hypothesized to be what would be found in Earth's core.

Cosmic rays damage the minerals in meteorites while they are traveling in space. This can be measured and correlated to the length of time the object was in space. Because exposure age is younger than the age of formation of the parent body, this suggests that these objects were part of a larger body that has since broken up- possibly due to collision with another asteroid. Being part of this parent body would be what once shielded it from damage from cosmic rays. Meteorites that have fallen to Earth appear to be from parent bodies that broke up early in the solar system.

As meteorites fall through Earth's atmosphere, they from an outer crust due to partial frictional melting as a result of falling through the atmosphere.

In addition to determining size of the parent body from crystal size, cooling rates and thus size can be calculated using fission tracks (for more information on this, see Asteroids, Meteorites, and Comets by Linda Elkins-Tanton)

Some meteorites have been matched to their parent bodies by matching composition. Some achondrites have been found to have originated on Mars, the Moon and Vesta and were broken off by meteorite impacts.

Activity Description and Teaching Materials:

Students will be measuring crystal sizes of iron nickel meteorites and correlating this to the cooling rate and the size of the meteorite's respective parent body.

In-class activities:

1. Students should be grouped into groups of 2-3.

2. Meteorites (real or photocopied) and other materials should be provided to each group. Student lab images (if not using actual samples) (Microsoft Word 2007 (.docx) 324kB Apr3 12)

3. Students should write down their general observations about these meterorites. (They should know the difference between an observation and an inference).

4. After making observations, students should be provided with the background information below: (the remainder of the background information can be given for homework)

You become curious about meteorites and go on a meteorite hunting expedition where you find the meteorites pictured. You read some articles/ letters/ books with fancy names like:

Iron meteorites: Crystallization, thermal history, parent bodies, and origin by J.I. Goldsteina, E.R.D. Scottb, N.L. Chabot

Iron meteorite evidence for early formation and catastrophic disruption of protoplanets by Jijin Yang, Joseph I. Goldstein & Edward R. D. Scott

Asteroids, Meteorites, And Comets by Linda T. Elkins-Tanton

<p style="margin-left:.25in;"> From your reading, you find that scientists estimate different cooling rates for iron meteorites. For example, one class of these meteorites has been calculated to have cooled extremely quickly at a rate of 210⁰ F (100⁰C) per hour with the corresponding parent body having a size of 24 miles (40 km) in radius. Another class has been calculated to have cooled at 9⁰F (5⁰C) per year. Overall, the iron meteorites that have been studied seem to come from parent bodies ranging from 6-105 mile (10-170 km) radius.

5. After reading this background information, students should be guided in their inquiry so that they discover that they could make measurements of the crystals using the guided questions below:

a. How can you use this background information to make inferences about your iron meteorites? (Hint: think back to what you know about crystal sizes in igneous rocks, like granite, on Earth).

b. What further observations might you make to gather more information? (students should puzzle this out a while and then, if needed, be prompted by the teacher to measure the crystals and to create a data table)

6. Students should create a data table to record the meteorite number, the trial number (10 or more measurements recommended), the measured crystal width, and average crystal width for each meteorite.

7. They should measure the width of at least 10 crystals per meteorite and then calculate an average.

Sample data table

Trial number	Crystal width measurement: Meteorite A	Crystal width measurement: Meteorite B
1.
-
10.
Average Crystal Width

8. Students should then answer the following questions:

a. What are some potential problems with your data? (sources of error)

b. What, if anything, can you infer about the cooling rate of each of the parent bodies from which these meteorites came? Why did you make this inference?

c. What, if anything, can you infer about the size of each of the parent bodies from which these meteorites came? Why did you make this inference?

d. What, if anything, can you infer about from what part of a parent body these meteorites came? Why did you make this inference?

e. In order to a parent body to differentiate, the more dense materials need to sink to the center. What can you infer about the state of matter of a parent body during differentiation?

f. What are some other inferences you can make after observing these meteorites?

g. What additional data could you gather?

9. Students should be directed to read the remainder of the background information (and/or research meteorites on their own) for homework.

10. The following class period, a general discussion should be held to go over the lab work and the homework questions. This should be designed to allow students to solidify their understanding and help clear up confusion that may have arisen during the inquiry portion of the lab.

At-home assignments

Student homework:

In class, you looked at iron-nickel meteorites. Read the provided background information. Answer the following questions and be prepared to answer your classmates' questions and your teacher's questions about your observations and inferences:

a. Scientists cannot sample cores of planets. How could the data you collected in class today be useful to the study of other planets?

b. Can you calculate a cooling rate and parent body size from your data? If so, make the calculations here. If not, explain what additional information you would need and explain where you would get this information.

c. List the categories of meteorites in addition to the iron meteorites you observed in class.

d. List one interesting fact about each of the classes of meteorites.

Materials

-Two or three iron nickel meteorites per group- these can be actual samples or photocopied photographs with scale. (The meteorites should each have different crystal widths) Student lab images (if not using actual samples) (Microsoft Word 2007 (.docx) 324kB Apr3 12)

<p class="MsoNormal"> This activity will be field-tested in May 2012, but one common student misconception is that most meteorites burn up in the atmosphere. This is true for sand-sized meteorites, but anything larger does not loose a significant portion (unless it is so large as to vaporize on impact).

<p class="MsoNormal">

Assessing student completion of the student lab worksheet and the homework questions can be used to determine students' general lab work/participation and their management of lab data. After students have been provided time, discussion and additional information to cement the knowledge gained from the lab, students could be assessed on a more formal test or quiz.

Books for additional information/photographs about meteorites:
Meteorites and their Parent Planets by Harry Y. McSween Jr
Asteroids, Meteorites, And Comets by Linda T. Elkins-Tanton
Websites for iron meteorite images and additional meteorite information: http://www.museumwales.ac.uk/en/837/
http://www.kgs.ku.edu/Publications/PIC/pic26.html