Planetary Interiors Lesson Plan

An Astronomy Earth Science Classroom Activity on Density of Planets

May 20, 2007 Paul A. Heckert

A low cost science classroom activity that teaches students about density and how it applies to deducing the interior composition of Earth and other planets.

For the Teacher

This activity uses the concept of density to show how we deduce the interior composition of Earth and other planets. Students will measure the density of rock and metal samples. A shorter alternative version has students compute, but not actually measure, densities. For the first version, use rock samples from a gravel road or driveway. Use steel ball bearings, or small pieces of scrap iron or steel for the metal samples. This activity is adaptable to any level from middle school to college.

Central question

How do we know the interior composition of the Earth and other planets?

Introduction

One way is by density. Density is mass divided by volume. (Density=Mass/Volume) Taking the ratio cancels out the amount of material. A large box of feathers may have more mass than a small box of cement, but the density of the feathers or of cement will be completely independent of how much cement or feathers one has. We can therefore distinguish between feathers and cement by measuring the density. The amount won't matter.

Mass is usually measured in kilograms and volume in cubic meters, so density is then in kilograms per cubic meter. The density of water is 1000 kilograms per cubic meter (=1 gram per cubic centimeter) so we can, if desired, express the density as a multiple of the density of water. For example a rock with a density of 3500 kilograms per cubic meter is 3.5 times the density of water. This is called the specific gravity.

Activity

You have some rocks and samples of iron or steel. Measure the density of two rock and two metal samples. Use balances to measure their masses in grams. To find their volumes, use a graduated cylinder or beaker. Put in a carefully calibrated amount of water, then gently drop in the sample and read how much the water level increases. Note that the beaker might be calibrated in milliliters, which are the same as cubic centimeters. Then divide the mass by the volume to get the density in grams per cubic centimeter. Multiply by 1000 to get the density in kilograms per cubic meter.

Make a table of your results, including the type of material, mass, volume, and density, on a separate sheet of paper. Before you continue, your rock samples should have densities of approximately 2000 to 3000 kilograms per cubic meter. You should get about 7000 to 9000 kilograms per cubic meter for the iron or steel samples. Repeat the density measurements if necessary. Round your answers to two figures.

Alternative Activity

For the computations below, show your calculation and round your answer to two figures.

Lets say that at a construction site a large steel (mostly iron) beam has a mass of 5000 kilograms. This beam also has a volume of 0.63 cubic meters. What is the density of iron?

Now lets say that to excavate the site they crush a boulder into gravel. A two cubic meter load of gravel has a mass of 5200 kilograms. What is the density of rock?

Application to planets

Compute the density of the Earth, using the following values: M=5.97e24 kilograms, and V=1.01e21 cubic meters. (Here the e indicates the exponent in scientific notation.) Compare the density of the Earth to the densities of the rock and iron samples. The rocks are fairly typical of the rocks near Earth's surface. What can you conclude about the density and composition of the Earth's core?

Look up the densities the other planets. As a rule of thumb, gaseous or icy materials have densities about the same as water, rocky materials about 2-3 times the density of water, and metallic materials about 7 to 9 times the density of water. Intermediate densities indicate mixtures of materials. Infer the compositions of the planets.

Make another table showing each planet, its density, and its inferred composition. Can you divide the planets into two groups based on their densities/compositions? Do so. Compare your grouping with the groups in your textbook.

Write a brief conclusion.

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