Exoplanet density

Students use iron and sand to model the composition of the Earth and estimate what fraction of the Earth is occupied by its iron core.

Apparatus and materials

(per group of 2 to 4 students)

• Balance
• Measuring cylinder
• Steal ball bearing or steel block approx. 2 or 3 cm across
• Sand 

Each student will also require a photocopy of the instructions and worksheet

Health & safety and technical notes

If using ball bearings, remind students that if any fall on the floor they must be picked up promptly so that so no-one slips on them. Give each group a dish to keep them in. A little bit of tissue paper on the balance will stop them rolling off.

The practical activity

1. Introduce the activity by showing a steel ball (to represent the Earth’s core) and some Plasticine. Discuss their different densities. Explain how to calculate density and introduce units. 
2. Wrap a layer of Plasticine around the ball to represent the mantle and crust. What can be said about the average density?
3. You could measure mass and volume of the ball + Plasticine by immersing the ball in water in a measuring cylinder on a balance and then add increasing amounts of Plasticine. However, sand is a better material to represent rock as its density is closer to that of the rock found on the Earth’s surface.
4. They should find that the average density decreases from that of steel as more sand is added.
5. After the activity you may want to discuss the composition of the Earth. Explain that although the crust is of a similar density to sand, the rock in the mantle has a higher density