# Planetary Densities

Density is defined as the ratio of the mass of an object to the volume of space the object takes up. Mathematically, we would say D = M/V. Density will determine if you sink or float when put into a liquid, like water.

Take 8 same-size plastic Easter eggs that come apart ( so you can fill them). This gives you 8 equal volumes. To determine the volume of your egg, fill it completely with water (top and bottom) and then pour the water into a graduated cylinder. For water, 1 mL = 1 cm3. (For example, our eggs held 70 mL of water, so the volume of the egg was 70 cm3.) To create the average densities of the 8 planets in our solar system, we just need to fill the eggs with the right amount of material so they have the correct mass for the given volume. The average densities for each planet and the required mass for a 70 cm3 egg are given in the Table below. Remember, Required Mass = Average Density multiplied by Egg Volume, so if your eggs have a different volume than 70 cm3, just multiply the average densities in the Table by your egg volume to find the required masses for each of your eggs. Small lead balls (such as lead shot or BBs) work best as a filler!

Planet Average Density (gm/cm3) Required Mass for 70 cm3 (gm)
Mercury 5.4 378.0
Venus 5.2 364.0
Earth 5.5 385.0
Mars 3.9 273.0
Jupiter 1.3 91.0
Saturn 0.7 49.0
Uranus 1.3 91.0
Neptune 1.6 112.0

Now you are ready to drop all 8 eggs in a giant tub of water and see what happens!

Explanation: If the density of the object is less than that of the water (which is 1 gm per cubic cm), the object floats. If density is greater, the object sinks. Saturn is mainly composed of the lightest two gases known, hydrogen and helium. It is the only planet in our solar system whose density is less than water. So Saturn will float while all the other planets sink!

This activity is part of the StarChild site.

http://starchild.gsfc.nasa.gov/