Density of the Terrestrial Planets

Directions: Choose a terrestrial world with the tabs at the top of the window: Earth, Venus, Mars, Mercury, Moon (listed in order of largest to smallest). Then, use the slider to change the relative volume of rock and metal to match the Measured Density to the Calculated Density for all five worlds.

Change the fraction of the planet filled with rock or iron metal with the slider control on the right. Sliding the pointer up increases the volume of the iron core of the planet and increases the planet's density. Sliding the pointer down increases the volume of rock in the planet and decreases the planet's density.

The Measured Density displays the uncompressed density of the selected world. This measure of a planet's density removes the effects of gravitational compression due to the weight of the planet squeezing the interior. This density is based on spacecraft measurements of the planet's mass and radius combined with laboratory measurements of the densities of various types of rock and metal.

The Calculated Density is the density of the selected planet calculated from the chosen ratio of rock and metal. For the purposes of this simulation, a pure-metal planet has a density of 8 grams per cubic centimeter (the density of iron) and a pure-rock planet has a density of 3 grams per cubic centimeter (an average value for silicate rock).

Questions to Consider:

1. How large are the cores of the 5 terrestrial worlds in this simulation? That is, what fraction of the interior of each world is filled with metal? (Click for Answer)

By matching the Measured Density to the Calculated Density, you are likely to find values similar to the following: Earth has a core filling about 65% of its interior. Venus: 65%, Mars: 40%, Mercury: 80%, The Moon: 40%.

2. Which world has the greatest difference between its observed density (based on its observed radius and mass) and its uncompressed density? Which world has the smallest difference between its observed density and its uncompressed density? (Click for Answer)

Earth has the largest mass and strongest gravity of the 5 terrestrial worlds. This means that Earth's interior is the most compressed of the terrestrial worlds. The metal at Earth's center has a higher density than the same metal sitting at Earth's surface. The observed density of Earth is 5.5 grams per cubic centimeter, but its uncompressed density is only 4.2 grams per cubic centimeter.

Earth's Moon has the smallest mass and weakest gravity of the 5 terrestrial worlds. This means that the Moon's interior is the least compressed. The metal at the Moon's center is only slightly more dense than the same metal on the Moon's surface. The observed density of the Moon is 3.36 grams per cubic centimeter. The uncompressed density of the Moon is only slightly less, 3.35 grams per cubic centimeter.

3. Based on the chemistry of Earth and Moon rocks, it appears that both worlds formed at the same distance from the Sun. But the Moon has a much lower uncompressed density than Earth. What does this imply about the interiors of Earth and Moon? (Click for Answer)

For Earth to have a higher uncompressed density, it must contain a much larger volume of metal than the Moon. You can determine this yourself by finding the proper metal-rock ratio for Earth and the Moon.

If both worlds formed at the same distance from the Sun, one would expect them to form out of the same fraction of rock and metal. To explain the observed difference in the two worlds, astronomers have proposed that the very early Earth suffered a massive collision with another world. The ejected material from that collision was mostly rock from Earth's mantle and crust. The giant impact hypothesis proposes that the Moon formed from the material splashed out of the Earth.

4. Which world(s) has the highest uncompressed density? What does this indicate about that world's internal composition? (Click for Answer)

Flip through the 5 terrestrial planets and notice that Mercury has the highest uncompressed density, 5.40 grams per cubic centimeter. (Mercury's observed density of 5.43 is essentially the same, since it is a small world with weak gravity.)

Determine the ratio of rock to metal that matches the uncompressed density of Mercury. You should get a proportion close to 80% metal and 20% rock. This is the largest fraction of metal to rock in any of the other terrestrial worlds.

5. Why do scientists model the interior of the terrestrial planets using primarily rock and metal? Why not other substances? (Click for Answer)

The short answer is that scientists know the temperature, density, and chemical composition of Earth's interior very well. This information comes from samples of the Earth's surface and shallow interior from direct observation and from molten rock from volcanoes around the world. In addition, the deep interior of Earth is understood from seismic waves produced by earthquakes. The surfaces of the other terrestrial planets are also rocky, so it is likely that they also have iron cores similar to Earth.

The other answer is that astronomers know the abundance of the chemical elements throughout the Universe. After hydrogen and helium, the most common elements are carbon, oxygen, and nitrogen, as well as silicon and iron. High-density elements like lead, gold, and uranium are very rare cosmically. The elements CNO are too light to account for the density of the interiors of the terrestrial planets, so silicate minerals and iron metal are likely components. This picture fits together nicely with the information scientists have discovered about Earth's interior.

Created by Kevin Healy, 2007