1. Density
In the solar system

2. Density of air 0.0012 g/cm3 (sea level at 15° C)
Density on Earth
Density of air g/cm3 (sea level at 15° C)
These are the densities of pure water and dry air. Changes in temperature, humidity and salinity can change density. A hot air balloon demonstrates that increasing air temperature leads to a decrease in air density.
Density of water 1.0 g/cm3
Changes in temperature, humidity and salinity can change the density of water and air

3. Density on Earth Density of rocks 2.2-3.3 g/cm3 Igneous rocks
Approximate density values (in grams per cubic centimeter): Continental Crust: 2.7 to 3.0; Oceanic Crust: 3.0 to 3.3
Density of rocks
g/cm3
Sedimentary rocks
Metamorphic rocks

4. Density of Earth Average Density 5.5 g/cm3
Given the density of materials near Earth’s surface and the average density of Earth, what can we infer about the density of Earth’s interior?
Average Density 5.5 g/cm3

5. Density of Earth
Given the density of materials near Earth’s surface and the average density of Earth, what can we infer about the composition of Earth’s interior?

6. Inside Earth
Seismic waves from earthquakes bend (refract) when they move across a boundary between materials with different densities, just as light bends (refracts) when it moves across a boundary between different materials, like air and glass.
Geologists infer that Earth has layers with different densities due to the way seismic waves from earthquakes bend as they pass through Earth’s interior.

7. Inside Earth
Using these and other pieces of evidence, geologists have been able to create a model of Earth’s internal structure. 15 10 5
Density g/cm3

8. Inside Earth Inner Core Outer Core Mantle
This graph is based on the Preliminary Reference Earth Model (PREM) of density as a function of depth in the Earth. (After Oziewonski and Anderson, 1981)
Mantle
Seismic waves from earthquakes passing through Earth provide important information about the size and characteristics of each layer.

9. Comparison of Earth and Moon
The mass and volume of Earth are much larger than the mass and volume of the Moon.
Earth’s is mass is 81.3 times the mass of the Moon. Earth’s volume is 49.3 times the volume of the Moon.
What about density?

10. Comparison of Earth and Moon
What can we infer about the composition and internal structure of the Moon based on its density compared to Earth?
Moon must have a smaller iron core relative to its volume than Earth. A small iron core is consistent with its lower density.
Moon
Average Density 3.4 g/cm3
What about density?
Earth
Average Density 5.5 g/cm3

11. Mercury vs. Ganymede Average Density 5.43 g/cm3 Average Density
Mercury is the smallest planet. Ganymede is Jupiter’s (and the solar system’s) largest moon. The images correctly depict the relative sizes of Mercury and Ganymede.
Average Density
5.43 g/cm3
Average Density
1.94 g/cm3
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Credit: NASA

12. Mercury vs Ganymede
What can we infer about the compositions of Mercury and Ganymede based on their densities?
Mercury is thought to have a large iron core. Ganymede is thought to be made primarily of rock and ice.
Average Density
5.43 g/cm3
Average Density
1.94 g/cm3
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Credit: NASA

13. Mercury vs. Ganymede Rock surrounding a large iron core
Mercury’s large iron core. Ganymede is thought to have a small metallic core and rocky mantle and an ice-rich outer shell
Rock surrounding a large iron core
Icy shell surrounding a small rocky mantle and small metallic core
Image courtesy of Windows to the Universe
Credit: NASA