Average Density
Average density—the mass of an object divided by that object’s volume—is a useful quantity for describing the differences between planets in our solar system. This same quantity has many applications here on Earth.
A rock tossed into a lake sinks to the bottom, while an air bubble produced at the bottom of a lake (for example, by the air tanks of a scuba diver) rises to the top. These are examples of a general principle: An object sinks in a fluid if its average density is greater than that of the fluid, but rises if its average density is less than that of the fluid. The average density of water is 1000 kg/m3, which is why a typical rock (with an average density of about 3000 kg/m3) sinks, while an air bubble (average density of about 1.2 kg/m3) rises.
At many summer barbecues, cans of soft drinks are kept cold by putting them in a container full of ice. When the ice melts, the cans of diet soda always rise to the top, while the cans of regular soda sink to the bottom. Why is this? The average density of a can of diet soda—which includes water, flavoring, artificial sweetener, and the trapped gas that makes the drink fizzy—is slightly less than the density of water, and so the can floats. A can of regular soda contains sugar instead of artificial sweetener, and the sugar is a bit heavier than the sweetener. The extra weight is just enough to make the average density of a can of regular soda slightly more than that of water, making the can sink. (You can test these statements for yourself by putting unopened cans of diet soda and regular soda in a sink or bathtub full of water.)
The concept of average density provides geologists with important clues about the early history of Earth. The average density of surface rocks on Earth, about 3000 kg/m3, is less than Earth’s average density of 5515 kg/m3. The simplest explanation is that in the ancient past, Earth was completely molten throughout its volume, so that low-density materials rose to the surface and high-density materials sank deep into Earth’s interior in a process called chemical differentiation. This series of events also suggests that Earth’s core must be made of relatively dense materials, such as iron and nickel. A tremendous amount of other geological evidence has convinced scientists that this picture is correct.