Earth, Which Is Mostly Rock and Iron, Has a Density

Multiple Choice

Earth, which is mostly rock and iron, has a density of 5515 kg/m³. Ganymede, Jupiter's largest satellite, contains much water and water ice and has a density of 1940 kg/m³. Io has a density of 3570 kg/m³. What conclusions have astronomers drawn from this?

A) Io must be composed of some new material other than rocks, iron, water, or ice.
B) Io is composed of compressed water and water ice.
C) Io is mostly rock and iron, but because of its smaller mass it cannot compress the material as much as Earth can.
D) Since Io is smaller than Earth, it is expected to have a smaller density regardless of composition.