Read These Notes:

They have low masses and high density (average density: 5.03 grams/cubic cm.). Their high density indicates that they are composed of heavy elements and are solid, rocky planets. Their high density and their slow rotational velocity help to explain why their low oblateness (def.) (average oblateness: .001).

The low masses of the terrestrials accounts for their low escape velocities (def.) (average: 7.75 km/sec). This might explain why the terrestrial planets have thin (if any) atmospheres.

The atmospheres (def.) of the terrestrial are of great interest for several reasons. They are much thinner than those of the jovian planets. They are composed of gasses which are atypical of the rest of the universe and solar system. They are composed of heavy elements (table 1), rather than hydrogen and helium. They also exhibit a lot of diversity. Mercury has no atmosphere. Venus and Mars have atmospheres composed of carbon dioxide (CO₂); and the Earth has an atmosphere that is unique in the solar system. It has the only atmosphere capable of supporting life as we know it.

Complete These Self-check Questions: These questions and their answers are designed to help you determine how well you understand this objective and to provide additional instruction.

• The Earth's atmosphere is composed of oxygen and nitrogen. It is more correct to say that it is composed of molecular oxygen and nitrogen. Most atoms have the tendency to bond with other atoms. Thus, it is rare to find single atoms of oxygen or nitrogen. The oxygen and nitrogen in the Earth's atmosphere is diatomic (def.), ) O₂. Use this periodic table of elements to determine the molecular weight of of diatomic oxygen. (ans.)