Figure 24.2: Planets and dwarf planets of the solar system. ( 2 )atmosphere. Earth’s atmosphere is unique in the solar system in that it contains just the
right amount of oxygen to support animal life. Therefore, Earth is the only planet in the
solar system on which life is found. The ‘Our Solar System’ chapter will discuss the features
of other planets in more detail.
When describing Earth, it’s useful to name and define the many components of our planet.
Since Earth is a sphere, the layers that make it up are also referred to as spheres (Figure
24.3).
They are:
- Atmosphere—the thin layer of air that surrounds the Earth.
- Hydrosphere–the part of Earth’s surface that consists of water.
- Biosphere—the part of the Earth that supports life. The biosphere includes all the
areas where life is found. - Lithosphere–the solid part of Earth. The lithosphere consists of mountains, valleys,
continents and all of the land beneath the oceans. Only one-fourth of Earth’s surface
is land, but solid rock makes up more than 99% of Earth’s total mass.
Earth’s layers all come into contact with each other and interact. Therefore, Earth’s surface
is constantly undergoing change.
Earth’s Gravity
We know that the Earth orbits the Sun in a regular path (Figure 24.4). The Earth’s
Moon also orbits the Earth in a regular path.Gravityis the force of attraction between all
objects. Gravity keeps the Earth and Moon in their orbits. Isaac Newton was one of the first
scientists to explore the idea of gravity. He understood that the Moon can only circle the
Earth because some force is pulling the Moon toward Earth’s center. Otherwise, the Moon
would continue moving in a straight line off into space. Newton also came to understand