Figure 12.6: An artist’s rendition of a baby star still surrounded by a protoplanetary disc in
which planets are forming. ( 10 )
common. We see evidence for heavy bombardment by planetesimals on the surfaces of the
moon and Mercury (Figure12.7andFigure12.8).
The same types of collisions would have occurred on the surface of the Earth, however erosive
processes have erased all except the most recent of these collisions. Pictured inFigure12.9
is a Meteor Crater in Arizona.
About 100 million years after the formation of the Sun, the gravity of the planets and moons
in our solar system had swept up most of the planetesimals. However, millions of these
objects still remain in gravitationally stable orbits in the main asteroid belt of the solar
system, in the Trojan asteroid belt, or out beyond Neptune and Pluto in the Kuiper belt.
Illustrated in the sketch below is the location of the largest reservoir of asteroids in our solar
system today (Figure12.10).
Earth is the only object in our solar system known to support life (Figure12.11). Today
there are over 1 million knownspeciesof plants and animals on Earth.
The materials that came together to form the Earth were made of several different chemical
elements. Each element has a different density, defined as mass per volume. Density
describes how heavy an object is compared to how much space the object takes up. After
Earth’s early formation, the denser elements sank to the center. The lighter elements rose
to the surface. You have probably seen something like this happen if you have ever mixed
oil and water in a bottle. The water is denser than oil. If you put both in a bottle, shake it
up, and then let it sit for a while, the water settles to the bottom and the oil rises up over
the top of the water.
Today, the Earth consists of layers that represent different densities (Figure12.12). Earth’s