Craters and Rays
Craters on the Moon range in size from as small as one inch to over 100 miles
across. Craters are formed when meteoroids hit the Moon. Meteoroids are large
rocks traveling through space traveling thousands of miles per hour. The energy
in the meteoroid blasts a large hole with a raised mountain of rock around its
outer edge. The crater Tycho is probably the most easily seen crater. Copernius
and Kepler are also noticeable.
The force of the meteoroid often splashes up rock that spreads across the
surrounding area in long, thin rays that all extend away from the crater. Rays
are especially visible in Tycho, where some rays reach out for more than 1,000
miles.
Maria and Mountains
Maria are the large, dark, hole-like features on the moon. Astronomers
believe they formed several billion years ago, just after the moon was created
and when it still had a thin crust. Large meteoroids hit the Moon with such
force that they broke through the crust, and the molten lava from below filled
shallow basins on the moon. The lava cooled and hardened into the large flat
areas we see as maria. The maria are darker than the surrounding areas because
the rock is basalt, a volcanic rock that is darker than the material making up
the mountains surrounding it. Ancient astronomers thought these dark spots on
the Moon were oceans, so they called them maria, which was Latin for
"seas."
Mountains are especially easy to see around the outer edge of most maria. The
mountains probably formed over four billion years ago when most of the moon was
still hot and molten. As the material cooled, the moon shrank slightly, making
the outer layer buckle. This pushed rocks up to form the mountain features we
see today. A lack of an atmosphere means that almost no erosion occurs. The moon’s
mountains have changed little since they were formed billions of years ago. This
is unlike the Earth’s mountains, which have changed greatly due to erosion.
This is what makes the Earth and the Moon’s surface features different.
