Download A crater is a large, bowl-shaped hole found in solid, rocky surfaces

A crater is a large, bowl-shaped hole found in solid,
rocky surfaces. Terrestrial planets, moons, and asteroids all
contain craters. Some craters form when meteoroids,
asteroids, or comets smash into the surface of a planet or
moon. These craters – called “impact craters” – are usually
circular. They range in size from tiny pits to huge basins
hundreds of meters across. Impact craters are the most
common geological feature in the solar system.
The Impact
The appearance of an impact crater depends on many
things. For example, the size and speed of the object
causing the crater affects its width or depth. With smaller or
slower impacts, the surface material is simply thrown out, like
sand that is thrown when a rock hits the surface of a beach.
However, when the impacting object is large and traveling at
higher speeds, it hits the surface with enormous force. The
extreme temperatures and pressures from the collision
cause the object to melt and mix with the surrounding rock.
Parts of a Crater
After impact, a crater forms with a high rim and central
peak (see the illustration below). Landslides may create
terraces around the rim. The floor of the crater is often
below the level of the surrounding terrain. Ejected materials,
Craters: Links to the Past
Craters can tell us about a planet or moon’s history.
The more craters on a planet or moon’s surface, the older
that part of the planet’s surface is. During the early
formation of our solar system, many meteoroids bombarded
the planets. The craters they caused can still be seen on the
Moon and Mercury.
This is because geological processes, such as wind
and water erosion,
stopped millions of years
ago on these bodies.
The craters remain much
as they were at the time
of their creation.
Left – craters on Mercury. Above– craters on the far side of the Moon. Gaseous planets have little or no evidence of impact
craters, even though meteoroids strike gaseous planets as
often as they strike rocky planets. Craters leave only a
temporary record in the gaseous atmosphere.
Craters on Earth
Earth also was heavily cratered during its formation,
and it still receives impacts today. Many craters on Earth
have been eroded by wind and water and destroyed by
earthquakes and volcanism. About 120 impact craters have
been identified on Earth. Some of Earth’s craters are
relatively young; for example, Barringer (Meteor) Crater in
Arizona (below) is only around 50,000 years old.
Manicouagan Crater in Quebec, Canada, is much older; it
was created about 214 million years ago. At 70 km in
such as dust, sand, and – if the temperature is high enough diameter, Manicouagan Crater is one of the largest impact
– liquid rock, fall back around the crater to form an area of
craters
debris that looks like spokes on a wheel. These spokes,
on the
called rays, radiate outward in all directions. Some rays can
surface
go for hundreds of kilometers beyond the point of impact.
of Earth.
Many smaller, secondary craters also form around the main
crater.
Craters on moons and planets can be seen most easily
when long shadows are cast on their surfaces. Craters
along the border between the Moon’s dark and light side are
most visible from Earth when the Moon is in is quarter phase.