|Why does the moon have so many craters?|
|Question Date: 2012-10-24|
This is an excellent question! The answer to
this question includes important clues about how
the Earth, moon, and other rocky planets were
formed and about how Earth and the moon are
geologically different. Impact craters form when
meteorites crash into the surface of a planet.
You have already made the good observation that
the moon has a lot of these craters.
Earth and other planets in our solar
system formed about 4.6 billion years ago
(that’s a long time). Early in the history of
our solar system, asteroid and meteorite impacts
were much more common. Our planet and other
planets were struck by meteorites thousands of
time more frequently than today. That is because
back then there was a lot more “stuff” (meteors
and planetoids) flying around in our solar
system. Over time, those chunks of rock
were “caught” by the gravity of big planets.
Every time a planet “catches” a meteorite, it
gets a crater. This is the same for the moon. In
fact, our moon formed when an object the size of
a small planet crashed into Earth and tore off a
chunk. That chunk was caught by the Earth’s
gravity and now orbits it as the moon.
So, if Earth was catching meteorites
just as often as the moon, why are there many
more craters on the moon? The answer is… PLATE
TECTONICS!!! Plate tectonics is movement and
interaction of “plates” of crust on the surface
of our Earth. Plate tectonics is responsible for
volcanoes, earthquakes, giant mountain ranges,
and the recycling of crust back into the mantle.
These processes shape and reshape the surface of
Earth very rapidly (geologically speaking, over
millions of years). Things like subduction (when
crust sinks into the mantle) and erosion
(wearing away of rocks at the surface) have
erased many of the impact craters on earth. Only
very young craters can still be seen. For
example, meteor crater in Arizona is about
50,000 years old (that’s really young compared
to our planet which is 100,000 times older than
The moon is tectonically “dead”.
Plate tectonics no longer happen on the moon, so
there is nothing to erase all of the craters. I
ended up talking more about the Earth than the
moon, but that is because plate tectonics is the
big reason that so many impact craters are
preserved on the moon.
Good question! If the moon and earth are hit
with meteors at the same
rate, a similar question is: why does the earth
have so few craters?
A couple of reasons:
1) the earth has a much denser atmosphere than
the moon. Incoming
meteors cause the air in our atmosphere to
compress, rapidly heating
it to the point of combustion. This causes the
light we see from
shooting stars, and cause most small meteors to
before they touch the surface. The moon, which
has very little
atmosphere, doesn't have the same protection.
2) the surface of the earth is constantly
changing. From water
erosion, plate tectonics, and weather, the
surface of the earth is
always relatively young on astronomical time
scales. In contrast, the
moon - with no water or molten core -
doesn't 'refresh' its surface at
the same rate. Therefore, impact craters from
millions of years ago
are still very visible.
Two reasons: lacking an atmosphere, the moon
is unprotected from external projectiles.
Second, since there’s little to no erosion on
the moon, nothing erases them once they form.
Earth would be much more pock-marked by craters,
erosion and weathering wasn’t rapidly filling
those craters in.
The moon's craters are most likely the
result of many asteroids and other space debris
(big rocks, etc.) colliding with the moon over
the past few billion years.
The many craters on the moon are formed
mainly because of the collision of asteroids,
meteorites with the moon´s surface. Asteroids
strike the surface of the moon average speed of
12 miles per second. Earth´s surface also has
craters but not nearly as many as the moon
because the earth is surrounded by an atmosphere
which causes asteroids to burn up before they
can reach the surface. The moon does not have an
atmosphere so small rocks can collide with its
surface. As a result, the moon has many more
craters than the earth. The moon´s craters also
stay there permanently (or until they are
covered by a new crater caused by the collision
of another asteroid) as without an atmosphere,
there is no wind, rain, erosion, etc. on the
moon. The earth´s craters are often deteriorated
or washed away by all of these forces.
The moon gets pelted by objects in the solar
system, and unlike the Earth, the moon has no
weather and no plate tectonics that erodes
craters away. The Earth would have just as many
craters but for the fact that the forces of
weather and plate tectonics on Earth fills
craters in and destroys them.
In the case of the Earth, literally BILLIONS
of rocks of all sizes from ones you can hold in
your hand to ones the size of Mars! crashed into
the PROTO Earth and allowed the Earth to achieve
its present day mass!!!!!
The craters we see on the MOON are the last
bits of evidence of the FINAL major collisions
that occurred on the MOON. The same thing
happened on EARTH but because the Earth has an
atmosphere rain and wind, and because the Earth
has active volcanoes and plate tectonics, most
of the evidence of these impacts is gone. The
Earth does have 150 craters but they are all
relatively young... the older craters have been
On the moon, that is not the case. We see
craters on the Moon that formed almost four and
a half billion years ago!!!!
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