Sponge

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Sponge What is a synchronous orbit?
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Surface Features - Maria - oceans or seas.
(Galileo thought they were oceans when he saw
them through his telescope.) Maria are large,
dark, flat areas that are probably the result of
lava flows in the Moons past.
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There are 14 maria, all roughly circular. Most
of the maria are on the side of the moon that
faces the earth. The far side has almost no
maria.
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Terrae - highlands. (Galileo thought these were
the land masses of the moon.) Terrae are
elevated several kilometers above the maria.
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Craters bowl-shaped depressions, mostly due to
meteoric impact.
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The largest craters are 100s of km in diameter,
the smallest are microscopic. Craters are found
all over the Moons surface, but are more
prevalent in the highlands.
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The far side of the Moon is almost entirely
highlands, no maria. This fact affects theories
of the Moons formation.
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The Moon does undergo erosion despite the lack
of an atmosphere. This erosion is due to
collisions with interplanetary debris.
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These particles, large or small, all collide with
the surface since there is no atmosphere to
cause them to burn up.
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These particles strike at several km/sec. Even
small particles carry tremendous energy at these
speeds.
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A 1 kg meteorite hitting at 10 km/s releases as
much energy as 10 kg of TNT.
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This blasts out a crater with a diameter about
10 times that of the meteorite itself.
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The depth of the crater produced is about twice
that of the meteorites diameter.
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A 10 cm meteorite would produce a crater 1 m
across and 20 cm deep.
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The material thrown out surrounds the crater in
an ejecta blanket.
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Objects hit the Moon all the time, from
meteorites to protons from the solar wind.
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The larger the crater, the less often they are
formed. There just are not many large pieces
still floating randomly around the solar system.
Most of the large objects have already collided
with something.
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The larger craters are mostly all older, while
smaller craters are relatively young.
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The Orientale crater
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If the Orientale crater was facing Earth,
we would think a big eye was looking at us.
Creepy!
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The Moons average erosion rate has been
calculated to be about 10,000 times slower than
Earths.
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This is primarily due to the absence of running
water and atmospheric wind.
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The crater density is greater in the highlands.
This leads scientists to conclude that the
highlands are older than the maria. (We will use
this standard to judge the oldest parts of a
planet or moon thoughout the solar system.)
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Moon rocks brought back from the Apollo missions
have allowed the different areas of the moon to
be dated.
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Highlands are typically more than 4 billion years
old maria are from 3.2 to 3.9 billion years.
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This information on age combined with the crater
density indicates that the Moon experienced a
sudden drop in meteoric bombardment about 3.9
billion years ago and that this slower rate of
collisions has been constant since that time.
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The highlands solidified and received most of
their cratering before 3.9 billion years ago, the
maria solidified afterward.
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