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LUNAR
METEORITES
We know we have meteorites from the Moon because they are
very similar to rocks returned by the Apollo and Luna missions
to the Moon.
We do not know exactly where lunar meteorites come from, but
some probably come from the far side of the Moon, which astronauts
have not yet visited. Meteorites give us a more complete picture
of the Moon's geology than the Apollo rocks alone.
From 1969–1972, the Apollo astronauts collected samples
from near the equator on the near side of the Moon. In the 1990s
and 2000s, the Clementine and Lunar Prospector missions orbited
and mapped the whole globe of the moon. These later missions
showed that the areas where the astronauts picked up samples
were unique places on the Moon. We rely on the lunar meteorites
to learn about large parts of the lunar surface that the Apollo
missions did not sample.
| [pictured
above] Lunar meteorite NWA 482 was found in Northwest
Africa. It is a rock from the lunar highlands, the white
part of the Moon. The rock is anorthosite, and is mostly
made up of feldspar. It is brecciated, which means it is
made of a lot of small fragments. |
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BLACK
AND WHITE
When you look up at the Moon, you can see dark and light areas.
You are looking at two different kinds of rocks.
The light areas of the Moon are the "highlands." These
light colored rocks form mountain ranges and they are heavily
cratered. The rock is mostly made of the mineral plagioclase (a
feldspar) and the rocks are called anorthosites. These rocks are
the original crust of the Moon.
The dark areas of the Moon are called "mare" (Latin
for oceans). They were made when lava flowed into low areas on
the lunar crust. The dark rocks of lunar lava flows are basalt.
We have samples of both highlands and mare rocks from the Apollo
missions, and we also have both types of rocks in the form of
meteorites. Many lunar rocks are breccias, or collections of small
broken rock fragments that have become stuck together over time.
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 MAGMA
OCEAN
For the first few million years of its lifetime, the Moon
was so hot that rock melted. The surface of the Moon was covered
in a deep ocean of molten rock, called magma, so we say that the
Moon had a "magma ocean" at this time.
As the liquid magma ocean cooled down, it started to crystallize.
Heavy minerals like olivine and pyroxene sank to the bottom of
the magma ocean, while the light mineral plagioclase floated to
the surface and formed a crust. Later, lava flows erupted
from the partly-cooled interior. The Moon's structure, with a
crust and a mantle, is similar to the Earth. But we still don't
know whether the moon has a core made of metal, like the Earth
does.
The magma ocean started to crystallize about 4.56 billion years
ago. We know this by measuring the age of the oldest lunar anorthosites,
part of the original crust. But the Moon didn't have enough heat
to sustain geologic activity for long — lunar lava flows
ended around 3 billion years ago.
| [pictured
top] Astronauts on the Apollo 17 mission noticed
a patch of orange on the grey lunar surface and collected
a sample for study in the laboratory. Photo courtesy of
the Lunar and Planetary Institute.
[pictured
above] The orange colored material is made up of
tiny beads of orange glass. The glass was produced during
an eruption that was like a fire fountain, 4 billion years
ago. The lava that erupted was produced deep down in the
Moon's mantle. In this light microscope image, each bead
is about 1 mm across. |
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|  COLOSSAL
COLLISION
Imagine the power of a planet the size of Mars smashing into
the Earth. We think that the Moon formed after a giant collision
of this sort that happened soon after the Earth formed.
In the early stages of Solar System history, as the planets were
forming, there were many minor planets in unstable orbits. One
of these stray bodies, already the size of Mars, smashed into
the Earth in a giant impact. Most of the material from the impactor
got trapped on the Earth. A small amount of material was
completely vaporized and was flung into orbit around the Earth.
As it cooled down, this debris coalesced into the rocky Moon that
now orbits the Earth. The material that the Moon was made from
had been so hot that no water remained, and so the Moon's rocks
are completely dry.
[pictured
above] Artist's impressio of a giant impact.
Courtesy of Bill Hartman. |
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|  ANCIENT
SURFACE
The Moon has been battered by impacts over and over again.
You can see craters all over the Moon's ancient highlands surfaces.
The mare are younger than the highlands so they don't have as
many craters on them. Since the Moon doesn't have plate tectonics
or erosion like the Earth does, even craters that were made billions
of years ago are still visible. The deepest and oldest craters
in the inner solar system are located on the Moon. Some very large
craters, or basins, are thousands of kilometers across and many
kilometers deep.
Most basins on the Moon are filled with mare lava flows, but
the deepest one is not. The South Pole-Aitken Basin punched a
hole 7 km into the Moon's crust on the far side of the Moon, giving
us a window into the Moon's deep crust. Along the rim of this
basin, at the South Pole of the Moon, are two interesting features.
There are some small craters that never receive sunlight, so water
left behind by comet impacts may stay there, frozen, for millions
of years. There are other places where there is always sunlight,
making them good spots for a lunar base.
| [pictured
above] The Moon, imaged by the Clementine mission.
The colors indicate altitude. The circle in the center that
looks like a target is the Orientale Basin, which is more
than 1000 km across. The purple region on the lower left is
the South Pole-Aitken basin. Photo courtesy of the Lunar and
Planetary Institute. |
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|  METEORITE
LAUNCH!
Lunar meteorites are proof that meteorites can come from other
planets.
For a long time, scientists believed that all meteorites came
from the asteroid belt. Then, in 1982, an interesting meteorite
was discovered in Antarctica. When scientists examined meteorite
ALH 81005, they realized it was different from any other type
of meteorite. It had chemical compositions, isotope ratios, minerals,
and textures similar to lunar samples collected by the Apollo
astronauts. ALH 81005 was the first meteorite scientists recognized
as being from the Moon.
Before this discovery, most scientists thought that meteorites
couldn't come from large planets like Mars or the Moon, because
they have strong gravity. It was thought that any impact large
enough to launch rock from those bodies would pulverize (smash)
the rocks so we would never find a rock big enough to land as
a meteorite on Earth. After the discovery of lunar meteorites,
scientists had to think again! New models for impacts were made.
We now understand how meteorites can be launched from large planets.
| [pictured
above] Lunar meteorite ALH81005 was found in Antarctica
in 1981. It was the first piece of the Moon to be recognized
on the Earth. The meteorite is a piece of rock from the lunar
highlands — the white part of the Moon. |
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