- The Moon is not a primordial object; it is an evolved terrestrial
planet with internal zoning similar to that of Earth.
Before Apollo, the state of the Moon was a subject
of almost unlimited speculation. We now know that the Moon is made
of rocky material that has been variously melted, erupted through
volcanoes, and crushed by meteorite impacts. The Moon possesses
a thick crust (60 km), a fairly uniform lithosphere (60-1000 km),
and a partly liquid asthenosphere (1000-1740 km); a small iron core
at the bottom of the asthenosphere is possible but unconfirmed.
Some rocks give hints for ancient magnetic fields although no planetary
field exists today.
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The Moon is ancient and still preserves an early
history (the first billion years) that must be common to all terrestrial
planets.
The extensive record of meteorite craters on the
Moon, when calibrated using absolute ages of rock samples, provides
a key for unravelling time scales for the geologic evolution of
Mercury, Venus, and Mars based on their individual crater records.
Photogeologic interpretation of other planets is based largely on
lessons learned from the Moon. Before Apollo, however, the origin
of lunar impact craters was not fully understood and the origin
of similar craters on Earth was highly debated.
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The youngest Moon rocks are virtually as old
as the oldest Earth rocks. The earliest processes and events that
probably affected both planetary bodies can now only be found on
the Moon.
Moon rock ages range from about 3.2 billion years
in the maria (dark, low basins) to nearly 4.6 billion years in the
terrae (light, rugged highlands). Active geologic forces, including
plate tectonics and erosion, continuously repave the oldest surfaces
on Earth whereas old surfaces persist with little disturbance on
the Moon.
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The Moon and Earth are genetically related and
formed from different proportions of a common reservoir of materials.
The distinctively similar oxygen isotopic compositions
of Moon rocks and Earth rocks clearly show common ancestry. Relative
to Earth, however, the Moon was highly depleted in iron and in volatile
elements that are needed to form atmospheric gases and water.
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The Moon is lifeless; it contains no living organisms,
fossils, or native organic compounds.
Extensive testing revealed no evidence for life,
past or present, among the lunar samples. Even non-biological organic
compounds are amazingly absent; traces can be attributed to contamination
by meteorites.
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All Moon rocks originated through high-temperature
processes with little or no involvement with water. They are roughly
divisible into three types: basalts, anorthosites, and breccias.
Basalts are dark lava rocks that fill mare basins;
they generally resemble, but are much older than, lavas that comprise
the oceanic crust of Earth. Anorthosites are light rocks that form
the ancient highlands; they generally resemble, but are much older
than, the most ancient rocks on Earth. Breccias are composite rocks
formed from all other rock types through crushing, mixing, and sintering
during meteorite impacts. The Moon has no sandstones, shales, or
limestones such as testify to the importance of water-borne processes
on Earth.
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Early in its history, the Moon was melted to
great depths to form a "magma ocean." The lunar highlands
contain the remnants of early, low density rocks that floated to
the surface of the magma ocean.
The lunar highlands were formed about 4.4-4.6 billion
years ago by flotation of an early, feldspar-rich crust on a magma
ocean that covered the Moon to a depth of many tens of kilometers
or more. Innumerable meteorite impacts through geologic time reduced
much of the ancient crust to arcuate mountain ranges between basins.
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The lunar magma ocean was followed by a series
of huge asteroid impacts that created basins which were later filled
by lava flows.
The large, dark basins such as Mare Imbrium are gigantic
impact craters, formed early in lunar history, that were later filled
by lava flows about 3.2-3.9 billion years ago. Lunar volcanism occurred
mostly as lava floods that spread horizontally; volcanic fire fountains
produced deposits of orange and emerald-green glass beads.
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The Moon is slightly asymmetrical in bulk form,
possibly as a consequence of its evolution under Earth's gravitational
influence. Its crust is thicker on the far side, while most volcanic
basins -- and unusual mass concentrations -- occur on the near side.
Mass is not distributed uniformly inside the Moon.
Large mass concentrations ("Mascons") lie beneath the
surface of many large lunar basins and probably represent thick
accumulations of dense lava. Relative to its geometric center, the
Moon's center of mass is displaced toward Earth by several kilometers.
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The surface of the Moon is covered by a rubble
pile of rock fragments and dust, called the lunar regolith, that
contains a unique radiation history of the Sun which is of importance
to understanding climate changes on Earth.
The regolith was produced by innumerable meteorite
impacts through geologic time. Surface rocks and mineral grains
are distinctively enriched in chemical elements and isotopes implanted
by solar radiation. As such, the Moon has recorded four billion
years of the Sun's history to a degree of completeness that we are
unlikely to find elsewhere.
