1. 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.
2. 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
3. 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.
4. 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.
5. 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.
6. 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, testifying
to the importance of water-borne processes on Earth.
7. 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.
8. The lunar magma ocean was followed by a series of huge
asteroid impacts that created basins which were later filled by lava
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.
9. 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.
10. 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.
In 1993, more than 60 research laboratories throughout the
world continued studies on the Apollo lunar samples. Many new analytical
technologies, which did not exist in 1969-72 when the Apollo missions
were returning lunar samples, were being applied by the third generation
of scientists. The deepest secrets of the Moon remain to be revealed.
document was developed by the staff of the Curator for Planetary Materials
office at the Johnson Space Center. The Curator for Planetary Materials
is responsible for the curation of both lunar samples and meteorites
and distributes these items for scientific study.
September 5, 2012
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