Lunar Exploration

Today's Moon:
a view from Apollo 17

First spacecraft impact on the Moon: Luna I (USSR), 1959.

Discovery that the lunar farside consists almost entirely of highland regions, with no maria (large dark basins): Luna 3, 1959.

Investigation of the details of the lunar surface by the U.S. Ranger 7, 8, and 9 spacecraft in 1964-1965 revealed a gently rolling terrain with no sharp relief; there is a layer of powdery rubble, with rocks and craters down to at least one meter in diameter everywhere.

Luna 9 and Surveyor I landed on the moon in 1966, found that the surface is firm and capable of supporting machines and astronauts.

Surveyor 5, in 1967, found that the surface chemical composition of the maria resembles that of terrestrial basalt lava.

Surveyor 7, in 1968, found that the highlands composition differs from that of the maria and is aluminum rich.

photo of an astronaut and his lunar rover next to a large lunar rock

Rock hunting on the Moon.

Data from five Lunar Orbiters showed in 1968 that mascons or concentrations of excess mass exist under circular maria. This showed that the lunar crust must be sufficiently cool and strong to support the extra mass.

The manned Apollo missions to the Moon from 1969 to 1972 succeeded in collecting and returning rock and soil samples, emplacing instruments, including sensors for long-term measurements, and performing remote sensing from lunar orbit. From 1969 to the present time, the samples and data have been the subject of numerous scientific studies on Earth.

Nature and history of the Moon

The Moon was found to be a complex, evolved planet, with three basic rock types: (1) volcanic lavas in the maria; (2) aluminum-rich rocks in the high lands; (3) unusual rocks (called KREEP basalts) that are enriched in silica and radioactive elements.

Evidence was found relating to the early history of planetary develop ment (4.6 billion to 3.0 billion years ago): extensive primordial melting, catastrophic meteorite impacts, and major volcanic eruptions. The Moon was formed 4.6 billion years ago, along with the Earth and the rest of the solar system.

It appears that the lunar surface has been basically quiet and unchanging over the past 3 billion years.

No life, past or present, was found on the Moon.

The lunar surface material, or "soil," is a layer of powdery rubble, 10 to 100 meters (about 33 to 330 feet) deep, formed by meteorite impacts over billions of years.

photo of Apollo 16 in high orbit above the moon

Apollo 16 command module
in orbit above the Moon.

The lunar surface composition is fairly uniform over large areas, judged from the 20 percent of the Moon that has been analyzed from orbit. There is a basic division between iron- and magnesium-rich lavas in the maria and calcium- and aluminum-rich rocks in the highlands.

The Rocks

All of the lunar rocks are igneous (formed by cooling from molten lava) or derived from igneous rocks. There are no sedimentary (derived from water-deposited sediments) rocks.

The rocks are very fresh and chemically unaltered, due to the lack of water.

The rocks are generally like those of Earth in chemistry and minerals, but are deficient in volatile elements such as hydrogen, sodium, and potassium.

Three new minerals, never found on Earth, were discovered in the Moon rocks: tranquillityite, armalcolite, and pyroxferroite.

The Moon rocks range in age from 3.0 billion to 4.6 billion years; the older ones thus are older than any remaining rocks on Earth.

The lunar Interior

A dead, cratered Moon,
seen by Apollo 8.

photo of the dimpled and cratered surface of the moon

The huge lunar crater Langrenus,
photographed in stereo by Apollo 8.

The Moon was found to be slightly egg-shaped, with the small end pointing toward Earth.

The interior of the Moon consists of a crust, a mantle, and perhaps a core. The possible core may be metallic.

The thick, rigid, outer portion of the Moon, the lithosphere, was found to lack the plate tectonic motions that occur on Earth.

It was found that the Moon is not seismically active. There are weak, infrequent quakes, some triggered by tidal forces.

Fossil magnetism was found in lunar rocks, although the Moon has no magnetic field. The source of the fossil magnetism is still unexplained.

A large magnetic anomaly was found on the lunar farside, near the crater Van de Graff.

There is some outgassing underway from the lunar interior, as shown by the detection of radon from lunar orbit.

Tiny crystals line a
crack in a lunar rock.

Solar History and Space Environment Discoveries

Magnified photo of depression caused by impact of cosmic dust

Tiny microcrater, made when a particle of cosmic dust struck a small bead of lunar glass on the Moon's surface.

Bombardment by cosmic dust (which produces microcraters on the lunar rocks) seems to have occurred at a constant rate over the last few million years.

Rocks have remained exposed on the lunar surface for periods as long as 500 million years without being destroyed.

Specific impact craters on the Moon have been dated: Copernicus was formed 900 million years ago; Tycho, 100 million years ago; smaller craters at the Apollo landing sites, 2 million to 50 million years ago.

The solar wind striking the Moon was found to have a higher hydrogen/helium ratio than the Sun itself.

Lunar soil analyses show that major variations in the amount and isotopic composition of solar wind nitrogen have occurred during the past 2.5 billion years.

There do not appear to have been major changes in the intensity of solar flares and the composition of particles erupted from them over the past 100,000 years.

astronaut performing solar experiment on the lunar surface

Capturing the Sun:
an aluminum panel (the "windowshade") traps
atomic particles from the Sun during the Apollo 11 mission.

Study of the Surveyor 3 television camera lens, which was retrieved from the Moon by the Apollo 12 crew, revealed that there is a higher iron/hydrogen ratio in solar flare particles than in the Sun as a whole.

The flux of galactic cosmic rays has apparently been constant on the Moon over the past 1 billion years.