The top ten discoveries of the Apollo Missions are as follows:13
1. The Moon is not a primordial14 object, but rather 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 lithosphere15 (to a depth of approximately 1,000 km), and a partly liquid asthenosphere16 (1,000-1,740 km). A small iron core at the
14 Primordial means primitive, fundamental, or original. Some of the asteroids may be primordial objects; appearing now as they were originally formed, and not subject to the processes of change (heating, volcanic activity, and mountain building) that have affected Earth, Mars, and the Moon.
15 Lithosphere: in plate tectonics, a layer of strength relative to the underlying asthenosphere. It includes the crust and part of the upper mantle. On Earth, the lithosphere is about 100 km thick. On the Moon, the lithosphere may be much thicker. Some research remains to be done to establish this.
16 Asthenosphere: the layer or shell below the lithosphere, which is relatively weak and in which magmas may be generated and seismic waves are strongly attenuated.
bottom of the asthenosphere is possible but unconfirmed. Some rocks give hints for ancient magnetic fields, although no planetary magnetic field exists today. Compasses won't help you on the Moon.
2. The Moon is ancient and still preserves a record of the first 1,000 million years of its history. That same history must be common to all terrestrial planets. Craters and their relationships to each other, when calibrated using absolute ages of rock samples, provide a key for unraveling time scales for the geologic evolution of Mercury, Venus, and Mars based on their individual crater records. Photo 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.
3. The youngest Moon rocks are about the same age 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 3,200 million years in the maria (dark, low basins) to nearly 4,600 million years in the terrae (light, rugged highlands). On Earth, active geologic forces, including plate tectonics17 and erosion, continuously repave the oldest surfaces, whereas on the Moon, old surfaces persist with little disturbance.
4. The Moon and Earth are related and formed from different proportions of a common reservoir of materials. The distinctively similar oxygen isotopic18 compositions of Moon rocks and Earth rocks clearly show common ancestry. Relative to Earth, however, the Moon is highly depleted in iron and in volatile19 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 the giant basins; they generally resemble, but are much older than, lavas that comprise
17 Plate tectonics: a theory that the Earth's crust is divided into a series of vast, plate-like parts that move or drift as distinct masses. Sometime referred to as the theory of "continental drift''.
18 Many elements have isotopes - individual atoms that have a different number of neutrons in the nucleus. Typically, an oxygen atom has eight protons and eight neutrons, which makes the most common isotope, oxygen-16. Sometimes there are nine neutrons, which makes oxygen-17, while ten neutrons makes oxygen-18. There are "natural abundances'' of these isotopes on the Earth, such that an "average mass'' can be calculated for the oxygen atom. It was learned from the Viking missions to Mars and from sampling meteorites that there are different oxygen isotope ratios on different planets. In fact, the isotope ratios of many elements vary from one planet to another. The planetary origin of a rock can thus be determined from the isotope ratios of oxygen, which makes up about 40% of all rocks everywhere.
19 Volatile: elements or compounds that evaporate quickly or are given off as vapor during combustion. These include light elements, as well as the typical gases and liquids such as water, ammonia, carbon dioxide, carbon monoxide, methane, etc.
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 because water is required for the formation of these rock types.
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 4,400 million to 4,600 million 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 a thick layer of boulders and rubble.
8. 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 3,200 million to 3,900 million years ago. Lunar volcanism occurred mostly as lava floods that spread horizontally. However, in some cases volcanic fire fountains produced deposits of orange and green glass beads.
9. The Moon is slightly non-symmetrical in shape, possibly as a consequence of the Earth's gravitational influence. The lunar 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 many large lunar basins which may 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 for 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 4,000 million years of the Sun's history to a degree of completeness that we are unlikely to find elsewhere.20
The Apollo Program marked the beginning of a global program of human exploration; the comprehensive in-situ investigation of the maria, mountains, rilles, lava tubes, and craters awaits the permanent return of humankind to the Moon.
20 Since the Moon has no atmosphere and no magnetic field, solar radiation can reach the surface of the Moon much more easily than it can reach the surface of Earth, Mars, or Venus.
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