Surface Properties
of the Moon

The surface of the Moon has two hemispheres with rather asymmetric properties; as a consequence the nature of the Lunar surface that we can see from the Earth is substantially different from the surface that is always hidden from the Earth.

The Near Side

The face of the Moon turned toward us is termed the near side (image at right). It is divided into light areas called the Lunar Highlands and darker areas called Maria (literally, "seas"; the singular is Mare). The Maria are lower in altitude than the Highlands, but there is no water on the Moon so they are not literally seas ( Recent evidence from the Clementine spacecraft suggests that there may be some water on the Moon, contrary to previous assumptions). See also here . The dark material filling the Maria is actually dark, solidified lava from earlier periods of Lunar volcanism. Both the Maria and the Highlands exhibit large craters that are the result of meteor impacts. There are many more such impact craters in the Highlands.

The Far Side

The side of the Moon unseen from the Earth is called the far side. One of the discoveries of the first Lunar orbiters is that the far side has a very different appearance than the near side. In particular, there are almost no Maria on the far side, as illustrated in the image shown to the left of a portion of the far side surface. In this figure a number of meteor impact craters are visible.

Cratering Density

The amount of cratering is usually an indication of the age of a geological surface: the more craters, the older the surface, because if the surface is young there hasn't been time for many craters to form. Thus, the Earth has a relatively young surface because it has few craters. This is because the Earth is geologically active, with plate tectonics and erosion having obliterated most craters from an earlier epoch. In contrast the surface of the Moon is much older, with much more cratering. Further, different parts of the surface of the Moon exhibit different amounts of cratering and therefore are of different ages: the maria are younger than the highlands, because they have fewer craters.

The oldest surfaces in the Solar System are characterized by maximal cratering density. This means that one cannot increase the density of craters because there are so many craters that, on average, any new crater that is formed by a meteor impact will obliterate a previous crater, leaving the total number unchanged. Some regions of the moon exhibit near maximal cratering density, indicating that they are very old.

The Lunar Surface Material

The bulk density of the Moon is 3.4 g/cc, which is comparable to that of (volcanic) basaltic lavas on the Earth (however, the bulk density of the Earth is 5.5 g/cc, because of the dense iron/nickel core). The Moon is coverered with a gently rolling layer of powdery soil with scattered rocks that is called the regolith; it is made from debris blasted out of the Lunar craters by the meteor impacts that created them. Each well-preserved Lunar crater is surrounded by a sheet of ejected material called the ejecta blanket.

Geological Composition

One striking difference between the Lunar surface material and that of Earth concerns the most common kinds of rocks. On the Earth, the most common rocks are sedimentary, because of atmospheric and water erosion of the surface. On the Moon there is no atmosphere to speak of and little or no water, and the most common kind of rock is igneous ("fire-formed rocks"). Geologically, the Lunar surface material has the following characteristics:
  1. The Maria are mostly composed of dark basalts, which form from rapid cooling of molten rock from massive lava flows.

  2. The Highlands rocks are largely Anorthosite, which is a kind of igneous rock that forms when lava cools more slowly than in the case of basalts. This implies that the rocks of the Maria and Highlands cooled at different rates from the molten state and so were formed under different conditions.

  3. Breccias, which are fragments of different rocks compacted and welded together by meteor impacts, are found in the Maria and the Highlands, but are more common in the latter.

  4. Lunar Soils contain glassy globules not commonly found on the Earth. These are probably formed from the heat and pressure generated by meteor impacts.
The Anorthosites that are common in the Lunar Highlands are not common on the surface of the Earth (The Adirondack Mountains and the Canadian Shield are exceptions). They form the ancient cores of continents on the Earth, but these have largely been obliterated by overlying sedimentary deposits and by plate tectonic activity.

Chemical Composition

The Lunar rocks may also be examined according to the chemicals that they contain. Such analysis indicates:
  1. They are rich in refractory elements, which are elements such as calcium (Ca), Aluminum (Al), and Titanium (Ti) that form compounds having high melting points.

  2. They are poor in the light elements such as hydrogen (H).

  3. There is high abundance of elements like Silicon (Si) and Oxygen (O).
The high concentration of rare metals like Titanium, and the availability of abundant amounts of Silicon and Oxygen has led to serious proposals about mining and manufacturing operations in the future for the Moon.

Age of Lunar Material

The abundances of radioactive elements in rock samples can be used to tell the age of the rock in a process called Radioactive Dating. When such techniques are applied to the Lunar rock samples, one finds the following:
  1. Samples from Mare Imbrium and the Ocean of Storms brought back by Apollo 11 and Apollo 12 are about 3.5 billion years old, which is comparable to the oldest rocks found on the surface of the Earth.

  2. The ejecta blanket from the Imbrium Basin (which was formed by a gigantic meteor impact) was returned by Apollo 14 and found to be about 3.9 billion years old.

  3. Lunar Highlands rocks returned by Apollo 16 are about 4 billion years old. The oldest Lunar rock found was located by Apollo 17 and appears to be about 4.5 billion years old.
Thus, the oldest material from the surface of the Moon is almost as old as we believe the Solar System to be. This is more than a billion years older than the oldest Earth rocks that have been found. Thus, the material brought back from the Moon by the Apollo missions gives us a window on the very early history of our Solar System that would be difficult the find on the Earth, which is geologically active and has consequently has obliterated its early geological history.