MOON PROPERTIES

The Near Side It is divided into light areas called the Lunar Highlands and darker areas called Maria (literally, "seas"; the singular is Mare).

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

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.

Different Types of Impact Craters

Most of the craters on the Moon are circular. The few craters that are not circular, like Messier and Messier A (pictured at the left) in the Mare Fecunditatis, are an enigma. Scientists do not know exactly how these oddly-shaped craters were formed

Most craters on the Moon that have diameters less than about 15 kilometers have a simple, bowl-like form.

Moltke crater, a simple crater with a diameter of 4.3 miles (7 km).

Simple Crater

Complex Crater

Euler crater, a complex crater with a diameter of 17 miles (28 km) and a depth of 1.5 miles (2.5 km).

Simple Impact Craters:

Simple impact craters have bowl-shaped depressions, mostly with smooth walls. This type of crater generally has a diameter less than 9 miles (15 km). Their depth is about 20% of the diameter.

Complex Impact Craters:

Complex impact craters have a single or multiple peaks in the middle of the crater. These craters have diameters between about 12 and 110 miles (20 and 175 km), and the central uplift is usually one or a few peaks. Craters with a diameter over 110 miles (175 km) can have more complex, ring-shaped uplifts within the crater.

Impact Basin: A impact basin is an impact crater that has a rim diameter greater than 185 miles (300 km). There are over 40 impact basins on the Moon. These catastrophic impacts produce faulting and other crust deformations. Material ejected from impact basins is distributed over wide areas.

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 MaterialThe 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:

The Maria are mostly composed of dark basalts, which form from rapid cooling of molten rock from massive lava flows.

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.

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.

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.

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:

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.

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.

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.

The Structure of the Interior

Although there is a small amount of geological activity on the Moon, it is largely dead geologically (the energy associated with the Earth's seismic activity is about 10^14 times larger than that of the Moon). Most Lunar seismic activity appears to be triggered by tidal forces induced in the Moon by the Earth.

Geological History of the Moon

The weight of the evidence is that the Moon was active geologically in its early history, but the general evidence suggests that the Moon has been essentially dead geologically for more than 3 billion years. Based on that evidence, we believe the chronology of Lunar geology was as follows:

The Moon was formed about 4.6 billion years ago; maybe hot or maybe cold. The surface was subjected continuously to an intense meteor bombardment associated with debris left over from the formation of the Solar System.

By about 4.4 billion years ago the top 100 km was molten, from original heat of formation and from heat generated by the meteor bombardment.

By 4.2 billion years ago the surface was solid again

As the intense meteor bombardment associated with debris left over from the formation of the Solar System continued, most of the craters that we now see on the surface of the Moon were formed by meteor impact.