Geology of the Moon: Encyclopedia II - Geology of the Moon - Composition

Geology of the Moon - Composition

More than 4.5 billion years ago, the surface of the Moon was a liquid magma ocean. Scientists think that one component of lunar rocks, KREEP (K-potassium, Rare Earth Elements, and P-phosphorus), represents the last chemical remnant of that magma ocean. KREEP is actually a composite of what scientists term "incompatible elements": those which cannot fit into a crystal structure and thus were left behind, floating to the surface of the magma.

The lunar crust is composed of a variety of primary elements, including uranium, thorium, potassium, oxygen, silicon, magnesium, iron, titanium, calcium, aluminum and hydrogen. The overall composition of the Moon is believed to be similar to that of the Earth other than a depletion of volatile elements and of iron.

Geology of the Moon - Surface

The Apollo program brought back 381.7 kg (841.5 lb) of lunar surface material, most of which is stored at the Lunar Receiving Laboratory in Houston, Texas. These rocks have produced additional insights into the geological processes on the Moon. Material scattered by formation of the impact basins was collected among the soil samples, producing data for a other portions of the surface.

Due to the impact history of the Moon, particularly the largest basins, the upper crust to a depth of several kilometers is composed primarily of deposits of ejecta blankets from these impacts. This broken and intermixed rock is called breccia, and the surface layer of this rock is the megaregolith. The regolith is the name given to such unconsolidated debris overlaying the bedrock in the highland regions and covering parts of the mares.

Hypervelocity impacts of micrometeorites on the lunar surface release energy, which has the effect of fusing rock together to form larger, glassy bodies called agglutinates. (The accumulation of these bodies darkens the soil, causing high albedo features such as ray systems to fade.) The impacts also grind down materials into ever finer particles, until a fine powder is formed. Over time an equilibrium is reached between the formation of the larger glassy rocks and the powder, resulting in the regolith soil that covers much of the surface.

Among the minerals found on the surface are Armalcolite, Ilmenite, Olivine, Plagioclase Feldspar, Pyroxene, and Quartz. Armalcolite, a mineral composed of iron and titanium oxide, was named for Armstrong, Aldrin, and Collins, the three members of the Apollo 11 crew.

The mares are composed primarily of basalts that are relatively rich in iron. The highland regions are iron-poor, and are thought to be composed primarily of ferroan anorthosite, a plagioclase feldspar that is rich in aluminum and calcium. Another significant component of the crust are the igneous rocks called the Mg-suite, of which troctolite is made of equal portions of olivine and Plagioclase. The norite is an igneous rock found in the lunar crust, consisting of plagioclase feldspar and pyroxene.

Composite rocks on the lunar surface often appear in the form of breccias. Of these, the subcategories are called fragmental, granulitic, and impact-melt breccias, depending on how they were formed. The aluminous melt group of breccias are impact-melt rocks with a high proportion of aluminum. Conglomerates are lunar rocks made from other rocks that have previously become rounded from impact wear.

Finally there is the LKFM, or low-K Fra Mauro rocks. These were formed by impact-melt, but have a higher proportion of iron and magnesium than is normal for upper crust rocks.

Geology of the Moon - Lunar surface

The surface of the Moon is colored gray and presents a large amount of fine sediment as a result of innumerable impacts by meteorites. This "dust" receives the name of lunar regolith, a term coined to describe layers of sediments produced by mechanical effects on the surfaces of the planets. The thickness of the regolith varies between 2 meters, on the younger maria, up to 20 meters in the oldest surfaces of the lunar highlands. The lunar regolith is composed of the material of the rocks found in the region, but also contains traces of materials expelled by distant impacts, which makes the regolith a rock of high scientific value.

The regolith contains rocks, fragments of minerals from the original bedrock, and glassy particles formed during impacts. In most of the lunar regolith, half of the particles are made of mineral fragments fused by the glassy particles; these objects are called agglutinates. The chemical composition of the regolith varies according to its location; the regolith in the highlands is rich in aluminium, just as the rocks in those regions. The regolith in the maria is rich in iron and magnesium, as the basaltic rocks from which it is made of.

The lunar regolith is very important because it also stores information about the history of the Sun. The atoms that compose the solar wind—mostly helium, neon, carbon and nitrogen—hit the lunar surface and insert themselves into the mineral grains. Upon analyzing the composition of the regolith, particularily its isotopic composition, it is possible to determine if the activity of the Sun has changed with time.

The gases of the solar wind could be useful for future lunar bases, since the oxygen, hydrogen (water), carbon and nitrogen are not only essential to sustain life, but are also very useful in the production of fuel. There is a large quantity of oxygen stored in silicates, which compose almost 50% of lunar minerals by volume, and the solar wind provides the rest.

Adapted from the Wikipedia article "Composition", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki