Lunar soil is dry and airless, covered in a fine layer of electrically charged dust. It is mainly composed of oxygen and silicon dioxide, with other compounds such as iron oxide and calcium oxide. The composition varies depending on the lunar highlands and marias. Moon rocks were brought back for analysis, totaling about 900 pounds.
As you might expect, lunar soil (or regolith) is completely dry and airless. Before astronauts and probes visited the Moon, some scientists thought parts of the surface might be covered in a thin layer of dust up to a mile deep, but this has been found to be false. The lunar soil is only a couple of centimeters deep. It is covered with an extremely fine layer of dust which is electrically charged. This dust gets into everything and can be highly irritating to human skin.
The composition of the lunar soil is mainly oxygen, just like the earth’s crust. Also, like the earth’s crust, the most common substance is silicon dioxide, also known as silica, the main component of sand, glass and concrete. Broken down by element, about 43% of lunar soil is made up of oxygen, 21% silicon, 13% iron, 8% calcium, 6% aluminum, 5% magnesium, and 4% other elements. Other compounds found in lunar soil include titanium dioxide, aluminum oxide (alumina, a gray substance that gives the Moon much of its color), iron oxide (rust), magnesium oxide, and calcium oxide (lime).
The composition of the lunar soil varies greatly depending on whether we consider the lunar highlands (which appear lighter) or the marias (the lunar “seas”), which are dark. The lunar highlands are primarily anorthositic rock, which is characterized by a mixture of 90% plagioclase feldspar (a class of silicate minerals) with a 10% metallic component such as iron or magnesium. The feldspar component consists of calcium, aluminum, silicon and oxygen. These rocks are reflective and create moonlight when they reflect sunlight back to the night Earth.
The lunar maria, the darkest part of the Moon, is made up of basaltic minerals from ancient volcanic eruptions that occurred between 3.5 and 3 billion years ago. These minerals flowed into nearby basins (the far side of the Moon has barely any sea) and cooled. Compared to terrestrial basalts they have a higher viscosity and a higher iron content. In general, basalts are relatively rich in magnesium and calcium oxides and have lower levels of silicon dioxide.
In total, about 900 pounds (400 kg) of Moon rock was brought back from the Moon for analysis by scientists, about a pound from Soviet lunar missions, about 100 pounds from asteroids lifted from the Moon’s surface that fell in Antarctica or Earth’s deserts where they could be recovered, and the rest from the Apollo missions in the 1960s and 1970s.
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