CHEMICAL COMPOSITION OF THE SURFACE
The moon appears to have formed from the same chemical elements, although in somewhat different
proportions, as those that formed the earth. It has less iron, more of the substances that are hard to melt (refactory materials) - such as calcium, aluminum, and titanium-and less of the easily vaporized atoms and molecules (volatile compounds)-such as sodium and potassium-than the earth does.
The most common surface rocks are anorthosites (silicates mainly of aluminum and calcium), iron-rich basalts from the maria, and thorium-rich and uraniumrich rocks. No traces of water and no organic compounds, the indicators of living processes, were discovered in any of the lunar samples. I n fact the Apollo lunar rocks contain only tiny amounts of carbon and the carbon-based compounds from which life originates. With no water or oxygen present the minerals in the lunar rocks could not react with water to form clays or rust, nor did the iron react with oxygen to forrn oxides. The lunar highlands, which cover about four-fifths of the lunar surface, are the oldest preserved terrain. They consist mostly of material rich in aluminum and silicon and lighter than the mare basalts.
THE FIRST 2 BILLION YEARS OF THE MOON
Radioactive dating of the lunar rocks points to an origin much like that of the earth. Most of the lunar material probably came from accreting planetesimals (small solid bodies) that were part of the contracting material forming the solar system. Nearly all the original crust was lost when the moon underwent a global melting to a depth of at least several hundred kilometers, followed by chemical differentiation or separation shortly after the moon's formation. A few of the rock samples retu rned to the earth are about 4.6 bi Ilion years old, the same age as the earth (even though earth rocks no longer exist that are older than 3.5 to 3.8 billion years).
The highland areas are apparently about 4.0 to 4.3 billion years old. After its formation, the global crust of the moon was continually modified by the impact of material from elsewhere in the solar system. The cratering record preserved in the early crustal units represents a distinct phase of intense cratering, which began to decline rapidly about 3.8 billion years ago. Although volcanic processes may have operated during this early period, the surface history of the moon is primarily that due to cratering. As mentioned earlier, this phase in the earth's history has been almost completely erased. Impact cratering continues today, but at a drastically reduced rate from what it must have been billions of years ago.
The next stage in lunar history was dominated by the formation of dark mare plains, which cover about 17 percent of the lunar surface, favoring the earthward side. These structures are relatively thin ponds of basaltic lava totaling less than 1 percent of the volume of the crust. Apollo rocks from maria suggest that the major outpouring of lava occurred between 3.9 and 3.2 billion years ago. Although some mare deposits may be as young as 2 billion years, there appears to have been no extensive igneous activity on the lunar surface for the last 3 billion years. Thus the shaping of the present lunar terrain is almost the opposite of that of the earth's-the moon dominated by cratering and the earth by volcanic and tectonic activity.
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