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A BRIEF HISTORY OF THE MOON We know the general outlines of what happened to the Moon after it was formed by a giant impact. The first notable event, which may have been a consequence of the giant impact, was the formation and crystallization of the magma ocean. Nobody knows how deep it was, but the best guess is that it was at least 500 km deep. The first minerals to form in this mind-boggling magmatic system were the iron and magnesium silicates olivine and pyroxene. They were denser than the magma, so they sank, like rocks in a pond, though not as fast. Eventually, plagioclase feldspar formed, and because it was less dense than the magma, began to float to the top, like bubbles in cola. It accumulated and produced mountains of anorthosite, producing the first lunar crust. The magma ocean phase ended by about 4.4 billion years ago. [See the 'Differentiation” activity on Pages 57-60.] Almost as soon as the crust had formed, perhaps while it was still forming, other types of magmas that would form the norites and troctolites in the highlands crust began to form deep in the Moon. A great mystery is where inside the Moon and how deep. Many lunar specialists believe the magmas derived from unmelted Moon stuff beneath the magma ocean. In any case, these magmas rose and infiltrated the anorthosite crust, forming large and small rock bodies, and perhaps even erupting onto the surface. Some of the magmas reacted chemically with the dregs of the magma ocean (KREEP) and others may have dissolved some of the anorthosite. This period of lunar history ended about 4.0 billion years ago. All during these first epochs, left-over projectiles continued to bombard the Moon, modifying the rocks soon after they formed. The crust was mixed to a depth of at least a few kilometers, perhaps as much as 20 km, as if a gigantic tractor had plowed the lunar crust. Though not yet proven, the rate of impact may have declined between 4.5 and 4.0 billion years ago, but then grew dramatically, producing most of the large basins visible on the Moon. This cataclysmic bombardment is postulated to have lasted from 4.0 to 3.85 billion years ago. [See the 'Impact Craters' activity on Pages 61-70, and the 'Regolith Formation' activity on Pages 47-52.] Once the bombardment rate had settled down, the maria could form. Basalts like those making up the dark mare surfaces formed before 3.85 billion years ago, but not as voluminously as later, and the enormous bombardment rate demolished whatever lava plains formed. However, between 3.7 and about 2.5 billion years ago (the lower limit is highly uncertain), lavas flowed across the lunar surface, forming the maria and decorating the Moon’s face. Along with the basalts came pyroclastic eruptions, high fountains of fre that launched glowing droplets of molten basalt on flights up to a few hundred kilometers. Since mare volcanism ceased, impact has been the only geological force at work on the Moon. Some impressive craters have been made, such as Copernicus (90 km across) and Tycho (85 km). These flung bright rays of material across the dark lunar landscape, adding more decoration. In fact, some of the material blasted from Tycho caused a debris slide at what would become the Apollo 17 landing site. Samples from this site indicate that the landslide and some associated craters formed about 110 million years ago. This, therefore, is the age of the crater Tycho. It is a triumph of geological savvy that we were able to date an impact crater that lies over 2000 km from the place we landed! The impacts during the past billions of years also have mixed the upper several meters of crust to make the powdery lunar regolith. The Sun has continued to implant a tiny amount of itself into the regolith, giving us its cryptic record and providing resources for future explorers. And recently, only seconds ago in geologic time, a few interplanetary travelers left their footprints here and there on the dusty ground. THE MOON AND EARTH: INEXORABLY INTERTWINED The Moon ought to be especially alluring to people curious about Earth. The two bodies formed near each other, formed mantles and crusts early, shared the same post-formational bombardment, and have been bathed in the same flux of sunlight and solar particles for the past 4.5 billion years. Here are a few examples of the surprising ways in which lunar science can contribute to understanding how Earth works and to unraveling its geological history. Origin of the Earth-Moon System. No matter how the Moon formed, its creation must have had dramatic effects on Earth. Although most scientists have concluded that the Moon formed as a result of an enormous impact onto the growing Earth, we do not know much about the details of that stupendous event. We do not know if the Moon was made mostly from Earth materials or mostly projectile, the kinds of chemical reactions that would have taken place in the melt-vapor cloud, and precisely how the Moon was assembled from this cloud. Magma oceans. The concept that the Moon had a magma ocean has been a central tenet of lunar science since it sprung from fertile minds after the return of the first lunar samples in 1969. It is now being applied to Earth, Mars, and asteroids. This view of the early stages of planet development is vastly different from the view in the 1950s and 1960s. Back then, most (not all) scientists believed the planets assembled cold, and then heated up. The realization that the Moon had a magma ocean changed all that and has led to a whole new way of looking at Earth’s earliest history. Early bombardment history of Earth and Moon. The thousands of craters on the Moon’s surface chronicle the impact record of Earth. Most of the craters formed before 3.9 billion years ago. Some scientists argue that the Moon suffered a cataclysmic bombardment (a drastic increase in the number of impacting projectiles) between 3.85 and 4.0 billion years ago. If this happened and Earth was subjected to the blitzkrieg as well, then development of Earth’s earliest crust would have been affected. The intense bombardment could also have influenced the development of life, perhaps delaying its appearance. Impacts, extinctions, and the evolution of life on Earth. The mechanisms of evolution and mass extinctions are not understood. One possibility is that some mass-extinction events were caused by periodic increases in the rate of impact on Earth. For example, the mass extinctions, which included the demise of the dinosaurs, at the end of the Cretaceous period (65 million years ago), may have been caused by a large impact event. Attempts to test the idea by dating impact craters on Earth are doomed because there are too few of them. But the Moon has plenty of craters formed during the past 600 million years (the period for which we have a rich fossil record). These could be dated and the reality of spikes in the impact record could be tested. How geologic processes operate. The Moon is a natural laboratory for the study of some of the geologic processes that have shaped Earth. It is a great place to study the details of how impact craters form because there are so many well-preserved craters in an enormous range of sizes. It is also one of the places where volcanism has operated, but at lower gravity than on either Earth or Mars.