The Apollo program demonstrated clear differences in composition between the lunar highlands and the maria. The highlands are made largely of rocks rich in aluminum, making them lighter in color and lower in density (2900 kg/m3). The maria's basaltic matter contains more iron, giving it a darker color and greater density (3300 kg/m3). Loosely speaking, the highlands represent the Moon's crust, while the maria are made of mantle material. Many of the rock samples brought back by the Apollo astronauts show patterns of repeated shattering and melting—direct evidence of the violent shock waves and high temperatures produced in meteoritic impacts.
Geologists believe that the type of rock found in the maria arose on the Moon much as basalt did on Earth, through the upwelling of molten material through the lunar crust. The great basins that comprise the maria are thought to have been created during the final stages of the heavy meteoritic bombardment just described, between about 4.1 and 3.9 billion years ago. Subsequent volcanic activity filled the craters with lava, creating the formations we see today. In a sense, then, the maria are oceans—ancient seas of molten lava, now solidified.
Not all these great craters became flooded with lava, however. One of the youngest craters is the Orientale Basin (Figure 8.15a), which formed about 3.9 billion years ago. It did not undergo much subsequent volcanism, and so we can recognize its structure as an impact crater rather than as another mare. Similar "unflooded" basins are seen on the lunar far side.
Meteoroid collisions with the Moon are the main cause of the layer of pulverized ejecta—also called lunar dust, or regolith (meaning "fine rocky layer")—that covers the lunar landscape to an average depth of about 20 m. This microscopic dust has a typical particle size of about 0.01 mm. In consistency, it is rather like talcum powder or ready-mix dry mortar. Figure 8.18 shows an astronaut's bootprint in the regolith. Owing to the very low rate of lunar erosion, even these shallow bootprints will remain intact for millions of years. The regolith is thinnest on the maria (10 m) and thickest on the highlands (over 100 m deep in places).
Figure 8.18 Photograph of an Apollo astronaut's bootprint in the lunar dust. The astronaut's weight has compacted the regolith to a depth of a few centimeters.
In contrast to Earth's soil, the lunar regolith contains no organic matter like that produced by biological organisms. No life whatsoever exists on the Moon. Nor were any fossils found in Apollo samples. Lunar rocks are barren of life and apparently always have been. NASA was so confident of this that the astronauts were not even quarantined on their return from the last few Apollo landings. Furthermore, all the lunar samples returned by the U.S. and Soviet Moon programs were bone dry. Apart from the regolith near the poles (see Interlude 8-1), lunar rock doesn't even contain minerals having water molecules locked within their crystal structure. Terrestrial rocks, by contrast, are almost always 1 or 2 percent water.
Only a few decades ago, debate raged in scientific circles as to the origin of lunar craters, with most scientists holding the opinion that the craters were the result of volcanic activity. We now know that almost all lunar craters are in fact meteoritic in origin. However, a few of them apparently are not. For example, Figure 8.19 shows an intriguing alignment of several craters in a crater-chain pattern so straight that it is very unlikely to have been produced by the random collision of meteoroids with the surface. Instead, the crater chain probably marks the location of a subsurface fault—a place where cracking or shearing of the surface once allowed molten matter to well up from below. As the lava cooled, it formed a solid "dome" above each fissure. Subsequently, the underlying lava receded and the centers of the domes collapsed, forming the craters we see today. Similar features have been observed on Venus by the orbiting Magellan probe (see Chapter 9).
Figure 8.19 This "chain" of well-ordered craters was photographed by an Apollo 14 astronaut. The largest crater, called Davy, is located on the western edge of Mare Nubium. The entire field of view measures about 100 km across.
Many other examples of lunar volcanism are known, both in telescopic observations from Earth and in the close-up photographs taken during the Apollo missions. Figure 8.20 shows a volcanic rille, a ditch where molten lava once flowed. There is good evidence for surface volcanism early in the Moon's history, and the volcanism explains the presence of the lava that formed the maria. However, whatever volcanic activity once existed on the Moon ended long ago. The measured ages for rock samples returned from the Moon are all greater than 3 billion years. (Recall from Chapter 7 that the radioactivity clock doesn't start "ticking" until the rock solidifies.) More Precisely 7-2 Apparently, the maria solidified over 3 billion years ago and the Moon has been dormant ever since.
Figure 8.20 A volcanic rille, photographed from the Apollo 15 spacecraft orbiting the Moon, can be seen clearly here (bottom and center) winding its way through one of the maria. Called Hadley Rille, this system of valleys runs along the base of the Apennine Mountains (lower right) at the edge of the Mare Imbrium (to the left). Autolycus, the large crater closest to the center, spans 40 km. The shadow-sided, most prominent peak at the lower right, Mount Hadley, rises almost 5 km high. (See also the chapter—opening image)
