THE LUNAR SURFACE
Because of its small mass, the moon's history has been vastly different from the earth's. With a small mass comes a weak gravitational attraction; as a result the moon retains almost no atmosphere. It has no surface water, either free or chemically combined in the rocks (as in earth rocks), although some water may be trapped under its surface. It also has no general magnetic field, but its rocks suggest that a strong one existed in the very distant past. However, the moon is far from a simple, featureless satellite.
Galileo's subdivision of the lunar surface into maria, the low-lying, almost circular dark regions in Figure 7.15, and terrae, the rough, crate red highlands, is still significant in terms of lunar history and terrainshaping processes. The maria are covered with layers of basaltic lava similar to the lavas that erupt from terrestrial volcanoes in Iceland, Hawaii, and elsewhere. The highlands are a lighter-colored rock that are older than the rocks of the maria.
Even with earth-based telescopes observers over the years have recognized a variety of other surface features, such as a range of sizes of impact craters, rugged mountain ranges, and deep winding canyons, or rilles.
Over eons of time small meteoroids have pulverized the lunar surface, leaving a dusty layer, some 1 to 20 meters deep, covering the lunar terrain. Known as the regolith,' it is the lunar "soil" on which the astronauts left their footprints. Since this soil contains no water or organic matter, it is totally different from solids formed on the earth by water, wind, and life. It could only have been formed over billions of years on the surface of an airless body.
More than just bits of ground-up lunar rocks, the regolith has also been exposed to cosmic-ray particles, subatomic matter flowing from the sun, and a fine dust from interplanetary space. Without an atmosphere to shield it the layers of the regolith contain both the record of lunar events and that of events in the larger solar system.
