On Earth, the combined actions of wind and water erode our planet's surface and reshape its appearance almost on a daily basis. Coupled with the never-ending motion of Earth's surface plates, this means that most of the ancient history of our planet's surface is lost to us. The Moon, on the other hand, has no air, no water, no plate tectonics, and no ongoing volcanic or seismic activity. Consequently, features dating back almost to the formation of the Moon itself are still visible today. The lunar surface is not entirely changeless, however. There is ample evidence for erosion—for example, the soft edges of the craters visible in the foreground of Figure 8.13. In the absence of erosion, those features would still be as jagged and angular today as they were just after they formed. Something must have worn them down to their present condition.
Figure 8.13 Despite the complete lack of wind and water on the airless Moon, the surface has still eroded a little under the constant "rain" of impacting meteoroids, especially micrometeoroids. The twin tracks were made by the Apollo lunar rover.
The primary source of erosion on the Moon is interplanetary debris, in the form of small meteoroids that collide with the lunar surface. This material, much of it rocky or metallic in composition, is strewn throughout the solar system. 
(Sec. 6.5) It orbits the Sun in interplanetary space, perhaps for billions of years, until it happens to collide with some planet or moon. On Earth, most meteoroids burn up in the atmosphere, producing the streaks of light known as meteors, or "shooting stars." But the Moon, without an atmosphere, has no protection against this onslaught. Large and small meteoroids just zoom in and collide with the surface, sometimes producing huge craters. Over billions of years, these collisions have scarred, cratered, and sculpted the landscape. Craters are still being formed today—even as you read this—all across the lunar surface.
Meteoroids generally strike the Moon at speeds of several kilometers per second. At these speeds, even a small piece of matter carries an enormous amount of energy—for example, a 1-kg object hitting the Moon's surface at 10 km/s releases as much energy as the detonation of 10 kg of TNT. As illustrated in Figure 8.14, impact of a meteoroid with the surface causes sudden and tremendous pressures to build up, heating the normally brittle rock and deforming the ground like heated plastic. The ensuing explosion pushes previously flat layers of rock up and out, forming a crater.
Figure 8.14 Several stages in the formation of a crater by meteoritic impact. (a) A meteoroid strikes the surface, releasing a large amount of energy. (b, c) The resulting explosion ejects material from the impact site and sends shock waves through the underlying surface. (d) Eventually, a characteristic crater surrounded by a blanket of ejected material results.
The diameter of the eventual crater is typically 10 times that of the incoming meteoroid; the crater depth is about twice the meteoroid's diameter. Thus, our 1-kg meteoroid, measuring perhaps 10 cm across, would produce a crater about 1 m in diameter and 20 cm deep. Shock waves from the impact pulverize the lunar surface to a depth many times that of the crater itself. The material thrown out by the explosion surrounds the crater in a layer called an ejecta blanket. The ejected debris ranges in size from fine dust to large boulders. Figure 8.15(a) shows the result of one particularly large meteoritic impact on the Moon. As shown in Figure 8.15(b), the larger pieces of ejecta may themselves form secondary craters.
Figure 8.15 (a) A large lunar crater, called the Orientale Basin. The impact that produced this crater thrust up much surrounding matter, which can be seen as concentric rings of cliffs called the Cordillera Mountains. The outermost ring is nearly 1000 km in diameter. (b) Two smaller craters called Reinhold and Eddington sit amid the secondary cratering resulting from the impact that created the 90-km-wide Copernicus crater (near the horizon) about a billion years ago. The ejecta blanket from crater Reinhold, 40 km across, and in the foreground, can be seen clearly. View is looking northeast from the lunar module during the Apollo 12 mission.
In addition to the bombardment by meteoroids with masses of a gram or more, a constant "rain" of micrometeoroids (debris with masses ranging from a few micrograms up to about 1 gram) also eats away at the structure of the lunar surface, contributing to the overall erosion process. Some examples can be seen in Figure 8.16, a photomicrograph of glassy "beads" brought back to Earth by the Apollo astronauts. The beads themselves were formed during the explosion following a meteoroid impact, when surface rock was melted, ejected, and rapidly cooled. However, several of them also display fresh miniature craters caused by micrometeoroids that struck the beads after they had cooled and solidified.
Figure 8.16 Craters of all sizes litter the lunar landscape. Some shown here, embedded in glassy beads retrieved by Apollo astronauts, measure only 0.01 mm across. (The scale at the top is in millimeters.)
The rate of cratering decreases rapidly with crater size—fresh large craters are scarce, but small craters are very common. The reason for this is simple: there just aren't very many large chunks among the interplanetary debris, so their collisions with the Moon are rare. At the present average rates, one new 10-km (diameter) lunar crater is formed roughly every 10 million years, a new meter-sized crater is created about once a month, and centimeter-sized craters are formed every few minutes.
Despite this constant barrage from space, the Moon's present-day erosion rate is still very low—about 1/10,000 that on Earth. Wind and water on Earth are far more effective erosive agents than is meteoritic bombardment on the Moon. For example, the Barringer Meteor Crater (shown in Figure 8.17) in the Arizona desert, one of the largest meteoroid craters on Earth, is only 25,000 years old but has already undergone noticeable erosion. It will probably disappear completely in a mere million years, quite a short time geologically. If a crater that size had formed on the Moon even 4 billion years ago, it would still be plainly visible today.
Figure 8.17 The Barringer Meteor Crater, near Winslow, Arizona, is 1.2 km in diameter and 0.2 km deep. Geologists think a large meteoroid made it about 25,000 years ago. The meteoroid probably was about 50 m across and weighed around 200,000 tons.
When the Apollo astronauts visited several lunar sites and brought back rock samples, it became possible to measure the ages of the highlands and the maria using radioactive dating techniques, and astronomers can now use the known ages of Moon rocks to estimate the rate of cratering in the past. The highlands are typically more than 4 billion years old, whereas the maria have ages ranging from 3.2 to 3.9 billion years. Thus the much more heavily cratered highlands are indeed older than the less cratered maria, but the difference in cratering is not simply a matter of exposure time. Astronomers now believe that the Moon, and presumably the entire inner solar system, experienced a sudden sharp drop in meteoritic bombardment about 3.9 billion years ago. The highlands solidified and received most of their craters before that time, while the maria solidified afterward. The rate of cratering has been roughly constant ever since.
