5.6 The Surface of the Moon

Figure 5.14 Full Moon, Near Side
A photographic mosaic of the full Moon, north pole at the top. Some prominent maria are labeled. (UC/Lick Observatory)
On Earth, the combined actions of air, water, and geological activity erode our planet’s surface and reshape its appearance almost daily. As a result, most of the ancient history of our planet’s surface is lost to us. The Moon, though, has no air, no water, no ongoing volcanic or other geological activity. Consequently, features dating back almost to the formation of the Moon are still visible today. For this reason, studies of the lunar surface are of great importance to Earth geologists, and have played a major role in shaping theories of the early development of our planet.

Large-Scale Features

The first observers to point their telescopes at the Moon noted large, roughly circular, dark areas that resemble (they thought) Earth’s oceans. They called these regions maria, a Latin word meaning "seas" (singular: mare). The largest of them (Mare Imbrium, the "Sea of Showers") is about 1100 km in diameter. Today we know that the maria are actually extensive flat plains that resulted from the spread of lava during an earlier, volcanic period of lunar evolution. In a sense, then, the maria are oceans—ancient seas of molten lava, now solidified.

Early observers also saw light-colored areas that resembled Earth’s continents. Originally dubbed terrae, from the Latin word for "land," these regions are now known to be elevated several kilometers above the maria. Accordingly, they are usually called the lunar highlands. Both types of region are visible in Figure 5.14, a photographic mosaic of the full Moon. These light and dark surface features are also evident to the naked eye, creating the face of the familiar "Man-in-the-Moon."

Figure 5.15 Full Moon, Far Side The far side of the Moon, as seen by the Clementine military spacecraft. The large, dark region at center bottom outlines the South Pole-Aitken Basin. the largest and deepest impact basin known in the solar system. This image shows only a few small maria on the far side. (Defense Department)
Based on studies of lunar rock brought back to Earth by Apollo astronauts and unmanned Soviet landers, geologists have found important differences in both composition and age between the 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. Maria rock is quite similar to terrestrial basalt, and geologists believe that it arose on the Moon much as basalt did on Earth, through the upwelling of molten material through the crust. Radioactive dating indicates ages of more than four billion years for highland rocks, and from 3.2 to 3.9 billion years for those from the maria. 2

Until spacecraft flew around the Moon, no one on Earth had any idea what the Moon’s hidden half looked like. To the surprise of most astronomers, when the far side of the Moon was mapped first by Soviet and later by American spacecraft, no major maria were found. The lunar far side (Figure 5.15) is composed almost entirely of highlands.

 Cratering

Because the smallest lunar features we can distinguish with the naked eye are about 200 km across, we see little more than the maria and highlands when we gaze at the Moon. Through a telescope, however (Figure 5.16), we find that the lunar surface is scarred by numerous bowl-shaped craters (after the Greek word for "bowl").

Figure 5.16 Moon, Close Up (a) The Moon near third quarter. Surface features are visible near the terminator, the line separating light from dark, where sunlight strikes at a sharp angle and shadows highlight the topography. (b) Magnified view of a region near the terminator, as seen from Earth through a large telescope. Crater Copernicus is at bottom left, and the central dark area is Mare Imbrium, ringed at the bottom right by the Apennine mountains. (c) Enlargement of the lower right portion of (b).The smallest craters visible here have diameters of about 2 km, about twice the size of the Barringer crater shown in Figure 4.13. (UC/Lick Observatory; California Institute of Technology)

Figure 5.17 Meteoroid Impact Stages in the formation of a crater by meteoritic impact. (a) The 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.
Most craters formed eons ago as the result of meteoritic impact. (Sec. 4.2) 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 would release as much energy as the detonation of 10 kg of TNT. As illustrated in Figure 5.17, impact by a meteoroid causes sudden and tremendous pressures to build up on the lunar surface, heating the normally brittle rock and deforming the ground. The ensuing explosion pushes previously flat layers of rock up and out, forming a crater.

The material thrown out by the explosion surrounds the crater in a layer called an ejecta blanket, the ejected debris ranging in size from fine dust to large boulders. The larger pieces of ejecta may themselves form secondary craters. 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.

Lunar craters come in all sizes, reflecting the range in sizes of the impactors that create them. The largest craters are hundreds of kilometers in diameter, the smallest microscopic. Because the Moon has no protective atmosphere, even tiny interplanetary fragments can reach the lunar surface unimpeded. Figure 5.18(a) shows the result of a large meteoritic impact on the Moon, Figure 5.18(b) a crater formed by a micrometeoroid.

Craters are found everywhere on the Moon’s surface, but the older highlands are much more heavily cratered than the younger maria. Knowing the ages of the highlands and maria, researchers can estimate the rate of cratering in the past. They conclude that the Moon, and presumably the entire inner solar system, experienced a sudden sharp drop in meteoritic bombardment rate about 3.9 billion years ago. The rate of cratering has been roughly constant since that time.

This time—3.9 billion years in the past—is taken to represent the end of the accretion process through which planetesimals became planets. (Sec. 4.3) The lunar highlands solidified and received most of their craters before that time. The great basins that formed the maria are thought to have been created during the final stages of heavy meteoritic bombardment between about 4.1 and 3.9 billion years ago. Subsequent volcanic activity filled the craters with lava, creating the formations we see today.

Lunar Erosion

Meteoritic impact is the only important source of erosion on the Moon. Over billions of years, collisions with meteoroids, large and small, have scarred, cratered, and sculpted the lunar landscape. At the present average rates, one new 10-km-diameter lunar crater is formed every 10 million years, one new 1-m-diameter crater is created about once a month, and 1-cm-diameter craters are formed every few minutes. In addition, a steady "rain" of micrometeoroids also eats away at the lunar surface (Figure 5.19). The accumulated dust from countless impacts (called the lunar regolith) covers the lunar surface to an average depth of about 20 m, thinnest on the maria (about 10 m) and thickest on the highlands (more than 100 m in places)

Despite this barrage from space, the Moon’s present-day erosion rate is still very low—about 10,000 times less than on Earth. For example, the Barringer Meteor Crater (Figure 4.13) in the Arizona desert, one of the largest meteor craters on Earth, is only 25,000 years old, but it is already decaying. 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 a billion years ago, it would still be plainly visible today.

Figure 5.18 Lunar Craters (a) 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, in the foreground, can be seen clearly. (b) 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 beads themselves were formed during the explosion following a meteoroid impact, when surface rock was melted, ejected, and rapidly cooled. (NASA)

Figure 5.19 Lunar Surface The lunar surface is not entirely changeless. 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. Note the soft edges of the craters visible in the foreground of this image. In the absence of erosion, these features would be as jagged and angular today as they were when they formed. (The twin tracks were made by the Apollo lunar rover.) (NASA)

2Radioactive dating compares the rates at which different radioactive elements in a sample of rock decay into lighter elements. The "age" returned by this technique is the time since the rock solidified.

Concept Check

Describe two important ways in which the lunar maria differ from the highlands.