As our closest celestial neighbor, the Moon is a familiar and inspiring object to investigate using a small telescope, binoculars, or even photographs or one of the many high-quality maps available online (see "On the web"). The wondrously varied surface of the Moon--filled with craters, mountains, volcanic flows, scarps, and rilles--makes the Moon an excellent context for students to explore the methods scientists use to date geologic features, while learning about scientific observation and inference. This activity includes a unique application of the law of superposition that allows students to explore the relative ages of key lunar features and their origins.
Even with an unaided eye, two types of terrain seem to dominate the Moon's surface: the relatively light, very heavily cratered highlands and the dark, nearly smooth maria (Figure 1, page 36). The maria are extensive basaltic lava flows that cover about 16% of the lunar surface. As on Earth, the different types of terrain on the Moon have different ages. The relative ages of the Moon's highlands and maria can be determined by counting the number of craters per unit area superimposed on them (see sidebar at right). The older highlands have built up a larger number of impact craters than the smooth maria, which have not been exposed as long to bombarding meteoroids, asteroids, and comets (Wilhelms 1987).
Most craters on the Moon were caused by objects impacting the surface and explosively releasing their kinetic energy. In a typical collision, the impacting body burrows into the surface and is nearly instantly vaporized. The kinetic energy of the impactor is converted into shock waves that pulverize and launch the target material outward from the impact point, creating the crater (Shoemaker 1963). On the Moon, a typical impact crater is about 10 times the diameter of the impactor.
The material launched from the crater comes crashing down on the surrounding surface, creating a halo of debris called the ejecta blanket. Material launched higher on ballistic trajectories impacts the surface farther from the crater, creating secondary craters. In many cases, these secondaries cluster together, and the cluster may be oriented to point back to the primary crater.
An important tool of the geologist is the law of superposition: In general, younger formations are found atop older formations (see sidebar, page 37). While originally formulated for stratigraphy on Earth, the law of superposition can also help us understand the geologic history of many of the planets and moons in our solar system. Just as the history of the Earth has been divided into geological eons, eras, periods, and so on by studying the layers of rocks found on Earth, geologic periods of the surface of the Moon can be discerned by studying the layers evident in photographs of the lunar surface (Shoemaker and Hackman 1962).
Until the mid-20th century, the history of the surface of the Moon was widely argued. Some believed the craters had a volcanic origin. Others said they were impact craters. Such debates are common in science when data are inconclusive, especially in the early stages of a research program. Eugene Shoemaker largely settled the debate in 1959 (Wilhelms 1993). Shoemaker had studied the geology of the Great Meteor Crater in Arizona and a number of large craters in Nevada carved by nuclear weapons. He realized that the similarity of the craters to those on the Moon demonstrated that most lunar craters resulted from impacts. Thus, observations about craters on Earth led to inferences about the origin of craters on the Moon.
In 1962, Shoemaker and Hackman were the first to define the five geologic periods of the surface of the Moon by studying its northwest quadrant (Figure 2, page 38). This activity allows your students to follow in Shoemaker's and Hackman's footsteps by using the law of superposition to determine the relative ages of some of the Moon's key features. …