Surface Features

Though the Moon appears bright in the night sky, its surface is actually dark.The albedo of the lunar surface is 0.07, meaning that it absorbs 93 percent of the sunlight that strikes it. (Albedo is a measure of the light reflected by an object as a fraction of the light shining on an object; mirrors have high albedo, while charcoal has low albedo.) The

Astronaut Harrison (Jack) Schmitt stands by a lunar boulder while on the Apollo 17 mission. (NASA/Apollo 17/ NSSDC)

Moon simply appears bright from Earth because it is so relatively large and close, and stands in comparison to the deep dark of space.

The surface of the Moon consists of rock in various stages of pulverization by impacts. The outer shell of the Moon's crust is regolith, a region some one to six miles (2 to 10 km) deep made of giant blocks of bedrock broken by the Late Heavy Bombardment covered by a three- and 60-foot-deep (1- and 20-m-deep) regolith made of small rock fragments from impacts and volcanic glass beads, all altered by gardening from the solar wind. It is called regolith rather than soil, though, since it has none of the water or organic components so crucial to Earth's soil. The Moon's soil was created by millennia of meteorite bombardments.

The wide size range of rock fragments that make up the lunar regolith can be seen in this photograph of lunar module pilot Harrison Schmitt standing in front of a large boulder on the Moon, taken by Eugene Cernan, commander of Apollo 17. The lunar rover is in the foreground at left. Apollo 17, launched on December 7, 1972, was the last of the Apollo Moon landing missions.

Astronaut Harrison (Jack) Schmitt stands by a lunar boulder while on the Apollo 17 mission. (NASA/Apollo 17/ NSSDC)

This soil sample from the Apollo 15 mission shows rounded and broken beads of volcanic glass, some with skeletal olivine crystals, and soil agglutines caused by melting. (Linda T. Elkins-Tanton/NASA/JSC)

While the image of Jack Schmitt shows a large boulder making up part of the lunar regolith, the photomicrograph here shows the other size extreme in the lunar regolith. The image shows round volcanic glass beads, sharp-cornered broken fragments of glass and rock, and swirly mixtures of materials created by heat of impact into the surface. A bead of volcanic glass in the upper right corner of the image also shows dark bar-shaped crystals of olivine, indicating that the glass cooled slowly enough to allow crystals to form.

All the rocks on the lunar surface are igneous, the cooled result of rock heated enough to be completely liquid. No rocks on the Moon are sedimentary; without atmosphere and water, the Moon lacks the processes that create sedimentary rocks. No rocks on the Moon are metamorphic, either, since the Moon lacks the tectonic processes to bury rocks under heat and pressure and then bring them back to the surface in their changed form. Knowledge of lunar surface compositions comes from orbital data, material returned by the Apollo and Soviet Luna missions, and from a small collection of lunar meteorites that have landed on Earth, the result of large impacts on the Moon spalling off smaller pieces.

The more that is learned the more complicated the lunar surface appears. Tom Prettyman, David Vaniman, and their colleagues at Los Alamos National Laboratory have used special data processing techniques to determine the concentrations of major elements from Lunar Prospector gamma-ray data.They show that the most abundant compositions on the surface are matched by some of the rarer rocks in collections. Lunar meteorites, for example, seem to be more characteristic of the lunar highlands than samples of the surface soil collected by Apollo astronauts.

Even more curiously, rocks with high ratios of magnesium to iron and low concentrations of thorium may be quite abundant in some regions of the Moon, especially in the North Polar regions. The anorthosite-rich highlands rocks are by comparison unusually low in magnesium and so are completely different from the high magnesium rocks spotted in orbital data. Not everyone is convinced that the magnesium determination is calibrated sufficiently, but these preliminary results are interesting and, if they survive careful scrutiny, may have important implications for the origin of the lunar crust. Characterization of the rocks in these areas will have to wait for future missions to test the magnesium concentration by independent techniques (for example, orbital X-ray spectrometry and sample return missions; for more, see the sidebar "Remote Sensing" on page 156), but it shows that the Moon's crust is compositionally diverse.

Though the lunar surface is continually bombarded by the solar wind, this is the only continuous process acting to change the surface. With no wind, rain, ice, or volcanic activity, the Moon's craters are the cleanest in the solar system, and they make a convenient natural laboratory for Earth scientists to study cratering.

Two of the youngest craters on the Moon are Copernicus and Tycho, each of which can be seen on the near side. Tycho, with a diameter of 53 miles (85 km), is particularly prominent because of its bright white ejecta rays that reach almost around the Moon.Tycho is visible on the near side of the Moon near its South Pole. Tycho's exceptional brightness is due to its young age. One such ray crosses the Apollo 17 landing site, about 1,300 miles (2,000 km) from Tycho. The impacts of this ejected material from Tycho are thought to have triggered a landslide near the Apollo 17 landing site. Laboratory analysis of returned samples from this landslide suggests that Tycho's age is

Was this article helpful?

0 0

Post a comment