While impact cratering on the Moon can be a very destructive process, it also has the capacity to throw new samples of rock our way. As an impact crater is being excavated, some rocky material can be thrown out of the crater with enough velocity to escape the Moon's gravity and fall to Earth. Since impact craters occur at random locations, lunar meteorites provide a set of samples from all portions of the Moon, including the farside and polar regions, which Apollo astronauts and Luna spacecraft were unable to visit.Thus far, 15 lunar meteorites have been discovered (see table below). Many of these have been recovered in Antarctica, where they can be preserved in glacial ice for thousands of years after they fall. Some lunar meteorites, however, have also been found in other parts of the world. Generally these are hot desert regions where the meteorites are preserved because there is so little rain to affect them.
Even though the lunar meteorites sample different regions of the Moon than the Apollo and Luna programs, they have some similarities. One common type of lunar meteorite is an anorthositic breccia. The word "anorthositic" indicates the rock contains lots of bright white fragments of anorthosite, the type of rock found in the lunar highlands. A "breccia" is a rock that contains the broken fragments of older rocks. These breccias are usually produced by the impact processes, which crush rock, move it around the surface of the Moon, and mix it with broken fragments of other types of rock. There are different types of impact breccias, including fragmental breccias, polymict breccias, and regolith breccias.
Anorthositic Breccia
This is a picture of lunar meteorite MacAlpine Hills 88105, which is commonly abbreviated MAC 88105. This meteorite was found in Antarctica by a joint National Science Foundation and National Aeronautics and Space Administration (NASA) expedition. The numbers in the name of the meteorite indicate it was found in 1988 (88) and that it was sample number 105 in the expeditions collection. The number 11 indicates that this is split 11 from the original mass. For scale the cube is 1 centimeter square. The letters on the cube indicate the orientation of the sample when it was found on the ice. The white fragments or clasts in the sample are composed of anorthosite and were produced in the highland regions of the Moon.Some lunar meteorites come from the vast plains of basalt, or maria, on the Moon, which are the dark gray areas one can see on the surface. Sometimes the meteorites can be pristine samples of the basalt, but sometimes they are breccias composed of basalt fragments. Like breccias from the lunar highlands, these basaltic breccias were produced by impact cratering processes that constantly churn the surface of the Moon.Image courtesy of NASA's Antartcic Meteorite Laboratory at the Johnson Space Center.
These meteorites can be used to understand the geologic processes that shaped the Moon. One of the most dramatic events was the lunar cataclysm, which was an intense period of bombardment that appears to have resurfaced most of the Moon. Material in the meteorites also record other geologic processes, like volcanic fire-fountaining. When molten rock erupts on the surface of a planet, it sometimes shoots above the surface in a fountain of lava. Like a sheet of water thrown through the air, the lava can break up into a lot of molten droplets. These droplets often solidify before they fall back on the ground, forming a blanket of glassy beads. The Apollo astronauts discovered green and orange glass beads. During the Apollo 17 mission, the astronauts found a deposit of orange glass beads that was so thick it formed a carpet of orange soil. These fascinating little baubles have also been found in lunar meteorites.
Basalt
Other web sites:
Lunar NewsAdditional reading:
Cosmic Pinball, Carolyn Sumners and Carlton Allen (eds.), McGraw-Hill, New York, 190 p., 2000.Lunar Sourcebook, Grant H. Heiken, David T. Vaniman, and Bevan M. French (eds.), Cambridge
University Press, Cambridge, 736 p., 1991.Planetary Science: A Lunar Perspective, Stuart Ross Taylor, Lunar and Planetary Institute, Houston, Texas, and Research School of Earth Sciences, Canberra, Australia, 481p., 1982.
The Geologic History of the Moon, Don E. Wilhelms, U.S. Geological Survey, United States Government Printing Office, Washington, 301p., 1987.
