The nearside (left) and farside of the moon. The difference in cratering is dramatic, and gives the farside the appearance of another body altogether. Photo (left) by Luc Viatour, right by the Apollo 16 astronauts and NASA
Most of the lunar seas, so-called because people thought they resembled the smooth, dark surfaces of the open sea, are concentrated on the nearside of the moon, while the farside is almost totally saturated with craters. We only see the nearside because the moon rotates on its axis at the same rate as it revolves around the Earth, always keeping one side to us. Only the Apollo astronauts have had the opportunity to see the battered farside firsthand.
Scientists still aren’t certain why there are so many more maria or seas on "our" side of the moon. We do know they were created around four billion years ago by impacting asteroids and comets during the moon’s formation. The seas were originally enormous craters which later filled with hot, igneous lavas that welled up through cracks in the crust. The lavas, rich with iron, were not only darker colored than the surrounding highlands (white, cratered areas) but they flowed onto the surface after much of the early bombardment of the moon had ceased. That’s why they appear relatively unscared compared to the highlands. Perhaps the reason the seas are concentrated on the nearside has to do with the crust being thinner there, allowing the lavas to reach the surface more easily.
A basalt moon rock collected by the Apollo 15 astronauts. The Swiss cheese holes are called vesicles, caused by gas bubbling out of the rock as it cooled. Photo: NASA
The 12 Apollo astronauts who walked on the moon collected 2,415 moon rocks weighing 842 lbs. In addition, three Russian Luna sample-return missions netted an additional 3/4 lb. Once on Earth, the rocks were analyzed and dated using the radioactive age dating technique. They range from 3.1 billion years for the dark lavas found in the maria to 4.5 billion years for highland rocks. Ancient either way you look at it.
Professional meteorite hunter Mike Farmer of Tucson, Ariz. holds a lunar meteorite he found in 2005 in the Dhofar desert region of Oman. The talon-shaped meteorite, called Dhofar 1180, weighs 115.2g. Photo courtesy: Robert Woolard
There is one more way lunar material has been delivered to Earth — through impacts on the moon itself. Some of the material blasted from the surface has come to our planet over time as meteorites. As of this year, independent meteorite hunters searching the deserts of the world, and scientific crews scouring Antarctica in snowmobiles, have discovered about 50 lunar meteorites with a total weight of almost 88 lbs. Scientists determine a meteorite is lunar rather than the more common variety from the asteroid belt, by studying its age, composition, mineral structure and percentage of certain isotopes (alternate forms) of elements like oxygen. Every potential lunar meteorite is sent to a lab for analysis before verified as truly originating from the moon. Rocks from the Apollo missions has helped researchers know what to look for.
Fragments of rock are welded together inside Farmer’s Dhofar 1180 lunar meteorite, which originated in the heavily-cratered, lunar highlands. Scientists describe this texture as brecciated (BRECH-che-ayt-ed) or fragmented. Photo courtesy: Mike Farmer
Many lunar rocks contain countless shattered fragments of rock busted and ground up during the long period of intense asteroid bombardment in the early days of the solar system when the planets and moons were forming. Farmer’s piece shows this beautifully. Lunar meteorites have greatly increased our sampling and understanding of the moon’s crust and evolution. They’re also a lot cheaper to obtain than launching a full-blown lunar mission. We just have to be clever and persistent enough to find them.
Another brecciated lunar meteorite called NWA (Northwest Africa) 2995. Feldspar is a common mineral in meteorites like this one from the lunar highlands. Feldspars are also found in Earth’s crust. Photo: Randy Korotov
A good place to learn more about moon meteorites is Randy Korotov’s excellent site at the Washington University of St. Louis. Be sure to click on this List of Lunar Meteorites link as well to see lots of photos and descriptions. To learn more about the Apollo rocks, check out this site.
The sky looks like it will clear late this evening. When it does, give our old friend the moon a fresh look. Does a moon rock lie in your future? Tomorrow we’ll look at meteorites in more depth and how you might obtain one.