Moonrocks and Maria

The Moon is a very different world than the Earth. It has no atmosphere to speak of and has probably forever been a cold and lifeless world. Because the moon does not have a "plastic" mantle like the Earth, it does not have plates like the Earth. There are mountains created by plates colliding, or giant trenches where new crust is being created. The moon has been largely unchanged over its lifetime.

The mountains that are on the moon, are ones blasted out of the surface when large bodies slammed into the moon. There are a few small volcanoes, but they are long since dead, having spewed forth their lava in the Moon's early days.

Because of its small size, the moon could never retain an atmosphere. Its surface gravity is only 0.2 that of Earth's and gas molecules simply escaped into space. Without an atmosphere, there are no winds or rain to erode the surface. The only factors that change the surface at all are heating and cooling of the rocks and the occasional impact.

The lunar surface can be divided into two distinct regions, the lunar highlands and the lunar lowlands. The lowlands are the smooth dark plains which have few craters. We call these the lunar Maria, Mare from the Latin word for sea, because it was first thought that these smooth areas were lunar seas. They are really lowlands filled with ancient floods of dark lava. The first Apollo mission landed on the Mare, and they found the surface covered with a thick lava dust. The samples returned indicated that the lava solidified about 3.2

The lunar highlands are about 3 km higher in elevation than the lowlands. These are the lighter colored heavily cratered areas. It was because of the higher elevation that these areas were not flooded with the basaltic lava. These areas are therefore older than the lowlands. The last two Apollo missions landed in these areas, though landing here was much trickier. These rocks proved to be different in composition from the basaltic lava. Rocks here were anorthosite, a mixture of aluminum and calcium silicates, less dense than basalt.

Many lunar rocks have been altered from the extensive cratering. These rocks, called breccias, have been shattered fragments cemented back together. It is interesting to note that many people think that moon rocks must be light in color, no doubt because the moon looks so bright in the sky. In reality, all moon rock types are dark grayish brown in color!