The shapes of the mineral grains and how they are intergrown in mare basalts indicate that these rocks formed in lava flows, some thin (perhaps a meter thick), others thicker (up to perhaps 30 meters). This is not unusual for basalt flows on Earth. Many lunar mare basalts also contain holes, called vesicles, which were formed by gas bubbles trapped when the lava solidified. Earth basalts also have them. On Earth, the abundant gases escaping from the lava are carbon dioxide and water vapor, accompanied by some sulfur and chlorine gases. We are not as sure what gases escaped from lunar lavas, although we know that water vapor was not one of them because there are no hints for the presence of water or water-bearing minerals in any Moon rock. The best bet is a mixture of carbon dioxide and carbon monoxide, with some sulfur gases added for good measure.

On Earth, when the amount of gas dissolved in magma (the name for lava still underground) becomes large, it escapes violently and causes an explosive eruption. In places such as Hawaii, for example, the lava erupts in large fountains up to several hundred meters high. The lava falls to the ground in small pieces, producing a pyroclastic deposit. This also happened on the Moon, producing the dark mantle deposits. One of these was sampled directly during the Apollo 17 mission. The sample, called the “orange soil,” consists of numerous small orange glass beads. They are glass because they cooled rapidly, so there was not enough time to form and grow crystals in them.

Small samples of pyroclastic glasses were also found at other sites. Some are green, others yellow, still others red. The differences in color reflect the amount of titanium they contain. The green have the least (about 1 weight percent) and the red contain the most (14 weight percent), more than even the highest titanium basalt.

Experiments conducted on mare basalts and pyroclastic glasses show that they formed when the interior of the Moon partially melted. (Rocks do not have a single melting temperature like pure substances. Instead they melt over a range of temperatures: 1000-1200°C for some basalts, for example.) The experiments also show that the melting took place at depths ranging from 100 to 500 km, and that the rocks that partially melted contained mostly olivine and pyroxene, with some ilmenite in the regions that formed the high-titanium basalts. An involved but sensible chain of reasoning indicates that these deep rocks rich in olivine and pyroxene formed from the lunar magma ocean: while plagioclase floated to form anorthosites in the highlands crust, the denser minerals olivine and pyroxene sank. So, although the anorthosites and mare basalts differ drastically in age and composition, the origins are intimately connected.

What's next?

In the meantime, Nature has provided a bonus: samples from the Moon come to us free of charge in the form of lunar meteorites. (See companion volume Exploring Meteorite Mysteries.) Thirteen separate meteorites have been identified so far, one found in Australia and the rest in Antarctica. We are sure that they come from the Moon on the basis of appearance and chemical and isotopic composition, but of course we do not know from where on the Moon they come. These samples have helped support the magma ocean idea. Most important, knowing that meteorites can be delivered to Earth by impacts on the Moon lends credence to the idea that we have some meteorites from Mars.

BACK | NEXT