5.6 The Surface of the Moon
The first observers to point their telescopes at the Moon noted large, roughly circular, dark areas that resemble (they thought) Earths oceans. They called these regions maria, a Latin word meaning "seas" (singular: mare). The largest of them (Mare Imbrium, the "Sea of Showers") is about 1100 km in diameter. Today we know that the maria are actually extensive flat plains that resulted from the spread of lava during an earlier, volcanic period of lunar evolution. In a sense, then, the maria are oceansancient seas of molten lava, now solidified.
Early observers also saw light-colored areas that resembled Earths continents. Originally dubbed terrae, from the Latin word for "land," these regions are now known to be elevated several kilometers above the maria. Accordingly, they are usually called the lunar highlands. Both types of region are visible in Figure 5.14, a photographic mosaic of the full Moon. These light and dark surface features are also evident to the naked eye, creating the face of the familiar "Man-in-the-Moon."
Until spacecraft flew around the Moon, no one on Earth had any idea what the Moons hidden half looked like. To the surprise of most astronomers, when the far side of the Moon was mapped first by Soviet and later by American spacecraft, no major maria were found. The lunar far side (Figure 5.15) is composed almost entirely of highlands.
Because the smallest lunar features we can distinguish with the naked eye are about 200 km across, we see little more than the maria and highlands when we gaze at the Moon. Through a telescope, however (Figure 5.16), we find that the lunar surface is scarred by numerous bowl-shaped craters (after the Greek word for "bowl").
The material thrown out by the explosion surrounds the crater in a layer called an ejecta blanket, the ejected debris ranging in size from fine dust to large boulders. The larger pieces of ejecta may themselves form secondary craters. Many of the rock samples brought back by the Apollo astronauts show patterns of repeated shattering and meltingdirect evidence of the violent shock waves and high temperatures produced in meteoritic impacts.
Lunar craters come in all sizes, reflecting the range in sizes of the impactors that create them. The largest craters are hundreds of kilometers in diameter, the smallest microscopic. Because the Moon has no protective atmosphere, even tiny interplanetary fragments can reach the lunar surface unimpeded. Figure 5.18(a) shows the result of a large meteoritic impact on the Moon, Figure 5.18(b) a crater formed by a micrometeoroid.
Craters are found everywhere on the Moons surface, but the older highlands are much more heavily cratered than the younger maria. Knowing the ages of the highlands and maria, researchers can estimate the rate of cratering in the past. They conclude that the Moon, and presumably the entire inner solar system, experienced a sudden sharp drop in meteoritic bombardment rate about 3.9 billion years ago. The rate of cratering has been roughly constant since that time.
This time3.9 billion years in the pastis taken to represent the end of the accretion process through which planetesimals became planets. (Sec. 4.3) The lunar highlands solidified and received most of their craters before that time. The great basins that formed the maria are thought to have been created during the final stages of heavy meteoritic bombardment between about 4.1 and 3.9 billion years ago. Subsequent volcanic activity filled the craters with lava, creating the formations we see today.
Meteoritic impact is the only important source of erosion on the Moon. Over billions of years, collisions with meteoroids, large and small, have scarred, cratered, and sculpted the lunar landscape. At the present average rates, one new 10-km-diameter lunar crater is formed every 10 million years, one new 1-m-diameter crater is created about once a month, and 1-cm-diameter craters are formed every few minutes. In addition, a steady "rain" of micrometeoroids also eats away at the lunar surface (Figure 5.19). The accumulated dust from countless impacts (called the lunar regolith) covers the lunar surface to an average depth of about 20 m, thinnest on the maria (about 10 m) and thickest on the highlands (more than 100 m in places)
Despite this barrage from space, the Moons present-day erosion rate is still very lowabout 10,000 times less than on Earth. For example, the Barringer Meteor Crater (Figure 4.13) in the Arizona desert, one of the largest meteor craters on Earth, is only 25,000 years old, but it is already decaying. It will probably disappear completely in a mere million years, quite a short time geologically. If a crater that size had formed on the Moon even a billion years ago, it would still be plainly visible today.
2Radioactive dating compares the rates at which different radioactive elements in a sample of rock decay into lighter elements. The "age" returned by this technique is the time since the rock solidified.
Describe two important ways in which the lunar maria differ from the highlands.