At the outset, it is important to note that we assume that the physical and chemical laws that govern nature are constant. For example, we use observations about how chemical reactions occur today, such as the combination of oxygen and hydrogen at specific temperatures and pressures to produce water, and infer that similar conditions produced the same results in the past. This is the basic assumption of all sciences. Moreover, much of what we know about the planets, as in all science, is a mixture of observation and theory---a mixture that is always subject to change. Scientific knowledge is pieced together slowly by observation, experiment, and inference. The account of the origin and differentiation of planets we present is such a theory or model it explains our current understanding of facts and observations. It will certainly be revised as we continue to explore the solar system and beyond, but the basic elements of the theory are firmly established. When Galileo first observed the Moon through a telescope, he discovered that its dark areas are fairly smooth and its bright areas are rugged and densely pockmarked with craters.
Radiometric Dating Methods Uses amp the Significance of
He called the dark areas maria, the Latin word for seas, and the bright areas terrae (lands). These terms are still used today, although we know the maria are not seas of water and the terrae are not geologically similar to Earth's continents. The maria and terrae do, however, represent major provinces of the lunar surface, each with different structures, landforms, compositions, and histories. The maria and terrae can even be distinguished from Earth by the naked eye. As shown in Figure 9.
6, the maria on the near side of the Moon appear to be dark and smooth, with only a few large craters. Some maria occur within the walls of large circular basins such as Crisium, Serenitatis, and Imbrium, whereas others such as Oceanus Procellarum occupy much larger, irregular depressions. We know from lunar rock specimens and surface features that the maria are vast layers of thin basaltic lava, which flowed into depressions and flooded large parts of the lunar surface. The terrae, or highlands, constitute about two-thirds of the near side of the Moon and exhibit a wide range of topographic relief. This is the highest and most rugged topography on the Moon, where local relief in many areas is up to 5555 meters.
Problems U Pb Radioisotope Dating Methods in Genesis
An important characteristic of the lunar highlands is that they contain abundant craters, many of which range from 55 to 6555 km in diameter. For example, craters larger than 65 km are about 55 times more abundant on the highlands than on the maria. The far side of the Moon was totally unknown until photographs were first taken by a Soviet spacecraft in 6959. It was a total surprise to learn that, although the details were poorly defined, the far side of the Moon was composed almost entirely of densely cratered highlands. Later, orbiting satellites launched by the United States completely photographed the far side of the Moon with definition sharp enough to map the surface in considerable detail (Figure 9.
7). These photographs confirmed that the far side of the Moon is densely cratered only a few large craters contain mare lavas. Why the maria are largely restricted to the near side of the Moon remains a fundamental problem of lunar geology. Results from several Apollo experiments demonstrate other fundamental differences between the highlands and the maria. Remote sensing measurements of the composition of the lunar surface indicate that the maria and highlands are composed of distinctly different rock types.
Rocks collected by Apollo astronauts show the highlands to be mostly composed of anorthosite and other feldspar-rich rocks, in contrast to the basaltic maria. Tracking Moon-orbiting spacecraft from Earth has confirmed the elevation differences. The highlands may be as much as 5 km above the mean radius of the Moon, whereas the maria lie almost 5 km below the mean radius. (As the Moon has no water, its mean radius can be used as a reference level from which relative elevations or depressions can be measured). Studies of the velocities of seismic waves have shown that the highland crust is also much thicker, in some cases up to 655 km thick, while the crust under some of the near side maria is only around 95 km thick.
The maria and highlands not only represent different types of terrain, they broadly represent two different periods in the history of the Moon. The highlands, which occupy about 85 percent of the entire lunar surface, are composed of rocks that formed very early in the Moon's history.