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. 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.
Problems U Pb Radioisotope Dating Methods in Genesis
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. The entire outer portion of the Moon is thought to have been molten at the time the highlands crust began to form 9. 6 billion years ago. As light silicate minerals accumulated at the top of this magma ocean, they formed a crust, which soon became densely cratered by a contemporaneous intense bombardment of meteorites. This initial high cratering rate declined rapidly but the Moon's surface, one of the oldest in the solar system, became covered with craters. The maria were formed by the extrusion of vast amounts of lava that accumulated in the lowlands of large craters or basins and, in places, overflowed and spread over parts of the lunar highlands. The maria are thus relatively young features of the lunar surface, even though they began to form four billion years ago. Impact cratering is a rare event on Earth today, but it has been a fundamental process in planetary development. The Moon is pockmarked with literally billions of craters, which range in size from microscopic pits on the surfaces of rock specimens to huge, circular basins hundreds of kilometers in diameter. The same is true for the surfaces of Mercury, Mars, the asteroids, and most of the satellites of the outer planets as well. Indeed, impact cratering was undoubtedly the dominant geologic process on Earth during the early stages of its evolution. Earth was once heavily cratered like the Moon today. Evaluation of the impact process can provide an important interpretive tool for understanding planets and their development.
Although it is difficult to imagine the magnitude of these enormous impact events, which excavated millions of cubic meters of material in seconds, the results of the photographic missions to the planets and studies of experimental craters produced in laboratories have greatly increased our understanding of what happens when a meteorite strikes a planet. The formation of a crater is generally divided into three stages: compression, excavation, and modification (Figure 9. 8). During the compression stage, the meteor's kinetic energy is transferred through the ground by a shock wave that expands in a spherical pattern away from the point of impact. The pressures experienced by the target materials are so high that the rock behaves like a fluid for a short time. During this stage, only a small amount of material is removed by vapor jets around the side of the meteorite the main mass of the material remains to be excavated. During the final stage, after much ejecta has settled to the surface, the crater becomes modified from its initial, excavated form. The floor of the crater may rapidly rebound upward to compensate for the material removed during excavation. Terraces form a nearly concentric natural staircase to the crater floor. The modification stage probably continues over a long period as the crust reestablishes a stable configuration in the planet's gravity field (Figure 9. 5). Impact craters are typically circular, in contrast to many volcanic craters, which are frequently asymmetrical or elongate.
When an impacting body strikes the surface at an angle greater than 65, the crater will nonetheless be circular, because the shock waves spread out with equal velocity in all directions from the point of impact and the rarefaction waves move back toward that point. As can be seen in Figure 9. 5, the rim of the crater is built of deformed bedrock that has been heaved upward and outward, bent back, and even overturned. The material thrown out of the crater accumulates around the crater rim as an ejecta blanket. This is thickest near the crater and thins outward, becoming discontinuous and patchy at a distance of about 6. 5 crater diameters from the rim. Larger blocks thrown out from the main crater may impact with enough speed to form secondary craters, which tend to be irregular in shape and are typically grouped in clusters or chains. Many overlap and form distinct linear patterns. Fine powdered material and melted material, which resolidifies into glass beads, is thrown farther and accumulates as a system of long, bright, splashlike rays. The floor of an impact crater is commonly covered with a lens-shaped deposit of fragmented rock (breccia) and small amounts of lava produced by shock melting during impact. The bedrock below is highly fractured down to a depth equal to about three times the depth of the crater. It is important to emphasize that the impacts of meteorites produce landforms (craters) and new rock bodies (ejecta blankets), and are therefore similar to other rock-forming processes (such as volcanism and sedimentation) that operate at the surface of a planet. In each process, energy is transferred, material is transported and deposited to form a new rock body, and a new landform (crater, volcanic cone or flow, or delta) is created.
The rock-forming processes associated with impact include: fragmentation, ballistic transportation, and deposition of rock particles, in addition to rock modifying processes of shock metamorphism, partial melting, and vaporization. The surface of the Moon (Figure 9. 6) has been described as being covered by a forest of craters, and at first glance all craters may look the same.