The Planet Earth - Astronomy
The interior of the earth. The shape of earth is an oblate spheroid with the larger diameter in the equatorial plane. This equatorial bulge is a result of the rotation of the earth. In addition to bulges of solid portions of the earth's crust, the moon's gravitational attraction produces two tidal bulges in the oceans and atmosphere. The moon's braking effect on the earth is also slowing earth's rotation. The study of seismic waves reveals that the interior of the earth is not rigid. The hot, highly compressed inner core of solid iron and nickel is surrounded by a molten outer core of liquid iron and nickel. The outer envelope surrounding the core is the mantle. A thin layer-the crust- overlies the mantle and contains the continents and ocean basins.
Surface of planet earth. Earth's crust and outermost part of the mantle constitute the lithosphere. Underlying it is the asthenosphere, a partially molten layer that flows under pressure. The lithosphere resting on the asthenosphere is thus not rigid but is actually broken into a dozen major lithospheric plates that have been rafted across the earth's mantle over the 4.6 billion years of earth's history. The plate boundaries are regions of intense earthquake and volcanic activity.
The present conformation of land masses is the result of the breakup of a single supercontinent about 200 million years ago. Because plate movements are driven by the outflow of heat from the earth's interior, over the next 2 billion years or so, when the earth has cooled further, the plate movements will slow and eventually cease. Other forces that shaped the surface features of earth are impact cratering by meteorites early in the earth's history. That phase has been virtually erased by more recent thermal-tectonic activity.
Earth's atmosphere. Earth's atmosphere insulates the earth by the greenhouse effect and blocks harmful electromagnetic and particle radiation from reaching the surface. The atmosphere has several layers, each with distinctive thermal, chemical, physical, and electrical properties. The chemical composition of the lowest 90 kilometers of the atmosphere is primarily nitrogen (77 percent) and oxygen (21 percent). Above 90 kilometers, there is a chemical separation with the heavier elements settling and the lighter molecules rising. The relatively large concentration of oxygen now in the atmosphere is the result of oxygenproducing plants, beginning about 2 billion years ago. The amount of each constituent is controlled by various production and destruction processes.
Magnetosphere of the earth. The earth acts as an electromagnet. Motion of electrical currents through the earth's outer core generates the magnetic field. Magnetic lines of force run between the poles and extend outward from the earth, forming the magnetosphere. This magnetic zone controls the motion of high-energy particles of the solar wind and cosmic rays, trapping many in the Van Allen radiation belts.
The moon. The lunar exploration program has given scientists samples of lunar material to study and strengthened the belief that the earth and moon are the same age, about 4.6 billion years. The lunar surface presently differs from earth's in having no atmosphere, no water, no general magnetic field, no active volcanoes. The moon's surface is marked predominantly by two types of physical featu res: lava-flooded basins and mountainous highlands. The lunar terrain is pock-marked by hundreds of thousands of impact craters of all sizes.The face of the far side of the moon differs from that of the near side.
Evolution of the lunar surface. Although the same elements appear on earth and moon, the proportions are different. The original crust was lost when the moon underwent global melting. Intense cratering, occurring later, declined significantly about 3.8 million years ago. Maria formed in the next phase when lava outpourings occurred 3.9-3.2 million years ago.
Internal structure of the moon. Seismic activity is occasional, but of a low order of magnitude. Most volcanic activity ceased about 3 million years ago. The conclusion is that the lunar interior is rigid and lacks sufficient thermal circulation requi red for the dynamic events of earth's interior. The moon mayor may not possess a small iron-rich core.
