Earth is the fifth largest planet in the solar system and the third out from the sun. Its position relative to the sun is such that the overall surface temperature is neither too hot nor too cold for life to exist upon it. Also, its mass and gravity are of the right size to provide a layer of gases, the atmosphere, which screen out harmful solar radiation.
The earth itself is basically a slightly flattened sphere, although satellite observations have shown its shape to be somewhat more complex, with the polar radius being 26 miles (42 kilometers) less than the equatorial radius. It rotates on its axis once every 23.9 hours and _ orbits the sun once every 365 days, 6 hours, 9 minutes, and 9.54 seconds; the axis is tilted by 23.44° to this orbit. The earth’s atmosphere reflects back into space about 25 per cent of the light it receives from the sun, so that to an observer near the sun, the earth would appear as a point of light to magnitude —3.8.
Only 30 per cent of the earth’s surface crust is visible, the rest being covered by liquid water. The rigid crustal layer is composed mainly of oxygen and silicon and is segmented into a number of plates. These are only about 20 miles (32 kilometers) thick, but thousands of square miles in extent. They are in constant motion, “floating” on the underlying mantle. Seismic evidence and the existence of a weak magnetic field around the earth suggests that its center consists of a nickel-iron core.
The atmospheric zones
The earth’s atmosphere is composed of a mixture of gases and can be considered as a series of layers that extend from the surface up to a nominal height of about 1,000 miles (1,600 kilometers). Its overall chemical composition is 78 per cent molecular nitrogen and 21 per cent oxygen, the balance being made up of traces of gases such as carbon dioxide and water vapor. Although vast in extent, the low density of the atmospheric constituents implies that its mass makes up only one-billionth of the earth’s total mass.
The lowest level, in which our weather occurs, is the troposphere, which varies in depth from less than 6 miles (10 kilometers) near the poles to 10 miles (16 kilometers) at the tropics. Above this layer is the stratosphere, which contains the ultraviolet-absorbing ozone layer. The temperature in this layer is about —67° F. ( — 55° C) near the tropopause, at a height of around 6 miles (10 kilometers) above the earth’s surface, but rises to nearly 28° F. ( —2° C) at about 50 miles (80 kilometers) above the surface, in the mesosphere. Above the mesosphere, from an altitude of about 50 miles (80 kilometers), are the rarefied layers of the thermosphere, so called because of the higher temperature of this zone. Ionization takes place at these heights; it is caused by the most energetic photons from the sun, such as X rays, being absorbed by gas molecules and releasing free electrons. This results in the observed rise in temperature. The free electrons in the ionosphere, between about 150 and 450 miles (240 and 720 kilometers) above the earth’s surface, are capable of reflecting radio waves of more than a few meters in wavelength, and this phenomenon permits radio transmissions over large distances on the earth.
The surface and interior of the earth
The surface layer of the earth, the crust, has an average thickness of 20 miles (32 kilometers) over the continental areas. But it is much thinner (only about 5 miles 18 kilometers!) in the regions under the oceans. The crust is composed chiefly of igneous rocks, with an average density of about 190 pounds per cubic foot, which is about 40 per cent less than that of the underlying layer, the mantle. Details of the earth’s interior have been deduced by seismological studies that indicate the existence of a boundary between the crust and mantle known as the Mohorovicic discontinuity. Below this boundary, the density steadily increases to about 300 pounds per cubic foot. Although solid, the mantle is also reasonably plastic. There is a region known as the asthenosphere that may well be fluid enough to transport, by means of convection currents, some of the heat produced within it by radioactive decay. These currents may, in turn, be responsible for the movement of the crustal plates over the surface, in the process called plate tectonics.
The mantle comes to an abrupt end at the Weichert, or Gutenberg, discontinuity, which lies at a depth of 1,800 miles (2,900 kilometers). At this level, the density suddenly increases to 625 pounds per cubic foot, which marks the boundary of the earth’s outer core. Seismic data indicate that it is composed of an extremely dense liquid. Finally, the innermost central core of the earth occurs at a depth of around 3,200 miles (5,150 kilometers) from the surface and is thought to consist of solid nickel and iron with a temperature of around 9000° F. (5000° C).
The earth’s magnetic field
The most likely process responsible for the earth’s weak magnetic field is called the selfexciting dynamo effect It is generally believed that a combination of the earth’s rotation and the convective currents within it provide a means whereby the molten iron outer core can generate electric currents. These produce the observed magnetic field. The magnetic field has, however, shown substantial changes over geological time. Analyses of ocean-floor igneous rocks reveal that the north magnetic pole and the south magnetic pole periodically reverse over a time scale of a few hundred thousand years. On a shorter time scale, in the order of a few centuries, the magnetic poles “wander” slightly and change position. It is postulated that the liquid nature of the outer core may also be responsible for these motions.
The earth’s magnetic field also affects highly charged particles from deep space called cosmic rays. These particles are trapped by the magnetic field, forming two barrel-shaped regions. The boundaries of these belts are not sharp but the inner one extends from about 600 miles (1,000 kilometers) to 3,000 miles (5,000 kilometers) above the earth, and the outer one extends from about 9,300 miles (15,500 kilometers) to about 15,000 miles (25,000 kilometers). They are called the Van Allen belts, after the scientist who discovered them in 1958.
The pressure of the particles from the sun squashes the earth’s magnetic field into a hemispherical cap on the sunward side, trailing it off on the opposite side into a long tail that stretches back beyond the moon’s orbit. This region is known as the magnetosphere, and it is only beyond this protective sheath that interplanetary space begins.