The Planets: An Overview

Careful examination of the Table above shows that the planets fall quite nicely into two groups: the terrestrial (Earth-like)planets (Mercury, Venus, Earth, and Mars) and the Jovian (Jupiter-like) planets (Jupiter, Saturn, Uranus, and Neptune). Pluto is not included in either category because its great distance from Earth and its small size make this planet's true nature a mystery.

The most obvious difference between the terrestrial and the Jovian planets is their size. The largest terrestrial planets (Earth and Venus) have diameters only one-quarter as great as the diameter of the smallest Jovian planet (Neptune). Also, their masses are only 1/17 as great as Neptune's. Hence, the Jovian planets are often called giants. Because of their relative locations, the four Jovian planets are referred to as the outer planets, whereas the terrestrial planets are called the inner planets As we shall see, there appears to be a correlation between the positions of these planets and their sizes.

Other dimensions along which the two groups markedly differ include density, chemical makeup, and rate of rotation. The densities of the terrestrial planets average about 5 times the density of water, whereas the Jovian planets have densities that average only 1.5 times that of water. One of the outer planets, Saturn, has a density only 0.7 times that of water, which means that

Saturn would foat if placed in a large enough water tank!

1. The gases, hydrogen and helium, are those with melting points near absolute zero (0 degrees kelvin or -237 degrees C), the lowest possible temperature.
2. the rocks are principally silicate minerals and metallic iron, which have melting points exceeding 700 degrees C.
3. The ices include ammonia (NH3), methane (CH4), carbon dioxide (C02), and water (H20). They have intermediate melting points (for example, H20 has a melting point of 0 degrees C).

The Terrestrial planets are mostly rock. they are dense, rocky, and metallic, with minor amounts of gases. The Jovian planets, on the other hand, contain a large percentage of gases (hydrogen and helium), with varying amounts of ices (mostly water, ammonia, and methan). This accounts for their low densities. (The outer planets may contain as much rocky and metallic material as the terrestrial planets, but this material would be concentrated in their small central cores.)

The Jovian planets have very thick atmospheres of hydrogen, helium, methane, and ammonia. By contrast, the terrestrial planets, including Earth, have meager atmospheres at best. The reason is that a planet's ability to retain an atmosphere depends on its mass and temperature.

Simply stated, a gas molecule can "evaporate" from a plantet if it reaches a speed known as the escape velocity. For Earth, this velocity is 11 kilometers (7 miles) per second. Any material, including a rocket, must reach this speed before it can escape Earth's gravity and go into space.

The Jovian planets, because of their greater surface gravities, have higher escape velocities (21 to 60 kilometers per second) than the terrestrial planets.

Consequently, it is more difficult for gases to "evaporate" from them. Also, because the molecular motion of a gas is termperature-dependent, at the low temperatures of the Jovian planets even the lightest gases are unlikely to acquire the speed needed to escape.

In contrast, a comparatively warm body with a small surface gravity, like our Moon, is unable to hold even heavy gasses, like carbon dioxide and radon, and thus lacks an atmosphere. The slightly larger terrestrial planets of Earth, Vens, and Mars retain some heavy gases such as carbon dioxide, but even their atmospheres make up only an infinitesimally small portion of their total mass.

The primordial cloud of dust and gas from which all the planets are thought to have condensed may have had a composition to Jupiter's. However, unlike Jupiter, the terrestrial planets today are nearly devoid of light gases and ices.