Comparative Planetology

There are two ways of studying objects in the solar system. One approach is to compare and contrast one feature of all similar objects, such as all of the atmospheres or all of the surfaces of planets, and then move on to the next feature. This method has the advantage of showing the “big picture” for each feature as it arises. The other approach is to take an object and explore all of its properties before moving on to the next object. This method has the advantage of connecting all of the properties of each object directly to each other, so that you do not inadvertently connect, say, the atmosphere of Venus with the surface of Mercury. Because both approaches are valuable, we do both in this book.

5-10 Comparisons among the eight planets show distinct similarities and significant differences

The planets that emerged from the accretion of planetesimals in the young solar system were Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Before exploring the properties of the individual planets in the upcoming chapters, it is instructive to compare a variety of their orbital and physical properties.

Orbits

Table 5-1 lists some orbital characteristics of the eight planets. As shown in Figure 5-10, the four inner planets—Mercury, Venus, Earth, and Mars—are crowded close to the Sun. In contrast, the orbits of the four large outer planets—Jupiter, Saturn, Uranus, and Neptune—are widely spaced at greater distances from the Sun. Kepler’s laws (see Chapter 2) showed us that all of the planets have elliptical orbits. Even so, most of their orbits are nearly circular. The exception is Mercury, whose orbit is noticeably oval.

Size

The planets fall into three size groups (Figure 5-11). The four inner planets form one group, Jupiter and Saturn form the second group, and Uranus and Neptune compose the third, intermediate-size group. Jupiter is the largest planet, with a diameter about 11 times bigger than that of Earth, whereas Mercury, with about a third of Earth’s diameter, is the smallest planet. Indeed, two moons, Ganymede and Titan, are larger than Mercury. Neptune and Uranus are each about 4 times larger in diameter than Earth, the largest of the four inner planets. The diameters of the planets are given in Table 5-2.

Figure 5-11: The Sun and the Planets This figure shows the eight planets drawn to size scale in order of their distances from the Sun (distances are not to scale). The four planets that orbit nearest the Sun (Mercury, Venus, Earth, and Mars) are small and made of rock and metal. The next two planets (Jupiter and Saturn) are large and composed primarily of hydrogen and helium. Uranus and Neptune are intermediate in size and contain roughly equal amounts of ices, hydrogen and helium, and terrestrial material.

Mass

Mass, a measure of the total amount of matter an object contains, is another characteristic that distinguishes the inner planets from the outer planets. Planetary masses are determined using Kepler’s third law or related equations by measuring the periods of moons’ orbits and their distances from the planet. For planets without moons, the deflection by the planets of passing objects, like comets or satellites, provides the information needed to calculate their masses. The four inner planets have small masses compared to the giant outer ones. Again, first place goes to Jupiter, whose mass is 318 times greater than Earth’s (see Table 5-2). Uranus and Neptune, ice giants, have masses intermediate between the terrestrial and the gas giant planets.

Density

Size and mass can be combined in a useful way to provide information about the chemical composition of a planet (or any other object). Matter composed of heavy elements, such as iron or lead, has more particles (protons and neutrons) and hence more mass packed into the same volume than does matter composed of light elements, such as hydrogen, helium, or carbon. Therefore, objects made primarily of heavier elements have a greater average density than objects composed primarily of lighter elements, where average density is given by the equation

The chemical composition (kinds of elements) of any object, therefore, determines its average density—how much mass the object has in a unit of volume. For example, a kilogram of copper takes up less space (is denser) than a kilogram of water, even though both have the same mass (Figure 5-12). Average density is expressed in kilograms per cubic meter. To help us grasp this concept, we often compare the average densities of the planets to the density of something familiar, namely liquid water, which has a density of 1000 kg/m³.

Figure 5-12: The Volumes of Objects with Different Densities All of the objects in this image have the same mass. However, the chemicals from which they form have different densities (mass per volume), so they all take up different amounts of space (volume).

The four inner planets have high average densities compared to water (see Table 5-2). In particular, the average density of Earth is 5520 kg/m³. Because the density of typical surface rock is only about 3000 kg/m³, this high density implies that Earth contains a large amount of material inside it, namely iron and nickel, that is much denser than surface rock. We will explore the terrestrial planets in Chapters 6 and 7.

In sharp contrast, Jupiter, Saturn, Uranus, and Neptune have relatively low average densities (see Table 5-2). Indeed, Saturn’s average density is less than that of water. The low densities of the giant outer planets suggest that they contain significant amounts of the lightest elements, hydrogen and helium, as discussed earlier in this chapter. As we saw in Section 5-3, Jupiter and Saturn share similar compositions, being primarily hydrogen and helium, while Uranus and Neptune both contain vast quantities of water and ammonia, as well as much hydrogen and helium. These four worlds are sometimes called Jovian planets (the Roman god Jupiter was also known as Jove) because, like Jupiter, they all have relatively low densities and high masses compared to Earth. However, the differences in the chemistries of Jupiter and Saturn compared to those of Uranus and Neptune make the name giant planets (giant compared to Earth) more appropriate in collectively describing these four. As noted earlier, Jupiter and Saturn are specifically called gas giants, while Uranus and Neptune are ice giants. We will explore the giant planets as two groups in Chapter 8.

Spectra

Further information about the chemical composition of bodies in the solar system is obtained from their spectra. For solar system objects, spectra are provided primarily by sunlight scattered off the surface or clouds that surround each object. As we saw in Chapter 4, spectra provide us with details of an object’s surface (or atmospheric) chemical composition and rotation rate. From their spectra, we can confirm that the outer layers of the giant planets are primarily hydrogen and helium, and that the surface of Mars is rich in iron oxides, among many other things.

Albedo

The surfaces or the upper cloud layers of the various planets scatter (send in all directions) different amounts of light. The fraction of incoming light returning directly into space is called a body’s albedo. An object that scatters no light has an albedo of 0.0; for example, powdered charcoal has an albedo of nearly 0.0. An object that scatters all of the light that strikes it (a high-quality mirror comes close) has an albedo of 1.0. Multiplying the albedo of an object by 100 gives the percentage of light directly scattered off that body. Three planets (Mercury, Earth, and Mars) have albedos of 0.37 or less. Such albedos result from dark, dry surfaces, or from a mixture of light (water and clouds) and dark surfaces (continents) exposed to space. In contrast, Venus, Jupiter, Saturn, Uranus, and Neptune have albedos of 0.47 or more. High albedos like these imply bright materials exposed to space. The albedos of Venus, Jupiter, Saturn, Uranus, and Neptune are high because these planets are all completely enshrouded by clouds, which are very good reflectors of sunlight.

[img id='dtu10e-ch5-image-35' src='asset/img_ch5/s4.jpg'/] Moons

Every planet, except Mercury and Venus, has moons. At least 173 planetary moons are known to exist in the solar system (up from 99 known in 2001), and more are still being discovered. (Neptune’s 14th moon was discovered just as this book was going to press.) Table 5-3 lists the numbers of moons orbiting each planet. Unlike our Moon, most moons are irregularly shaped, and look more like potatoes than spheres. We will discover that there is as much variety among moons as there is among planets.