This is a Geotours question. To answer it, please install Google Earth ™ and ope

Question

This is a Geotours question. To answer it, please install Google Earth™ and open the Geotours file. Files and download instructions are available here.

In Google Earth™, open the 2. Exploring Geology Using Geotours > A. Earth & Sky folder. Check and double-click items associated with each problemto travel to the appropriate location with the prescribed perspective/zoom.

Turn on the Scaled Solar System folder by clicking the check box next to it. Double-click the folder icon to zoom out to space to see the solar system scaled from Los Angeles, CA (Sun) to New York, NY (Neptune). The placemarks in the folder contain numerical information about original and/or scaled parameters (e.g., radius, orbital distance, distance between objects).

All of the terrestrial planets scale to reside entirely in California

.All of the terrestrial planets scale to reside entirely outside California

.The distance between previous objects increases for the Sun, Mercury, Venus, and Earth (i.e., distance between objects increases from Sun to Mercury, Mercury to Venus, and Venus to Earth).

Explanation / Answer

A terrestrial planet, telluric planet, or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun, i.e. Mercury, Venus, Earth, and Mars. The terms "terrestrial planet" and "telluric planet" are derived from Latin words for Earth, as these planets are, in terms of structure, "Earth-like". Terrestrial planets have a solid planetary surface, making them substantially different from the larger giant planets, which are composed mostly of some combination of hydrogen, helium, and water existing in various physical states All terrestrial planets in the Solar System have the same basic type of structure, such as a central metallic core, mostly iron, with a surrounding silicate mantle. The Moon is similar, but has a much smaller iron core. Io and Europa are also satellites that have internal structures similar to that of terrestrial planets. Terrestrial planets can have canyons, craters, mountains, volcanoes, and other surface structures, depending on the presence of water and tectonic activity. Terrestrial planets have secondary atmospheres, generated through volcanism or comet impacts, in contrast to the giant planets, whose atmospheres are primary, captured directly from the original solar nebula.

On large scales, the solar system presents us with a sense of orderly motion. The planets move nearly in a plane, on almost concentric (and nearly circular) elliptical paths, in the same direction around the Sun, at steadily increasing orbital intervals. However, the individual properties of the planets themselves are much less regular.A clear distinction can be drawn between the inner and the outer members of our planetary system based on densities and other physical properties. The inner planets—Mercury, Venus, Earth, and Mars—are small, dense, and rocky in composition. The outer worlds—Jupiter, Saturn, Uranus, and Neptune (but not Pluto)—are large, of low density, and gaseous. Because the physical and chemical properties of Mercury, Venus, and Mars are somewhat similar to Earth’s, the four innermost planets are called the terrestrial planets. (The word terrestrial derives from the Latin word terra, meaning “land” or “earth.”) The larger outer planets—Jupiter, Saturn, Uranus, and Neptune—are all similar to one another chemically and physically (and very different from the terrestrial worlds). They are labeled the jovian planets, after Jupiter, the largest member of the group. (The word jovian comes from Jove, another name for the Roman god Jupiter.) The jovian worlds are all much larger than the terrestrial planets, and quite different from them in both composition and structure.

The four terrestrial planets all lie within about 1.5 A.U. of the Sun. All are small and of relatively low mass, and all have generally rocky composition and solid surfaces. Beyond that, however, the similarities end:

All four terrestrial planets have atmospheres, but the atmospheres are about as dissimilar as we could imagine, ranging from a near-vacuum on Mercury to a hot, dense inferno on Venus.

Earth alone has oxygen in its atmosphere and liquid water on its surface.

Surface conditions on the four planets are quite distinct from one another, ranging from barren, heavily cratered terrain on Mercury to widespread volcanic activity on Venus.

Earth and Mars spin at roughly the same rate—one rotation every 24 (Earth) hours—but Mercury and Venus both take months to rotate just once, and Venus rotates in the opposite sense from the others.

Earth and Mars have moons, but Mercury and Venus do not.

Earth and Mercury have measurable magnetic fields, of very different strengths, whereas Venus and Mars have none.

Finding the common threads in the evolution of these four diverse worlds is no simple task! Comparative planetology will be our indispensable guide as we proceed through the coming chapters.

Comparing the average densities of the terrestrial planets allows us to say something about their overall compositions. However, before making the comparison, we must first take into account how the weight of overlying layers compresses the interiors of the planets to different extents. When we do this, we find that the uncompressed densities of the terrestrial worlds—the densities they would have in the absence of any compression due to their own gravity—decrease as we move outward from the Sun: 5300, 4400, 4400, and 3800 kg/m3 for Mercury, Venus, Earth, and Mars, respectively. The amount of compression is greatest for the most massive planets, Earth and Venus, and much less for Mercury and Mars. Based in part on these figures, planetary scientists conclude that Earth and Venus are quite similar in overall composition. Mercury’s higher density implies that it contains a higher proportion of some dense material—most likely nickel or iron. The lower density of Mars probably means that it is deficient in that same material.

Yet for all their differences, the terrestrial worlds still seem very similar when compared with the jovian planets. Perhaps the simplest way to express the major differences between the terrestrial and jovian worlds is to say that the jovian planets are everything the terrestrial planets are not. Table 6.2 compares and contrasts some key properties of these two planetary classes.

The terrestrial worlds lie close together, near the Sun; the jovian worlds are widely spaced through the outer solar system. The terrestrial worlds are small, dense, and rocky; the jovian worlds are large and gaseous, being made up predominantly of hydrogen and helium (the lightest elements), which are rare on the inner planets. The terrestrial worlds have solid surfaces; the jovian worlds have none (their dense atmospheres thicken with depth, eventually merging with their liquid interiors). The terrestrial worlds have weak magnetic fields, if any; the jovian worlds all have strong magnetic fields. The terrestrial worlds have only three moons among them; the jovian worlds have many moons each, no two of them alike and none of them like our own. Furthermore, all the jovian planets have rings, a feature unknown on the terrestrial planets. Finally, all four jovian worlds are thought to contain large, dense “terrestrial” cores some 10 to 15 times the mass of Earth. These cores account for an increasing fraction of each planet’s total mass as we move outward from the Sun.

Beyond the outermost jovian planet, Neptune, lies one more small world, frozen and mysterious. Pluto doesn’t fit well into either planetary category. Indeed, there is ongoing debate among planetary scientists as to whether it should be classified as a planet at all. In both mass and composition, it has much more in common with the icy jovian moons than with any terrestrial or jovian planet. Many astronomers suspect that it may in fact be the largest member of a newly recognized class of solar system objects that reside beyond the jovian worlds (see Chapter 14). In 1999 the International Astronomical Union, which oversees the rules for classifications in astronomy, decided that Pluto should, for now at least, still be called a planet.

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