Planetary Science

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PLANETARY SCIENCE

CONCEPT

The term planetary science encompasses a whole range of studies involving a combination of earth sciences and astronomy. Sometimes known as planetology or planetary studies, these disciplines are concerned primarily with the geologic, geophysical, and geochemical properties of other planets. They also draw on areas of astronomy, such as cosmology, a fascinating discipline devoted to the study of the origin, structure, and evolution of the universe. As always when considering realms beyond our Earth, there are many surprises. Indeed, the more one learns about Earth's relationship to the rest of the cosmos, the harder it is to say which is more intriguing: the many factors that make Earth different or the myriad ways that our home planet is just like the rest of the known universe.

HOW IT WORKS

Earth and the Cosmos

Most of us spend our daily lives without devoting a great deal of thought to what lies beyond Earth. People who live outside cities are perhaps more attuned to the cosmos than are their urban counterparts, simply because they see the vast oceans of stars that cover the sky on a clear night. But a person who lives in the city, where bright lights and smog conspire to cover all but the brightest heavenly bodies, rarely finds a reason to look up into the night sky.

One reason people spend little time thinking about the cosmos is that to do so ultimately fills one with a sense of awe bordering on dread. We know that Earth is but one planet of nine, revolving around an average-sized star, the Sun, somewhere between the center and the edge of a galaxy called the Milky Way—itself just one of many galaxies in the universe. This awareness naturally makes a person feel small and almost inevitably raises questions about the nature of the soul, divinity, and the afterlife.

RELIGION, PHILOSOPHY, MYSTICISM, AND SCIENCE.

Such questions are a natural accompaniment to of our feeling that if one person is so truly insignificant in this vast cosmos, there must be something else that gives meaning to the structure of reality. These vast issues, of course, are properly addressed not by science but by theology and philosophy. Science, on the other hand, is concerned simply with the facts of how the universe emerged and how Earth fits into the larger picture.

Yet it is easy to see how ancient peoples would have perceived no distinction between religion and science where the study of the cosmos was concerned. The Babylonians, for instance, had no concept of any difference between scientific astronomy and astrology, which today is recognized as a superstitious and thoroughly unscientific pursuit. The Greeks modeled the cosmos on their philosophical systems, which provided a hierarchy of material forms and an ordered arrangement of causes and substances. And the Judeo-Christian tradition depicts a universe fashioned by a loving, all-powerful creator who designed the human being in his own image.

In the belief systems of Judaism and Christianity, handed down through the Bible, the cosmos is depicted as the setting of a vast spiritual drama centered around the themes of free will, sin, and redemption. The Bible never says that Earth is the center of the physical universe, but it clearly presents it as the center of the spiritual one. This is understandable enough, especially if human beings truly are the only intelligent life-forms; unfortunately, these spiritual ideas eventually informed an erroneous cosmology that depicted Earth as the physical center of the cosmos.

Cosmology

In fairness to Christianity, it should be said that most religious, philosophical, and even scientific traditions before about 1500 depicted Earth as the center of the universe. Indeed, it required a great feat of insight to discern that Earth is not the center. The same is true of many other discoveries about the cosmos, where nothing is as it appears when simply gazing into the night sky.

In a scene from his great novel The Adventures of Huckleberry Finn (1884), Mark Twain aptly illustrated the impossibility of understanding the universe simply on the basis of unaided intellect. Huck and the runaway slave Jim have just finished supper and are lying on their backs and staring up at the stars, speculating as to their origins. One of them comes up with a theory that seems altogether plausible on the face of it: the Moon, because it looks larger than the stars, must have laid them like eggs. A similar scene occurs in the children's movie The Lion King (1994), in which one character postulates that the stars have become stuck to the sky like flies on flypaper. When another character, the warthog Pumbaa, correctly suggests that the stars are actually great balls of burning gas billions of miles away, his companions laugh this off as preposterous.

ARISTARCHUS AND HIPPARCHUS.

Although they lacked telescopes, the Greeks developed rather sophisticated (though in many cases wrong) ideas concerning the arrangement of the cosmos. Most notable among these early thinkers was the astronomer Aristarchus of Samos (ca. 320-ca. 250 b.c.), who proposed that Earth rotates on its axis once every day and revolves with other planets around the Sun. He also correctly suggested that the Sun is larger than Earth.

Unfortunately, the astronomer Hipparchus (146-127 b.c.) rejected this heliocentric, or Sun-centered, cosmology in favor of a geocentric, or Earth-centered, model. Among Hipparchus's later followers was the Alexandrian Ptolemy (ca. a.d. 100-170), destined to become the most influential astronomer of ancient and medieval times, who established geocentric cosmology as a guiding principle of astronomy.

THE PTOLEMAIC SYSTEM.

The influence of Ptolemy's erroneous ideas is partly an accident of history. He lived, as it turned out, in the last great era of civilization: ten years after his death came that of the Roman emperor Marcus Aurelius (a.d. 121-180), whose passing marked the beginning of Rome's decline over the next three centuries. Learning in western Europe virtually ceased until about 1200, and even though the Muslim world produced several thinkers of note during this period, most of them worked within the tradition established by Ptolemy. Muslim thinkers' respect for Ptolemy is reflected in the name that Arab translators gave to his most important writing: al-majisti or "majesty." When this work made its way to Europe, it became known as the Almagest.

The Ptolemaic system proves that it is possible to prove anything, if one creates a methodology elaborate enough. Of course, as we know now, Earth is not the center of the universe, but pure observation alone did not reveal this, and Ptolemy's cosmology worked because he developed mathematics and ideas of planetary motion that made it workable. For instance, not only did planets orbit around Earth in Ptolemy's cosmology, but they also moved in circles around the paths of their own orbits. Of course, they do revolve on their axes, but that was not part of Ptolemy's model. In fact, it is hard to find an analogy in the real world, with the exception of some bizarre amusement park ride, for the form of motion Ptolemy was describing.

He was trying to explain retrograde motion, or the fact that other planets seem to speed up and slow down. Retrograde motion makes perfect sense once one understands that Earth is moving even as the other planets are moving, thus creating the optical illusion that the others are changing speeds. Since the Ptolemaic system depicted a still Earth in the middle of a moving universe, however, the explanation of retrograde motion required mental acrobatics.

CHALLENGING PTOLEMY.

Although it is incorrect, the Ptolemaic system was a creation of genius; otherwise, it could not have survived for as long as it did. Even with the recovery of learning in Europe during the late Middle Ages, scientists continued to uphold Ptolemy's ideas. Instead of discarding his system, or at least calling it into question, astronomers simply adjusted the mathematics and refined their ancient forebear's physical model to account for any anomalies.

The revolution against Ptolemy began quietly enough in the fifteenth century, when the Austrian astronomer and mathematician Georg Purbach (1423-1461) noted the inaccuracies of existing astronomical tables and the need for better translations of Greek texts. Purbach attempted to produce a revised and corrected version of the Almagest, but he died before completing it. The job fell to his student, Johann Müller, who was known as Regiomontanus (1436-1476).

The Epitome of the Almagest (1463), begun by Purbach and completed by Regiomontanus, proved to be a turning point in astronomy. Like their medieval predecessors, the two men started out working in the Ptolemaic tradition, but by showing the errors in Ptolemy's work, they actually were criticizing him. Their discoveries were not lost on a young Polish astronomer named Nicolaus Copernicus (1473-1543).

THE COPERNICAN REVOLUTION.

The story of the Copernican Revolution, the opening chapter in a larger movement known as the Scientific Revolution, is among the greatest sagas in the history of thought. It was a watershed event, marking the birth of modern science as such, but the change in thought patterns created by this revolution was not so much the work of Copernicus as it was of the Italian astronomer Galileo Galilei (1564-1642). Although he often is given less attention than Copernicus and the other most noted figure of the Scientific Revolution, the English natural philosopher Isaac Newton (1642-1727), Galileo was a thinker of the first order who took Copernicus's discoveries much further.

Copernicus had been concerned with how the planets move as they do, and in the course of his work he showed that all of them (Earth included) move around the Sun. Galileo, on the other hand, set out to discover why the planets revolve around the Sun, and in so doing he discovered the principles of inertia and gravitational acceleration that would influence Newton. He made numerous other contributions, such as the discovery that Jupiter had moons, but by far his greatest gift to science was his introduction of the scientific method.

Thanks to Galileo and others who later refined the method, thinkers would no longer be content to let mere conjecture guide their work. Before his time, scientists generally had followed a pattern of absorbing the received wisdom of the ancients and then seeking evidence that confirmed those suppositions. The new scientific method, on the other hand, required rigorous work: detailed observation, the formation of hypotheses, testing of hypotheses, formation of theories, testing of theories, formation of laws, testing of laws—and always more observation and testing.

REAL-LIFE APPLICATIONS

Gravity, the Sun, and Earth

A fourth key figure in the Scientific Revolution was the German astronomer Johannes Kepler (1571-1630), whose laws of planetary motion directly influenced Newton's laws of gravitation and motion. Thanks to Kepler, we know that planets do not make circular orbits around the Sun; rather, those orbits are elliptical. As Newton later showed, the reason for this is the gravitational pull exerted by the Sun.

Gravitational force explains why Earth, the Sun, and all celestial bodies larger than asteroids are round—but also why they cannot be perfectly round. As to the latter issue—the fact that Earth bulges near the equator—it is a consequence of its motion around its axis. Because it is spinning rapidly, the mass of the planet's interior responds to the centripetal (inward) force of its motion, producing a centrifugal, or outward, component. If Earth were standing still, it would be much nearer to the shape of a sphere.

MASS AND SPHERICITY.

Now to the larger question: Why is Earth round? The answer is that the gravitational pull of its interior forces a planetary body to assume a more or less uniform shape. Furthermore, the larger the mass of an object, the greater its tendency toward roundness. Earth's surface has a relatively small vertical differential: between the lowest point and the highest point is just 12.28 mi. (19.6 km), which is not a great distance, considering that Earth's radius is about 4,000 mi. (6,400 km).

An object of less mass is more likely to retain a shape that is less than spherical. This can be shown by reference to the Martian moons Phobos and Deimos, both of which are oblong, and both of which are tiny, in terms of size and mass, compared with Earth's Moon. Mars itself has a radius half that of Earth, yet its mass is only about 10% of Earth's, and therefore it is capable of retaining a less perfectly spherical shape.

There is also the possibility of more pronounced differences in elevation, and thus it should not be surprising to learn that Mars is also home to the tallest mountain in the solar system. Standing 15 mi. (24 km) high, the volcano Olympus Mons is not only much taller than Earth's tallest peak, Mount Everest (29,028 ft., or 8,848 m), it is also 22% taller than the distance from the top of Mount Everest to the lowest spot on Earth, the Mariana Trench in the Pacific Ocean (-36,198 ft., or-10,911 m).

Why Earth Is Special

With regard to gravitation, a spherical object behaves as though its mass were concentrated near its center. Indeed, 33% of Earth's mass is at is core (as opposed to the crust or mantle), even though the core accounts for only about 20% of the planet's volume. Geologists believe that the composition of Earth's core must be molten iron, which creates the planet's vast electromagnetic field.

Certain particulars of Earth's core lead us to answering another great question about our home planet: Why is it alone capable of sustaining life—as far as we can tell—while the other planets of our solar system are either hellish worlds of fire or frigid, forbidding realms of ice crystals and liquefied gas?

DENSITY OF EARTH'S INTERIOR.

At first glance, Earth seems to have few distinctions other than its ability to support life: it is neither the largest nor the smallest planet in the solar system, positions held by Jupiter and Pluto, respectively. (Earth ranks fifth.) Earth has a moon, but that is hardly a distinction: Saturn has 18 moons. And not only does Olympus Mons tower over Everest, but the gaseous oceans of Jupiter also are much deeper than the Mariana Trench. In the lists of planetary superlatives, Earth has only one: it is the most dense.

The only bodies that come close are Mercury and Venus, which along with Earth and Mars are designated as terrestrial planets. (Earth's Moon often is considered along with the terrestrial planets because its composition is similar to them and because it is a relatively large satellite.) The terrestrial planets are small, rocky, and dense; have relatively small amounts of gaseous elements; and are composed primarily of metals and silicates. This is in contrast to the Jovian planets, which are large, low in density, and composed primarily of gases. (The Jovian planets usually are designated as the four giants Jupiter, Saturn, Uranus, and Neptune. Pluto, the smallest of all nine planets, has a density higher than any Jovian planet.)

Density is simply the ratio of mass to volume, meaning that Earth packs more mass into a given volume than any other body in the solar system. Saturn, least dense among the planets, has a mass 95.16 times as great as that of Earth, yet its volume is 764 times greater, meaning that its density is only about 12% of Earth's. But whereas Saturn and other Jovian planets are composed primarily of gases surrounding small, dense cores, Earth—beneath its atmospheric layer and it waters—is a hard little ball. Its core, composed of iron, nickel, and traces of other elements, including uranium, is relatively heavy.

That gives it a strong gravitational pull and, in combination with the comparatively high speed of the planet's rotation, causes Earth to have a powerful magnetic field. It is also important to note the significance of planetary mass in making possible the formation of an atmosphere. Because of their mass, larger planetary bodies exert enough gravitational pull to retain gases around their surfaces; by contrast, the Moon and Mercury are too small and have no atmosphere. Of course, Earth is the only planet whose atmosphere is capable of sustaining life as we know it, and this is a result of activity beneath the planet's surface.

A VOLATILE PLANET.

Earth is the only terrestrial planet on which the processes of plate tectonics, or the shifting of plates beneath the planetary surface, take place. The other terrestrial planets have crusts of fairly uniform thickness, suggesting that they have never experienced the internal shifting that has helped give our planet its unique topography. Earth also has a relatively thin lithosphere—the upper layer of the planetary surface, including the crust and the brittle portion at the top of the mantle—which helps make it a particularly volatile body.

Of the terrestrial planets, the only ones still given to volcanic activity are Earth and Venus. Mars seems to have experienced volcanic activity at some point in the past billion years, while Mercury and the Moon have not had volcanoes for several billion years. This is also an important factor in determining Earth's capacity to support living things, because volcanoes—which transport gases from the planet's interior to its atmosphere—have been crucial to the creation of the conditions necessary for sustaining life.

The heat generated by internal volatility is also a component influencing the sustainability of life on Earth. At the time Earth and other planets were formed, some 4.5 billion years ago, the planets experienced such heat that they melted, causing a separation of chemical compounds. The heavier compounds, mostly containing iron, sank to the core of the planet, where they remain today, while the lighter ones rose to the surface. Included in these lighter substances were oxygen and other elements essential to the sustenance of life. Even now Earth and Venus, because of their volcanic activity, are cooling at rates slower than those of the other planets, and this has facilitated the separation of elements.

THE CREATION AND SUSTENANCE OF LIFE.

Aside from the distinctive features of its core, Earth's position relative to the Sun has helped make it possible for life to take root on this planet. For decades scientists believed that Earth is unique in possessing that life-sustaining compound of hydrogen and oxygen, H₂O or water; but now we know that even Jupiter—not to mention Venus and Mars—have water on their surfaces. The problem is that Venus's water is too hot, existing as vapor in the upper atmosphere, while the water on Mars and Jupiter takes the form of ice crystals. Earth is uniquely placed to sustain liquid water.

The existence of liquid water made it possible for the first microorganisms to form on Earth, leading over hundreds of millions of years to the development of the complex biosphere known today. The existence of life in simple forms promoted the development of the atmosphere and geosphere, because these life-forms took in carbon dioxide and water, processed them, and returned them to the environment as oxygen and organic materials.

The Solar System and Beyond

The reader may have noticed that earlier in this essay, we ceased discussing progress in cosmology after about 1650. This is not because nothing happened after that time; on the contrary, the centuries that have elapsed since then have seen the greatest progress in astronomical study since the dawn of civilization. To give this topic the coverage it warrants, however, would require a lengthy discussion—one that would take us away from the earth sciences and toward the sister science of astronomy.

Up until the Scientific Revolution, the earth sciences hardly existed, except inasmuch as various people over the millennia had recorded data concerning Earth and made sometimes unscientific speculations regarding its origin and composition. As the oldest of the physical sciences, astronomy was much more mature, but even it could progress only so far under the restrictions of the Ptolemaic system. Unfettered, it began to progress rapidly, and the result has been an unfolding vision of the universe that is at once more clear and more complex.

THE SIZE OF THE UNIVERSE.

One of the dominant themes in astronomy from Galileo's time to the present day is astronomers' quite literally expanding vision of the universe. Up until 1781, when the German-born English astronomer William Herschel (1738-1822) discovered Uranus, scientists had known only of the five other planets visible to the naked eye, all of which had been discovered in prehistoric times. (Neptune was discovered in 1846 and Pluto not until 1930.)

In the seventeenth century, astronomers still regarded what we call the solar system as the entire universe, but Herschel was instrumental in ascertaining that Earth is part of a bright band ofstars called the Milky Way. Just as Earth once hadbeen believed to be the center of the "universe," or solar system, astronomers then came tobelieve it was at the center of the Milky Way. Onlysince 1920 has it been known that our solar system is, in fact, somewhere between the center and the edge of the vast galaxy. Even the Milky Way, composed of several hundred billion stars and about 120,000 light-years in diameter, is not the entire universe; it is only one of many hundreds of galaxies or "island universes."

As discussed at the beginning of this essay, such a scale is almost too much for the human mind to comprehend, particularly inasmuch as Earth is the only planet known to sustain intelligent life. As the British science-fiction writer Arthur C. Clarke (1917-) has observed, either there are other intelligent life-forms out there in the universe, or there are not—and either possibility is mind-boggling.

THE BIG BANG AND THE SOLAR SYSTEM.

Not only has astronomers' understanding of the universe expanded, along with their idea of its size; it also appears that the universe itself is expanding. Today the most widely accepted model regarding the formation of the universe is the big bang theory, first put forward by the Belgian astrophysicist Georges Édouard Lemaître (1894-1966) in 1927. According to this theory, an explosion 10-20 billion years ago resulted in the rapid creation of all matter in the universe, and that matter is continuing to move outward, expanding the frontiers of the universe.

Our own solar system appears to be about five billion years old, meaning that the Sun is a relatively young star. It seems that the future solar system was just one of many great balls of gas, rotating as they moved outward, that were scattered around the universe as a result of the big bang. Just as these balls of gas exploded from the center, the material of the various stars emerged from the center of the ball that became our solar system.

FORMATION OF THE PLANETS.

The proto-solar system we have described here was a great rotating cloud, and though it has long since ceased to be a cloud, it continues to rotate—only now it is in the form of planets turning around a sun at the center. The hottest portion of the cloud, at the center, became the Sun, while cooler portions at the fringes became planets. The Sun itself is composed primarily of hydrogen and helium, the two most plentiful elements in the universe. In the extraordinarily high temperatures on the Sun, atoms of hydrogen (which has one proton in its nucleus) experience nuclear fusion, becoming atoms of helium, which has two protons. It appears that continued fusion resulted in the creation of the heavier elements (for instance, nitrogen, carbon, oxygen, and silicon) of which the planets—in particular, our own—are composed.

Earth's elemental makeup is discussed elsewhere in this book, as is the structure of its interior. So, too, is the Sun's effect on Earth. These matters are not unrelated. In studying the solar system and the planets that make it up, one is confronted again and again with the fact that a planet's destiny is governed by its position relative to the Sun. Ultimately, the planets in our solar system are ruled by the same principle that drives the sale of real estate: location, location, location!

This is true not only of the atmosphere and temperature of planets but also of their relative density. It is no mistake that the terrestrial planets are closer to the Sun: their internal composition is as it is because these bodies became the destination of most of the heavier elements that emanated from it. Many of the lighter elements continued to move outward, where they gathered around rocky centers to become the mostly gaseous Jovian planets.

WHERE TO LEARN MORE

Astronomy and Cosmology (Web site). <http://www.netlabs.net/hp/tremor/>.

Beatty, J. Kelly. The New Solar System. New York: Sky, 1999.

Cambridge Cosmology (Web site). <http://www.damtp.cam.ac.uk/user/gr/public/cos_home.html>.

Canup, Robin M., and Kevin Righter. Origin of the Earth and Moon. Tucson: University of Arizona Press, 2000.

The Cosmology: Explore the Largest Mystery (Web site). <http://library.thinkquest.org/28181/>.

Gallant, Roy A. Earth's Place in Space. New York: Benchmark Books, 1999.

Lambert, David, and Martin Redfern. The Kingfisher Book of the Universe. New York: Kingfisher, 2001.

Llamas Ruiz, Andrés. The Origin of the Universe. Illus. Luis Rizo. New York: Sterling Publishers, 1997.

Ned Wright's Cosmology Tutorial (Web site). <http://www.astro.ucla.edu/~wright/cosmolog.htm>.

Skinner, Brian J., Stephen C. Porter, and Daniel B. Botkin. The Blue Planet: An Introduction to Earth System Science. 2d ed. New York: John Wiley and Sons, 1999.

KEY TERMS

ATMOSPHERE:

In general, an atmosphere is a blanket of gases surrounding a planet. Unless otherwise identified, however, the term refers to the atmosphere of Earth, which consists of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon, which together comprise 0.07%.

BIOSPHERE:

A combination of all living things on Earth—plants, mammals, birds, reptiles, amphibians, marine life, insects, viruses, single-cell organisms, and so on—as well as all formerly living things that have not yet decomposed. Typically, after decomposing, a formerly living organism becomes part of the geosphere.

CENTRIFUGAL:

A term describing the tendency of objects in uniform circular motion to move outward, away from the center of the circle. Though the term centrifugal force often is used, it is inertia, rather than force, that causes the object to move outward.

CENTRIPETAL FORCE:

The force that causes an object in uniform circular motion to move inward, toward the center of the circle.

COMPOUND:

A substance made up of atoms of more than one element chemically bonded to one another.

COSMOLOGY:

The study of the origin, structure, and evolution of the universe.

COSMOS:

The universe.

ELECTROMAGNETIC ENERGY:

A form of energy with electric and magnetic components that travels in waves and which, depending on the frequency and energy level, can take the form of long-wave and short-wave radio; microwaves; infrared, visible, and ultraviolet light; xrays, and gamma rays.

ELEMENT:

A substance made up of only one kind of atom. Unlike compounds, elements cannot be chemically broken into other substances.

GEOCENTRIC:

Earth-centered.

GEOCHEMISTRY:

A branch of the earth sciences, combining aspects of geology and chemistry, that is concerned with the chemical properties and processes of Earth.

GEOLOGY:

The study of the solid earth, in particular, its rocks, minerals, fossils, and land formations.

GEOPHYSICS:

A branch of the earth sciences that combines aspects of geology and physics. Geophysics addresses the planet's physical processes as well as its magnetic and electric properties and the means by which energy is transmitted through its interior.

GEOSPHERE:

The upper part of Earth's continental crust, or that portion of the solid earth on which human beings live and which provides them with most of their food and natural resources.

HELIOCENTRIC:

Sun-centered.

HYDROSPHERE:

The entirety of Earth's water, excluding water vapor in the atmosphere but including all oceans, lakes, streams, groundwater, snow, and ice.

HYPOTHESIS:

An unproven statement regarding an observed phenomenon.

INERTIA:

The tendency of an object in motion to remain in motion and of an object at rest to remain at rest.

JOVIAN PLANETS:

The planets between Mars (the last terrestrial planet) and Pluto, all of which are large, low indensity, and composed primarily of gases.

LAW:

A scientific principle that is shown always to be the case and for which no exceptions are deemed possible.

LITHOSPHERE:

The upper layer of Earth's interior, including the crust and the brittle portion at the top of the mantle.

MANTLE:

The layer, approximately 1,429 mi. (2,300 km) thick, between Earth's crust and its core.

MASS:

A measure of inertia, indicating the resistance of an object to a change in itsmotion. (By contrast, weight—which people tend to think of as similar to mass—is a measure of gravitational force, or mass multiplied by the acceleration due to gravity.)

PLANETARY SCIENCE:

The branch of the earth sciences, sometimes known as planetology or planetary studies, that focuses on the study of other planetary bodies. This discipline, or set of disciplines, is concerned with the geologic, geophysical, and geochemical properties of other planets but also draws on aspects of astronomy, such as cosmology.

PROTON:

A positively charged particle in the nucleus of an atom.

SCIENTIFIC METHOD:

A set of principles and procedures for systematic study that includes observation; the formation of hypotheses, theories, and laws; and continual testing and reexamination.

SCIENTIFIC REVOLUTION:

A period of accelerated scientific discovery that completely reshaped the world. Usuallydated from about 1550 to 1700, the Scientific Revolution saw the origination of the scientific method and the introduction of such ideas as the heliocentric (Sun-centered) universe and gravity.

TERRESTRIAL PLANETS:

The four inner planets of the solar system: Mercury, Venus, Earth, and Mars. These planets are all small, rocky, and dense; have relatively modest amounts of gaseous elements; and are composed primarily of metals and silicates. Compare with Jovian planets.