Chapter 15: Comparison of the Planets
This has been an extraordinary period of exploration. We have landed on the Moon and sampled its rocks. We have mapped the surface of Mars, tested its soil for evidence of life, and explored its huge canyons and grand volcanoes. We have surveyed the diverse landscapes of the moons of Jupiter and Saturn and have discovered that small, icy satellites can have complex histories of impact, volcanism, and tectonic deformation. We have photographed the exotic surfaces of the moons of Uranus and Neptune, which present some new and exciting insights into the origin of planetary bodies. We have begun to acquire detailed information about rocky asteroids and icy comets. In short, we discovered new worlds and looked back at Earth from space with greater understanding of how our planet functions and why it is unique. Every object in the solar system contains part of a record of planetary origin and evolution. These new worlds are important to us on Earth because they tell us a great deal about the phenomena and forces that shape and control planetary environments and vividly show how things on our own planet might have been different. They also show how things might develop in the future. By understanding the history of the planets and the reasons for their diversity, we have greatly increased our understanding of Earth, our home.
1. The solar system consists of the Sun and its family of planetary bodies. They were formed at the same time and from the same nebula and, although each body is unique, they all have much in common.
2. The great variety of surface features, compositions, and internal structures of the planets was produced and modified by geologic processes that can be understood as the results of the interaction of planetary matter and energy.
3. Geologic processes that acted on almost all planetary bodies of the solar system include impact cratering, internal differentiation, volcanism, and tectonism. Physical and chemical interactions with atmospheres and other surface fluids were important on some planets and moons.
4. The terrestrial planets (Mercury, Venus, Earth, the Moon, and Mars) are composed mostly of rocky materials surrounding metallic cores. Although they probably started out much the same with impact-dominated surfaces, they evolved along different paths because of differences in size, composition, internal thermal energy, and distance from the Sun.
5. The satellites of the outer planets and Pluto are composed mostly of ice surrounding cores of rock or mixtures of rock and ice. Most have changed very little since they originated, but a few have had sufficient internal energy to develop crustal deformation and volcanism after the period of heavy bombardment. The giant outer planets are large bodies of gas and ice.
The exploration of the solar system reveals a family of planets of great diversity, which we have attempted to summarize graphically in Figures 15.1, 15.2, and 15.3. The Moon and Mercury are small, rocky bodies with ancient surfaces marked by an abundance of impact craters and smooth plains only slightly wrinkled by tectonic processes. Mars has a variety of landforms including giant volcanoes, huge canyons, and large terrains eroded by catastrophic floods of water. Earth has continents and ocean basins, and its very young surface is continually washed with an ocean of water, modified by volcanic activity, and deformed by a mobile lithosphere. Venus is also a dynamic planet with rifts, volcanoes, "continental" highlands, and eolian processes, but it has no liquid water. Even the small, mostly icy moons of the outer solar system present evidence for a vast array of geologic phenomena. Io has volcanoes erupting sulfur. Titan has a hazy secondary atmosphere of nitrogen that hides hydrocarbon seas. There are the complex, grooved terrains of Ganymede, the surprising rifts and plains of Enceladus, the dark terrain of Iapetus, the coronae of Miranda, and the ancient cratered surfaces of Callisto, Tethys, Oberon, and others. The giant outer planets are radically different from the other bodies of the solar system in that they lack solid surfaces and consist mostly of hydrogen and helium inherited from the ancient solar nebula---the cocoon from which all of the planets emerged 4.6 billion years ago. Delicate rings of small particles have formed around the giant planets. Tiny methane-frosted Pluto, has more in common with the moons of the outer solar system than with the other outer planets.
The surface features of the planets and moons in the solar system are shown here at the same scale. The dominant landform of the planets is the impact crater. Many of these craters are ancient and date from a period of heavy bombardment. Impact-related fractures cross the surfaces of some of the smallest bodies. Volcanic plains and shields are apparent on some planets, even at this small scale. Tectonic modifications represent the response of the lithosphere to stresses derived from the thermal or tidal evolution of the planet. Large domes and rifts are apparent on several bodies.
The internal structures of the planets and moons are dominated by concentric layers of diverse compositions and mechanical properties. The inner planets and Io probably have dense cores of iron metal and thick mantles and crusts of silicates. In contrast, the other moons of the outer planets and Pluto may have cores of silicates surrounded by mantles of water ice. Although internal differentiation was an important result of accretionary heating in many planets, moons, and asteroids, some small icy satellites of Saturn, Uranus, and Neptune may not be differentiated. The interiors of these small objects may consist of more-or-less homogeneous mixtures of ice and silicate rock.
The compositions of the surfaces of the moons and planets are emphasized in this diagram showing their relative brightness or albedo. The inner planets, their moons, and the asteroids are composed of dark, rocky materials. The satellites of the outer planets have surfaces that are either bright and dominated by water ice or dark and dominated by methane ice. Ices of nitrogen, and liquid and solid hydrocarbons may be important on a few of these bodies.
Aside from the differences apparent on their surfaces, there are also dramatic variations in density, internal structure, and albedo (Figures 15.2 and 15.3) among the planets and their satellites. This diversity reflects systematic variations in their compositions. The albedo, or brightness, is caused by the composition of the surface materials; water ice is bright, and silicates and methane ices are dark. The density reflects the proportions of dense silicates, moderate-density ices, and light gases in the interiors of the planets.
Here, then, is a system of planets that at first seems bewildering in its variety. Indeed, a grand theme revealed by the exploration of the planets is their great diversity. Yet all of the planetary bodies were formed in the same little corner of the universe, at the same time, and from the same materials. Why are they so different? Why have some planets remained geologically active for 4.6 billion years, whereas others have remained essentially unchanged since they formed? Why do some have atmospheres, oceans, and continents, whereas others are giant spheres of gas or tiny balls of ice? Why is the balance of matter and energy just right for life to evolve on Earth and so hostile on others? Are there a few fundamental laws that govern planetary development? Our purpose in this closing chapter is to reemphasize a few basic processes (Table 15.1) that can explain the diversity of the various planetary bodies as we review some differences and similarities among them.
Answers to some questions posed above may be found by analyzing the type and amount of material that makes up a planet, and the nature and magnitude of energy available at its surface or in its interior. One of the general principles to emerge from this venture into comparative planetology is that the science of geology is an exploration of the interaction of planetary matter (in its various forms) and energy (in its various forms).
Planetary matter can be described by its chemical composition, mass, and size. These properties of matter help determine at which temperature the material melts or freezes, how strong it is, how it transfers heat, and how quickly it cools. Moreover, a planet's composition determines its abundance of long- and short-lived radioactive elements, which affects how much internal energy it possesses.
The chemical compositions and sizes of the planets are not random, but are strongly controlled by their distances from the Sun. As we saw in Chapter 2, our solar system was spawned 4.6 billion years ago in a cold, diffuse cloud of gas and dust deep within a spiral arm of the Milky Way galaxy. The huge cloud was made up largely of the two lightest elements, hydrogen and helium, and only small concentrations of the other elements. Under the force of gravity, the giant cloud collapsed and assumed the shape of a rotating disk. Solid material within the disk was segregated according to composition. In the center of the disk, near the hot proto-Sun, only relatively refractory compounds, such as silicates and iron, condensed from the gaseous nebula to form solid particles. The outer part of the cloud was naturally colder, so that other, more voluminous substances such as water, ammonia, methane, and so forth also solidified. Thus, early in the history of the solar system, there was a separation and differentiation of solid matter that exerted a profound control on the nature of the future planets. Denser particles that condensed at high temperatures were the only solids in the central region near the Sun; whereas low-temperature, low-density ices dominated the condensates near the fringes of the nebular disk. The planets formed by collisional accretion of the particles that were present at a particular distance from the Sun. The silicates and iron in the inner solar system accreted to form the inner planets. The ices in the outer part of the nebula eventually formed the cores of the giant worlds---Jupiter, Saturn, Uranus, and Neptune. Because of the large size of their icy cores, huge masses of nebular gas, mostly hydrogen, were gravitationally attracted to these planets. In contrast to the planets of the inner solar system, subsequent T-Tauri winds were unable to strip this gas away from the giant planets; the features we see on these planets are all in their primary atmospheres. Therefore, the outer planets are larger, but less dense compared to the rocky inner planets. In addition, most satellites of the outer planets have large proportions, of water ice, with smaller amounts of silicate and metal. Whereas the inner planets have, at best, discontinuous caps of polar ice or shallow oceans of molten water on spheres dominated by silicates and metals. This same sequence of icy low-density to rocky high-density bodies is seen in the four Galilean satellites (Figure 15.2), which are thought to have formed in a temperature gradient focused on Jupiter. Even the rings of icy particles that encircle these outer planets are apparently remnants of icy fragments that did not accrete to form sizable moons, or the result of recent impact fragmentation of small satellites.
Thus, although all of the planets in the solar system formed from the same solar nebula, the differentiation of the nebula caused the composition of the inner and outer planetary bodies to be distinctly different. The trend in the compositions of the planets supports the fundamental idea that condensation in a thermally zoned nebula resulted in the dichotomy between the rocky inner planets and the icy outer planets and their satellites.
After the planets were formed, their subsequent histories were controlled largely by the amount and type of energy operating on the planet. Without energy there can be no change. Three major forms of energy are important in this context: (1) internal heat (including that derived from radioactivity, differentiation, and tidal friction), (2) impact of meteorites and comets, and (3) thermal energy from the Sun. In a planet, as energy interacts with matter, a new rock body is formed and a landform or structure is produced. For example, when internal heat causes melting and volcanic eruption of lava or ash, new rock bodies (lava flows and plutons) and new surface features (volcanoes) are created. Similarly, when a meteorite collides with a planet, its kinetic energy produces an ejecta blanket (a new rock body) and a crater (a new landform). Tectonic deformation of the crust also produces new rock bodies and terrain types (rifts and mountain belts). In addition, solar energy drives the hydrologic systems found on several planets, allowing water to erode new landscapes and deposit sediments to form new layers of rock.
The amount and type of energy that drive geologic processes on and within planets varies greatly from planet to planet, and has varied throughout the history of each planet. For example, the thermal energy of a planet and the geologic activity it produces are very sensitive to the planet's size. Most planets formed relatively hot because of accretion. From this initial state, small planetary bodies tended to cool rapidly because of their large surface areas compared to their masses. Cooling may be moderated by the production of heat from differentiation and from radioactivity. Nonetheless, small planets, such as the rocky Moon or icy Mimas, cooled rapidly and had short thermal histories. As a result, ancient surface features are well preserved on these bodies because, soon after formation, their internal heat was rapidly radiated to space. Larger bodies retain their internal heat longer and, as a result, have prolonged periods of volcanism and crustal deformation that obliterated ancient cratered terrains.
But other factors also affect a planet's thermal evolution. If the missions to the outer planets had not revealed the tumultuous volcanic eruptions of Io or the smooth plains of Enceladus, we might not have considered tidal heating, caused by the effects of a distinctive orbital environment, to be important to the thermal history of a planet. We have also learned that a planet's accretion history (slow and cool versus fast and hot) is important in a planet's thermal history.
The relationship of composition, size, and energy in a planet's evolutionary history can be shown in a three-dimensional diagram (Figure 15.4). The vertical axis represents the composition of a planetary body and ranges from silicates at the base to icy materials at the top. The present thermal state, shown along the back of the diagram, ranges from cold and inactive planetary bodies to those that are still hot and experiencing tectonic deformation and volcanism. The size of the planet is depicted along the remaining axis with small bodies at the back of the diagram and large ones at the front. Within the diagram are the positions of various planetary bodies that have solid surfaces. These three parameters describe the most important aspects of a planet's available energy and the matter acted upon.
The size, composition, and thermal evolution of a planetary object are its fundamental characteristics and determine the nature of its surface and interior. The three-dimensional chart shown here uses the vertical axis to represent the composition of the planetary body and ranges from rocky silicates at the bottom of the cube to icy objects at the top. The present thermal state, shown along the back of the cube, ranges from cold and inactive bodies to moons and planets that are still hot and experiencing volcanism. The size of the planet is shown on the remaining axis with small bodies at the back of the cube and large ones at the front. In terms of these three factors, various planets and moons are shown within the cube. In general, larger bodies are more thermally evolved and show signs of recent volcanic activity (Earth and Venus), whereas smaller bodies have ancient surfaces unmodified by recent volcanism (Mimas, Oberon). The correlation between size and evolution implies that these bodies derive their internal heat energy from the decay of radioactive elements and that their rates of heat loss are controlled by the surface area to mass ratio. However, several moons of the outer solar system diverge strongly from this trend, showing signs of a high degree of thermal evolution in spite of their small sizes (Io and Enceladus). These bodies appear to have a size-independent source of energy related to tidal heating.
In general, larger bodies are more thermally evolved and show signs of recent tectonic and volcanic activity (Earth and Venus), whereas smaller bodies have ancient surfaces unmodified by recent tectonism or volcanism (Mimas, Oberon). For example, contrast the history of Mars with the other inner planets. Mars has a diameter about half that of Earth or Venus, but it is almost half again larger than Mercury, and about twice as large as the Moon. It has thus retained its internal heat longer and has been more tectonically and volcanically active than the Moon and Mercury, but less than Earth and Venus. It is believed that Mars started much like Venus and Earth, with a cratered surface, an atmosphere, and active volcanism. However, because of its smaller size and more rapid loss of heat, the volcanic and tectonic activity of Mars was limited by comparison with Earth. Large lithospheric domes and rifts developed over mantle plumes, but lithospheric plates did not develop, shift, and recycle as on Earth.
The correlation between size and evolution of the planets implies that the rates of heat loss on many planets and satellites are controlled by their surface area to mass ratios. Consequently, an important source of heat must be the decay of radioactive elements---a quantity related to the mass and total composition of the planet. However, several moons of the outer solar system diverge strongly from this trend of increasing tectonic evolution and size. Io, Europa, and Enceladus show signs of a high degree of thermal evolution in spite of their small sizes. These bodies appear to have a source of energy independent of size---a source of energy related to tidal interaction in their respective satellite systems and the creation of heat by friction as they are gravitationally flexed along their slightly elliptical orbital paths.
In brief, size, composition, and energy content control a planet's thermal evolution, including the rate of volcanism, the extent of atmospheric degassing, the intensity of internal differentiation, and the rate at which the lithosphere thickens. Lithospheric thickness, density, and strength control volcanic and tectonic processes on a planet, how and when rifting may occur, if mountain belts or volcanoes can form, and if recycling of the lithosphere back into the mantle is possible. As a result, many geologic differences among the planets can be explained by (1) different amounts of internal energy, as expressed by the rate of heating or cooling, and (2) different kinds of planetary materials, including silicates, ices, liquid water, and atmospheric gases. These differences drove the fascinating histories of the moons and planets of the solar system along unique paths.
With these basic ideas about planetary matter and energy in mind, let us consider the important geologic processes that have shaped the planets---impact cratering, internal differentiation, volcanism, tectonism, and the flow of surface fluids. The characteristics of each geologic process, and the landforms and rocks they create, depend on the nature of the energy and of the material that this energy acts upon.
Cratering by meteorite or comet impact has been one of the most pervasive geologic process in the solar system. Indeed, the planets grew to their present sizes by collisional accretion from smaller bodies. Most of the planetary bodies we have studied retain an imprint of an early episode of intense bombardment, which declined rapidly during the first several hundred million years of the solar system's history (4.6 to 3.9 billion years ago). Mercury and the Moon are excellent examples and remain as fossils of this early stage in planetary development. They provide a valuable record of this first chapter in the history of the solar system. Most of the small, icy bodies in the outer solar system are also dominated by impact craters. Besides this similarity, a tremendous diversity in crater shapes and sizes is apparent.
Impact cratering is the result of the transfer of a projectile's kinetic energy to a planet's surface. As energy is transferred, a crater is formed whose features depend not only on the amount of energy the projectile had but also on the composition and nature of the surface materials. As a result, impact craters do not all look the same; large craters are distinctly different from small craters in their general geometry. Small craters are bowl-like with smooth walls and floors; larger, complex craters develop terraced walls and central peaks; and still larger basins develop multiple concentric rings. The morphologic changes that accompany changes in crater size are reflections of the amount of energy expended by a projectile as it collides with a planet's surface---energy that is proportional to the mass and velocity of the falling projectile.
Changes in crater features are also affected by the manner in which the target materials respond to the passage of the shock wave and subsequent relaxation to normal temperatures and pressures. Thus, a projectile with a small size or low velocity excavates a simple depression, scattering the ejected debris around the crater rim. At higher energies, a deeper crater is excavated, which enhances the likelihood for crustal rebound to form central peaks or rings. Simultaneously, failure occurs at the crater rim to form terraces and outer rings. The diameter of the crater at which these features appear on different planets varies with the size or strength of the gravitational field of the body (Figure 15.5). Craters up to 100 km in diameter on Amalthea and 30 to 40 km on Mimas do not have central peaks; on Earth, in contrast, the appearance of central peaks may occur at diameters as small as 3 to 5 km. Apparently, the process of rebound is accentuated on planets with large sizes and gravitational forces.
The occurrence of central peaks in impact craters depends on the size of the planet or moon, and also the energy of the impact. Even small craters on the relatively large Earth develop central peaks, whereas the largest craters seen on Amalthea lack central peaks.
In other details as well, the response to impact cratering stems from the physical and chemical characteristics of a planet's surface. For example, the production of large volumes of impact melt is facilitated on icy bodies with their low temperatures of melting, and is hindered on silicate crusts because of their higher melting temperatures. Groundwater also greatly affects crater morphology. As we saw in Chapter 6, the water-rich regolith on Mars creates ejecta that flows like mud, rather than arching away in ballistic trajectories. In addition, the longer-term response of a surface is affected by the planet's chemical and physical properties, especially its strength or viscosity, which are measures of a material's ability to flow. Simple bowl-shaped craters on icy satellites of the outer planets are systematically 20 to 40% shallower than on the rocky terrestrial planets. Apparently, the rebound of crater floors is easier in ice than silicate rock. The low-strength icy lithosphere of Ganymede, for example, allowed crater excavations to become almost totally erased by viscous flow of the ice. Stronger silicate lithospheres will support larger craters without relaxing, but (as we suggested for the early history of Mercury or present-day Venus) when silicate lithospheres are warm and weak, they too may respond in a similar viscous fashion.
The major role of large impacts in the evolution of the planets has become appreciated only recently. Several important planetary features believed to be the result of giant impacts serve to illustrate this idea. For example, the high density of Mercury may be the result of a giant impact that stripped away parts of the outer silicate layers of the already differentiated planet, leaving it enriched in the dense iron that had drained to form the core. A late impact on Venus with a Mars-sized object may have slowed its spin and reversed its rotation direction compared to that of all other planets. Moreover, without large collisions, all of the planets should have spin axes that are perpendicular to the ecliptic. A large collision may, therefore, be required to explain the tilt of both Earth and Mars. The origin of the Moon may be rooted in the collision of a Mars-sized object with Earth. Fragments of this collision later reaccreted in Earth orbit to form the Moon. The global dichotomy on Mars can be traced back to a giant, impact basin in the northern hemisphere. The shapes of the asteroids are the result of fragmentation during large impacts. Perhaps, the small, icy satellites of Saturn and Uranus were fragmented several times only to reaccrete later. Moreover, the rings that encircle the outer planets may be created by the collisional fragmentation of small, icy moons. At the very least, several satellites sustained massive impacts that created global fracture systems and large craters. In addition, planetary collisions with large bodies in the outer solar system may have tipped Uranus on its side and fragmented Pluto to form a double-planet system. Collisions with large bodies late in the accretion histories of the planets can be understood by remembering that just as the planets grew larger by accretion, so did the impacting planetesimals. Therefore, as the planets approached their final sizes, they were probably struck by larger and larger planetesimals.
Impact cratering has been important over the entire 4.6-billion-year history of the solar system. But has cratering influenced life on Earth? Many scientists think so. An impact that occurred 65 million years ago, at the end of the Cretaceous Period, may have caused a mass extinction of many forms of Earth life, including dinosaurs. Others suggest that throughout much of geologic time, periodic extinctions have been caused by impacts on Earth. Even today the human species is not isolated from its cosmic environment and may be vulnerable to the catastrophic effects of large impact events.
Planetary differentiation is another fundamental geologic process. The interiors of all of the planets and most of their satellites are layered as the result of thermally and gravitationally driven separations of elements with distinctive chemical affinities. The magnitude and significance of this geologic process are dramatically displayed in the many variations of composition and internal structure found in the planets. Figure 15.2 shows this diversity and emphasizes other basic themes as well.
The planets, larger satellites, and even many asteroids became differentiated during their very early histories. The most important source of thermal energy was probably accretionary heating, caused by the conversion of kinetic energy of a meteorite or comet to thermal energy stored in the planet. The extent of differentiation as a result of accretionary heating is critically dependent on how quickly a planet accretes. Planets that accrete quickly, store the heat in the growing mass; those that accrete slowly, radiate the impact-created heat before it can be buried by subsequent additions of matter. Other important heat sources may have been the decay of short-lived radioactive isotopes and energy released by differentiation itself. Nonetheless, small bodies, like the icy moons of Saturn, Uranus, and Neptune, may not have acquired enough energy during accretion for complete internal differentiation. Most of the moons in the outer solar system are made largely of water ice with smaller amounts of silicate and metal; the interiors of these bodies may consist of icy rinds surrounding undifferentiated mixtures of rock and ice. In Figure 15.2, the cores shown at the center of the moons of the outer planets represent only one possible configuration and are shown to emphasize the proportions, not exact distributions, of rock and ice in the smaller bodies.
Moreover, heat, derived from accretion and from the decay of radioactive elements in the rocky fractions of the small icy satellites, was rapidly transmitted to their surfaces and radiated to space. Consequently, most of these moons lacked sufficient internal energy to generate substantial tectonic systems that could change their surfaces. Instead they retain densely cratered surfaces, like those of Mercury and the Moon, and record only ancient events because of their rapid thermal evolutions. Callisto, the outermost Galilean moon of Jupiter, may be considered typical of this class of satellites. Although each of the moons of the outer solar system has a distinctive geologic history and surface features, many can be grouped with Callisto because their rapid thermal evolutions soon terminated their internal dynamics. Their surfaces have changed little since the period of intense bombardment. This class of satellites includes Mimas, Tethys, Dione, Rhea, and Iapetus (moons of Saturn), Oberon, Titania, Umbriel, and Ariel (moons of Uranus), Proteus and Nereid (moons of Neptune).
Most of the solid planets and their moons experienced volcanism during their histories. Volcanic landforms can be as dramatic as the giant volcanoes on Mars, the large ash-flow calderas of Earth and Io, or as subtle as the smooth plains of Enceladus that apparently lack features related to vents. In addition, the composition of the volcanic materials varies from planet to planet depending on the planet's composition. Eruptions may consist of lavas of molten silicates, such as those on Earth, Venus, Mars, the Moon, some asteroids, and probably Mercury and Io; or they may be volcanic flows of molten water mixed with other volatiles, such as those on Ganymede, Europa, Ariel, Triton, and many others; or they may even be exotic lavas made of molten sulfur as on Io. In spite of these variations, an important generalization is that volcanic activity is a sensitive indicator of the mechanism of heat loss and the thermal state of the interior of a planet. Thus, when the rate of volcanism is compared with the age of a planet, it usually shows the progressive cooling of the planet. Moreover, most small bodies have short histories of volcanism.
Mercury and the Moon, for example, were too small to retain much internal energy. They heated initially and became differentiated with metal cores, and rocky mantles and crusts. Oceans of magma may have entirely enveloped these bodies for a short time. Internal heat also produced short episodes of volcanic activity, during which lava was extruded and covered some early formed, densely cratered terrain. The Moon's lithosphere thickened rapidly during its early history; presently it is 1000 km thick---10 times thicker than Earth's. This thick rigid shell makes it nearly impossible for molten lava to reach the surface and prohibits lateral movements like those that produce continental drift on Earth. Mercury is similar to the Moon in many ways. A period of extensive volcanism is recorded on Mercury's surface but, like the Moon, volcanic activity stopped early in its history. A rigid lithosphere must have developed well before the end of the period of intense bombardment. Thus, on Mercury and the Moon there has been little or no geologic activity after the volcanic event that produced floods of lava several billion years ago.
In the outer solar system, Io and Europa are notable exceptions to these generalities about small planetary bodies in that each has a prolonged volcanic history resulting from a unique source of internal energy. Io, the innermost moon of Jupiter, is a small body that is tremendously hot on the inside but frigid on the outside. Ordinarily, because of its small size, it would be expected to be a cold, dead world like the Moon, but internal heat is generated in Io because of the tidal actions of Jupiter and its large satellites. As a result, Io is the most volcanically active body in the solar system and has a very young surface continually being resurfaced by volcanism. Water and other volatiles lighter than sulfur, which were extruded during its prolonged volcanic history, have escaped into space because Io's small gravitational field could not hold them. Sulfur, on the other hand, is too heavy to escape Io's gravitational field and has been concentrated on the surface. Many scientists believe that sulfur functions on Io much like water does on Earth---melting and erupting like geysers. Deeper sources may produce molten silicate lava flows. The volcanic history of Io, created by geologic processes unknown before the space program, is dominated by its distinctive energy source and the distinctive composition of its surface materials.
Europa is smaller and farther from Jupiter than Io. Europa's almost perfectly smooth icy surface is marred with sets of tan streaks, which are similar to fracture systems in sea ice in the polar regions of Earth. Europa's internal heat produced magmas of slushy water that extruded through fissures in the crust and coated the surface with fresh ice. The process must be similar to fissure eruptions of silicate lava on the Moon, Mercury, Mars, and Earth; but on Europa the lava is water. The near absence of impact craters on Europa indicates that the surface is very young, and the resurfacing processes of water eruptions have continued up to quite recent times. Like Io, much of Europa's internal energy is derived by tidal flexing of the planet.
Not only are volcanic histories different from planet to planet, but the volcanoes themselves are strikingly different. The various types of volcanoes reflect the style of eruption, volume of magma erupted, composition of the magma, and the characteristics of the path taken by the magma to the surface. The style of eruption varies from quiet eruptions of lava (the small eruptions of martian low shields) to violent gas-driven explosions that devastate large areas (the ash-flow calderas of Earth's continents). The volume of magma in a single eruption or in a group of related eruptions also helps to shape a volcanoes' features and, just as importantly, reflects the amount of energy inside a planet. Individual eruptions range from fractions of a cubic kilometer to huge eruptions that may pour thousands of cubic kilometers of magma across a planet's surface in just a few days. Small cinder cones, low lava shields, great shield volcanoes with collapse calderas, sheets of flood lavas, and huge ash-flow shields result partly from variations in the supply of magma. Magma composition controls the amount of volatiles and the way the magma flows or fragments. Eruptions of water, like those that occurred on the icy outer satellites, produce vast thin sheets; silicate magmas may form similar large sheets, but also may pile up around their vents to produce cinder cones, shield volcanoes, or other types of edifices. Finally, the way that magma rises to the surface also affects the final appearance of a volcano. A central, long-lived conduit may produce a large shield volcano, like Olympus Mons on Mars, or a stratovolcano, like Mt. St. Helens on Earth; whereas magma sources spread over a large area create fields of small isolated volcanoes with short lives, like those on Earth's Snake River Plain.
Fundamental controls on volcanic activity then are (1) a source of heat to partially melt the interior of a planet, (2) the composition of the magma source, (3) a pathway to the surface, and (4) the nature of the vent and its environment. In the simplest cases, volcanic processes are driven by thermal energy that, if it increases locally, causes partial melting of rock or ice. The amount of liquid depends on the amount of heat and the composition of the rock. If the liquid produced is less dense than its surroundings, then it can rise to the surface and erupt. If it is more dense, as happens with liquid iron in a silicate planet, it may sink toward the core of the planet. Thus, volcanic processes are part of the overall differentiation of a planet.
The large scale deformation that produces the structure or architecture of a planet's lithosphere is the result of tectonism. Here again, internal heat is the driving force, but the response (e.g., flowing, buckling, folding, fracturing) of the crust depends on its strength. The strength of crustal rocks, in turn, depends mostly on their composition and temperature. Gravitational potential energy is also important in shaping some tectonic features (e.g., folded mountains of Venus and crater palimpsests on Ganymede).
As we have emphasized, the tectonic history of a planet is strongly related to its thermal history. Planetary lithospheres that are cool, rigid, and thick do not yield to tectonic stresses because of their strength. In contrast, warm-and-ductile or thin-and-brittle lithospheres are weak, and they fold or fracture when stresses are sufficient. Perhaps the simplest forms of tectonics are those of global extent that result from planetary heating and expansion, or cooling and contraction. The global pattern of thrust-fault systems on Mercury is an example. The Moon is another; it cooled rapidly to develop a thick lithosphere. As it cooled, it contracted and its surface was deformed as a result. Subsequently, the lithosphere has not deformed for almost 3 billion years. In this way, the Moon is similar to many other small planets composed of ices or silicates. The only apparent tectonism, not related to impact, is the result of planetary or local expansion or contraction that occurred long ago. In contrast, the surfaces of Earth, Venus, Io, Europa, Ganymede, Enceladus, and Triton have been dramatically reshaped by tectonic processes.
Ganymede is an example of the tectonically more complex bodies; it has a thick outer shell of water ice, but its surface is unique in that it consists of a baffling array of structural features unlike any in the solar system. Many features appear to result from breaking and lateral movement of crustal fragments, a system similar in some respects to the tectonic plates on Earth. This has produced two distinct terrain types. The older is dark and is nearly saturated with craters, but it has been fractured and split apart and many fragments have shifted about. Found between these fragments is the younger terrain, brighter, smoother, and probably created by eruptions of water. It is criss-crossed by a series of complex grooves and stripes, features that result from deformation and cracking of an icy crust.
Enceladus, a tiny, icy moon of Saturn, also has a complex, grooved terrain similar in some ways to the terrain on Ganymede, showing unexpected tectonism on such a small icy body. It is clear from the large tracts of very smooth, crater-free plains that tectonism on Enceladus must be very young. Some source of heat is necessary to soften the interior of Enceladus and cause the volcanic activity that resurfaced the satellite to create the smooth plains. What type of energy activates the surface of this tiny ball of ice? Can tidal heating be sufficient to warm it and produce crustal deformation and volcanic activity?
The ages of rocks and surfaces on the inner planets show variations that reflect the amount of internal energy available to drive geologic activity such as volcanism and tectonism. Most small bodies, like asteroids, have very old ages because they lost their heat shortly after they formed. Their geologic features are dominated by impact craters formed during the intense bombardment that tailed off about 3.8 billion years ago. Larger planets like Venus and Earth, lack surfaces and rocks that are ancient. Most of their surfaces are young and post-date the period of intense bombardment.
A convenient measure of the duration and intensity of active tectonism on a planet's surface is the proportion of the surface that is not heavily cratered. All of the planets experienced an early bombardment by meteoritic projectiles, but only those with sufficient internal energy have been resurfaced by destruction and burial of the cratered terrains (Figure 15.6). For example, some small, icy moons of Jupiter, Saturn, Uranus, and Neptune (e.g., Callisto, Rhea, Mimas, and Umbriel) and the asteroids have surfaces unmarked by tectonic or volcanic processes younger than the early intense bombardment. Other planetary bodies, both silicate and icy, such as Mercury, the Moon, Tethys, and Dione, have surfaces dominated by heavily cratered terrain with only small areas buried by younger volcanic flows, cut by rifts, or folded to form ridges. Mars, Ganymede, and Enceladus have lost about 50 percent of their ancient cratered terrain to tectonic processes and have an intermediate status. The planets with the most sustained thermal evolution---Earth, Venus, Io, and Europa---have been completely resurfaced many times and retain no heavily cratered regions.
The Role of Surface Fluids
The geologic processes related to the movement of fluids on the surface of a planet can completely resurface a planet many times. These processes derive their energy from the Sun and the gravitational forces of the planet itself. As these fluids interact with surface materials, they move particles about or react chemically with them to modify or produce new materials. The energy, the nature of the fluid, and the composition and environment (temperature and pressure) of the surface materials determine the patterns of modification.
On solid planets with atmospheres and hydrospheres only tiny fractions of their masses flow as surface fluids. Yet the movements of these fluids have dramatically altered their appearances. To emphasize the importance of surface fluids, let us consider Venus, Earth, and Mars---the terrestrial planets that have atmospheres.
Venus and Earth are commonly considered twins, but not identical twins. They are about the same size, composed of roughly the same mix of materials, and may have been comparably endowed with carbon dioxide and water. However, the twins evolved differently, largely because of their distance from the Sun and the resultant differences in solar energy that they receive. With a significant amount of internal heat, Venus may continue to be geologically active with volcanoes, rifting, and folding. However, it lacks any sign of a hydrologic system; there are no streams, lakes, oceans, or glaciers. Space probes suggest that Venus may have started with as much water as Earth, but was unable to keep its water in liquid form. Receiving more heat from the Sun, water, outgassed from the interior, evaporated and rose to the upper atmosphere where the Sun's ultraviolet rays broke the molecules apart. Much of the freed hydrogen escaped into space and Venus lost its water. Without water, Venus became less and less like Earth and kept an atmosphere filled with carbon dioxide. The carbon dioxide acts as a blanket creating an intense greenhouse effect and driving surface temperatures high enough to melt lead and to prohibit the formation of carbonate minerals. Volcanoes continually vented more carbon dioxide into the atmosphere. On Earth, liquid water removes carbon dioxide from the atmosphere and combines it with calcium, from rock weathering, to form carbonate sedimentary rocks. Without liquid water to remove carbon from the atmosphere, the level of carbon dioxide in the atmosphere of Venus remains high. During the early years of the solar system, temperatures at the surface of Venus were only slightly warmer than those on Earth. Can a few tens of degrees make such a difference? We must understand the answer to this question in order not to stress the environment on Earth beyond its limits. Moreover, what role does water play in the internal dynamics of a planet? Is the apparent lack of continents and plate tectonics on Venus a result of its water-poor interior and high surface temperatures?
Like Venus, Earth is large enough to be tectonically active and for its gravitational field to hold an atmosphere. Unlike Venus, it is just the right distance from the Sun so that temperature ranges allow water to exist as a liquid, a solid, and a gas. Water is thus extremely mobile and moves rapidly over the planet in a continuous hydrologic cycle. Heated by the Sun, the water moves in great cycles from the oceans to the atmosphere, and over the landscape in river systems, and ultimately back to the oceans. As a result, Earth's surface has been continually changed and eroded into delicate systems of river valleys---a remarkable contrast to other planetary bodies where impact craters dominate. Few areas on Earth have been untouched by flowing water. As a result, river valleys are the dominant feature of its landscape. Similarly, wind action has scoured fine particles away from large areas, depositing them elsewhere in sheets of loess or as vast sand seas dominated by dunes. These fluid movements are caused by gravity flow systems energized by heat from the Sun. Other geologic changes occur when the gases in the atmosphere or water react with rocks at the surface to form new chemical compounds with different properties. An important example of this process was the removal of most of Earth's carbon dioxide from its atmosphere to form waterlaid carbonate rocks that now form extensive layers covering the stable platforms or folded into mountain belts. However, if Earth were a little closer to the Sun, our oceans would evaporate; if it were farther from the Sun, the oceans would freeze solid. Because liquid water was present, self-replicating molecules of carbon, hydrogen, and oxygen developed life early in Earth's history and have radically modified its surface, blanketing huge parts of the continents with greenery. Life thrives on this planet and helped create its oxygen- and nitrogen-rich atmosphere and moderate temperatures. Indeed, if alien scientists were to study Earth with a passing spacecraft, the composition of its unique atmosphere would be one of the most important evidences of the existence of life.
Although Mars has only a thin atmosphere, the effects of moving surface fluids are abundant. Distinctive wind-formed streaks and plumes cross its surface. A vast sea of sand dunes encircles its north polar cap. Sheets of loess may be common. The surface of Mars reveals river channels, large and small, produced by flowing water. These channels seem to require that water participated in some type of hydrologic cycle over a long period. Mars may have developed a dense atmosphere very early in its history and possessed liquid water and a moderate climate warmed by a carbon dioxide greenhouse. Rainfall and flooding may have produced significant erosion at this time. But Mars is smaller than Earth, has less internal heat, and may not have outgassed as much water or carbon dioxide. In any case, eventually atmospheric pressure and temperature dropped and the water froze; Mars was left cold and dry. Today, great dust storms rage on Mars altering its surface and atmospheric temperatures. Earth's atmosphere acts not only as a window for sunlight, but as a blanket for heat. Unlike carbon dioxide, which thickens the blanket and creates a warming greenhouse, dust thrown into the atmosphere closes the window, blocking solar energy from reaching the surface below. Mars, therefore, remains cold with all its water locked up in its ice caps or frozen in the pore spaces of its rocks and soil. Important questions persist about the evolution of the fluid envelope around Mars. Under what conditions did liquid water flow in small rivulets or great floods across the surface? Did life evolve when surface water was more abundant? What role do dust storms play in atmospheric conditions and surface temperatures of the planet?
A comparison of the terrestrial planets has awakened our appreciation for the susceptibility of a planetary surface to large environmental changes. Perhaps, if nothing more, our studies of the diversity of compositions and conditions of solar system bodies should remind us of the delicate balance of energy and evolution that allows us to exist at all.
Extreme examples of fluid- or gas-rich planets are the giant outer planets---Jupiter, Saturn, Uranus, and Neptune---which present only the tops of their thick colorful atmospheres to the view of telescopic observers and passing spacecraft. At the other end of the spectrum lie most of the solar system's planetary bodies that lack atmospheres and the consequent movement of fluids. None of the planetary satellites of the inner solar system, the asteroids, or Mercury have significant atmospheres. There are at least three reasons why these bodies lack atmospheres. Some were poor in volatile elements to begin with (Mercury and the Moon); others were too small to hang on to released gas (some asteroids); and still others were too small to differentiate and release volatiles from their interiors (Phobos, Deimos, and some asteroids). Without atmospheres and hydrospheres, their surfaces are modified only by impact today.
Small planetary bodies in the outer solar system also lack atmospheres and surface features resulting from the flow of fluids. (Here we specifically exclude water magmas because of their high temperatures compared with the surface temperature.) The absence of fluids on the surfaces of these planets is not because they are poor in volatiles, or that they are too small to have become differentiated. They are quite simply too cold. Water is an important constituent on these bodies, but was released to their surfaces and frozen to form icy shells. The energy from the Sun that reaches these bodies is insufficient to maintain water in its molten, erosive state; in a solid form, water ice behaves like rock. Another complicating factor is the generally small size of these bodies and their resultant inability to gravitationally retain light volatile substances like hydrogen, helium, or nitrogen gas. Obvious exceptions are Titan with its nitrogen atmosphere and a liquid-gas methane cycle. The spectacular images returned by Cassini show a landscape carved by flowing water, a multitude of shallow seas, and vast expanses of wind-blown dunes. All this is possible because Titan has just the right temperature and pressure for a different compound to be a liquid--methane. Methane only condensed at very low temperatures and so it is not common in or on the moons of Jupiter.
By studying other planetary bodies we gain a greater understanding of our own Earth. Its size and composition are just right for the development of a tectonic system that even today, 4.6 billion years after Earth formed, recycles the lithosphere, creates continents and ocean basins, and concentrates ores and minerals. Earth appears to have accreted some solids that condensed at low enough temperatures to contain significant amounts of water. During its differentiation, some of this water was released from the solids and accumulated at the surface. Earth's gravitational field was strong enough to hold this water and an atmosphere, which was also extruded from its interior. Earth is just the right distance from the Sun so that water at the surface can exist as solid, liquid, and vapor and can move in a hydrologic cycle. If Earth were a little closer to the Sun, the oceans would evaporate; if farther from the Sun, they would freeze solid. Because liquid water is present, huge volumes of carbon dioxide were removed from Earth's atmosphere and concentrated in layers of rock. As a result, greenhouse warming of Earth is only moderate compared to the inferno present on Venus.
Another result of Earth's composition and distance from the Sun was the evolution of life. Life, and especially human life, is not a passive presence on the planet; life modifies and drives a variety of geologic processes on Earth. For example, the evolution of plants enriched the atmosphere in free oxygen and allowed a radiation-filtering ozone layer to form in the upper atmosphere. Billions of years later, humans are upsetting this delicate balance by infusing the atmosphere with carbon dioxide created by burning fossil fuels and risking greenhouse warming. Moreover, we have created complex compounds that are destroying the protective ozone layer. The studies of other planets have taught us that Earth is a small place, an oasis in space, a home that we are still trying to understand. By exploring the worlds in our solar neighborhood, we are beginning to understand how Earth works, and why it is unique, why we have an atmosphere, moderate climates, continents, and ocean basins, and why we alone have life. Will the intelligence that allowed us to explore the planets and begin to understand them give us the ability to live within the limits of our own natural system?
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