Morphology of the Rocky Members of the Solar System

In the vastness of space, nine planets, 27 moons having diameters larger than 100 km, and thousands of smaller bodies orbit around a star that is not much different from many other stars in the Universe. The third planet from the Sun appears quite blue from space as a result of its extensive hydrosphere, an envelope that made possible the creation of life. Streaks of white depict the circulation of its atmospheric cover. Since inception, its biosphere has deeply influenced the Earth’s development, so much so that it can be conceived as being the prime agent of the Earth’s morphological evolution. The geohistory of Earth includes the effects of another agent — massive lateral tectonic transport of crustal plates induced by convection circulation within the planet’s interior — a tectonic regime that has led to changes in its surface morphology, its climate, and even its biosphere as the latter adapted to ever-changing habitats. In this inquiry we look at Earth’s rocky companions in the Solar System to determine how similar or different was their development from that of the Earth, then we attempt to ascertain the causes of these differences. Such a goal has been made possible by recent observations of the planets via unmanned satellites and actual landings by man and vehicles; these observations have yielded many publications during the past two decades. As our own geological field experience has been restricted to our planet, we have made extensive use of these publications.

In the broadest sense, we consider that the morphologies of rocky members of the Solar System (Table 1, listed in order of distance from their host) can be grouped into three categories: endogenic, exogenic, and exotic. Endogenic provinces are ones that were produced by internal forces that caused plate movements and interplate and intraplate magmatic/tectonic activities. Exogenic provinces are ones that owe their origin to external processes such as weathering, erosion, transportation, and deposition from earlier rock surfaces; therefore, they require the presence of a hydrosphere (or other liquid) and an atmosphere (Table 2, listed in order of decreasing diameter). Exotic provinces are ones created by bombardment of the planets or satellites by planetesimals and comets; this bombardment has produced large regions of terrae (impact craters), planitias (flat plains of debris ejected from impact sites), and impact melts (country rock melted by the kinetic energy of impact on rocky planets and moons).

Accretion of the Earth and its companions from chondritic planetesimals appears to have been completed by about 4.55 Ga, 100 Ma after formation of the solar nebula, and according to Condie (1989, p. 29) 1 Ma before the Sun went through a T-Tauri stage and the resulting strong solar winds blew away the most volatile elements from the inner Solar System and the early atmospheres of the inner planets. In the three-dimensional model described by Boss (1990) compressional heating during nebula formation caused the gross depletion of volatiles on Earth relative to their solar abundances. The inner nebula may have experienced temperatures of 1500 K that were regulated by vaporization of iron grains. Remote sensing data indicate that planetesimals of the inner Asteroid Belt underwent post-accretionary heating during the first few million years of Solar System history, a heating that led to extensive melting and magmatic differentiation (Gaffey 1990). This heating, which may have been caused by electrical induction during the T-Tauri stage or by short-lived radioisotope activity, also may have affected the planetesimals in the inner Solar System whose accretion produced the rocky planets. Such planetesimal differentiation must have influenced the accretionary and post-accretionary geohistory of the rocky planets. Accretions from volatile-depleted planetesimals differentiated into metallic cores and silicate mantles and allowed the Earth’s core to form during accretion (Taylor and Norman 1990). Some investigators suggested that formation of the Earth may have been homogeneous, involving simultaneous condensation and accretion of compounds from a hot nebula as it cooled (Condie 1989, pp. 26–29). Others proposed an inhomogeneous model with an iron core and with the silicate mantle condensing sequentially, and still others believed that giant impacts influenced the evolution of the Earth, including the ejection of a primitive atmosphere, a source for melting the Earth, and a component for the mantle and core (Ahrens 1990; Melosh 1990; Newson and Sims 1991). Ahrens and O’Keefe (1989) also stated that possibly the core settled out at the same time that accretion occurred; thus there were some deviations from homogeneity during accretion of the planet. In a scenerio described by Benz and Cameron (1990), impact of the Earth by a giant planetesimal caused iron from the core of the impactor to penetrate the mantle and settle atop the Earth’s core where it was heated to several ten thousand degrees. Most of the mantle of the impactor and a considerable volume of the Earth’s mantle were ejected, but later some fell back onto the proto-earth. There was some heating of all parts of the interior of the proto-earth, and the surface layers reached temperatures of 16 000 K and were vaporized. It is this intense surface heating coupled with the presence of an orbiting disk of rock vapors and magmas that Benz and Cameron (1990) believed caused ejection of the early terrestrial atmosphere.

The nearest extraterrestrial body to Earth is the Moon (only 356 410 to 406 697 km distant center-to-center, or about 30 Earth diameters). Earth’s Moon, with its diameter of 3476 km (27% that of the Earth’s diameter), is rather unique in that it has an unusually large ratio of satellite mass to planet mass, 0.12. Its mean density is much lower than that of Earth, 3.34 versus 5.52 g/cm³. The higher density on Earth results from the large amount of iron and nickel concentrated in its core. Siderophile elements appear to be deficient in the Moon, volatile elements also are depleted, but the refractory elements are much richer than on Earth. This distribution indicates that the Moon was very hot during the past and its volatile elements were vaporized and escaped into space. Five models have been proposed for the origin of the Moon. They are: (1) fission from Earth, (2) collision ejection due to impact of a Mars-size planetesimal with Earth, (3) disintegration by tidal forces of a planetesimal that entered Earth’s Roche Limit; the resulting debris formed an orbit around the Earth and served as the source for accumulation of the Moon, (4) coaccretion from planetesimals, and (5) capture (Brush 1986; Wood 1986).

The innermost planet of the solar system, Mercury, was known to the Sumerians about 2500 BC, and to the Egyptians it was known as Sebegu (Strouhal, 1992, p. 240). Its speed in orbit (revolution of 88 days, rotation of 59 days, diameter of 4879 km; Table 1) and difficulty of observation (maximum angle of only 27°45′ from the Sun) may have been the reason for attributes to Hermes (Greek) and Mercury (Roman) with winged sandals and hat and identification as a fleet messenger for Zeus (Greek) and a patron of business (Roman). In the Middle Ages astrologers assigned to those persons under the influence of the planet a lively or mercurial temperament, and alchemists named the metal mercury (quicksilver) after it. Details of Mercury’s topography could not be examined from the Earth by telescope or radar because of its small size, great distance, and angular nearness to the Sun. In fact, the most detailed maps of Mercury made prior to about 1970 (Murray et al., 1972) bear no resemblance to the truth. Only in 1965 was even its period of rotation found to be exactly two-thirds of its period of revolution about the Sun. The only spacecraft to visit it has been Mariner 10 — in three passes during orbits around the Sun (29 March 1974, 21 September 1974, and 24 March 1975) after which contact was lost. During these three encounters, only the same half of Mercury was sunlit with the rest in darkness (Moore 1990), so that we know the general morphology of less than half of the planet but have insufficient information to draw contours of elevation (Fig. 57). Names assigned by NASA to most of the topographic features, especially to craters and their adjacent ejecta aprons and the plains, were intended to commemorate authors and composers even though few, if any, of them are known to have had any interest in the topography of Mercury or of any other planet or moon.

Venus, the second planet from the Sun, is the brightest object in the Solar System after the Sun and Moon. Because of its prominence in the evening sky after sunset and its brightness at dawn Venus also is known as the Evening Star and the Morning Star. Venus (Latin) also was known as Aphrodite (Greek), the Goddess of Love. The planet was known as Astarte to the Phoenicians, as Ishtar to the Babylonians, and as Tai-pe (the Beautiful One) to the Chinese. Observations of the planet go back as far as 1900 B.C. when they were recorded by the Babylonians on the cuniform Venus Tablets (Saunders and Carr 1984). With the aid of a telescope Galileo determined in A.D. 1610 that Venus had lunar-like phases, lending support to the Copernican Sun-centered concept of the Solar System (Greeley 1985, p. 132). Some early observers using a telescope reported that Venus was featureless, but others later noted that the planet displayed bright patches. These patches were used to determine the length of a venusian day. In 1897 Lowell even published a map of canals that he believed he saw on Venus (Saunders and Carr 1984). Another feature noted by Father Johannes Riccoli during the seventeenth century was the Ashen Light. This faint phosphorescence of the night hemisphere when Venus appears as a thin crescent is believed to be due to extensive twilight or to electrical phenomena. Early observers were particularly interested in timing the transit of Venus across the Sun; the most famous attempt probably is the one made by Captain James Cook of HMS Endeavour, and he was given the responsibility for making these measurements by the Royal Society. The observations were made on 3 June 1769 from Tahiti; Bligh, future captain of HMS Bounty, was a member of Cook’s crew. Speculations regarding a possible atmosphere on Venus go as far back as 1796 (Schroter in Saunders and Carr 1984). During the midnineteenth century astronomers noted that Venus displayed a dark halo when silhouetted against the Sun, an observation that led Lomonosov to infer that the planet had an atmosphere. Visual and photographic spectroscopic data obtained during the third decade of the twentieth century led to the conclusion that the principal component of the venusian atmosphere is carbon dioxide. Infrared measurements by Kuiper (1962) showed nearly 40 carbon dioxide absorption bands, and 300 times the concentration of the gas in the venusian atmosphere than in the Earth’s atmosphere; thus, Venus is in a stage of experiencing an intense greenhouse effect.

The planet Mars was well recognized in very ancient times because of its brightness and red color. About 1000 B.C. it was named Nergal by the Babylonians after their god of death and pestilence, and Horus the Red by the Egyptians. Later, the Greeks named it Ares for their god of battle (who supported the Trojans), and the Romans renamed it Mars for their god of war (thus the term, martial). The symbol for Mars is a shield and spear. Mars’ two moons, discovered in 1877, are known appropriately as Phobos (Fear) and Deimos (Panic) from the two horses of Mars’ warchariot. Perhaps because of their similar brightness, Mars and Venus are associated in mythology with Harmony, born of the union of strife and love; similarly, Ares and Aphrodite became the parents of Eros (Cupid). Mars has a mean equatorial diameter of 6794 km (0.51 times that of the Earth), a mass of 6.418 x 10²⁶ g, a mean density of 3.93 g/cm³ (Earth has a density of 5.52 g/cm³), an elliptical orbit of 780 Earth days, a daily rotation rate of 24 h and 37 min, and perihelion and aphelion distances from the Sun of 1.381 and 1.666 AU, respectively (Table 1; Carr 1984). Mars’ present obliquity of 25.1° may have changed during the past because of waxing and waning of polar ice caps (Rubincam 1992). These glacial advances and retreats tend to change Mars’ dynamical flattening, which is out of phase with the Sun. This in turn produces an annual solar torque on Mars that varies the angle between the equatorial and orbital planes. Rubincam estimated that climatic changes in martian history were capable of secularly increasing the planet’s obliquity by 1 ° or 2° since the Solar System was formed. Such a change, in turn, would enhance the martian seasons. Additional change in the obliquity of Mars may have been caused by creation of the Tharsis bulge, an increase of + 7° (Ward 1979). Bills (1990), however, indicated that until we know the dynamical history of the planets we cannot know the exact effect that the bulge had on the obliquity of Mars.

In contrast with the rocky planets, the gaseous planets beyond the Asteroid Belt consist of a rocky core mantled by thick ices of water, methane, and ammonia, liquid metallic hydrogen, and liquid molecular hydrogen (Hartmann 1983, p. 294). Jupiter may be experiencing an anomalously slow cooling because of condensation and settling of helium-rich droplets from a hydrogen-helium mixture; a similar process also may be occurring on Saturn (Klepeis et al. 1991). The satellites orbiting the gaseous planets have various origins ranging from accretion from a disk of gas from a previous larger satellite, to fragmentation, and reconsolidation (a process that may have occurred more than once), and capture. The internal structures displayed by these satellites include a total molten stage model except for a rigid thin solid crust on Io. However, Schubert et al. (1986) believed that there are problems with this molten model of Io and have proposed instead a model consisting of an Fe-S core, a solid mantle, a molten or partly molten asthenosphere, and a rigid crust. Models proposed by Schubert et al. for the other satellites include mostly rock (hydrated or dry silicates) covered by water and an ice crust (Europa), and differentiated bodies having rock cores covered by ice mantles and undifferentiated structures of homogenous ice rock mixtures. Thus, the tectonics and internal dynamics of these satellites are controlled by the rheology and viscosity of high-pressure and low-temperature forms of ice (Poirier 1982).

Intellectually, this grand cruise across the Solar System has been one of the most stimulating voyages we have taken. One could not have asked for more informed guides than the geologists who have been involved in the many National Aeronautical and Space Administration projects on the Solar System. Their numerous reports have provided us with concise views rather than a maze of unrelated images of that part of the cosmos which we inhabit. It is obvious from their descriptions that the cosmos is not only well organized, but also it is beautiful. In the early eighteenth century the Dictionnaire de l’Academie Française defined the literary form ‘romance’ as a light verse that tells an ancient tale. This short book is our version of a romance of the Solar System. It is a tale that roams within the vastness of the cosmos with our gaze concentrating on one aspect of the cosmos, then turning abruptly to another phenomenon. Our discourse seems to show a cosmos that is accumulating tension, leading to some unknown climax as it expands ever outward since the Big Bang. Sometimes it is difficult to avoid the feeling that our study of the Solar System and its place in the Universe is rather superficial as though an understanding of this immense canvas is beyond our comprehension. To rephrase the words that Fuentes (1992, p. 53) used to describe the great mosque at Cordoba, the cosmos is a vision of the infinite where the creation searches for the creator to complete the task. We believe that this task is to make the unknown known. It is an image of the cosmos where different events are occurring at different places at the same time, and yet we believe that all these alterations form part of the same event. This tale of ours must not be looked upon as the ultimate explanation of the Solar System. On reading this essay errors will be found, errors of omission and/or commission, and there will be some readers who will strongly disagree with what we say. So be it. We hope that those who read this book will find reading it as enjoyable as we found writing it, and we hope that we have given proper credit to all.