Planetary geology is the study of the origin, evolution, and distribution of solid matter condensed in the form of planets, satellites, comets, and asteroids. The term geology is used here in its broadest sense to mean the study of the solid parts of the planets. Aspects of geophysics, geochemistry, geodesy, cartography, and other disciplines concerned with the solid planets are all included in the general term.
This report is concerned with the kinds of experiments, observations, and studies that can be made through direct exploration of the entire solar system, although the discussion focuses on the terrestrial planets and the satellites of the outer planets. Planetary atmospheres are discussed only in relation to their evolution by the release of volatile gases through geologic processes and their influence on surface processes through time.
1.1. Relevance of Planetary Geology
The National Academy of Sciences (1966) identified three principal goals for the exploration of space: (1) to determine the origin and evolution of the solar system, (2) to determine the origin and evolution of life, and (3) to clarify the nature of the processes that shape our terrestrial environment. These objectives form part of the basic charter of space exploration of the National Aeronautics and Space Administration (NASA); planetary geology is an important component of each of these goals.
1.1.1. Origin and Evolution of the Solar System
Study of the geology of other planetary bodies leads directly to a better understanding of the origin and evolution of the solar  system. There are two ways of attacking the problem: one is to model the evolution from an initially assumed state to the observed present condition; the other is to examine the present state of the solar system and deduce its history by moving backward in time. Both methods are valuable, but it is with the latter that the geologist is concerned. Generally, the surface of a planetary body preserves two records: one of its interactions with the external environment; the other is the result of processes intrinsic to the planet itself. Both records are fragmentary, and it is the task of the geologist to reconstruct the history of the planet from whatever record survives. This historical aspect of geology distinguishes it from many other disciplines, which usually are more concerned with characterizing and understanding the processes that are occurring at the present time.
The success with which the history of a planet can be deduced depends on how well the record is preserved and how clever one is in analyzing it. In the early stages of exploration, when only remote sensing data are available, emphasis is on interpretation of the topographic landforms and the spectral, photometric, and thermal properties of the surface. At a later stage of exploration, the record....
....preserved in the petrology, mineralogy, and chemistry of the surface can be examined. For many objects in the solar system (Uranus, Neptune, Pluto, and their satellites, as well as for the asteroids and comets), not even the first of these two stages has begun. Only for Earth and the Moon has the second stage been reached.
The degree of preservation of the record varies from planet to planet and with it the extent to which the history of the planet can be reconstructed On Mercury the record of interaction with the external environment is relatively well preserved because the cumulative effect of internal activity over the last 3 to 4 billion years has been small. In contrast, the topography of the volcanically active  satellite, Io, appears to result entirely from internal processes. Moreover, due to the intensity of the volcanism, the record of only relatively recent events is preserved. Fortunately, for most planetary bodies the record that survives covers a more extended period, and geologic histories can be reconstructed more fully.
Geophysics is concerned largely with documenting and interpreting structural heterogeneities and physical processes inside a planet. The chemistry of internal materials cannot be determined directly; nevertheless, inferences are possible from remotely measured physical properties. Although the value of any particular physical parameter may not be important in itself, its significance derives from the fact that it may provide information on internal chemical composition, conditions, and processes.
Stratigraphy is concerned with documentation of crustal heterogeneity. Even though surface rocks constitute a minute part of the total mass of the planet, it is the part most accessible to measurement. For this reason, an understanding of surface materials is crucial. We must know how variegated the surface rocks are and how they were formed. The mode and sequence of formation are particularly important, for different processes vary in timing and in the extent to which they cause chemical change. Consequently, to understand the broader implications of surface analyses, we should know the distribution and mode of formation of the rocks studied as well as the degree to which they typify the materials of the planet.
1.1.2. Origin and Evolution of Life
The geology of planetary surfaces bears directly on the origin of life in our solar system. Biologists concerned with understanding the evolution of life must know how a planet's environment has changed with time. This characterization includes a knowledge of the composition of rocks and minerals on and near the surface, the nature of the atmosphere and climate, and the nature and intensity of different geologic processes. These factors all involve aspects of planetary geology.
This report was written largely with the assumption that, in our solar system, life exists only on Earth, but that many important clues about the chemical precursors of life and the conditions necessary for life to evolve can be obtained by studying other solar system objects. Should life be found elsewhere, this strategy will  need drastic revision. Paleontology, which has been ignored in this strategy, would become a study of paramount importance, and different types of missions would be required to meet its needs.
1.1.3. Our Terrestrial Environment
An inevitable, important result of planetary exploration is an increased understanding of Earth. Many fundamental geologic problems are illuminated by detailed comparison of Earth with other planetary bodies. The relative effects of size and original composition and the presence of an atmosphere, hydrosphere, and biosphere on the evolution of Earth are of particular geologic importance; comparison of Earth with other bodies allows the importance of these effects to be assessed.
From the point of view of our terrestrial environment, the most significant results of planetary exploration will probably be an improved understanding of the early history and deep interior of Earth. Because the geologic record of events and processes that took place early in Earth's history has been largely destroyed by more recent events, very little is known of the first billion years of Earth's history. Features on Earth's surface are subject to relatively rapid destruction and modification as a result of the erosive action of water, and of tectonic and volcanic activity. Fortunately, the early histories of both the Moon and Mercury have been preserved on their surfaces, partly because both objects have been relatively inactive internally through much of geologic time, and partly because neither has an atmosphere, so that erosion rates have been much slower than those on Earth. The Moon and Mercury appear to have had similar early histories involving severe external bombardment, and there is growing evidence that this situation also extended to the other terrestrial planets, Mars, Venus, and Earth. It is not farfetched to assert that the chances of understanding the development of continents and ocean basins on Earth are low if we do not understand the initial conditions which prevailed 4 billion years ago. An analysis of the geologic record preserved on the more primitive bodies may be the only way to arrive at an understanding of Earth's early history.
1.2. The Planetary Geology Approach
How can the three objectives discussed above be met? In the geologic exploration of the solar system, certain basic questions can  he asked about any planet. 'These questions relate to the following: (1) the present geologic state of the planet (2) how the present state differs from those that obtained in the past and (3) how the planet's present and past geologic conditions differ from those of other solar system bodies.
1.2.1. Present Geologic State
The present state of a planet concerns knowledge of its surface morphology the composition of the surface and interior the physical nature of the interior and whether or not there are active processes such as volcanism. These topics are interrelated and knowledge of one often provides clues to the others. Investigations of surface morphology involve the identification of the type and distribution of individual landforms such as basins mountain chains and volcanoes as well as structural elements such as fault scarps. The presence of these features often signals specific geologic processes. For example the identification of volcanoes on Mars indicates melting in the interior thus placing constraints on various models of the planet's internal structure and composition. Observations of variable surface features that can be related to specific eolian landforms suggest that wind processes play an important role in modifying the planet's surface. Determination of surface compositions and mapping of the distribution of rock types as has been accomplished in part for the Moon, can provide direct information about the chemical differentiation of a planet.
1.2.2. Geological Evolution of Planets
One of the primary goals in geology is to derive the geological history of a body that is to determine the sequence of events and processes that have occurred form the time of formation of the planet to the present Specific questions about geological history include:
These profound questions are extremely difficult and in some cases probably impossible to answer completely. However geochemical data geologic mapping and analyses of specific surface processes can provide important clues.
1.3. Comparative Planetology
Comparative planetology is the study of the differences and similarities among planets and satellites. The last two decades have seen a tremendous expansion of knowledge of the inner solar system through successful lunar and planetary missions. We are beginning to perceive Earth as an extremely active planet with a thin lithosphere that slides over a plastic asthenosphere. At the other end of the activity scale is the Moon with its very thick static lithosphere. With respect to many geologic processes Mars has turned out to be more Earthlike, whereas Mercury snares many  attributes with the Moon. Venus, because of its dense cloud cover, remains largely unexplored; less is known about its present state and evolution than about any other Earthlike planet.
Earth, its Moon, and the meteorites remain important reference points for comparative studies in planetology and will continue to be for the foreseeable future. A much more comprehensive set of basic data is available for Earth than for any other object. As a highly differentiated object that possesses a core, mantle, and crust, and whose internal thermally driven processes are still active, Earth is a natural laboratory for investigation of these processes. The Moon is the only body sampled and studied in detail on which particles and radiation interact directly with the surface without the shielding of an intervening atmosphere. The Moon also represents a body in a stage of planetary evolution different from that of Earth and one which, in terms of size, is similar to an important class of objects that includes Mercury and many of the satellites of the giant planets.
Meteorites supply the greatest variety of "returned samples" from the solar system and provide important clues about the very early conditions and processes in the solar nebula, variations in chemical composition among solid objects, and the lifetimes and collision frequency of planetary debris. Experience in studying the Earth, Moon, and meteorites has highlighted the fact that one measurement, or even one set of data, may not yield answers to the major questions associated with the formation and evolution of these objects. Many types of data must be obtained to understand the important processes active in the formation, evolution, and present state of each object in the solar system as well as that of the solar system itself. These data include global surveys from spacecraft, analyses of returned samples, and the results of theoretical modeling, laboratory experiments, and investigation of specific features on Earth that may provide clues about geologic processes on other bodies.
The chapters that follow describe the major aspects of planetary geology and discuss means of obtaining relevant data. It is not our intention to assess the relative merits of various instruments that could be employed on future planetary missions, but rather to present the types of measurements that should be made and to discuss their relevance to answering fundamental questions about our solar system.