In addition to spectroscopy, planetary geoscience uses some other tools familiar to most geologists, and some tools that are either unique or involve new twists in how they are employed. We explain how stratigraphic principles are adapted for planets (using strata produced by impacts), how the density of craters can be quantified to derive relative ages of geologic units, and how radioisotope measurements on samples, where available, give absolute ages. We explain how images from orbiting and landed spacecraft are used, along with chronologic and remote-sensing data, to make planetary geologic maps at different scales. We consider various geophysical techniques that are used on spacecraft to obtain information about planetary potential fields, interior structure, and surface topography. We summarize the kinds of extraterrestrial materials that are available for laboratory investigations, and briefly describe the analytical techniques used to characterize their mineralogy, petrology, and geochemistry. We also examine some techniques that are adapted as remote sensing tools for analyses of rocks and soils on planetary surfaces.

We describe the near-surface wind profile, its relation to environmental conditions, and how it can be quantified. The freestream wind speed can be converted to a friction wind speed, which relates to the flow at the atmosphere–surface interface and thus to the entrainment of sediment. The minimum wind speed for entrainment of aeolian sediment depends on gravity and grain size, so that threshold wind speed differs for varying planetary conditions. The difference in transport mechanism for grains leads to different depositional morphologies, which provide clues to the wind speed, wind direction, and sediment availability. Erosional landforms likewise provide information on near-surface atmospheric processes and surface sediments, as well as bedrock lithologies. The study of aeolian landforms thus informs our understanding of the atmosphere, surface geology, and sedimentology on other planets.

The surfaces of terrestrial planets and icy satellites have enjoyed deformation marked by faults and folds. We use these geologic structures not only to characterize the morphology of the surfaces, but also to describe the motions, stresses, and deformation processes that created the structures. Ultimately, we can sum these structural data and interpretations to infer the tectonic deformation for large portions of, or even entire, planets and satellites. As we will see, understanding deformation at this large tectonic scale enables us to investigate what is driving overall planet or satellite development. We will also learn that while the expected will happen, conundrums exist too. For example, the rocks of terrestrial planets deform quite differently from the icy shells of the satellites of the gas giants, yet the magnitude of these differences and their causes can surprise us. On the other hand, Venus and Earth are quite similar in many planetary characteristics but have strikingly different tectonic histories, which challenges us to understand why.

Heavily cratered surfaces emphasize the important geologic role of impacts on almost all planetary bodies. Large impacts (referred to as “hypervelocity” to indicate velocities of tens of kilometers per second) produce micro-, meso-, and macroscale deformations that can influence the structures of planetary crusts. Crater morphologies are described as simple, complex, and multi-ring, and correlate with crater size and inversely with gravity of the target body. Crater formation is envisioned in three stages: contact/compression, excavation, and modification, each characterized by different processes and geologic features. Shock metamorphism has affected all Solar System bodies, producing breccias containing planar deformation features, high-pressure polymorphs, and melts. Craters provide the basis for planetary stratigraphy and chronology. Massive impacts on the Earth have potential consequences for damaging the planetary ecosystem and biosphere.

With our preliminary survey of planetary bodies in the Solar System complete, we next turn our attention to developing an understanding of the tools that are used by planetary geologists to study these bodies. In this chapter, we will learn about some of the remote sensing techniques that are most commonly used in planetary geologic exploration. We will focus greatest attention on methods that employ light as the carrier of information about the remote target, but we will also consider some methods that employ the detection of other types of carriers. We will describe techniques that can be used from aircraft flying above the Earth, from spacecraft orbiting planets, and from landed and roving platforms on other worlds. We will examine both active and passive remote sensing techniques, provide guidance on how to select the right type of remote sensing method for the desired scientific outcome, and discuss the role of a successful ground campaign in the analysis of remote-sensing data when that option is available.

We explain how heat is produced by radioactive decay, segregation and exothermic crystallization of metallic cores, impacts, and tidal forces. Planetesimals in the early Solar System were most affected by the decay of short-lived radionuclides. Larger, rocky planets were heated primarily by large impacts and core segregation. Because rocks are poor conductors, heat retention in rocky bodies is a function of planet size. Large-scale melting to produce magma oceans was likely a common process facilitating differentiation to form cores, mantles, and crusts. Metallic liquids are probably necessary for core segregation. Primary crusts, formed during planetary differentiation, are rarely preserved. Mantles are residues from the extraction of silicate crustal melts and core materials. Differentiation of the giant planets was driven by density variations in high-pressure forms of gases, ices, and rock more than by heating and melting. The importance of the various planetary heat sources changes over time; in modern planets the effective heat sources are decay of long-lived radioisotopes and, for the Earth, exothermal crystallization of the liquid outer core.

Planetary bodies dynamically respond to applied stresses. Heat transfer out of the interior commonly leads to stresses that affect the surface. For quantitative analysis of geodynamics, numerical techniques are generally required and are applied looking at the material as a continuum. Rocks and ice in planetary bodies ultimately want to be in equilibrium with applied stresses. Equilibrium can be assessed by computing whether the stress gradients balance the applied force. The material response to stress is strain, which can be calculated from displacement gradients throughout the material. Stress and strain in a solid are related through intrinsic material properties (e.g., Young’s modulus and Poisson’s ratio). The material properties of rock and ice are similar enough that the icy lithospheres of the moons of the outer planets undergo the same basic processes as the rocky lithospheres of the terrestrial planets. Large lithospheric blocks are supported isostatically, floating in the asthenosphere. Topography can also be supported by the strength of the lithosphere, in which case some amount of flexure occurs as a result of the load on the surface. The distribution of mass in the subsurface can be inferred from measurements of the gravity field. From such measurements, it is possible to discern if a feature such as a mountain or volcano has a large root, or if a large mass lies beneath a surface with no topography (e.g., lunar mascons). Surface temperature is controlled for most planetary surfaces by solar heating, the effect of which generally only penetrates a few meters into the surface. Heat flows through the brittle lithosphere by conduction, but the deeper asthenosphere transfers heat through convection. The asthenosphere behaves like a fluid on geologic timescales, and its response to stress must be investigated in terms of fluid mechanics. The exact response to stress, or the rheology, depends on many factors, including temperature, composition, grain size, and the magnitude of stress. The ductile behavior of the interior is coupled to the surface, enabling geodynamicists to use observations of the surface to infer properties of the interior.