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Asteroids 1 Ceres and 4 Vesta are the two most massive asteroids in the asteroid belt, with mean diameters of 946 km and 525 km, respectively. Ceres was reclassified as a dwarf planet by the International Astronomical Union as a result of its new dwarf planet definition which is a body that (a) orbits the sun, (b) has enough mass to assume a nearly round shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a moon. Scientists’ understanding of these two bodies has been revolutionized in the past decade by the success of the Dawn mission that visited both bodies. Vesta is an example of a small body that has been heated substantially and differentiated into a metallic core, silicate mantle, and basaltic crust. Ceres is a volatile-rich rocky body that experienced less heating than Vesta and has differentiated into rock and ice. These two contrasting bodies have been instrumental in learning how inner solar system material formed and evolved.

For religious institutions in Latin western Europe of the 16th century, Earth was the center of the universe, and the orderly and predictable motion of the heavenly planets about the Earth (which included the Sun) reflected divine will and an inducement to moral improvement. The discovery by Copernicus that the Earth was not at the center of the universe, but was itself a planet orbiting the Sun, was revolutionary. The invention of the telescope resulted in the discovery of more planets by Galileo and others, initially thought to be planets orbiting planets. All planets were expected to feature geological processes seen on Earth. It was even speculated that these other worlds also supported intelligent life. The search for and discovery of a predicted “missing planet” at the beginning of the 19th century opened the door to a rapidly growing number of new small planets, which appeared as points of light in the sky, and were also referred to as “asteroids,” meaning “star-like.” All planets at this time, which then included asteroids, were thought to have formed from a disk of nebular dust and gas surrounding the early Sun. While in the 19th century it was thought by some that asteroids may have arisen from the breakup of a larger planet, it was not until the 1950s when the smaller members of this population were shown to be collisional fragments that there was a paradigm shift in the scientific literature away from their being considered a type of planet. Near the end of the 20th century, planets around other stars were discovered. These planets now number in the many thousands, greatly expanding the diversity of planet characteristics and solar system architectures. Since then, a growing number of small planets have been discovered in the Solar System (often referred to as ice dwarfs), many of which are hypothesized to have or have had subsurface oceans. Dwarfs are also satellites of planets. Ice dwarfs are the most common type of planet in the Solar System and are hypothesized to be the most common type of planet around other stars. If life can arise in subsurface oceans of these worlds, it raises the question of whether life might be common in the universe. The use of the term “planet” today in the scientific literature continues to reflect its heritage from the time of Galileo, that is, planets are geophysical objects, regardless of their orbits. So, by definition, planets would be those objects large enough to be gravitationally round, in generally hydrostatic equilibrium, at which point differentiation commences and geophysical processes, similar to those observed on Earth, are observed to “turn on.”

Lunar and planetary geology can be described using examples such as the geology of Earth (as the reference case) and geologies of the Earth’s satellite the Moon; the planets Mercury, Mars and Venus; the satellite of Saturn Enceladus; the small stony asteroid Eros; and the nucleus of the comet 67P Churyumov-Gerasimenko. Each body considered is illustrated by its global view, with information given as to its position in the solar system, size, surface, environment including gravity acceleration and properties of its atmosphere if it is present, typical landforms and processes forming them, materials composing these landforms, information on internal structure of the body, stages of its geologic evolution in the form of stratigraphic scale, and estimates of the absolute ages of the stratigraphic units. Information about one body may be applied to another body and this, in particular, has led to the discovery of the existence of heavy “meteoritic” bombardment in the early history of the solar system, which should also significantly affect Earth. It has been shown that volcanism and large-scale tectonics may have not only been an internal source of energy in the form of radiogenic decay of potassium, uranium and thorium, but also an external source in the form of gravity tugging caused by attractions of the neighboring bodies. The knowledge gained by lunar and planetary geology is important for planning and managing space missions and for the practical exploration of other bodies of the solar system and establishing manned outposts on them.

Landslides are mass movements of rock, earth, or debris. All of these surface processes occur under the influence of gravity, meaning that they globally move material from higher to lower places. On planets other than Earth, these structures were first observed in a lunar crater during the Apollo program, but mass movements have been found on many rocky worlds (solid bodies) in the Solar System, including icy satellites, asteroids, and comets. On Earth, landslides have the effect of shaping the landscape more or less rapidly, leaving a signature that is recognized through field surveys and visual analysis or automatic identification on ground-based, aerial, and satellite images. Landslides observed on Earth and on solid bodies of the Solar System can be classified into different types based on their movement and the material involved in the failure. Material is either rock or soil (or both), with a variable fraction of water or ice; a soil mainly composed of sand-sized or finer particles is referred as earth while debris is composed of coarser fragments. The landslide mass may be displaced in several types of movement, classified generically as falling, toppling, sliding, spreading, or flowing. Such diverse characteristics mean that the size of a landslide (e.g., area, volume, fall height, length) can vary widely. For example, on Earth, their area ranges up to 11 orders of magnitude, while their volume varies by 16 orders, from small rock fragments to huge submarine landslides. The classification of extraterrestrial landslides is based on terrestrial analogs having similarities and characteristics that resemble those found on planetary bodies, such as Mars. The morphological classification is made regardless of the geomorphological environment or processes that may have triggered the slope failure. Comparing landslide characteristics on various planetary bodies helps to understand the effect of surface gravity on landslide initiation and propagation—of tremendous importance when designing manned and unmanned missions with landings on extraterrestrial bodies. Regardless of the practical applications of such study, knowing the morphology and surface dynamics that shape solid bodies in the space surrounding the Earth is something that has fascinated the human imagination since the time of Galileo.