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Cometary nuclei, as small, spinning, ice-rich objects revolving around the sun in eccentric orbits, are powered and activated by solar radiation. Far from the sun, most of the solar energy is reradiated as thermal emission, whereas close to the sun, it is absorbed by sublimation of ice. Only a small fraction of the solar energy is conducted into the nucleus interior. The rate of heat conduction determines how deep and how fast this energy is dissipated. The conductivity of cometary nuclei, which depends on their composition and porosity, is estimated based on vastly different models ranging from very simple to extremely complex. The characteristic response to heating is determined by the skin depth, the thermal inertia, and the thermal diffusion timescale, which depend on the comet’s structure and dynamics. Internal heat sources include the temperature-dependent crystallization of amorphous water ice, which becomes important at temperatures above about 130 K; occurs in spurts; and releases volatiles trapped in the ice. These, in turn, contribute to heat transfer by advection and by phase transitions. Radiogenic heating resulting from the decay of short-lived unstable nuclei such as 26Al heats the nucleus shortly after formation and may lead to compositional alterations. The thermal evolution of the nucleus is described by thermo-physical models that solve mass and energy conservation equations in various geometries, sometimes very complicated, taking into account self-heating. Solutions are compared with actual measurements from spacecraft, mainly during the Rosetta mission, to deduce the thermal properties of the nucleus and decipher its activity pattern.

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.