Terrestrial planets in the context of "Planetary-mass moon"

An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth and other terrestrial planets.

Planetary geology, alternatively known as astrogeology or exogeology, is a planetary science discipline concerned with the geology of celestial bodies such as planets and their moons, asteroids, comets, and meteorites. Although the geo- prefix typically indicates topics of or relating to Earth, planetary geology is named as such for historical and convenience reasons; due to the subject matter, it is closely linked with more traditional Earth-based geology.

Planetary geology includes such topics as determining the properties and processes of the internal structure of the terrestrial planets, surface processes such as volcanism, impact craters, even fluvial and aeolian action where applicable. Despite their outermost layers being dominated by gases, the giant planets are also included in the field of planetary geology, especially when it comes to their interiors. Fields within Planetary geology are largely derived from fields in the traditional geological sciences, such as geophysics, geomorphology, and geochemistry.

The geology of Mercury is the scientific study of the surface, crust, and interior of the planet Mercury. It emphasizes the composition, structure, history, and physical processes that shape the planet. It is analogous to the field of terrestrial geology. In planetary science, the term geology is used in its broadest sense to mean the study of the solid parts of planets and moons. The term incorporates aspects of geophysics, geochemistry, mineralogy, geodesy, and cartography.

Historically, Mercury has been the least understood of all the terrestrial planets in the Solar System. This stems largely from its proximity to the Sun which makes reaching it with spacecraft technically challenging and Earth-based observations difficult. For decades, the principal source of geologic information about Mercury came from the 2,700 images taken by the Mariner 10 spacecraft during three flybys of the planet from 1974 to 1975. These images covered about 45% of the planet's surface, but many of them were unsuitable for detailed geologic investigation because of high sun angles which made it hard to determine surface morphology and topography. This dearth of information was greatly alleviated by the Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft which between 2008 and 2015 collected over 291,000 images covering the entire planet, along with a wealth of other scientific data. The European Space Agency's (ESA's) BepiColombo spacecraft, scheduled to go into orbit around Mercury in 2026, is expected to help answer many of the remaining questions about Mercury's geology.

A primary atmosphere, often also called a primordial atmosphere or proto-atmosphere, is an atmosphere of a protoplanet that forms by accretion of gaseous matter from the protoplanetary disk. Gas giant planets including Jupiter, Saturn, have primary atmospheres. Primary atmospheres are very thick compared to secondary atmospheres like the one found on Earth. The primary atmosphere was lost on the terrestrial planets of the Solar System due to a combination of surface temperature, mass of the atoms and the escape velocity of the planet.