Planetary core in the context of "Mantle (geology)"

The geology of the Moon (sometimes called selenology, although the latter term can refer more generally to "lunar science") is the structure and composition of the Moon, which is quite different from that of Earth. The Moon lacks a true atmosphere outside of a sparse layer of gas. Because of this, the absence of free oxygen and water eliminates erosion due to weather. Instead, the surface is eroded much more slowly through the bombardment of the lunar surface by micrometeorites. It does not have any known form of plate tectonics, along with having a lower gravity compared to Earth. Because of its small size, it cooled faster in the early days of its formation. In addition to impacts, the geomorphology of the lunar surface has been shaped by volcanism, which is now thought to have ended less than 50 million years ago. The Moon is a differentiated body, with a crust, mantle, and core.

Geological studies of the Moon are based on a combination of Earth-based telescope observations, measurements from orbiting spacecraft, lunar samples, and geophysical data. Six locations were sampled directly during the crewed Apollo program landings from 1969 to 1972, which returned 382 kilograms (842 lb) of lunar rock and lunar soil to Earth. In addition, three robotic Soviet Luna spacecraft returned another 301 grams (10.6 oz) of samples, and the Chinese robotic Chang'e 5 returned a sample of 1,731 g (61.1 oz) in 2020.

Having a mean density of 3,346.4 kg/m, the Moon is a differentiated body, being composed of a geochemically distinct crust, mantle, and planetary core. This structure is believed to have resulted from the fractional crystallization of a magma ocean shortly after its formation about 4.5 billion years ago. The energy required to melt the outer portion of the Moon is commonly attributed to a giant impact event that is postulated to have formed the Earth-Moon system, and the subsequent reaccretion of material in Earth orbit. Crystallization of this magma ocean would have given rise to a mafic mantle and a plagioclase-rich crust.

Geochemical mapping from orbit implies that the crust of the Moon is largely anorthositic in composition, consistent with the magma ocean hypothesis. In terms of elements, the lunar crust is composed primarily of oxygen, silicon, magnesium, iron, calcium, and aluminium, but important minor and trace elements such as titanium, uranium, thorium, potassium, sulphur, manganese, chromium, and hydrogen are present as well. Based on geophysical techniques, the crust is estimated to be on average about 50 km thick.

Ganymede is a natural satellite of Jupiter and is the largest and most massive moon in the Solar System. Like Saturn's largest moon Titan, it is larger than the planet Mercury, but has somewhat less surface gravity than Mercury, Io, or the Moon due to its lower density compared to the three. Ganymede orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with the moons Europa and Io, respectively.

Ganymede is composed of silicate rock and water in approximately equal proportions. It is a fully differentiated body with an iron-rich, liquid metallic core, giving it the lowest moment of inertia factor of any solid body in the Solar System. Its internal ocean potentially contains more water than all of Earth's oceans combined.

Triton is the largest natural satellite of the planet Neptune. It is the only moon of Neptune massive enough to be rounded under its own gravity and hosts a thin, hazy atmosphere. Triton orbits Neptune in a retrograde orbit—revolving in the opposite direction to the parent planet's rotation—the only large moon in the Solar System to do so. Triton is thought to have once been a dwarf planet from the Kuiper belt, captured into Neptune's orbit by the latter's gravity.

At 2,710 kilometers (1,680 mi) in diameter, Triton is the seventh-largest moon in the Solar System, the second-largest planetary moon in relation to its primary (after Earth's Moon), and larger than all of the known dwarf planets. The mean density is 2.061 g/cm, reflecting a composition of approximately 30–45% water ice by mass, with the rest being mostly rock and metal. Triton is differentiated, with a crust of primarily ice atop a probable subsurface ocean of liquid water and a solid rocky-metallic core at its center. Although Triton's orbit is nearly circular with a very low orbital eccentricity of 0.000016, its interior may still experience tidal heating through obliquity tides.

Callisto (/kəˈlɪstoʊ/ kə-LIST-oh) is the second-largest moon of Jupiter, after Ganymede. It is also the third-largest moon in the Solar System, following Ganymede and Saturn's moon Titan, and nearly as large as the planet Mercury. With a diameter of 4,821 km, Callisto is roughly a third larger than Earth's Moon and orbits Jupiter on average at a distance of 1.883 million km, which is about five times further out than the Moon orbiting Earth. It is the outermost of the four large Galilean moons of Jupiter, which were discovered in 1610 with one of the first telescopes, and is today visible from Earth with common binoculars.

The surface of Callisto is the oldest and most heavily cratered in the Solar System. Its surface is completely covered with impact craters. It does not show any signatures of subsurface processes such as plate tectonics or volcanism, with no signs that geological activity in general has ever occurred, and is thought to have evolved predominantly under the influence of impacts. Prominent surface features include multi-ring structures, variously shaped impact craters, and chains of craters called catenae and associated scarps, ridges and deposits. At a small scale, the surface is varied and made up of small, sparkly frost deposits at the tips of high spots, surrounded by a low-lying, smooth blanket of dark material. This is thought to result from the sublimation-driven degradation of small landforms, which is supported by the general deficit of small impact craters and the presence of numerous small knobs, considered to be their remnants. The absolute ages of the landforms are not known.Callisto is composed of approximately equal amounts of rock and ice, with a density of about 1.83 g/cm, the lowest density and surface gravity of Jupiter's major moons. Compounds detected spectroscopically on the surface include water ice, carbon dioxide, silicates and organic compounds. Investigation by the Galileo spacecraft revealed that Callisto may have a small silicate core and possibly a subsurface ocean of liquid water at depths greater than 100 km.

An iron–nickel alloy or nickel–iron alloy, abbreviated FeNi or NiFe, is a group of alloys consisting primarily of the elements nickel (Ni) and iron (Fe). It is the main constituent of the "iron" planetary cores and iron meteorites. In chemistry, the acronym NiFe refers to an iron–nickel catalyst or component involved in various chemical reactions, or the reactions themselves; in geology, it refers to the main constituents of telluric planetary cores (including Earth's).

Some manufactured alloys of iron–nickel are called nickel steel or stainless steel. Depending on the intended use of the alloy, these are usually fortified with small amounts of other metals, such as chromium, cobalt, molybdenum, and titanium.