Icy moons are a class of natural satellites with surfaces composed mostly of ice. An icy moon may harbor an ocean underneath the surface, and possibly include a rocky core of silicate or metallic rocks. [1] It is thought that they may be composed of ice II or other polymorph of water ice. [2] The prime example of this class of object is Europa.
Icy moons warmed by tides may be the most common type of celestial body in the galaxy to have liquid water, [3] and thus the most likely type of object to possibly have water-based life.
Some icy moons exhibit cryovolcanism, as well as geysers. The best studied example is Saturn's Enceladus.
Most known large icy moons belong to giant planets, whose orbits lie beyond the Solar System's frost line; the remainder (such as Charon and Dysnomia) formed around dwarf planets such as Pluto and Eris, typically in large impacts not unlike the impact thought to have formed Earth's moon. In the case of icy gas giant satellites, an additional requirement is that a moon did not form in the inner region of a proto-satellite disk, which is too warm for ices to condense.
Europa is thought to contain 8% ice and water by mass with the remainder rock. [4] Jupiter's outer two Galilean moons Ganymede and Callisto contain more ice since they formed further from the hot proto-Jupiter.
Saturn's moon Titan looks and behaves more like Earth than any other body in the Solar System. [5] Titan is known to have stable pools of liquid methane on the surface. [5]
The Galilean moons, or Galilean satellites, are the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto. They are the most readily visible Solar System objects after Saturn, the dimmest of the classical planets; though their closeness to bright Jupiter makes naked-eye observation very difficult, they are readily seen with common binoculars, even under night sky conditions of high light pollution. The invention of the telescope enabled the discovery of the moons in 1610. Through this, they became the first Solar System objects discovered since humans have started tracking the classical planets, and the first objects to be found to orbit any planet beyond Earth.
Callisto, or Jupiter IV, is the second-largest moon of Jupiter, after Ganymede. In the Solar System it is the third-largest moon after Ganymede and Saturn's largest moon Titan, and nearly as large as the smallest planet Mercury. Callisto is, with a diameter of 4,821 km, roughly a third larger than Earth's Moon and orbits Jupiter on average at a distance of 1,883,000 km, which is about six 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, being visible from Earth with common binoculars.
Europa, or Jupiter II, is the smallest of the four Galilean moons orbiting Jupiter, and the sixth-closest to the planet of all the 95 known moons of Jupiter. It is also the sixth-largest moon in the Solar System. Europa was discovered independently by Simon Marius and Galileo Galilei and was named after Europa, the Phoenician mother of King Minos of Crete and lover of Zeus.
Titan is the largest moon of Saturn and the second-largest in the Solar System. It is the only moon known to have an atmosphere denser than the Earth's and is the only known object in space—other than Earth—on which there is clear evidence that stable bodies of liquid exist. Titan is one of seven gravitationally rounded moons of Saturn and the second-most distant among them. Frequently described as a planet-like moon, Titan is 50% larger in diameter than Earth's Moon and 80% more massive. It is the second-largest moon in the Solar System after Jupiter's Ganymede and is larger than Mercury; yet Titan is only 40% as massive as Mercury, because Mercury is mainly iron and rock while much of Titan is ice, which is less dense.
A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate, rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, Venus, Earth and Mars. Among astronomers who use the geophysical definition of a planet, two or three planetary-mass satellites – Earth's Moon, Io, and sometimes Europa – may also be considered terrestrial planets. The large rocky asteroids Pallas and Vesta are sometimes included as well, albeit rarely. The terms "terrestrial planet" and "telluric planet" are derived from Latin words for Earth, as these planets are, in terms of structure, Earth-like. Terrestrial planets are generally studied by geologists, astronomers, and geophysicists.
A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body. Natural satellites are colloquially referred to as moons, a derivation from the Moon of Earth.
Ganymede, or Jupiter III, is the largest and most massive natural satellite of Jupiter, and in the Solar System. Despite being the only moon in the Solar System with a substantial magnetic field, it is the largest Solar System object without a substantial atmosphere. 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.
A cryovolcano is a type of volcano that erupts gases and volatile material such as liquid water, ammonia, and hydrocarbons. The erupted material is collectively referred to as cryolava; it originates from a reservoir of subsurface cryomagma. Cryovolcanic eruptions can take many forms, such as fissure and curtain eruptions, effusive cryolava flows, and large-scale resurfacing, and can vary greatly in output volumes. Immediately after an eruption, cryolava quickly freezes, constructing geological features and altering the surface.
The exploration of Jupiter has been conducted via close observations by automated spacecraft. It began with the arrival of Pioneer 10 into the Jovian system in 1973, and, as of 2024, has continued with eight further spacecraft missions in the vicinity of Jupiter and two more en route. All but one of these missions were undertaken by the National Aeronautics and Space Administration (NASA), and all but four were flybys taking detailed observations without landing or entering orbit. These probes make Jupiter the most visited of the Solar System's outer planets as all missions to the outer Solar System have used Jupiter flybys. On 5 July 2016, spacecraft Juno arrived and entered the planet's orbit—the second craft ever to do so. Sending a craft to Jupiter is difficult, mostly due to large fuel requirements and the effects of the planet's harsh radiation environment.
An ocean world, ocean planet or water world is a type of planet that contains a substantial amount of water in the form of oceans, as part of its hydrosphere, either beneath the surface, as subsurface oceans, or on the surface, potentially submerging all dry land. The term ocean world is also used sometimes for astronomical bodies with an ocean composed of a different fluid or thalassogen, such as lava, ammonia or hydrocarbons. The study of extraterrestrial oceans is referred to as planetary oceanography.
Extraterrestrial liquid water is water in its liquid state that naturally occurs outside Earth. It is a subject of wide interest because it is recognized as one of the key prerequisites for life as we know it and is thus surmised to be essential for extraterrestrial life.
The habitability of natural satellites is the potential of moons to provide habitats for life, though it is not an indicator that they harbor it. Natural satellites are expected to outnumber planets by a large margin and the study of their habitability is therefore important to astrobiology and the search for extraterrestrial life. There are, nevertheless, significant environmental variables specific to moons.
In astronomy, a regular moon or a regular satellite is a natural satellite following a relatively close, stable, and circular orbit which is generally aligned to its primary's equator. They form within discs of debris and gas that once surrounded their primary, usually the aftermath of a large collision or leftover material accumulated from the protoplanetary disc. Young regular moons then begin to accumulate material within the circumplanetary disc in a process similar to planetary accretion, as opposed to irregular moons, which formed independently before being captured into orbit around the primary.
A planetary-mass moon is a planetary-mass object that is also a natural satellite. They are large and ellipsoidal in shape. Moons may be in hydrostatic equilibrium due to tidal or radiogenic heating, in some cases forming a subsurface ocean. Two moons in the Solar System, Ganymede and Titan, are larger than the planet Mercury, and a third, Callisto, is just slightly smaller than it, although all three are less massive. Additionally, seven – Ganymede, Titan, Callisto, Io, Earth's Moon, Europa, and Triton – are larger and more massive than the dwarf planets Pluto and Eris.
An ice planet or icy planet is a type of planet with an icy surface of volatiles such as water, ammonia, and methane. Ice planets consist of a global cryosphere.
The Jupiter Icy Moons Explorer is an interplanetary spacecraft on its way to orbit and study three icy moons of Jupiter: Ganymede, Callisto, and Europa. These planetary-mass moons are planned to be studied because they are thought to have significant bodies of liquid water beneath their frozen surfaces, which would make them potentially habitable for extraterrestrial life.
Planetary oceanography, also called astro-oceanography or exo-oceanography, is the study of oceans on planets and moons other than Earth. Unlike other planetary sciences like astrobiology, astrochemistry, and planetary geology, it only began after the discovery of underground oceans in Saturn's moon Titan and Jupiter's moon Europa. This field remains speculative until further missions reach the oceans beneath the rock or ice layer of the moons. There are many theories about oceans or even ocean worlds of celestial bodies in the Solar System, from oceans made of liquid carbon with floating diamonds in Neptune to a gigantic ocean of liquid hydrogen that may exist underneath Jupiter's surface.
The Ocean Worlds Exploration Program (OWEP) is a NASA program to explore ocean worlds in the outer Solar System that could possess subsurface oceans to assess their habitability and to seek biosignatures of simple extraterrestrial life.
Planetary habitability in the Solar System is the study that searches the possible existence of past or present extraterrestrial life in those celestial bodies. As exoplanets are too far away and can only be studied by indirect means, the celestial bodies in the Solar System allow for a much more detailed study: direct telescope observation, space probes, rovers and even human spaceflight.
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