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 the unaided visible Saturn,the dimmest of the classical planets,allowing observation 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.
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.
Ganymede,or Jupiter III,is the largest and most massive natural satellite of Jupiter as well as the largest in the Solar System,being a planetary-mass moon. It is the largest Solar System object without a substantial atmosphere,despite being the only moon in the Solar System with a substantial magnetic field. Like Titan,Saturn's largest moon,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.
Volcanism,vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet,moon or asteroid,where lava,pyroclastics,and volcanic gases erupt through a break in the surface called a vent. It includes all phenomena resulting from and causing magma within the crust or mantle of the body,to rise through the crust and form volcanic rocks on the surface. Magmas that reach the surface and solidify form extrusive landforms.
The Discovery Program is a series of Solar System exploration missions funded by the U.S. National Aeronautics and Space Administration (NASA) through its Planetary Missions Program Office. The cost of each mission is capped at a lower level than missions from NASA's New Frontiers or Flagship Programs. As a result,Discovery missions tend to be more focused on a specific scientific goal rather than serving a general purpose.
Io,or Jupiter I,is the innermost and third-largest of the four Galilean moons of the planet Jupiter. Slightly larger than Earth's moon,Io is the fourth-largest moon in the Solar System,has the highest density of any moon,the strongest surface gravity of any moon,and the lowest amount of water of any known astronomical object in the Solar System. It was discovered in 1610 by Galileo Galilei and was named after the mythological character Io,a priestess of Hera who became one of Zeus's lovers.
The New Frontiers program is a series of space exploration missions being conducted by NASA with the purpose of furthering the understanding of the Solar System. The program selects medium-class missions which can provide high science returns.
Tidal heating occurs through the tidal friction processes:orbital and rotational energy is dissipated as heat in either the surface ocean or interior of a planet or satellite. When an object is in an elliptical orbit,the tidal forces acting on it are stronger near periapsis than near apoapsis. Thus the deformation of the body due to tidal forces varies over the course of its orbit,generating internal friction which heats its interior. This energy gained by the object comes from its orbital energy and/or rotational energy,so over time in a two-body system,the initial elliptical orbit decays into a circular orbit and the rotational periods of the two bodies adjust towards matching the orbital period. Sustained tidal heating occurs when the elliptical orbit is prevented from circularizing due to additional gravitational forces from other bodies that keep tugging the object back into an elliptical orbit. In this more complex system,orbital and rotational energy still is being converted to thermal energy;however,now the orbit's semimajor axis would shrink rather than its eccentricity.
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 2023,has continued with eight further spacecraft missions in the vicinity of Jupiter. All of these missions were undertaken by the National Aeronautics and Space Administration (NASA),and all but two 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.
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 thus surmised as essential for extraterrestrial life.
Volcanism on Io,a moon of Jupiter,is represented by the presence of volcanoes,volcanic pits and lava flows on the moon's surface. Its volcanic activity was discovered in 1979 by Voyager 1 imaging scientist Linda Morabito. Observations of Io by passing spacecraft and Earth-based astronomers have revealed more than 150 active volcanoes. Up to 400 such volcanoes are predicted to exist based on these observations. Io's volcanism makes the satellite one of only five known currently volcanically or cryovolcanically active worlds in the Solar System
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 lava planet is a type of terrestrial planet,with a surface mostly or entirely covered by molten lava. Situations where such planets could exist include a young terrestrial planet just after its formation,a planet that has recently suffered a large collision event,or a planet orbiting very close to its star,causing intense irradiation and tidal forces.
The exploration of Io,Jupiter's innermost Galilean and third-largest moon,began with its discovery in 1610 and continues today with Earth-based observations and visits by spacecraft to the Jupiter system. Italian astronomer Galileo Galilei was the first to record an observation of Io on January 8,1610,though Simon Marius may have also observed Io at around the same time. During the 17th century,observations of Io and the other Galilean satellites helped with the measurement of longitude by map makers and surveyors,with validation of Kepler's Third Law of planetary motion,and with measurement of the speed of light. Based on ephemerides produced by astronomer Giovanni Cassini and others,Pierre-Simon Laplace created a mathematical theory to explain the resonant orbits of three of Jupiter's moons,Io,Europa,and Ganymede. This resonance was later found to have a profound effect on the geologies of these moons. Improved telescope technology in the late 19th and 20th centuries allowed astronomers to resolve large-scale surface features on Io as well as to estimate its diameter and mass.
Europa Clipper is an interplanetary mission in development by NASA comprising an orbiter. Planned for launch in October 2024,the spacecraft is being developed to study the Galilean moon Europa through a series of flybys while in orbit around Jupiter.
Tidal heating of Io occurs through the tidal friction processes between Jupiter and its moon. Orbital and rotational energy are dissipated as heat in the crust of the moon. Io has a similar mass and size as the Moon,but Io is the most geologically active body in the Solar System. This is caused by the heating mechanism of Io. The major heating source of Earth and the Moon is radioactive heating,but the heating source on Io is tidal heating. As Jupiter is very massive,the side of Io nearest to Jupiter has a slightly larger gravitational pull than the opposite side. This difference in gravitational forces cause distortion of Io’s shape. Differently from the Earth’s only moon,Jupiter has two other large moons that are in an orbital resonance with it. Io is the innermost of this set of resonant moons,and their interactions maintain its orbit in an eccentric (elliptical) state. The varying distance between Jupiter and Io continually changes the degree of distortion of Io's shape and flexes its interior,frictionally heating it. The friction-induced heating drives strong volcanic activities on the surface of Io.
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 diamond in Neptune to a gigantic ocean of liquid hydrogen that may exist underneath Jupiter's surface.
The Europa Lander is a proposed astrobiology mission concept by NASA to send a lander to Europa,an icy moon of Jupiter. If funded and developed as a large strategic science mission,it would be launched in 2027 to complement the studies by the Europa Clipper orbiter mission and perform analyses on site.
FIRE is a concept mission to Jupiter's innermost major moon Io. The mission was first presented in 2012 for a possible future consideration by NASA's New Frontiers program.
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.
