An Etymological Dictionary of Astronomy and Astrophysics
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The third of Jupiter's known satellites orbiting at about 181,300 km from Jupiter with a period of about 12h. A mean diameter of 189 km makes it the fifth largest satellite of Jupiter. Amalthea was discovered by E. Barnard in 1892.

Amalthea, in Gk. mythology, the goat that suckled Zeus after his mother had him sent to Crete so his father would not eat him.

The thirteenth of Jupiter's known satellites discovered by S.B. Nicholson in 1951. It orbits the planet at a mean distance of 21,200,000 km, and has a diameter of about 30 km.

In Gk. mythology, Ananke is the personification of destiny, unalterable necessity and fate; she is also the mother of Adrastea,

The eighth of → Jupiter's known moons and the second brightest and the outermost of the four → Galilean satellites. With a diameter of 4800 km (0.38 Earths), Castillo is roughly the same size as Mercury. It orbits Jupiter in 16.689 days at a distance of 1,883,000 km from the planet, beyond Jupiter's main → radiation belts. It is the third largest moon in the entire solar system. Its mass is 10.76 × 10²² kg (about 1.5 Earth Moons) and its mean → surface temperature is -155 °C. The most prominent feature of Callisto is its craters, as it has the most craters of any object in the solar system. Due to its orbit being further away from Jupiter, it is not under the same → tidal heating influences as → Io, → Europa, or → Ganymede. Callisto's thin → atmosphere is composed of → carbon dioxide and likely some → molecular oxygen. Callisto is thought to have formed as a result of slow → accretion from the → protoplanetary disk of gas and dust that surrounded Jupiter after its formation.

Callisto, an attendant of Artemis in Greek mythology. Because of her love affair with Zeus, she was transformed into a bear by Artemis. According to another legend she was changed into a bear by the jealous Hera. Zeus transferred her to the heavens as the → constellation  → Ursa Major (great bear).

The fourteenth of Jupiter's known satellites; 40 km in size; → retrograde orbit. It was discovered by Nicholson in 1938.

In Gk mythology, Carme was a wife of Zeus, and the mother of Britomartis, a Cretan goddess.

The thirteenth known moon of Jupiter, discovered in 1905 by Charles Perrine.

In Gk. mythology, Elara was the mother by Zeus of the giant Tityus.

The sixth of → Jupiter's known moons and the fourth largest; it is the second of the → Galilean satellites. With a diameter of 3140 km, Europa is slightly smaller than Earth's Moon. Its mass is 4.80 × 10²² kg, i.e. 1.5 times less massive than Earth Moon. Its distance to Jupiter is 670,900 km, or about 9 Jovian radii. Its → orbital period is 3.55 Earth days which equals its → rotation period. Europa's density is 3.0 g cm^-3, typical of a mixture of rocks including → ice. Its high → albedo (0.67) suggests that its surface is mostly → water ice. The → surface temperature of Europa ranges between about 125 K (-150 °C) at the equator and about 50 K (-220 °C) at the poles. There are few → impact craters on Europa, because its surface is too active and therefore young. The most striking features of Europa's surface are structures called → lineae and → lenticulae. The thickness of the ice crust could range between a few kilometers to a few tens of kilometers. It is now believed that there is an ocean of salty water, up to 100 km deep, flowing under Europa's ice. Europa's ocean is kept liquid due to → tidal heating by Jupiter.

In Gk. mythology, Europa was a Phoenician princess (Agenor's daughter) abducted to Crete by Zeus, who had assumed the form of a white bull, and by him the mother of Minos, Sarpedon, and Rhadamanthys.

The seventh and largest of → Jupiter's known satellites. This → Galilean satellite has a diameter of 5270 km, slightly larger than Mercury, a mass about 1.48 × 10²³ kg (about 2 Earth Moons); an → orbital period of 7.155 days, and an → eccentricity of e = 0.0015. It was discovered by Galileo and Marius in 1610. The mean → surface temperature of Ganymede is -160 °C. It is the only moon known to have a → magnetosphere.

In Gk. mythology, Ganymedes, a unusually beautiful prince of Troy who was abducted to Olympus by Zeus and made the cup-bearer of the gods.

The tenth of Jupiter's known satellites, 186 km in diameter revolving at a mean distance of 11,480,000 km from Jupiter. Discovered in 1904 by the Argentine-American astronomer Charles Dillon Perrine (1867-1951).

Himalia was a nymph of the island of Rhodes. She was seduced by the god Zeus (Jupiter).

A giant, gaseous, Jupiter-like planet lying too close to its parent star and having an orbital period from a few days to a few weeks. The existence of hot Jupiters is usually interpreted in terms of planetary migration. These planets can, in principle, be formed at larger distances from their stars and migrate to the inner regions due to dynamical interaction with the proto-planetary disk.

→ hot; → Jupiter.

1) The fifth of → Jupiter's known moons and the third largest. It is the innermost of the → Galilean satellites. With a diameter of 3630 km, Io is slightly larger than Earth's Moon. It revolves at a mean distance of 422,000 km from Jupiter. Its mass is 8.93 x 10²² kg (about 1.2 Earth Moons) and its → orbital period 1.8 Earth days. The mean → surface temperature of Io is -155 °C. Io's yellow color derive from → sulfur and molten → silicate rock. The unusual surface of Io is kept very young by its system of active → volcanoes. The intense → tidal force of Jupiter stretches Io and damps wobbles caused by Jupiter's other Galilean moons. The resulting friction greatly heats Io's interior, causing molten rock to explode through the surface. Io's volcanoes are so active that they are effectively turning the whole moon inside out. Some of Io's volcanic lava is so hot it glows in the dark.
2) Also the name of an → asteroid numbered 85.

In Gk. mythology, Io was a maiden who was seduced by Zeus (Jupiter). When Hera came upon their rendez-vous, Zeus transformed the maiden into a white heifer.

The largest → planet in the → Solar System and the fifth from the Sun, lying at a mean distance of about 5.2 → astronomical units from the Sun. Jupiter is a → gas giant, mostly → hydrogen and → helium, with a mass of 1.898 × 10²⁷ kg, or about 0.001 → solar masses, or 318 times → Earth masses. It is more than twice as massive as all the other solar system planets combined. Jupiter's diameter measures 11 times that of Earth. Its → rotation period, 9.93 hours (Jupiter/Earth ratio = 0.41), is the shortest of all the solar system planets. Its → orbital period is 11.857 Earth years. Jupiter has an extensive family of → satellites (79 known) and a faint → ring system; → Jupiter's ring. Jupiter probably has a core of rocky material amounting to something like 10 to 15 Earth masses. Above the core lies the main bulk of the planet in the form of liquid → metallic hydrogen. This exotic form of the most common of elements is possible only at pressures about 3 million bars, as is the case in the interior of Jupiter (and Saturn). Under the extreme pressure found deep inside Jupiter, the electrons are released from the hydrogen molecules and are free to move about the interior. This causes hydrogen to behave as a metal; it becomes conducting for both heat and electricity. See also → Jupiter's atmosphere.

Jupiter "the king of ancient Roman gods, the ruler of Olympus," from L. Iupeter, from PIE *dyeu-peter- "god-father," from *deiw-os "god" (cf. Pers. div "devil, demon;" Mid.Pers. dêw; O.Pers. daiva- "evil god, demon;" Av. daēva- "evil spirit, false god;" Skt. deva-; Gk. Zeus "supreme god;" from *dei- "to gleam, to shine") + *peter "father" (cf. Pers. pedar "father;" O.Pers. pitā- "father;" Av. patar-, ptā-; Skt. pitár-; Gk. pater; L. pater, O.H.G. fater).

Hormoz, from Mid.Pers. Ohrmazd "name of the highest god in Zoroastrianism," from O.Pers. aura-mazdā-, Av. ahura-mazdā- "Wise Lord," from ahura- "lord, god;" cf. Skt. ásura- "god, lord;" Hittite hassu- "king;" M.H.G. Asen "name of a group of gods;" O.N. āss "god;" PIE *ansu- "spirit, demon" + mazdā- "wisdom," mazdāθa- "what must be borne in mind," mazdāh- "memory;" cf. Skt. medhā- "mental power, wisdom, intelligence;" Gk. mathein "to learn, to know" (root of → mathematics).

An interplanetary mission currently in development by the → European Space Agency planned for launch in 2020. It is aimed mainly at in-depth studies of three potentially ocean-bearing satellites, → Ganymede, → Europa, and → Callisto. JUICE will complete a unique tour of the Jupiter system including several flybys of each planet-sized world, culminating with orbit insertion around Ganymede, the largest moon in the Solar System, followed by nine months of operations in its orbit. JUICE will carry the most powerful scientific payload ever flown to the outer Solar System. It consists of 10 state-of-the-art instruments plus one experiment that uses the spacecraft telecommunication system with ground-based instruments.

→ Jupiter; → icy; → explorer; → moon.

A quantity of mass equal to 1.898 × 10²⁷ kg, about 0.000954 → solar masses, or 317.83 → Earth masses. Jupiter mass, M_J, is used as a → unit to describe masses of the → gas giant, such as the outer planets and → extrasolar planets. Similarly, → brown dwarf masses are expressed in terms of Jupiter mass.

→ Jupiter; → mass.

The gaseous envelope surrounding Jupiter. It is about 90% → hydrogen and 10% → helium (by numbers of atoms, 75/25% by mass) with traces of → methane, → water, and → ammonia. This is very close to the composition of the primordial → solar nebula from which the entire solar system was formed. Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium. The outermost layer is composed primarily of ordinary → molecular hydrogen and helium. Visually, Jupiter is dominated by two atmospheric features; a series of ever-changing atmospheric cloud bands arranged parallel to the equator and an oval atmospheric blob called the → Great Red Spot.

→ Jupiter; → atmosphere.

Any of several faint, dark, narrow rings around Jupiter. Jupiter's rings are so faint and tenuous that are only visible when viewed from behind Jupiter and are lit by the Sun, or directly viewed in the infrared where they faintly glow. Unlike → Saturn's rings full of large icy and rock chunks, they are composed of tiny rock fragments and dust. Jupiter's rings are continuously losing material and being resupplied with new dust from → meteorite impacts with Jupiter's four inner moons (→ Metis, → Adrastea, → Amalthea, and → Thebe). Jupiter's rings were discovered by NASA's Voyager 1 in 1979. They are composed of three parts: the → Main ring, a → Halo ring that orbits closer to Jupiter, and a very wide → Gossamer ring that extends far from Jupiter.

→ Jupiter; → ring.

The eleventh of Jupiter's known satellites; it is 36 km across and orbits Jupiter at a distance of about 11,720,000 km with a period of 259 days. It was discovered by S. Nicholson in 1938.

Lysithea was a daughter of Oceanus and one of Zeus' lovers.