Thebe (), also known as , is the fourth of Jupiter's moons by distance from the

planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...

. It was discovered by Stephen P. Synnott in images from the ''

Voyager 1 ''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and the interstellar medium, interstellar space beyond the Sun's heliosphere. It was launched 16 days afte ...

'' space probe taken on March 5, 1979, while making its flyby of

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...

. In 1983, it was officially named after the

mythological Myth is a genre of folklore consisting primarily of narratives that play a fundamental role in a society. For scholars, this is very different from the vernacular usage of the term "myth" that refers to a belief that is not true. Instead, the ...

nymph A nymph (; ; sometimes spelled nymphe) is a minor female nature deity in ancient Greek folklore. Distinct from other Greek goddesses, nymphs are generally regarded as personifications of nature; they are typically tied to a specific place, land ...

Thebe. The second largest of the inner satellites of Jupiter, Thebe orbits within the outer edge of the Thebe gossamer ring that is formed from dust ejected from its surface. It is irregularly shaped and reddish in colour, and is thought like Amalthea to consist of porous water ice with unknown amounts of other materials. Its surface features include large craters and high mountains—some of them are comparable to the size of the moon itself. Thebe was photographed in 1979 by the ''Voyager 1'' and ''2'' spacecraft, and later, in more detail, by the ''Galileo'' orbiter in the 1990s.

Discovery and observations

Thebe was discovered by Stephen P. Synnott in images from the ''

'' space probe taken on March 5, 1979, and was initially given the provisional designation . In 1983 it was officially named after the mythological

Thebe who was a lover of

Zeus Zeus (, ) is the chief deity of the List of Greek deities, Greek pantheon. He is a sky father, sky and thunder god in ancient Greek religion and Greek mythology, mythology, who rules as king of the gods on Mount Olympus. Zeus is the child ...

—the Greek equivalent of Jupiter. After its discovery by ''Voyager 1'', Thebe was photographed by the ''

Voyager 2 ''Voyager 2'' is a space probe launched by NASA on August 20, 1977, as a part of the Voyager program. It was launched on a trajectory towards the gas giants (Jupiter and Saturn) and enabled further encounters with the ice giants (Uranus and ...

'' space probe in 1979. However, before the ''Galileo'' spacecraft arrived at Jupiter, knowledge about it was extremely limited. ''Galileo'' imaged almost all of the surface of Thebe and helped clarify its composition.

Orbit

Thebe is the outermost of the inner Jovian moons, and orbits Jupiter at a distance of about 222,000 km (3.11 Jupiter radii). Its orbit has an

eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-Centre (geometry), center, in geometry * Eccentricity (g ...

of 0.018, and an

inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Eart ...

of 1.08° relative to the equator of Jupiter. These values are unusually high for an inner satellite and can be explained by the past influence of the innermost Galilean satellite, Io; in the past, several mean-motion resonances with Io would have passed through Thebe's orbit as Io gradually receded from Jupiter, and these excited Thebe's orbit. The orbit of Thebe lies near the outer edge of the Thebe gossamer ring, which is composed of the dust ejected from the satellite. After ejection, the dust drifts in the direction of the planet under the action of Poynting–Robertson drag, forming a ring inward of the moon.

Physical characteristics

Thebe is irregularly shaped, with the closest

ellipsoid An ellipsoid is a surface that can be obtained from a sphere by deforming it by means of directional Scaling (geometry), scalings, or more generally, of an affine transformation. An ellipsoid is a quadric surface; that is, a Surface (mathemat ...

al approximation being 116×98×84 km. Its surface area is probably between 31,000 and 59,000 (~45,000) km². Its bulk density and mass are not known, but assuming that its mean

density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

is like that of Amalthea (around 0.86 g/cm³), its mass can be estimated at 4.3 kg. Similarly to all inner satellites of Jupiter, Thebe rotates synchronously with its orbital motion, thus keeping one face always looking toward the planet. Its orientation is such that the long axis always points to Jupiter. At the surface points closest to and furthest from Jupiter, the surface is thought to be near the edge of the

Roche limit In celestial mechanics, the Roche limit, also called Roche radius, is the distance from a celestial body within which a second celestial body, held together only by its own force of gravity, will disintegrate because the first body's tidal force ...

, where Thebe's gravity is only slightly larger than the

centrifugal force Centrifugal force is a fictitious force in Newtonian mechanics (also called an "inertial" or "pseudo" force) that appears to act on all objects when viewed in a rotating frame of reference. It appears to be directed radially away from the axi ...

. As a result, the

escape velocity In celestial mechanics, escape velocity or escape speed is the minimum speed needed for an object to escape from contact with or orbit of a primary body, assuming: * Ballistic trajectory – no other forces are acting on the object, such as ...

in these two points is very small, thus allowing dust to escape easily after meteorite impacts, and ejecting it into the Thebe gossamer ring. Zethus is the largest (diameter about 40 km) crater on and the only named surface feature of

's moon Thebe. There are several bright spots at the rim of this crater. It is located on the far side of Thebe, facing away from Jupiter. It was discovered by the ''Galileo'' spacecraft. It is named for Zethus (Ζῆθος), the husband of the nymph Thebe in

Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of ancient Greek folklore, today absorbed alongside Roman mythology into the broader designation of classical mythology. These stories conc ...

. The surface of Thebe is dark and appears to be reddish in color. There is a substantial asymmetry between the leading and trailing hemispheres: the leading hemisphere is 1.3 times brighter than the trailing one. The asymmetry is probably caused by the higher velocity and frequency of impacts on the leading hemisphere, which excavates a bright material (probably ice) from the interior of the moon. The surface of Thebe is heavily cratered and it appears that there are at least three or four large

impact crater An impact crater is a depression (geology), depression in the surface of a solid astronomical body formed by the hypervelocity impact event, impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal c ...

s, each being roughly comparable in size to Thebe itself.

References

Explanatory notes Citations Cited sources * * * * * * * * *

External links

Thebe Profile
b
NASA's Solar System Exploration

Thebe nomenclature
from th
USGS planetary nomenclature page
{{DEFAULTSORT:Thebe (Moon) Moons of Jupiter 19790305 Discoveries by Stephen P. Synnott Moons with a prograde orbit