Thebe , also known as , is the fourth of

Jupiter's moons There are 82 known moons of Jupiter, not counting a number of moonlets likely shed from the inner moons. All together, they form a satellite system which is called the Jovian system. The most massive of the moons are the four Galilean moons: ...

by distance from the

planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...

. 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 interstellar space beyond the Sun's heliosphere. Launched 16 days after its twin ''Voyager 2'', ''Voya ...

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

Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth th ...

. In 1983, it was officially named after the

mythological Myth is a folklore genre consisting of narratives that play a fundamental role in a society, such as foundational tales or origin myths. Since "myth" is widely used to imply that a story is not objectively true, the identification of a narrati ...

nymph A nymph ( grc, νύμφη, nýmphē, el, script=Latn, nímfi, label=Modern Greek; , ) in ancient Greek folklore is a minor female nature deity. Different from Greek goddesses, nymphs are generally regarded as personifications of nature, are typ ...

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 or , , ; grc, Δῐός, ''Diós'', label=genitive Boeotian Aeolic and Laconian grc-dor, Δεύς, Deús ; grc, Δέος, ''Déos'', label=genitive el, Δίας, ''Días'' () is the sky and thunder god in ancient Greek religion, ...

—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, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, ''Voyager 1'', on ...

'' 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-center, in geometry * Eccentricity (graph theory) of a v ...

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 Earth ...

of 1.08° relative to the equator of Jupiter. These values are unusually high for an

inner satellite In astronomy, an inner moon or inner natural satellite is a natural satellite following a prograde, low-inclination orbit inwards of the large satellites of the parent planet. They are generally thought to have been formed ''in situ'' at the same ...

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 may be obtained from a sphere by deforming it by means of directional scalings, or more generally, of an affine transformation. An ellipsoid is a quadric surface; that is, a surface that may be defined as the z ...

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 substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...

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 forc ...

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

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

. As a result, the

escape velocity In celestial mechanics, escape velocity or escape speed is the minimum speed needed for a free, non- propelled object to escape from the gravitational influence of a primary body, thus reaching an infinite distance from it. It is typically s ...

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 A major branch of classical mythology, Greek mythology is the body of myths originally told by the ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the origin and nature of the world, the lives and activities of de ...

. The surface of Thebe is dark and appears to be reddish in color. There is a substantial asymmetry between 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 excavate 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

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

s that are very large, 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