Thebe (moon)

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Thebe
Thebe.jpg
Image of Thebe taken by the Galileo spacecraft on January 4, 2000.
Discovery
Discovered by Stephen P. Synnott / Voyager 1
Discovery date March 5, 1979
Designations
Adjective Thebean, Estontonian
Orbital characteristics
Periapsis 218000 km[a]
Apoapsis 226000 km[b]
Mean orbit radius 221889.0±0.6 km (3.11 RJ)[1]
Eccentricity 0.0175±0.0004[1]
Orbital period 0.674536±0.000001 d (16 h 11.3 min)[1]
Inclination 1.076°±0.003° (to Jupiter's equator)[1]
Satellite of Jupiter
Physical characteristics
Dimensions 116 × 98 × 84 km[2]
Mean radius 49.3±2.0 km[2]
Volume ≈ 500000 km3
Mass 4.3×1017 kg[c]
Mean density 0.86 g/cm³ (assumed)
Equatorial surface gravity 0.013 m/s² (0.004 g)[2][d]
Escape velocity 20–30 m/s[3][e]
Rotation period synchronous
Axial tilt zero
Albedo 0.047±0.003[4]
Temperature ≈ 124 K

Thebe (/ˈθb/ THEE-bee; Greek: Θήβη) also known as Jupiter XIV, is the fourth of Jupiter's moons by distance from the planet. It was discovered by Stephen P. Synnott in images from the Voyager 1 space probe taken on March 5, 1979, while making its flyby of Jupiter.[5] In 1983 it was officially named after the mythological nymph Thebe.[6]

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.[3] 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.[2]

Thebe was photographed in 1979 and 1980 by the Voyager 1 and 2 spacecraft, and later, in more detail, by the Galileo orbiter in the 1990s.[2]

Discovery and observations[edit]

Thebe was discovered by Stephen P. Synnott in images from the Voyager 1 space probe taken on March 5, 1979, and was initially given the provisional designation S/1979 J 2.[5][7] In 1983 it was officially named after the mythological nymph Thebe who was a lover of Zeus—the Greek equivalent of Jupiter.[6]

After its discovery by Voyager 1, Thebe was photographed by the Voyager 2 spaceprobe in 1980.[3] 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.[2]

Orbit[edit]

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 of 0.018, and an inclination of 1.08° relative to the equator of Jupiter.[1] These values are unusually high for an inner satellite and can be explained by the past influence of the innermost Galilean satellite, Io;[3] 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.[3]

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.[8]

Physical characteristics[edit]

Thebe is irregularly shaped, with the closest ellipsoidal 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 is like that of Amalthea (around 0.86 g/cm³),[2] its mass can be estimated at roughly 4.3 × 1017 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.[3] At the surface points closest to and furthest from Jupiter, the surface is thought to be near the edge of the Roche lobe, where Thebe's gravity is only slightly larger than the centrifugal force.[3] As a result, the escape velocity in these two points is very small, thus allowing dust to escape easily after meteorite impacts, and ejecting it into the Thebe Gossamer Ring.[3]

The surface of Thebe is dark and appears to be reddish in color.[4] 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.[4] The surface of Thebe is heavily cratered and it appears that there are at least three or four impact craters that are very large, each being roughly comparable in size to Thebe itself.[3] The largest (diameter about 40 km) crater is situated on the side that faces away from Jupiter, and is called Zethus (the only surface feature on Thebe to have received a name).[3][9] There are several bright spots at the rim of this crater.[2]

References[edit]

Explanatory notes

  1. ^ Calculated as a×(1 − e), where a is semimajor axis and e is eccentricity.
  2. ^ Calculated as a×(1 + e), where a is semimajor axis and e is eccentricity.
  3. ^ Estimate is based on the known volume and assumed density of 0.86 g/cm³.
  4. ^ The estimate from Thomas, 1998 was divided by 1.5 to account for the difference in assumed densities.
  5. ^ The estimate from Burns, 2004 was divided by 1.5 to account for the difference in assumed densities.

Citations

Cited sources

External links[edit]