Thebe (moon)
Thebe is a small inner moon of Jupiter orbiting between Amalthea and Io. It is the second-largest of the planet's four small inner moons. Thebe has an irregular shape and a heavily cratered surface. Dust ejected by micrometeoroid impacts supplies Jupiter's outer gossamer ring.[6][5]
Size and physical characteristics
Thebe has a volumetric mean radius of approximately 49 km. A sphere of equal volume would be about 99 km in diameter, but the moon itself is markedly nonspherical. It is substantially smaller than Amalthea, whose mean radius is 83.5 km, and far smaller than the Galilean moons.[1]
Its mass is estimated at approximately 4.5 × 1017 kg and its mean density at about 0.9 g/cm³. The uncertainties are large: JPL lists a density uncertainty of roughly 0.43 g/cm³. The values permit a porous internal structure but do not establish the proportions of rock, ice and void space.[1]
Gravity is extremely weak. The mass and mean radius give an acceleration of about 0.012 m/s² in a spherical approximation. Local gravity across the elongated, rotating body depends on position. The moon's weak attraction allows small impact fragments to escape readily.[1][2]
Orbit and rotation
Thebe orbits at a mean distance of approximately 222,000 km from the centre of Jupiter. One revolution takes about 16 hours 12 minutes, so it circles the planet almost three times in two Earth days.[3][4]
Its orbit has a small eccentricity of approximately 0.018. The inclination to the local Laplace plane is about 1.1°, greater than Amalthea's. This difference contributes to the different vertical thicknesses of the dust rings associated with the two moons.[3][7]
Thebe rotates synchronously and always keeps the same side facing Jupiter. Galileo's image from 4 January 2000 shows the anti-Jovian hemisphere, which permanently faces away from the planet.[5]
Surface and Zethus crater
Thebe has a dark, reddish surface marked by large impacts. Galileo images resolve features approximately 2 km across. They reveal the principal landforms but not smaller boulders or the fine structure of the surface layer.[2][5]
The most prominent named feature is Zethus, a crater approximately 40 km in diameter. Its width is about two fifths of the equal-volume diameter of the entire moon, making it a major scar that occupies a substantial part of the visible hemisphere.[8][1]
The International Astronomical Union approved the name in 2000. Zethus was the husband of Thebe in Greek mythology. The crater's centre is located at approximately 10° north latitude and 175° west longitude in the planetographic coordinate system.[8]
Thebe gossamer ring
Thebe supplies material to the Thebe gossamer ring, the outer of Jupiter's two broad gossamer rings. Small interplanetary bodies strike the moon and eject dust. Some fragments overcome Thebe's weak gravity and remain in orbit around Jupiter.[2]
The ring is extremely tenuous and consists of microscopic particles. It has a greater vertical thickness than the inner Amalthea gossamer ring because Thebe's orbit is more inclined to Jupiter's equatorial plane, carrying the source of the dust farther above and below that plane.[7]
Galileo images revealed the structure of the rings in detail. The association of the two outer gossamer rings with the orbits of Amalthea and Thebe demonstrated that small moons can continually replenish circumplanetary space with new material.[2][7]
Discovery and name
Stephen Synnott discovered Thebe in Voyager 1 images acquired on 5 March 1979 during the spacecraft's Jupiter encounter. The moon received the provisional designation S/1979 J 2 and is also known as Jupiter XIV.[6][9]
The name Thebe was approved in 1983 after a figure from Greek mythology associated with Zeus. The names of the moon and its largest crater form a related mythological pair.[6][8]
Spacecraft exploration
Both Voyager spacecraft observed Thebe in 1979. Galileo later obtained more detailed images, particularly in late 1999 and early 2000 when its trajectory provided closer views of Jupiter's inner moons.[6][5]
On 4 January 2000, Galileo photographed Thebe from approximately 193,000 km. Zethus crater is clearly visible. NASA released the image as part of a series showing Thebe, Amalthea and Metis at the same scale.[5]
On 1 May 2026, the Juno spacecraft photographed Thebe from a distance of about 5,000 km. The observation used Juno's sensitive Stellar Reference Unit navigation camera, which could image the moon under very low illumination.[10]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Planetary Satellite Physical Parameters // NASA Jet Propulsion Laboratory, Solar System Dynamics.
- ↑ 2.0 2.1 2.2 2.3 2.4 Galileo Finds Jupiter's Rings Formed by Dust Blasted Off Small Moons // NASA Jet Propulsion Laboratory. 1998.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Planetary Satellite Mean Elements // NASA Jet Propulsion Laboratory, Solar System Dynamics.
- ↑ 4.0 4.1 Jovian Satellite Fact Sheet // NASA Space Science Data Coordinated Archive.
- ↑ 5.0 5.1 5.2 5.3 5.4 5.5 Best Images Yet of Thebe, Amalthea and Metis, PIA02531 // NASA Science; NASA/JPL/Cornell University. 24 April 2000.
- ↑ 6.0 6.1 6.2 6.3 6.4 Thebe // NASA Planetary Data System, Planetary Science Institute.
- ↑ 7.0 7.1 7.2 Jupiter's Main and Gossamer Ring Structures // NASA Jet Propulsion Laboratory. 15 September 1998.
- ↑ 8.0 8.1 8.2 Zethus // Gazetteer of Planetary Nomenclature, USGS; International Astronomical Union.
- ↑ Thebe // NASA Science.
- ↑ NASA's Juno Mission Captures Jupiter Moon Thebe, PIA26751 // NASA Science; NASA/JPL-Caltech. 5 May 2026.