Thebe (moon)

Thebe /ˈθiːbiː/, 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.[7] In 1983, it was officially named after the mythological nymph Thebe.[8]

Thebe

Image of Thebe taken by the Galileo spacecraft on 4 January 2000
Discovery
Discovered by	Stephen P. Synnott / Voyager 1
Discovery date	5 March 1979
Designations
Pronunciation	/ˈθiːbiː/[1]
Named after	Θήβη Thēbē
Adjectives	Thebean /θiːˈbiːən/[2]
Orbital characteristics
Periapsis	218000 km[lower-alpha 1]
Apoapsis	226000 km[lower-alpha 2]
Mean orbit radius	221889.0±0.6 km (3.11 RJ)[3]
Eccentricity	0.0175±0.0004[3]
Orbital period	0.674536±0.000001 d (16 h 11.3 min)[3]
Average orbital speed	23.92 km/s (calculated)
Inclination	1.076°±0.003° (to Jupiter's equator)[3]
Satellite of	Jupiter
Physical characteristics
Dimensions	116 × 98 × 84 km[4]
Mean radius	49.3±2.0 km[4]
Volume	≈ 500000 km^3
Surface gravity	0.04 m/s^2 (0.004 g)[4][lower-alpha 3]
Escape velocity	20–30m/s[5][lower-alpha 4]
Rotation period	synchronous
Axial tilt	zero
Albedo	0.047±0.003[6]
Temperature	≈ 124 K

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.[5] 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.[4]

Thebe was photographed in 1979 by the Voyager 1 and 2 spacecraft, and later, in more detail, by the Galileo orbiter in the 1990s.[4]

Discovery and observations

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.[7][9] In 1983 it was officially named after the mythological nymph Thebe who was a lover of Zeus—the Greek equivalent of Jupiter.[8]

After its discovery by Voyager 1, Thebe was photographed by the Voyager 2 space probe in 1979.[5] 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.[4]

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 of 0.018, and an inclination of 1.08° relative to the equator of Jupiter.[3] These values are unusually high for an inner satellite and can be explained by the past influence of the innermost Galilean satellite, Io;[5] 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.[5]

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.[10]

Physical characteristics

Thebe imaged by the Galileo spacecraft in January 2000

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),[4] 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.[5] At the surface points closest to and furthest from Jupiter, the surface is thought to be near the edge of the Roche limit, where Thebe's gravity is only slightly larger than the centrifugal force.[5] 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.[5]

Zethus Crater is the largest (diameter about 40 km) crater on and the only named surface feature of Jupiter's moon Thebe. There are several bright spots at the rim of this crater.[4] 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, the husband of the nymph Thebe in Greek mythology.[11]

The surface of Thebe is dark and appears to be reddish in color.[6] 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.[6] 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.[5]

References

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. The estimate from Thomas, 1998 was divided by 1.5 to account for the difference in assumed densities.
4. The estimate from Burns, 2004 was divided by 1.5 to account for the difference in assumed densities.

Citations

1. Noah Webster (1884) A Practical Dictionary of the English Language
2. The adjectives "Theban" and sometimes "Thebean" (with stress on the first syllable) refer to the city of Thebes.
3. Cooper Murray et al. 2006.
4. Thomas Burns et al. 1998.
5. Burns Simonelli et al. 2004.
6. Simonelli Rossier et al. 2000.
7. Synnott 1980.
8. IAUC 3872.
9. IAUC 3470.
10. Burns Showalter et al. 1999.
11. "Planetary Names: Crater, craters: Zethus on Thebe". United States Geological Survey. October 3, 2006. Retrieved August 7, 2015.

Cited sources

• Burns, Joseph A.; Showalter, Mark R.; Hamilton, Douglas P.; Nicholson, Philip D.; de Pater, Imke; Ockert-Bell, Maureen E.; Thomas, Peter C. (14 May 1999). "The Formation of Jupiter's Faint Rings". Science. 284 (5417): 1146–1150. Bibcode:1999Sci...284.1146B. doi:10.1126/science.284.5417.1146. PMID 10325220.
• Burns, Joseph A.; Simonelli, Damon P.; Showalter, Mark R.; Hamilton, Douglas P.; Porco, Carolyn C.; Throop, Henry; Esposito, Larry W. (2004). "Jupiter's Ring-Moon System" (PDF). In Bagenal, Fran; Dowling, Timothy E.; McKinnon, William B. (eds.). Jupiter: The Planet, Satellites and Magnetosphere. Cambridge University Press. pp. 241–262. Bibcode:2004jpsm.book..241B. ISBN 978-0-521-81808-7.
• Cooper, N. J.; Murray, C. D.; Porco, C. C.; Spitale, J. N. (March 2006). "Cassini ISS astrometric observations of the inner jovian satellites, Amalthea and Thebe". Icarus. 181 (1): 223–234. Bibcode:2006Icar..181..223C. doi:10.1016/j.icarus.2005.11.007.
• Marsden, Brian G. (April 28, 1980). "Satellites of Jupiter". IAU Circular. 3470. Retrieved 2012-03-28.
• Marsden, Brian G. (September 30, 1983). "Satellites of Jupiter and Saturn". IAU Circular. 3872. Retrieved 2012-03-28.
• Simonelli, D. P.; Rossier, L.; Thomas, P. C.; Veverka, J.; Burns, J. A.; Belton, M. J. S. (October 2000). "Leading/Trailing Albedo Asymmetries of Thebe, Amalthea, and Metis". Icarus. 147 (2): 353–365. Bibcode:2000Icar..147..353S. doi:10.1006/icar.2000.6474.
• Synnott, S. P. (14 November 1980). "1979J2: The Discovery of a Previously Unknown Jovian Satellite". Science. 210 (4471): 786–788. Bibcode:1980Sci...210..786S. doi:10.1126/science.210.4471.786. PMID 17739548.CS1 maint: ref=harv (link)
• Thomas, P. C.; Burns, J. A.; Rossier, L.; Simonelli, D.; Veverka, J.; Chapman, C. R.; Klaasen, K.; Johnson, T. V.; Belton, M. J. S.; Galileo Solid State Imaging Team (September 1998). "The Small Inner Satellites of Jupiter". Icarus. 135 (1): 360–371. Bibcode:1998Icar..135..360T. doi:10.1006/icar.1998.5976.
• USGS/IAU. "Zethus on Thebe". Gazetteer of Planetary Nomenclature. USGS Astrogeology. Retrieved 2012-03-27.

External links

• Thebe Profile by NASA's Solar System Exploration
• Thebe nomenclature from the USGS planetary nomenclature page