Thebe (moon)
Thebe, also known as Jupiter XIV, is the fourth moon outward from the giant planet Jupiter. On the 1st of May 2026, the Juno spacecraft photographed it from only 5,000 kilometres away. That sharp image is a long way from the moon's first appearance in spacecraft pictures in 1979. Thebe is small, lumpy and battered, yet it has a ring of its own. Who spotted it, and why does its name bend a rule? What bent its orbit out of line? Where does its ring come from? And what is written on its scarred face?
On the 5th of March 1979, the Voyager 1 probe swept past Jupiter and took a series of images. Stephen P. Synnott found a previously unknown moon in those pictures. It was given the provisional label S/1979 J 2. Voyager 2 photographed the same moon later in 1979. After that, knowledge of it stayed extremely limited for years. In 1983, the moon was officially named Thebe, after a nymph from Greek mythology who was a lover of Zeus. Zeus is the Greek counterpart of Jupiter. The choice needed special permission. The IAU convention says that prograde moons of Jupiter must receive names ending in the letter a, and Thebe does not. The exemption was approved anyway. The real breakthrough came when the Galileo spacecraft arrived and imaged almost all of Thebe's surface.
Thebe circles Jupiter at a distance of about 222,000 kilometres, which equals 3.11 Jupiter radii. That makes it the outermost of the inner Jovian moons. Its orbit has an eccentricity of 0.018 and an inclination of 1.08 degrees relative to Jupiter's equator. Those numbers sound tiny, but they are unusually high for a satellite so close to its planet. The likely explanation involves Io, the innermost of the Galilean satellites. Io has gradually receded from Jupiter over time. As it moved outward, several mean-motion resonances with Io would have swept through the region where Thebe orbits. Each pass excited Thebe's orbit and left it slightly stretched and tilted. Today that same orbit lies near the outer edge of a faint ring that carries the moon's name.
The Thebe gossamer ring is made of dust that the moon itself has thrown off. Once the dust is free, a force called Poynting-Robertson drag pulls it slowly toward Jupiter. The grains therefore pile up inward of the moon, and that forms the ring. Thebe loses dust so easily because of how it sits in space. Like all the inner satellites of Jupiter, it rotates in step with its orbit, so one face always looks toward the planet. Its long axis always points at Jupiter. At the surface points nearest to and farthest from Jupiter, the moon's own gravity is only slightly larger than the centrifugal force. The surface there is thought to be near the edge of the Roche limit. Escape velocity at those two spots is very small. A meteorite impact can therefore lift dust off with ease and send it into the ring. The surface that supplies this dust turns out to be dark, red and heavily marked.
Thebe is the second largest of the inner satellites of Jupiter, and it is irregularly shaped rather than round. Its mean density is greater than that of its neighbour Amalthea. The lower bound is 1.0 grams per cubic centimetre, and the likelier figure is closer to 1.4. Like Amalthea, Thebe is thought to be made of porous water ice mixed with unknown amounts of other materials. The mass is known only to some extent. Measurements give a lower bound rather than a firm value. That uncertainty leaves open exactly what the rest of the moon contains. What can be seen directly is the outside, and the outside shows plenty of damage.
The largest crater on Thebe is Zethus, which measures about 40 kilometres across. Zethus is the only named surface feature on the whole moon. It sits on the far side, facing away from Jupiter, and several bright spots mark its rim. Galileo discovered it, and it takes its name from Zethus, the husband of the nymph Thebe in Greek mythology. The wider surface is dark and reddish, and it is heavily cratered. There appear to be at least three or four large impact craters, each roughly comparable in size to Thebe itself. High mountains also rise from the surface, and some are comparable to the size of the moon. One hemisphere is also noticeably brighter than the other. The leading hemisphere is 1.3 times brighter than the trailing one. Impacts there are faster and more frequent, so they probably dig out bright material, most likely ice, from the interior. Thebe's own bright interior shows through where the impacts hit hardest.
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Who discovered Thebe, the moon of Jupiter?
Stephen P. Synnott discovered Thebe in images taken by the Voyager 1 space probe on the 5th of March 1979. It first carried the provisional designation S/1979 J 2.
How did Thebe the moon get its name?
Thebe was officially named in 1983 after a nymph from Greek mythology who was a lover of Zeus. The name was approved as an exemption from the IAU convention that prograde moons of Jupiter must receive names ending in a.
How far is Thebe from Jupiter?
Thebe orbits Jupiter at a distance of about 222,000 km, or 3.11 Jupiter radii. It is the outermost of the inner Jovian moons and the fourth moon by distance from the planet.
What is Thebe made of?
Thebe is thought to consist of porous water ice with unknown amounts of other materials, like Amalthea. Its mean density has a lower bound of 1.0 g/cm3 and is more likely closer to 1.4 g/cm3.
Why does Thebe have a ring?
The Thebe gossamer ring forms from dust ejected from the moon's surface by meteorite impacts. Escape velocity is very small at the points nearest to and farthest from Jupiter, so dust escapes easily and drifts inward under Poynting-Robertson drag.
What is Zethus on Thebe?
Zethus is the largest crater on Thebe at about 40 km across, and it is the moon's only named surface feature. It lies on the far side facing away from Jupiter and is named for the husband of the nymph Thebe in Greek mythology.
All sources
16 references cited across the entry
- 1BookA Practical Dictionary of the English LanguageNoah Webster — 1884
- 2Thebe - NASA Science2017-11-30
- 4Planetary Names: Crater, craters: Zethus on ThebeUnited States Geological Survey — October 3, 2006
- 5BookSolar System MoonsJürgen Blunck — Springer Berlin Heidelberg — 2010
- 6JournalA Constraint on the Density of Jupiter's Moon Thebe from Primordial DynamicsIan R. Brunton et al. — 2025-09-01
- 7JournalThe Formation of Jupiter's Faint RingsJoseph A. Burns et al. — 14 May 1999
- 8Jupiter's Ring-Moon SystemJoseph A. Burns et al. — Cambridge University Press — 2004
- 9JournalCassini ISS astrometric observations of the inner jovian satellites, Amalthea and ThebeN. J. Cooper et al. — March 2006
- 10JournalIo and the Minor Jovian Moons – Prospects for JUICETilmann Denk et al. — 5 March 2026
- 11JournalSatellites of JupiterBrian G. Marsden — April 28, 1980
- 12JournalSatellites of Jupiter and SaturnBrian G. Marsden — September 30, 1983
- 13JournalLeading/Trailing Albedo Asymmetries of Thebe, Amalthea, and MetisD. P. Simonelli et al. — October 2000
- 14Journal1979J2: The Discovery of a Previously Unknown Jovian SatelliteS. P. Synnott — 14 November 1980
- 15JournalThe Small Inner Satellites of JupiterP. C. Thomas et al. — September 1998
- 16Zethus on ThebeUSGS/IAU — USGS Astrogeology