Thebe (moon)

Lore & Background

Thebe was discovered by Stephen P. Synnott in images from the Voyager 1 space probe taken on March 5, 1979, during its flyby of Jupiter. It was initially given the provisional designation S/1979 J 2. In 1983, it was officially named after the mythological nymph Thebe, a lover of Zeus—the Greek equivalent of Jupiter. This name was approved as an exemption from the IAU convention requiring prograde Jovian moon names to end in 'a'.

Thebe orbits Jupiter at about 222,000 km (3.11 Jupiter radii) with an eccentricity of 0.018 and an inclination of 1.08° relative to Jupiter's equator—values unusually high for an inner satellite. These are thought to be explained by past mean-motion resonances with Io as Io gradually receded from Jupiter. Thebe rotates synchronously with its orbital motion, keeping one face always toward Jupiter, with its long axis pointing to the planet. At the surface points closest to and furthest from Jupiter, gravity is only slightly larger than centrifugal force, making escape velocity very small and allowing dust to escape easily after meteorite impacts, feeding the Thebe gossamer ring.

Thebe is irregularly shaped, with a best-fit ellipsoid of 116 × 98 × 84 km. Its mean density has a lower bound of 1.0 g/cm³ and is likely closer to 1.4 g/cm³; its mass has a lower bound of 5 × 10¹⁷ kg. The surface is dark, reddish, and heavily cratered, with at least three or four large impact craters comparable in size to the moon itself. The leading hemisphere is 1.3 times brighter than the trailing one, likely because higher-velocity impacts on the leading side excavate bright material (probably ice) from the interior. The only named surface feature is Zethus, a crater about 40 km in diameter on the far side of Thebe, discovered by the Galileo spacecraft.

Reader's Guide

Thebe holds significance as the fourth moon of Jupiter by distance and the second largest of its inner satellites. Its discovery by Stephen P. Synnott from Voyager 1 images on March 5, 1979, added a new member to Jupiter's satellite system. Thebe's orbit lies at the outer edge of the Thebe gossamer ring, which is composed of dust ejected from its surface—a direct link between the moon and Jupiter's faint ring system. Its irregular shape, reddish colour, and composition of porous water ice with unknown other materials are characteristic of the inner Jovian moons. Thebe's unusually high orbital eccentricity and inclination for an inner satellite are attributed to past gravitational interactions with Io. The moon was photographed by Voyager 1 and 2 in 1979, and later in more detail by the Galileo orbiter in the 1990s and by Juno in May 2026. Its surface features large craters and high mountains, some comparable to the moon's own size, and the leading/trailing hemisphere brightness asymmetry provides clues about impact processes. Thebe's low escape velocity at certain points facilitates dust ejection into the gossamer ring, illustrating the dynamic interplay between a small moon and its planetary ring environment.

Did You Know?

• Thebe was discovered by Stephen P. Synnott in Voyager 1 images taken on March 5, 1979.
• Thebe is the second largest of Jupiter's inner satellites and orbits within the outer edge of the Thebe gossamer ring.
• Thebe's largest named crater, Zethus, is about 40 km in diameter and located on the far side of the moon.
• Thebe rotates synchronously with its orbit, keeping one face always toward Jupiter.