Mir (crater)
Small Ganymede crater with bright floor and dark rays.
Mir is a small impact crater on Ganymede, Jupiter’s largest moon, measuring about 8 kilometers across. Its floor is notably bright, while the surrounding rays of ejected material are dark.
The crater is named after a wind deity from Canaanite mythology. Some older sources identify Mir (or Mirmir) as the Chaldeo-Babylonian version of Hadad (also called Rimmon or Ramman), the supreme god of wind and sky in that tradition. The International Astronomical Union approved the name in 1985, following its convention that Ganymede’s craters be named after deities, heroes, or places from ancient Middle Eastern mythologies, including Canaanite and Babylonian ones. Another crater on Ganymede, Adad, shares the same deity.
Mir lies near the center of Melotte Regio, a dark, rugged, and ancient region on Ganymede. It is surrounded by several craters without rays and is cut across by younger, smoother grooved terrain. To the northwest is the crater Ombos; to the northeast, another dark-ray crater called Antum. Southeast lies the bright terrain Hursag Sulcus, and southwest is Kishar Sulcus. Directly south is Lakhamu Fossa. Mir sits within the Tiamat Sulcus quadrangle (designated Jg9) on the hemisphere of Ganymede that always faces away from Jupiter. Because the moon rotates synchronously, one hemisphere permanently faces Jupiter, so an observer at Mir would never see the planet in the sky.
Mir is a relatively small impact crater in a very dark, ancient region. Its estimated depth is about 0.7 kilometers. The bright floor contrasts sharply with the very dark ejecta forming its ray system. These dark rays are mostly discontinuous and extend mainly westward, where they overlay a bright, unnamed sulcus. Spacecraft images show the rays reaching up to 160 kilometers from the crater. Studies suggest that two nearby unnamed craters—one superimposed on Mir’s dark rays—may have contributed to the dark ejecta. Because high-resolution imagery is lacking, it is unclear whether the crater has a central peak or pit. Other examples of dark-ray craters on Ganymede include Antum, Kittu, and Khesnu.
Mir is a dark-ray crater formed on dark terrain but with a bright interior, indicating excavation through a stratified subsurface. Models suggest the impact hit a surface layer of dark ice overlying a brighter, ice-rich layer.
Quick Facts
- Diameter
- 8 km
- Depth
- 0.7 km
- Eponym
- Mir
Facts from the source article.
Lore & Background
Mir crater is located near the center of Melotte Regio, a dark, rugged, and ancient region on Ganymede. It is surrounded by non-ray craters and intersected by younger grooved terrain. To the northwest lies the crater Ombos, to the northeast is Antum, to the southeast is Hursag Sulcus, to the southwest is Kishar Sulcus, and directly south is Lakhamu Fossa. Mir lies on the hemisphere of Ganymede that always faces away from Jupiter, so an observer at Mir would never see Jupiter in the sky.
The crater's floor is very bright, while its ray system is dark and largely discontinuous, extending predominantly westward up to 160 km. Studies suggest two nearby unnamed craters may have contributed to the dark ejecta. Due to lack of high-resolution imagery, it is uncertain whether Mir possesses a central peak or pit. Voyager 2 first imaged Mir in July 1979, and Galileo imaged it in low resolution in May 1999. The Juice mission, arriving at Jupiter in July 2031, is expected to obtain the first close-up images of Mir in 2034.
Spectroscopy from Voyager 2 indicates the floor is rich in water ice with larger ice grains (average 2 mm), while the rays are more contaminated with non-ice material and have smaller grains (average 1 mm). The dark ejecta contain three distinct materials: water ice, hydrated non-ice material, and a less-hydrated reddish carbonaceous component estimated at 40–50% of the ejecta. This carbonaceous component is likely endogenous to Ganymede, supporting the interpretation that excavation of subsurface material at Mir accounts for its dark-ray ejecta.
Reader's Guide
Mir crater is significant as a key example of a dark-ray crater on Ganymede, demonstrating the stratified nature of the moon's subsurface. Its bright floor and dark rays indicate that the impact excavated a dark upper layer of ice overlying a brighter, ice-rich layer, with the maximum excavation depth of 0.7 km implying the dark terrain thickness does not exceed 1 km. This contrasts with larger craters that penetrate deeper and expose bright ice. Spectroscopic observations reveal that Mir's ejecta contain a carbonaceous component likely from Ganymede's interior, comprising about 40–50% of the ejecta, which supports the interpretation that the material is endogenous. The crater's location on the Jupiter-facing hemisphere's opposite side means it has only been imaged clearly by Voyager 2, with Galileo providing only low-resolution views. The upcoming Juice mission in the 2030s is expected to provide the first close-up images, potentially offering insights into dark ray crater formation and Ganymede's interior composition. Mir is one of several dark-ray craters on Ganymede, including Antum, Kittu, and Khesnu.
Did You Know?
- Mir crater has a bright floor but a dark ray system, contrasting sharply with each other.
- The crater's name was approved by the IAU in 1985, after a wind deity from Canaanite mythology.
- Mir is located on the hemisphere of Ganymede that always faces away from Jupiter, so Jupiter is never visible from the crater.
- Voyager 2 provided the only clear images of Mir as of 2026; Galileo imaged it only in low resolution.
More in Moons of Jupiter, Part 3 1-24
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