Achelous (crater)
A young impact crater with bright rays on Ganymede.
Achelous is a fairly young impact crater on Ganymede, Jupiter’s biggest moon, distinguished by its bright rays. It sits near a similar-sized crater called Gula, and both are encircled by a raised pedestal of material blasted out during their formation.
**Naming** The crater takes its name from Achelous, a river god in Greek mythology. According to Pseudo-Apollodorus and certain myths, he was the father of Callirhoe, who in turn was the mother of the Trojan prince Ganymede. The International Astronomical Union (IAU) adopted the name in 1979. Surface features on Ganymede are named after deities, heroes, and places from either ancient Middle Eastern mythologies or Greek myths tied to Ganymede and his homeland, Troy. Achelous falls into the latter group.
**Location** Achelous lies at 61.90°N, 11.78°W on Ganymede’s sub-Jovian hemisphere—the side that always faces Jupiter—roughly two-thirds of the way from the equator to the north pole. It sits north of Aquarius Sulcus and is surrounded by several other impact craters. To its north is Gula, to the east is Sebek, and to the southeast is the oval-shaped Adad. The crater is in the Perrine quadrangle (designated Jg2), in the northeastern corner of that quadrangle. Because Ganymede is tidally locked to Jupiter, an observer at Achelous would always see Jupiter fixed in the same spot in the sky.
**Geological characteristics** Achelous is an impact crater about 35 kilometers (22 miles) across and polygonal in shape. Instead of being perfectly round, its rim consists of seven straight segments. It lies within Ganymede’s bright grooved terrain, where the grooves run northeast–southwest. These grooves appear to have influenced the crater’s shape: two of the seven rim segments align with the grooves, while the other five are oriented roughly 45 degrees from them. Unlike Gula, Achelous lacks a central peak, though its center may hold a collapsed central peak or a central pit.
The ejecta around Achelous has a clear inner and outer layer. The inner raised pedestal extends about 17 kilometers (11 miles) from the rim and has a hummocky texture near the rim. This pedestal completely buries most preexisting features; only the most prominent grooves from the surrounding terrain extend into it. The outer, thinner ejecta layer does not obscure the underlying grooves as much and is generally smoother.
Quick Facts
- Diameter
- 35.0 km
- Eponym
- Achelous
Facts from the source article.
Lore & Background
Achelous is an impact crater roughly 35 km in diameter, polygonal in shape with a rim consisting of seven straight segments. It lies within Ganymede's bright grooved terrain, and the grooves appear to have influenced the crater's shape, with two rim segments aligned with the grooves and the other five oriented roughly 45° from them. Unlike the nearby crater Gula, Achelous lacks a central peak, though its center may be occupied by a collapsed central peak or a central pit. Its surrounding ejecta has a distinct inner raised pedestal extending about 17 km from the rim, with a hummocky texture, and an outer thinner, smoother layer. Beyond these are radially extending secondary craters and crater chains, most 1–2 km in diameter. The double-layered ejecta structure is similar to double-layered ejecta craters on Mars, and since Ganymede lacks a dense atmosphere, this suggests the layers formed due to subsurface volatile material at the impact site. High-resolution images from the Galileo spacecraft showed the ejecta deposits are highly textured, not smooth, providing no direct evidence that liquid water was involved; instead, they resemble large, circular landslide deposits.
Reader's Guide
Achelous is significant as one of only two bright-ray craters on Ganymede that the Galileo spacecraft imaged at close range, allowing planetary scientists to correct initial assumptions that it was a smooth crater and to find no direct evidence of liquid water in its ejecta formation. Its double-layered ejecta structure, similar to Martian double-layered ejecta craters, provides insight into the role of subsurface volatiles on Ganymede. The crater's polygonal shape, influenced by the surrounding grooved terrain, and its depth relative to the similar-sized Gula (which is about half as deep and has a central peak) highlight differences in impact crater morphology. Achelous was first observed by Voyager 1 in 1979, later imaged by Cassini, New Horizons, and most recently by Juno in 2021. The upcoming Juice mission, expected to enter orbit around Ganymede in 2034, will obtain extremely high-resolution images to study Achelous in far greater detail.
Did You Know?
- Achelous is located on the hemisphere of Ganymede that always faces Jupiter due to tidal locking.
- Its crater rim consists of seven straight segments, making it polygonal rather than perfectly round.
- The double-layered ejecta structure of Achelous is similar to double-layered ejecta craters on Mars.
- Galileo's images of Achelous are the best available as of 2026.
More in Moons of Jupiter, Part 2 1-24
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