Hershef (crater)
A bright ray crater with a central dome on Ganymede.
Hershef is a large, bright ray crater in the northern hemisphere of Ganymede, the biggest moon of Jupiter. It has a central dome, and its outer rim spans about 120 kilometers (75 miles) across. The crater's bright rays stretch for hundreds of kilometers in every direction.
The crater is named after Heryshaf, a minor Egyptian god of strength and a guardian deity of the Nile. He was depicted with a ram's head, and his cult was influential during the First Intermediate Period of Egypt. Some Ancient Greeks equated him with Heracles (Hercules in Latin) because both were associated with great power. The name follows the IAU convention for Ganymede, which uses deities, heroes, and places from Ancient Near Eastern mythology—including Egyptian religion. The name was approved in 2000.
Hershef sits in Ganymede's northern hemisphere, on the moon's trailing hemisphere. It is surrounded by other ray craters, only two of which are named: Ta-urt and Haroeris. To the west lies the grooved terrain Nun Sulci, and to the northeast is the concentric crater Lagamal. The crater lies on the boundary between the Philus Sulcus (Jg4) and Nun Sulci (Jg5) quadrangles. Because Ganymede is tidally locked to Jupiter, one hemisphere always faces the planet. Hershef is located almost exactly on the boundary between the Jupiter-facing and anti-Jovian hemispheres, so from the crater, Jupiter would always appear low near the horizon.
Hershef is one of many bright ray craters on Ganymede. It has a circular rim and a central dome, and its floor is filled with bright material. The ejecta rays are asymmetrical—they extend farther to the southwest than to the north or northeast. The crater and its ejecta sit on ancient, dark terrain (called regio). These features make it similar to other major ray craters like Osiris. However, ray craters on the trailing hemisphere, including Hershef, are noticeably darker than those on the leading hemisphere. Data from Galileo's NIMS instrument shows Hershef has much lower reflectance beyond 3 μm, indicating larger-grained water ice compared to Osiris, which has finer-grained ice. Scientists think this difference is due to the trailing hemisphere's environment, where charged-particle bombardment (sputtering) alters the ice, not because Hershef is older. Yet research by Shoemaker et al. suggests its darker shade does indicate it is slightly older than Osiris.
Quick Facts
- Diameter
- 120 km
- Eponym
- Hershef
Facts from the source article.
Lore & Background
Hershef is named after Heryshaf, a minor Egyptian god of strength and guardian deity of the Nile, depicted as a ram-headed deity. Some Ancient Greeks considered Heryshaf equivalent to Heracles (Hercules) due to their shared association with strength. The name follows the IAU convention of using deities, heroes, and places from Ancient Near Eastern mythology for Ganymede's surface features, and was approved in 2000.
The crater lies on Ganymede's trailing hemisphere, near the boundary between the Jupiter-facing and anti-Jovian hemispheres, so Jupiter would always appear low on the horizon from the crater. Hershef consists of a circular rim surrounding a central dome, with bright material filling its floor. Its ejecta rays are asymmetrical, longer in the southwestern direction than to the north or northeast. The crater and its ejecta sit on ancient dark terrain (regio).
Using Galileo's Near Infrared Mapping Spectrometer, scientists found Hershef has much lower reflectance beyond 3 μm compared to Osiris, indicating larger-grained water ice. This is attributed to charged-particle bombardment (sputtering) on the trailing hemisphere. However, research by Shoemaker et al. suggests Hershef's darker shade indicates it is slightly older than Osiris. The dome formed when impact heat melted ice, creating meltwater that refroze, expanded, and pushed up the crater's center.
Reader's Guide
Hershef is significant as one of Ganymede's major bright ray craters, offering insights into the moon's geological history and surface processes. Its central dome, formed by meltwater refreezing and expansion, exemplifies a formation mechanism common to craters wider than 60 kilometers. The crater's darker infrared appearance compared to ray craters on the leading hemisphere, such as Osiris, highlights the role of the trailing hemisphere environment in modifying surface ice through charged-particle bombardment. This difference in ice grain size and reflectance provides a natural laboratory for studying space weathering. The crater was first observed by Voyager 1 in March 1979 from a distance, but high-resolution imaging was prevented as Hershef moved into Ganymede's night side. Only the Galileo space probe has imaged it from moderate distance, during a flyby in February 1997. Hershef remains relatively understudied compared to other major ray craters like Tros and Osiris. The upcoming Juice mission, scheduled to enter low polar orbit around Ganymede in 2031, may obtain higher-resolution images that could help explain why its rays are darker than those of other ray craters.
Did You Know?
- Hershef's outer rim measures approximately 120 kilometers (75 miles) in diameter.
- The crater is named after the Egyptian god Heryshaf, associated with strength and the Nile.
- Its central dome formed when meltwater from the impact refroze, expanded, and pushed up the crater's center.
More in Moons of Jupiter, Part 3 1-24
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