Marius Regio
Ancient dark region on Ganymede, crisscrossed by bright sulci.
Marius Regio is a large, elongated surface feature on Jupiter’s moon Ganymede, composed of several dark, ancient areas that have been broken into lobes by younger, brighter sulci. It is considered one of the oldest parts of Ganymede, heavily saturated with craters, rifts, and troughs. The dark coloration likely comes from material deposited by asteroids and comets over billions of years. Close-up images show an extremely rugged terrain, similar to the neighboring Galileo Regio, indicating a violent past of impacts. Tectonic activity has resurfaced portions of the regio with fresher water ice, creating the brighter sulci. These dark regions are analogous in age to the Moon’s bright highlands, though Ganymede’s newer surfaces are made of water ice rather than volcanic basalt.
The feature is named after Simon Marius, the German astronomer who co-discovered Jupiter’s four largest moons with Galileo Galilei and suggested naming them after Zeus’s lovers. The International Astronomical Union approved the name in 1979, following its convention of naming Ganymede’s dark regiones after astronomers who contributed to the discovery of Jupiter’s moons.
Marius Regio’s boundaries are marked by several sulci: Philus Sulcus and Elam Sulcus to the north, Uruk and Nippur Sulci to the east (separating it from Galileo Regio), Mashu Sulcus to the west, Tiamat Sulcus to the southwest (separating it from Mellote Regio), and Sippar Sulcus to the south. Its northern lobe is further dissected by Byblus Sulcus and Akitu Sulcus, with the large crater Epigeus located southeast of this lobe. The central main lobe is split into three uneven segments by north-south running Erech Sulcus and Lagash Sulcus; the western segment, the largest, is partly split by Anshar Sulcus and surrounded by many furrows. The regio is saturated with craters, both bright and young as well as dark and old. Named craters include Geinos (a ray crater to the west), Lugalmeslam (an ancient deformed crater to the north), Melkart (a bright ray crater to the southwest), and Erichthonius (a strained, damaged crater to the south). The western side contains several faculae—bright circular features likely palimpsests, or “ghost craters” that have flattened over time. Named faculae include Busiris Facula and Buto Facula (confirmed palimpsests), and Akhmin, Coptos, Hermopolis, and Thebes Faculae (not yet fully confirmed).
Quick Facts
- Length
- 4940 km
- Eponym
- Simon Marius
Facts from the source article.
Lore & Background
Marius Regio is named after Simon Marius, a German astronomer considered co-discoverer of Jupiter's four largest moons, who suggested naming them after lovers of Zeus. The IAU approved the name in 1979. The regio's northernmost boundary is marked by Philus Sulcus and Elam Sulcus; to the east it is separated from Galileo Regio by Uruk and Nippur Sulci, while Mashu Sulcus marks its western boundary. To the southwest, Tiamat Sulcus separates it from Mellote Regio, and Sippar Sulcus marks its southern boundary. The northern part is separated into its own lobe by Mashu Sulcus, crisscrossed by narrower sulci such as Byblus Sulcus and Akitu Sulcus. The central main lobe is split into three uneven segments by Erech Sulcus and Lagash Sulcus. The regio is saturated with craters, both bright young and dark old ones, including named craters Geinos, Lugalmeslam, Melkart, and Erichthonius. The western side hosts several faculae, many probably palimpsests (ancient ghost craters), including Busiris Facula and Buto Facula. Marius Regio is located at the longitude of Ganymede that never faces Jupiter, so an observer there would never see Jupiter. The regio crosses into five quadrangles: Philus, Uruk, Tiamat, Osiris, and a small part of Memphis Facula.
Reader's Guide
Marius Regio is believed to be one of the oldest areas of Ganymede's surface, similar to other dark regions. Its dark shades likely result from dark material delivered by asteroids and comets over its long history. Closeup images reveal extremely rugged terrain saturated with impact craters and furrows, indicating a violent past. Tectonic activity has resurfaced many portions with younger, brighter sulci, where fresher water ice upwells. Dark regions on Ganymede are analogous to the brighter highlands on Earth's Moon in age, but the newer terrain on Ganymede is water ice rather than volcanic basalt. In 2020, planetary scientists Hirata, Suetsugu, and Ohtsuki proposed that Ganymede was struck by a massive impactor roughly 4 billion years ago, possibly producing a global multi-ring structure. They suggested that Marius, Barnard, Galileo, Nicholson, and Perrine Regiones are likely remnants of a single ancient basin, fragmented by tectonic processes and resurfacing. If confirmed, this impact would rank among the largest ever identified in the Solar System. Marius Regio has been photographed by Voyager 1, Voyager 2, and Galileo. Voyager 2 sent back clear images in July 1979; Voyager 1 could not observe it due to its trajectory. Galileo provided the highest resolution images as of 2025. The Juice probe, launched in April 2023, is scheduled to arrive at Jupiter in July 2031 and later enter low orbit around Ganymede, expected to see Marius Regio in great detail.
Did You Know?
- Marius Regio is named after Simon Marius, who co-discovered Jupiter's four largest moons and suggested naming them after lovers of Zeus.
- In 2020, scientists proposed that Marius Regio is part of a giant ancient impact basin, possibly one of the largest in the Solar System.
- The Galileo probe provided the highest resolution images of Marius Regio as of 2025.
More in Moons of Jupiter, Part 3 1-24
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