Barnard Regio

Barnard Regio is a dark, irregular patch on Ganymede, one of Jupiter’s moons. It is made up of several ancient, dark areas that are crossed by many younger, brighter grooves called sulci.

The dark regions on Ganymede, known as regiones, are named by the International Astronomical Union after astronomers who helped discover Jupiter’s moons. Barnard Regio honors Edward Emerson Barnard, the American astronomer who found Amalthea—Jupiter’s fifth-largest moon, its largest minor moon, and the first Jovian moon discovered since Galileo spotted the four Galilean moons almost 300 years earlier. The IAU approved the name in 1979.

Location and geography

This region covers most of the equatorial area on the side of Ganymede that always faces Jupiter. Like all dark areas on the moon, Barnard Regio is thought to be among the oldest parts of the surface. It is rugged and covered with ancient dark material, likely accumulated from dark fragments brought by impacting asteroids and comets.

To the north, Phrygia Sulcus separates Barnard Regio from Perrine Regio. An unnamed narrow sulcus divides it from Nicholson Regio to the south. Dardanus Sulcus cuts through the southern half from north to south, splitting it in two. The more complex and chaotic Mysia Sulci run from the northeast to the southwest, breaking the region into many separate segments that look like islands.

A few craters lie inside and around Barnard Regio, but only a handful have names. Within the region, three craters are named: the bright ray crater Ishkur, and two fainter ones, Danel and Isimu. Craters without rays are generally older than those with rays. To the north sits the major ray crater Tros; to the west, the prominent dome crater Serapis; to the northwest, the bright crater chain Nanshe Catena; and to the east, the rayless crater Misharu and the dark-ray crater Kittu.

Barnard Regio lies within the Dardanus quadrangle (Jg6) and the Misharu quadrangle (Jg10) of Ganymede. A small part crosses into the Nabu quadrangle (Jg11) to the southwest. The region also contains the spot where Ganymede’s prime meridian—the line of 0° longitude that always points toward Jupiter because the moon rotates synchronously—meets its equator.

Relationship to other regiones

A 2020 study by Hirata, Suetsugu, and Ohtsuki suggests that Ganymede may have been hit by a huge impactor about 4 billion years ago.