Arbela Sulcus

Background

Arbela Sulcus is entirely located within the darker, ancient region called Nicholson Regio. The brighter sulcus snakes its way into the middle of Nicholson Regio, beginning from the northeast near a crater called Misharu, extending towards the southwest and roughly bisecting the dark terrain diagonally. It then encounters another sulcus called Dardanus Sulcus approximately midway along its length before crossing over and continuing its westward course, terminating at Nicholson Regio's south-westernmost point near a dark-ray crater called Humbaba. The entire length of Arbela Sulcus is located on the side of Ganymede that always faces Jupiter due to the moon's synchronous rotation.

Like many of Ganymede's bright regions, Arbela Sulcus is a relatively brighter terrain characterized by multiple sets of ridges and grooves mostly running parallel to each other. According to a study by Collins, Head, Pappalardo, Nixon, Giese, Wagner and the Galileo SSI team, it is possible that Arbela Sulcus marks a zone where Ganymede's surface is progressively splitting apart the older and darker original crust before replacing it with a newer surface. Simulations and reconstruction suggest it likely formed through horizontal splitting of Nicholson Regio, followed by diagonal shear that displaced features by as much as 65 kilometres (40 mi), strongly indicating strike-slip faulting played a significant role. However, this reconstruction is not perfect, leaving conclusions regarding spreading still ambiguous.

High-resolution stereo imaging reveals Arbela Sulcus has a sigmoid shape composed of differently oriented segments, with a smooth, north-northeast-trending plank that crosscuts older, east-northeast-trending grooved terrain. The plank surface displays subdued ridges and troughs at 100–200 m scales and stands at a higher elevation than adjacent dark terrain. These characteristics suggest that if Arbela Sulcus originally formed through volcanic flooding, later tectonic processes must have modified its present appearance.

Spatial Division and Regional Boundaries

Arbela Sulcus plays a critical role in structuring the geography of Nicholson Regio. Running from the southwest toward the northeast, it slices through the massive dark feature and, together with Dardanus Sulcus, partitions the regio's dark terrain into three broad segments. Within the eastern lobe specifically, the sulcus acts as a dividing line, separating a northeastern portion from a southeastern portion. It also serves as the southwestern boundary of Nicholson Regio itself, defining where the dark regio ends.

Because Nicholson Regio is so vast that it spans four of Ganymede's mapped quadrangles—Dardanus, Misharu, Nabu, and Namtar, designated Jg6, Jg10, Jg11, and Jg14—Arbela Sulcus necessarily traverses multiple quadrangle boundaries as well. Its path is not a straight line but rather a meandering course that winds through the center of the regio, making it a dominant linear landmark in an otherwise complex and heavily deformed landscape. The sulcus thus functions simultaneously as an internal divider and an external boundary, shaping how geologists delineate and study the various sub-regions of this ancient terrain.