Pharos (crater)

Pharos is a massive impact basin on Neptune's moon Proteus, and as of 2026, it remains the only named surface feature on that world. The crater takes its name from the island of Pharos. Measuring roughly 250 kilometers across and between 10 and 15 kilometers deep, Pharos spans more than half of Proteus’s own diameter, carving a noticeable flat facet into the moon’s limb.

Observation and naming

The basin was first spotted during Voyager 2’s flyby of the Neptune system on August 25, 1989, making it one of the earliest recognizable landmarks on Proteus. (Proteus itself had been discovered just two months earlier, on July 7, 1989.) In a 1992 paper, Steven K. Croft proposed the name Pharos, after the ancient island whose only surviving remnant is the Ras el-Tin promontory in modern Alexandria, Egypt. The International Astronomical Union officially approved the name in 1994.

Geology

Pharos is a degraded crater, bounded by an incomplete outer scarp that has been broken up by later impacts. Its floor is domed but lacks a clear central peak, though a possible peak ring structure—about half the diameter of the outer rim—may exist. Surrounding the basin is a series of depressions, with the most prominent being a set of north-south oriented valleys along the southeast rim.

A ridge also runs parallel to the northeastern rim. These features might actually represent an even larger outer ring, which would bring Pharos’s total diameter to between 500 and 550 kilometers—significantly larger than Proteus itself. However, the available Voyager 2 imagery and shape models are too coarse to confirm this.

Despite the limited resolution, several internal structures have been identified. The most notable is a large impact crater about 100 kilometers wide on the southwestern floor, near 40° S, 20° W. On the northern floor, near 5° S, 0° E, lies a dome or hill roughly 20 kilometers across; its origin is unknown, but a volcanic explanation is considered unlikely given Proteus’s small size and geological history. Much of the western floor is occupied by a broad, winding valley resembling a graben, about 12 kilometers wide and at least 100 kilometers long. This valley cuts across the northern rim of the 100-kilometer crater and may extend farther north, though the terminator in Voyager 2 images obscures its full length.