Proteus (moon)

Proteus, also known as Neptune VIII, is the second-largest moon of the planet Neptune and its largest inner satellite. It was discovered by the Voyager 2 spacecraft in 1989 and is named after Proteus, the shape-changing sea god from Greek mythology.

Proteus orbits Neptune in a path close to the planet's equator, at a distance about 4.75 times the radius of Neptune's equator. Even though it is mostly made of ice and larger than 400 kilometers (250 miles) in diameter, Proteus does not have a round shape like most moons. Instead, it looks more like an irregular polyhedron with several slightly bent sides and tall ridges reaching up to 20 kilometers (12 miles).

The surface of Proteus is dark, with a neutral color, and it is covered in many craters from impacts. Its largest crater, called Pharos, is more than 230 kilometers (140 miles) wide. There are also long cracks, grooves, and valleys on its surface linked to these large craters. Scientists think Proteus may not have formed with Neptune but could have come together later from pieces left over when Triton, Neptune's largest moon, was captured.

Discovery and naming

Proteus was discovered using images from the Voyager 2 space probe two months before it flew by Neptune in August 1989. This discovery happened 40 years after Neptune's moon Nereid was found in 1949.

At first, Proteus was given a temporary name, S/1989 N 1. Stephen P. Synnott and Bradford A. Smith shared the news of its discovery on July 7, 1989. Later, on September 16, 1991, it was officially named after Proteus, the shape-changing sea god from Greek mythology, to match the theme of Neptune's moons being named after sea-related gods and creatures.

Orbit

Proteus orbits Neptune at a distance of about 117,647 kilometers, which is nearly 4.75 times the width of Neptune. Its path around Neptune is almost round and stays close to Neptune's equator. Proteus is always facing the same way toward Neptune as it travels.

At one time, Proteus might have moved in a special pattern with another moon called Larissa, but this pattern changed long ago as Proteus moved farther away from Neptune.

Physical characteristics

Proteus is the second-largest moon of Neptune and the largest of its regular moons that orbit close to the planet. It is about 420 km (260 mi) in diameter, larger than Nereid, Neptune's third-largest moon. It was not discovered by telescopes from Earth because it orbits so close to Neptune that it gets lost in the bright light reflected from the planet.

The surface of Proteus is dark, reflecting only about 10 percent of the light it receives from the Sun. Its color does not change much from violet to green light, but it reflects less in the near-infrared range, suggesting the presence of complex organic compounds like hydrocarbons or cyanides. While scientists think Proteus may contain water ice, there is no clear proof of it on the surface. However, observations from the James Webb Space Telescope suggest possible water ice or similar materials on its surface. Proteus's infrared reflectivity matches that of other dark objects in the Solar System, such as 2004 EW₉₅.

Proteus is heavily cratered, with no signs of recent geological activity. The largest crater, Pharos, is about 255 km wide and 10–15 km deep, with a small dome in the center. Pharos is the only named feature on Proteus. In addition to Pharos, there are several craters between 50 and 100 km wide, and many smaller ones.

Other features on Proteus include straight cliffs, valleys, and grooves. The most noticeable one runs along the equator west of Pharos. These features likely formed from huge impacts that created large craters or from the pulling forces of Neptune's gravity.

Origin

Proteus, like Neptune's other inner moons, probably did not form with the planet. Instead, it likely came from pieces left over after Neptune captured another moon called Triton. When Triton was first captured, its path around Neptune was uneven, causing chaos among the original moons. This led to collisions that broke apart many moons, creating a cloud of debris. Later, as Triton's orbit settled into a steady path, some of this debris clumped together to form the moons we see today, including Proteus.

Proteus may have started closer to Neptune and slowly moved outward over time due to the gravitational pull between Neptune and its moon. As Proteus moved away, collisions might have created large craters and sent pieces into orbit around Neptune. One of these pieces may have become Neptune's smaller moon, Hippocamp.