Janus (moon)

Janus is a small moon that orbits the planet Saturn. It was first seen by a scientist named Audouin Dollfus on December 15, 1966, but pictures of it had been taken earlier without anyone noticing. Janus has a special relationship with another moon called Epimetheus. They share almost the same path around Saturn, passing each other in a way that looks like a dance. This unusual arrangement was confirmed when the spacecraft Voyager 1 flew by Saturn in 1980. The moon is named after Janus, a figure from Roman myths who could see in two directions at once, much like how Janus and Epimetheus move in their shared orbit.

History

Discovery

Janus was identified by Audouin Dollfus on 15 December 1966. It had actually been photographed earlier by Jean Texereau on 29 October 1966, but he did not realize what he had captured. Later, in 1978, scientists realized that earlier observations were best explained by two objects, Janus and Epimetheus, sharing very similar orbits.

Observational history

Janus was seen many times after its discovery and given different temporary names. It was detected by Pioneer 11 in 1979 and later observed by several astronomers in 1980.

Name

Janus is named after the two-faced Roman god Janus. The name was suggested soon after its discovery in 1966 but was not made official until 1983.

Orbit

Janus shares its path around Saturn with another moon called Epimetheus. Janus is only about 50 kilometers closer to Saturn than Epimetheus. Because of this small difference, Janus completes its orbit around Saturn about 30 seconds faster each day. Over time, this causes the moons to slowly move toward each other.

When they get close, their gravity affects each other. This changes their distances from Saturn and their travel times. As a result, the moons switch places every few years. The last time they came close was in January 2006, then again in 2010, 2014, 2018, and 2022. This special path is called a horseshoe orbit because it looks like the shape of a horseshoe when viewed from one moon to the other. This is the only known example of such an orbit between moons in our Solar System.

Physical characteristics

Janus is covered in many craters, some bigger than 30 kilometers, and has few straight lines on its surface. Its surface looks older than that of Prometheus but younger than that of Pandora. Janus has a low density and reflects a lot of sunlight, called high albedo, which suggests it is made mostly of ice and holds together like a loose pile, known as a rubble pile object.

Features

Craters on Janus, like those on Epimetheus, are named after characters from the story of Castor and Pollux.

Interactions with rings

A thin dust ring surrounds the area where Janus and another moon called Epimetheus travel around Saturn. This was discovered in 2006 using special images from the Cassini spacecraft. The ring stretches about 5000 km and is made from tiny bits of material knocked off the moons by space rocks.

Together with Epimetheus, Janus helps keep the outer edge of Saturn's main ring, called the A Ring, very sharp. This happens because of a special pattern in their orbits, known as a 7:6 orbital resonance. The effect is strongest when Janus, the bigger moon, is closer to Saturn.