Janus (moon)

Janus is a small moon that goes around the planet Saturn. A scientist named Audouin Dollfus first saw it on December 15, 1966, but pictures of it were taken even earlier, before anyone realized what they were.

Janus has a special friend, another moon called Epimetheus. They travel around Saturn in almost the same path, passing each other in a way that looks like a dance. This special way of moving 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, just like how Janus and Epimetheus move in their shared orbit.

History

Discovery

Janus was found by Audouin Dollfus on 15 December 1966. It was also photographed earlier by Jean Texereau on 29 October 1966, but he did not know what he had taken. In 1978, scientists learned that earlier pictures showed two objects, Janus and Epimetheus, moving in very similar paths.

Observational history

After its discovery, Janus was seen many times and given different temporary names. It was spotted by Pioneer 11 in 1979 and watched by astronomers in 1980.

Name

Janus is named after the two-faced Roman god Janus. The name was suggested in 1966 after its discovery but became official in 1983.

Orbit

Janus travels around Saturn with another moon named Epimetheus. Janus is only about 50 kilometers closer to Saturn than Epimetheus. Because of this, Janus finishes its orbit around Saturn about 30 seconds faster each day. Over time, this makes the moons move closer to each other.

When they get near, their gravity changes how far they are from Saturn and how fast they travel. This makes the moons switch places every few years. The last time they came close was in January 2006, and then again in 2010, 2014, 2018, and 2022. This special path is called a horseshoe orbit because it looks like a horseshoe when you look 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 has many craters, some larger than 30 kilometers. 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. This suggests it is made mostly of ice and holds together loosely, 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 found in 2006 using special pictures from the Cassini spacecraft. The ring is about 5000 km long 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 paths around Saturn, called a 7:6 orbital resonance. The effect is strongest when Janus, the bigger moon, is closer to Saturn.