All about Janus Moon of Saturn

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Janus: A Half Moon of Saturn

Two Astronomers, One Discovery

In the winter of 1966, two separate Astronomers discovered two halves of an ancient moon that had once been whole but were now circling Saturn in a practically identical orbit. Within days of each other, Audouin Dolfus and Richard Walker happened to be studying the rings of Saturn and had the same idea. When the co-orbit of Janus and its then unknown twin Epimetheus were close enough to the rings of Saturn and the rings were right on the edge of the horizon of Earth, they would appear practically invisible. At that time, the moons orbiting within the rings themselves became visible to observers. When Janus and Epimetheus first became visible it was an historic discovery for both men. It was first thought, however, that the two bodies were a single moon.

Two Halves, One Origin

To understand Janus in its proper context, it is necessary to understand its ongoing relationship to its twin, Epimetheus. It is widely thought that the two moons were likely to have formed by the break-up of a single moon. However, considering the smooth edges of the craters that would have to have happened early in Saturn’s formation. The twins also have an interesting ongoing dance unto themselves as they orbit Saturn; and the trail of rocks and dust they leave behind them creates a faint ring of their own to hint at their constant promenade, although it is only visible to exceptionally powerful telescopes. Janus is likely to be composed of mainly rubble; ice and rocks because it is known to be consisting of mainly water ice and yet is much less dense.

Two Moons, One Orbit

At any given time, one of the moons orbits 50 kilometers (31 miles) higher and thus, will orbit somewhat faster than the other moon. With this small speed disparity, the lagging moon won’t catch up to the faster for about four Earth years. At that point, gravity will sling the inner moon up into the higher orbit pulling it faster. The faster moon will drop into a lower orbit and the drag will slow it down and the cycle will begin over again. Gravity keeps them from actually touching. The closest they ever come to each other is approximately 15,000 kilometers (6,200 miles). Their orbit of Saturn itself takes approximately 17 hours.

Two Discoveries,  One Story

The two Siamese moons were initially together named Janus, back when they were thought to be a single moon, because one side is locked, constantly facing Saturn and one side constantly facing the darkness. The satellite was thought to be appropriately titled as Janus, being the God of beginnings, and while associated with transitions and doorways, was more appropriately depicted as a two-faced god, each looking forward and back, representing a constant bridge from the past into the future.

Later it was discovered that it was, in fact, two moons in nearly identical orbits. This was an obviously incredible discovery. The concept that they were able to continuously maintain their orbit, not only with the planet, but with each other without colliding was extraordinary. Dolfus was eventually given credit for the discovery of Janus and Walker was given credit for Epimetheus, even though it would be twelve years before two other scientists, Stephen M. Larson and John W. Fountain, would realize the truth about the twin sons of Saturn. For more information feel free to read the excellent information provided at the NASA website and the information posted online by the Planetary Society.


 "Planets: Saturn: Moons: Janus." Solar System Exploration. 8 Nov. 2010. Web. 29 Jan. 2011.

 "Saturn's Moon Janus - Explore the Cosmos." The Planetary Society. 1993. Web. 29 Jan. 2011.


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