Saturn's moon Mimas [credit: Cassini mission]

Everybody recognizes Saturn's beautiful rings, but that isn't all that circles the planet. It also has some amazing moons.

Saturn has 145 known moons, most of them tiny.
If we add up the mass of all Saturn's moons and rings, Titan has 96% of the total mass. Poor Rhea, Iapetus, Dione, Tethys, Enceladus and Mimas share less than 4%. And the tiny moons plus rings? About 0.04%.

Titan could be a planet if it orbited the Sun instead of Saturn.
Titan [TYE-tun] is bigger than the planet Mercury and it has a thick atmosphere. Titan and Earth are the only Solar System bodies with nitrogen atmospheres and surface liquid. However, Titan's lakes are liquid methane, not water.

Rhea has an oxygen exosphere – an exosphere is a really, really thin atmosphere.
There was some excitement in 2010 when NASA announced that Rhea [REE-uh] had an oxygen-carbon dioxide atmosphere. But let's not get too excited. Titan has an atmosphere. Rhea has an exosphere so thin that it's five trillion times thinner than Earth's atmosphere.

One half of Iapetus is ten times brighter than the other half.
Iapetus [ee-AP-eh-tus] looks strange. Imagine a globe divided in half by a line circling the north and south poles to make two halves, called hemispheres. Like our Moon, Saturn's moon Iapetus always has the same side facing the planet. So as it orbits, one hemisphere always leads. The leading hemisphere of Iapetus is as dark as coal, and the other one is ten times brighter.

Dione is back to front.
The front side of a moon gets more craters because it runs into space rocks as the moon orbits. But the back of Dione [dye-ON-ee], which should be protected, has many more craters than the front. A collision probably turned it around, but we don't know what happened.

Tethys is made of ice.
Saturn's moons have a lot of ice in them. Tethys [TEETH-iss] is made almost completely of frozen water, including an icy surface that makes it very reflective. If it orbited Earth, and were where our Moon is, we'd see a body the size of the Moon but much, much brighter.

Enceladus has ice volcanoes and a hidden ocean.
Enceladus (en-SELL-uh-dus) is one of four Solar System bodies that we've seen erupting. It has many volcanoes at the south pole. But these aren't the kind of volcanoes we have on Earth shooting out hot lava. These volcanoes are called cryovolcanoes - “cold volcanoes”. They erupt liquids and vapors such as water, methane, or ammonia.

Mimas is the smallest known round body in the Solar System.
Gravity acts on matter. The amount of matter in a body is its mass. If it has enough mass, gravity pulls it into a sphere (ball). Saturn's larger moons are rounded, but the tiny ones aren't. Mimas (MY-mass) is the smallest natural body we know of that is round. Little Mimas has also been severely battered and is heavily cratered. Although its diameter is just 400 km (250 miles), it has an impact crater 140 (87 miles) across.

Some of Saturn's rings have shepherd moons.
Shepherds herd sheep and shepherd moons herd rings! Some small moons have orbits to one side of a ring and their gravitational pull helps maintain the ring shapes and the gaps between rings.

Christiaan Huygens discovered Saturn's biggest moon Titan in 1655.
The European Space Agency's (ESA) Huygens probe landed on Titan in January 2005. It was the first lander to set down in the outer Solar System.

Jean Dominique Cassini discovered four of Saturn's moons between 1671 and 1684.
NASA's Cassini spacecraft went into orbit around Saturn on July 1, 2004 to study the planet and its rings and moons. The spacecraft also carried the Huygens probe that later landed on Titan.

None of Saturn's moons had a name until the nineteenth century.
From the time Huygens found Titan, it took nearly two hundred years to name the moons. English astronomer John Herschel suggested names for them in 1847. Until then they just had Roman numerals, so Titan was Saturn I. In Greek mythology Saturn was Cronus. He was the ruler of the elder gods, the giants called Titans. Herschel named the largest moon Titan, and he gave the other moons names of Titans. Since then, astronomers have run out of Titans and Greek myths, so they now use other mythologies for the new discoveries.