Mimas (Saturn's Moon) - (Important Facts) {Part-44}

MIMAS, AT 198km/123mi, a smallest body that can be for its own gravity to caused it to settle into spherical shape.

Mimas

With a very low density, Mimas is largely composed of water ice, with a small quantity of rock.
The most striking feature of the moon is an enormous crater 130km/81mi across, giving it an appearance not unlike the 'Death Star' in the Star Wars movies. This is a coincidence as detailed photos of the moon were only three years after the movie was made.

Enceladus (Saturn's Moon) - (Important Facts) {Part-43}

ENCELADUS IS A GEOLOGICALLY ACTIVE MOON with a patchy surface showing evidence of ancient craters and relatively smooth plains with extensive grooving, the result of tectonic activity.

Enceladus

The Cassini probe discovered that the moon has active volcanoes and plumes of water rich material have been witnessed venting into space from Geysers, ending up in orbit around Saturn as part of one of its rings.

Structure of Enceladus

Although the surface of the moon is largely water ice, it features a large rocky core. Heating of this core by tidal forces and radioactive decay drives geological activity on the moon.
The plumes of water out-gassing from Enceladus have been found to contain salt, suggesting that the water is being heated underground and that it originates from large caverns or even an ocean of liquid salt water.

Plumes of water out-gassing from Enceladus

This discovery makes Enceladus one of the most favourable places in the Solar System to search for life.

Tethys (Saturn's Moon) - (Important Facts) {Part-42}

COMPOSED ALMOST ENTIRELY OF WATER ICE, Tethys is one of the most reflective objects in the Solar System, having been polished by ice particles in Saturn's rings. As the moon is tidally locked, the leading face is brighter due to this polishing.

Tethys

Apart from impact craters, Tethys features a huge chasm, 100km/62mi wide and 3km/19mi deep, that runs nearly three quarters of the way around the moon, 2,000km/1,243mi in length. This is caused by freezing liquids and expanding as the core cooled, cracking the surface.

Diameter comparison of the Saturnian moon Tethys, Moon, and Earth. Credit: NASA/JPL/USGS/Tom Reding

Like most of Saturn's major moons, it was created from the cloud of material left in orbit after Saturn formed.
With a mean radius of 531.1 ± 0.6 km and a mass of 6.1745 ×1020 kg, Tethys is equivalent in size to 0.083 Earths and 0.000103 times as massive. Its size and mass also mean that it is the 16th-largest moon in the Solar System, and more massive than all known moons smaller than itself combined. At an average distance (semi-major axis) of 294,619 km, Tethys is the third furthest large moon from Saturn and the 13th most distant moon over all.

Dione (Saturn's Moon) - (Important Facts) {Part-41}

DIONE FEATURES A LARGER ROCKY CORE, accounting for approximately one third of the moon's mass, with the rest composed largely or water ice.

Dione

The moon's surface is heavily cratered though unusually, most of the craters are on the trailing face of the moon, suggesting that at some point in its history, a significant impact spun it 180 degrees, leaving it effectively facing 'backwards' as it orbits Saturn.

.Dione is the fourth-largest moon of Saturn, with a diameter of 698 miles (1,123 kilometres).
 It is the 15th largest moon in the solar system.
Dione orbits around Saturn once every 2.7 Earth-days, at a distance of 234,000 miles (377,400 km), approximately the same distance from Earth to its moon.

Saturn's Moon

The trailing hemisphere of the moon is also riddled with bright ice cliffs, the result of huge tectonic fractures.

Rhea (Saturn's Moon) - (Important Facts) {Part-40}

THE SECOND LARGEST MOON IN THE SATURN SYSTEM, Rhea has a composition, appearance and brightness similar to Dione.

Rhea

Around 25% rocky material and 75% water ice, despite its mass giving it the status of the ninth largest moon in the Solar System, it is believed that Rheas's interior is not differentiated. Its structure is a mixture of rock and ice with no defined core.

Like Titan, Rhea has an atmosphere composed of oxygen and carbon dioxide, though it is so thin as to be barely visible. Oxygen in the atmosphere is the result of water ice on Rhea's surface breaking down, but it is not clear where the carbon dioxide originates.

A false colour image of Rhea

Titan (Saturn's Moon) - (Important Facts) {Part-39}

TITAN IS THE SECOND LARGEST MOON in the Solar System, the largest moon of Saturn and is the only moon with a thick atmosphere.

Titan

Its radius is nearly 50% larger than Luna, but it has a lower density, giving it a mass and surface gravity that is around 80% of our Moon.

Structure of Titan

The surface is shrouded in an atmosphere that is 98% nitrogen with clouds of methane, ethane and nitrogen-rich organic smog.
In December 2004, the Huygens lander successfully touched down on Titan revealing world that is strikingly similar to our own, but at a much colder temperature.

Huygens in Titan

Featuring seas of liquid ethane or methane shorelines, sand dunes composed of organic material and terrain that has been weathered by methane rains. Titan is also geologically active, with water ice volcanoes and tectonic activity.
Titan's current state is very similar to the young Earth, with the exception of it lacking water vapour in the atmosphere, making it a prime candidate in the search for life.

Iapetus (Saturn's Moon Series) - (Important Facts) {Part-38}

COMPOSED MOSTLY OF WATER ICE, Iapetus'  most striking features is an equatorial ridge, the dark colouring of its leading face and the light colour of its trailing face.

Iapetus

The colours are likely caused by evaporation of water ice in the leading face leaving behind a dark residue.
It is not known how the equator ridge formed or why it runs along the equator of the moon.

Mystery of Iapetus

View of Iapetus

One possibility is that the young Iapetus was spinning faster then it is now, and if it cooled unevenly while slowing down, then the equator could have frozen before the polar regions, which might preserved the flattened spherical shape it had due to the speed of its rotation.