By Luka Novak and Ivanna Subbotina

• Titan

     Titan was discovered on March 25, 1655, by the Dutch astronomer Christiaan Huygens. He named it Saturni Luna (Latin for "Saturn's moon"), After Giovanni Domenico Cassini published his discoveries of four more moons of Saturn between 1673 and 1686, astronomers started calling these moons as well as Titan "Saturn I" through "Saturn V". It has a diameter roughly 50% larger than Earth's moon and is 80% more massive. Titan is the second largest moon in the Solar System.            

                                                                        (Titan, our Earth, and our moon)     

     Titan orbits Saturn once every 15 days and 22 hours. Like the Earth's moon and many of the other gas giant satellites, its orbital period is identical to its rotational period; this makes Titan tidally locked in synchronous rotation with Saturn. It is inclined 0.348 degree relative to Saturn's equator.Titan's bulk composition is half water ice and half rocky material. When we take a closer look at Titan's composition, we know that it most likely differentiated into several layers with a 3400 km rocky center that is surrounded by several layers that are made up of different crystal forms of ice. Its interior may still be hot and there may be a liquid layer consisting of water and ammonia between the ice Ih crust and deeper ice layers made of high-pressure forms of ice.

     Another thing that distingushes Titan from the other moons of the Solar System is the fact that it is the only known moon that has a completely developed atmosphere which consists of more than just trace gases. Titan's atmosphere is very thick (thickness has been suggested ranging between 200 km and 880 km). Observations from the Voyager space probes have shown that Titan's atmosphere is denser than Earth's, with a surface pressure more than one and a half times that of Earth. It has opaque haze layers that block most visible light from the Sun and other sources, which leaves Titan's surface a mystery to the naked eye.

              (Titan's haze layers)

     Titan's atmosphere is made up of 98.4% nitrogen, and the remaining 1.6% is made up of methane and small amounts of other gases such as hydrocarbons, cyanoacetylene, hydrogen cyanide, carbon dioxide, carbon monoxide, cyanogen, argon and helium. The hydrocarbons are thought to form in Titan's upper atmosphere in reactions resulting from the breakup of methane by the Sun's ultraviolet light, producing a thick orange smog. Titan has no magnetic field and sometimes orbits outside Saturn's magnetosphere, This directly exposes it to solar wind.

Another interesting thing about Titan's atmosphere is that it is believed that methane gas is in some way replenished by a reservoir on or within Titan itself, released through eruptions from cryovolcanoes.

      Titan's surface is also remearkable. The first images from the Cassini  revealed a diverse geology, with both rough and smooth areas. There are features that seem to have been formed by volcanoes. There are also streaky features, some of them hundreds of kilometers in length, that appear to be caused by windblown particles. Yet the surface does appear to be relatively smooth;since the few impact craters appear to have been filled in. Titan's surface is marked by broad regions of bright and dark terrain. These include Xanadu, a large, reflective equatorial area that is about the same size as Australia. The convoluted region is filled with hills and cut by valleys and chasms. It is criss-crossed in places by dark lineaments, which are sinuous topographical features that look like ridges or crevices. These may be a sign of tectonic activity. Another possibility is that the lineaments may be liquid formed channels. This might mean that old terrain has been cut through by stream systems. The Cassini–Huygens misson discovered liquid hydrocarbon lakes in Titan's polar regions. This is significant because these are the only large, stable bodies of surface liquid known to exist anywhere other than Earth.

      Scientists believe that conditions on Titan are like those of early Earth. Evidence of volcanic activity suggests that temperatures are probably much higher in hotbeds, enough for liquid water to exist. Argon 40 detection in the atmosphere shows that volcanoes spew molten material of lava composed of water and ammonia. Volcanism is believed to be a significant source of the methane in the atmosphere. Titan's surface temperature is about 94 K (−179 °C, or −290 °F).Its atmosphere is nearly free of water vapor. The haze in Titan's atmosphere adds to the moon's anti-greenhouse effect by reflecting sunlight back into space. This makes its surface significantly colder than its upper atmosphere. When we look at the climate of this moon we see many similarities with Earth, including wind and rain which create surface features that are similar to those on Earth (such as sand dunes and shorelines). Another aspect of Titan's climate that makes it similar to Earth's is the fact that it is dominated by seasonal weather patterns and has liquids (both surface and subsurface).Because of all this, the satellite has been cited as a possible host for microbial extraterrestrial life. Researchers have suggested a possible underground liquid ocean might serve as a biotic environment.

     Titan was examined by bothVoyager 1 in 1980 and Voyager 2 in 1981. Voyager 2 was more successful becasue Voyager 1 could not get beyond the haze of Titans atmosphere. The infromation collected by Voyager 1 and Voyager 2 was not enough so The Cassini–Huygens spacecraft was sent to take radar images of Titan's surface. Huygens landed on Titan an discovered that many of the moon's surface features seem to have been formed by flowing fluids at some point in the past. Interestingly, Titan is the most distant body from Earth that has seen a space probe landing Titan is also the second moon in the solar system to have a man-made object land on its surface.

• Mimas

     Mimas is the innermost of Saturn's larger moons, and the 8th moon orbiting Saturn.Mimas measures 397 km across. It is mainly made up of water ice, with a small amount of rock.Due to the tidal forces acting on it, the moon is not perfectly spherical. One of the craters, named Herschel (after the discoverer of the moon), is surprisingly large in comparison to the size of the moon. The crater is 130 kilometers (80 miles) wide, that is one-third the diameter of Mimas! Herschel is 10 kilometers (6 miles) deep, with a central mountain almost as high as Mount Everest on Earth. This central peak rises 6 kilometers (4 miles) above the crater floor. This impact probably came close to disintegrating the moon. This is evident since fractures(chasmata) can be seen on the opposite side of Mimas that may have been created by shock waves from the impact travelling through the moon's body.There are smaller impact craters on Mimas, but none of them come close to the staggering size of Herschel.Although Mimas is heavily cratered, the cratering is not uniform. This suggests that some process removed the larger craters from these areas, or that something prevented larger stellar bodies from hitting the south polar region, where there are not many crater that are even 20 km (compared to the craters elsewhere on the moon, which are more than 40 km in diameter). Because of the tidal forces acting on it, Mimas is not perfectly spherical; its longest axis is approximately 10% longer than the shortest axis. This gives the moon an ovoid (three-dimensional egg) shape.

     Another interesting thing about Mimas is its relationship with the rings of Saturn. Mimas is responsible for clearing the material from the gap (known as the Cassini Division) between Saturn's two widest rings, A ring and B ring. Particles at the inner edge of this gap orbit twice for each orbit of Mimas. The repeated pulls by Mimas on the particles in this gap, always in the same direction in space, force them into new orbits outside the gap.

     Mimas orbits at a range of 185,520 kilometers (115,280 miles) from Saturn in a time of 22 hours and 37 minutes. This makes Mimas the closest major moon of Saturn. Mimas is tidally locked to Saturn with one side always facing Saturn. Mimas' close orbit means that Mimas probably receives several times the rate of collisions as the other moons of Saturn. This explains its heavily cratered surface.Mimas would probably have been more heavily cratered, but because it was closer to Saturn, Mimas was warmer, and softer, for a longer time so older craters have faded away. 

(Mimas and its many impact craters)

     Mimas has been imaged several times from moderate distances by the Cassini orbiter, the closest being at 63 000 km. Cassini's extended mission will include several non-targeted close approaches to Mimas. Improvements on the current best will occur during passes on October 24, 2008 and October 14, 2009. The closest will be on February 13, 2010 at 9 500 km.

• Rhea

     Rhea is the second-largest moon of Saturn and the ninth largest moon in the Solar System. It was discovered in 1672 by Giovanni Domenico Cassini. Rhea is named after the Titan Rhea of Greek mythology, "mother of the gods".

 Rhea is an icy body with a density of about 1.233 g/cm³. This low density is a sign that it is made of ~approximately 25% rocks (density 3.250 g/cm³) and approximately 75% water ice (density 1.000 g/cm³). Astronomers used to think that Rhea had a rocky core in the center, but measurements taken during a close flyby by the Cassini orbiter determined the axial moment of inertia coefficient as 0.4 kg·m². Such a value indicates that Rhea has almost homogeneous interior (with some compression of ice in the center) while the existence of a rocky core would imply a moment of inertia of about 0.34. The triaxial shape of Rhea is also suggests that Rhea has a homogeneous body in hydrostatic equilibrium.

     The temperature on Rhea is 99 K (−174°C) in direct sunlight and between 73 K (−200°C) and 53 K (−220°C) in the shade. Now lets take a look at Rhea's surface. Rhea is heavily cratered and has bright wispy markings on its surface. Its surface can be divided into two geologically different areas based on crater density; the first area contains craters which are larger than 40 km in diameter, whereas the second area, in parts of the polar and equatorial regions, has craters smaller than 40 km in diameter. This suggests that a major resurfacing event occurred some time during its formation. The leading hemisphere is heavily cratered and uniformly bright. The craters lack the high relief features seen on the Moon and Mercury. On the trailing hemisphere there is a network of bright swaths on a dark background and few visible craters. It had been thought that these bright areas may be material ejected from ice volcanoes early in Rhea's history when its interior was still liquid. However, recent observations seem to suggest that the bright streaks on the Rhean surface are ice cliffs. The January 17, 2006 distant flyby by the Cassini spacecraft yielded images of the wispy hemisphere at better resolution and a lower sun angle than previous observations.

(A color image of Rhea taken by Cassini shows the wispy trailing hemisphere)

     Rhea has been imaged several times from moderate distances by the Cassini orbiter. There was one close targeted fly-by during the primary mission, at a distance of 500 km on November 26, 2005. An additional close flyby at a distance of 5,750 km was performed on August 30, 2007. One targeted encounter is planned for Cassini's two-year extended mission, a 100 km pass on March 2, 2010.

• Iapetus

     Iapetus is the third-largest moon of Saturn, and eleventh in the solar system, discovered by Giovanni Domenico Cassini in 1671. Iapetus is best known for its dramatic 'two-tone' coloration, but recent discoveries by the Cassini mission have revealed several other unusual physical characteristics, such as an equatorial ridge that runs about halfway around the moon.

      (Iapetus, featuring the dark Cassini Regio

              and its border with the bright Roncevaux Terra)

     The low density of Iapetus indicates that it is mostly composed of ice, with only a small (approximately 20%) amount of rocky materials. Unlike most moons, its overall shape is neither spherical nor ellipsoid, but has a bulging waistline and squashed poles; also, its unique equatorial ridge is so high that it visibly distorts the moon's shape even when viewed from a distance. Because of these features,some astronomers classify Iapetus as walnut-shaped. Iapetus is heavily cratered, and Cassini images have revealed large impact basins in the dark region, at least five of which are over 350 km wide. The largest has a diameter over 500 km; its rim is extremely steep and includes a scarp over 15 km high.

(Iapetus' equatorial ridge)

 Iapetus is locked in synchronous rotation about Saturn, and one side of Iapetus is darker than the other. The leading hemisphere and sides are dark (albedo 0.03–0.05) with a slight reddish-brown coloring, while most of the trailing hemisphere and poles are bright (albedo 0.5-0.6, almost as bright as Europa). Thus, the apparent magnitude of the trailing hemisphere is around 10.2, whereas that of the leading hemisphere is around 11.9. The dark region is called Cassini Regio, and the bright region Roncevaux Terra.

     On September 10, 2007, the Cassini orbiter passed within 1640 kilometres (1000 miles) of Iapetus and demonstrated that both hemispheres are heavily cratered. There is dark material filling in low-lying regions, and light material on the pole-facing slopes of craters, but no shades of grey. The material is a very thin layer, only a few tens of centimeters thick in some areas. NASA scientists now believe that the dark material may be residue from the evaporation of water ice on the surface of Iapetus. Because of its slow rotation of 79 days (equal to its revolution and the longest in the Saturnian system), Iapetus probably had the warmest daytime surface temperature and coldest nighttime temperature in the Saturnian system. Near the equator, heat absorption by the dark material results in a daytime temperatures of 128 K in the dark Cassini Regio compared to 113 K in the bright Roncevaux Terra.

     Current triaxial measurements of Iapetus give it dimensions of 747.1 × 749 × 712.6 km, with a mean radius of 736 km. Another  mystery of Iapetus is the equatorial ridge that runs along the center of Cassini Regio, about 1,300 km long, 20 km wide, 13 km high. It was discovered when the Cassini spacecraft imaged Iapetus on December 31, 2004. Parts of the ridge rise more than 20 km above the surrounding plains. Within the bright Roncevaux Terra there is no ridge, but there are a series of isolated 10 km peaks along the equator. The ridge system is heavily cratered, which means that it is vert old. It is not clear how the ridge formed. 

     Temperatures on the dark region's surface reach 130 K (−143.2 °C or −226 °F) at the equator, as heating is made more effective by Iapetus's slow rotation. The brighter surfaces absorb less sunlight so temperatures there only reach about 100 K (−173.2 °C or −280 °F).The orbit of Iapetus is a little unusual. Although it's Saturn's third-largest moon, it orbits much farther from Saturn than the next closest major moon, Titan. It has also the most inclined orbital plane of the regular satellites; only the irregular outer satellites like Phoebe have more inclined orbits. The cause of this is unknown. Because of this distant, inclined orbit, Iapetus is the only large moon from which the rings of Saturn would be clearly visible; from the other inner moons, the rings would be edge-on and difficult to see. From Iapetus, Saturn would appear to be 1°56' in diameter (four times that of the Moon viewed from Earth). Iapetus has been imaged multiple times from moderate distances by the Cassini orbiter. However, its orbit makes close observation difficult. There has been one close targeted fly-by, at 1227 km on September 10, 2007; there are no plans for any others.

(A view of Saturn from Iapetus)

     Iapetus has been imaged multiple times from moderate distances by the Cassini orbiter. However, its orbit makes close observation difficult. There has been one close targeted fly-by, at 1227 km on September 10, 2007. 

Experiments 

Experiment # 1 - Rhea’s Possible Ring System

      On March 6, 2008 NASA announced that Rhea may have a tenuous ring system. This would mark the first discovery of rings about a moon. The rings' existence is inferred by observed changes in the flow of electrons trapped by Saturn's magnetic field as Cassini passed by Rhea. We would send a satellite orbit to inspect and confirm if Rhea actually has a ring system or not and maybe get a clue on how the rings were formed.

Experiment # 2 - Water Underneath Titans Crust

     Scientist propose the idea that oceans or some sort of water body may be found underneath Titan’s crust, but no experiments have been done to prove this theory. We propose using a Synthetic Aperture Radar to collect imaging data. SAR systems take advantage of the long-range propagation characteristics of radar signals and the complex information processing capability of modern digital electronics to provide high resolution imagery. Because of this the radar would see through Titan's thick, methane-rich atmospheric haze, providing us with never-before-seen surface features and establishing their locations on the moon's surface. Its quite possible that there could be lakes and rivers hidden underneath Titans surface. This information could be used to solve some puzzles about Earth’s early days, especially since Titan resembles an early and very primitive Earth.

• Sources:

               http://en.wikipedia.org/wiki/Titan_(moon)

               http://www.solarviews.com/eng/titan.htm

               http://www.solarviews.com/eng/mimas.htm

               http://en.wikipedia.org/wiki/Rhea_(moon)

               http://www.solarviews.com/eng/rhea.htm

               http://en.wikipedia.org/wiki/Iapetus_(moon)

• Images:

                http://upload.wikimedia.org/wikipedia/commons/f/f5/Titan_Earth_Moon_Comparison.png

                http://en.wikipedia.org/wiki/File:Titan-Complex_%27Anti-greenhouse%27.jpg

                http://nirahlee.com/iswwr/evidence/Mimas.jpg

                http://apod.nasa.gov/apod/image/0502/rhea_cassini_big.jpg

                http://www.jacksofscience.com/wp-content/uploads/2007/05/iapetus-real.jpg

                http://en.wikipedia.org/wiki/File:Iapetus_equatorial_ridge.jpg

                http://en.wikipedia.org/wiki/File:Saturn_from_Iapetus.jpg