The Major Moons of Saturn
Titan, Rhea, Iapetus, Dione, Tethys, and Enceladus
Beautiful ringed Saturn, is the most recognized planet in our solar system, yet Saturn is not alone, but accompanied by many moons. Here, we will learn about the 6 largest moons of glorious Saturn. You will learn about hazy Titan, wispy Rhea, two-faced Iapetus, fractured Dione, reflective Tethys, and tiger-striped Enceladus.
Titan is the second largest moon in the solar system, and Saturn's largest moon. Titan is actually larger than Mercury. Titan's surface temperature is somewhere around 289Â°F below zero. Brrr! Titan is the only moon to have a fully formed thick atmosphere and pools of liquid on it's surface. Titan's atmosphere is so thick with such low gravity that if humans attached man-made wings to their arms, they could fly through it. (Personally, I've always wanted to fly, maybe my next vacation will be on Titan.)
Titan has no magnetic field and sometimes orbits outside Saturn's magnetosphere, causing Titan to be buffeted by solar winds. These winds may ionize and carry away some molecules from the top of the atmosphere closer to the surface, thereby either causing or adding to the thick atmospheric haze. This hazy atmosphere can be seen in image to the right taken by the Cassini Spacecraft.
Titan was the first moon of Saturn's to be discovered. In 1655, Dutch Astronomer Christiaan Huygens was inspired by the improvements Galileo made to the telescope allowing him to discover the Galilean moons of Jupiter.
Titan is believed to have liquid lakes, clouds, an atmosphere, mountains, and cryovolcanoes.
Titans lakes are believed to be liquid ethane with dissolved methane, and range to small bodies of liquid to large seas. While scientists believe methane is located near the surface, it doesn't exist as lakes, however, Titan's methane, through photochemistry, may be converting to ethane, acetylene, ethylene, and (when combined with nitrogen) hydrogen cyanide, which is especially important because it is a building block amino acid, necessary for the formation of life. Besides our Earth, Titan is the only other object in the solar system with stable pools of liquid. The lakes in the Cassini image to the right are called the "Kissing Lakes", now how cute is that!
Clouds have also been seen on Titan. These clouds are believed to be made up of methane and ethane, and also produce wind and rain. This activity produces features akin to shorelines and sand dunes. These clouds attribute to seasonal patterns on Titan.
Titan's atmosphere is extremely thick, thicker than that of earth, and is made up of mostly nitrogen and other hydrocarbon elements, which are also the building blocks for amino acids. Scientists believe that Titan's environment may be similar to that of the Earth's before oxygen began appearing in the atmosphere. Closer to the surface, this atmosphere becomes a thick hazy, orange fog, which unfortunately, doesn't allow for the greenhouse effect like that on Earth.
There also seems to be raised 'mountains' on Titan's surface which may be evidence of cryovolcanic activity. A cryovolcano is a volcano that exists in extremely icy conditions and erupt with plumes of material such as water, ammonia, and methane instead of molten rock.
Below is an image taken by the Cassini Spacecraft of two of Saturn's moons: Titan and Epimetheus with Saturn's rings in the foreground.
Saturn and her moons
Rhea was discovered on December 23, 1672 by Giovanni Cassini. It is the ninth largest moon in the solar system, and Saturn's second largest moon, measuring 949 miles(1,528 km) across.
Cassini discovered 4 moons of Saturn: Tethys, Dione, Rhea and Iapetus.
Rhea is made of 25% rock and 75% water ice. It was once assumed that Rhea had a rocky core, but measurements taken by NASA's Cassini spacecraft indicates the Rhea has an almost homogeneous interior (meaning built of the same material throughout). The shape of Rhea backs up the homogeneous body hypothesis.
Rhea is marked with so many craters, that there are craters within craters. These craters are flatter than those seen on our moon. Rhea has two separate areas of craters; in one area, the craters are typically larger than 25 miles(40km) in diameter, and in the second area they are smaller.
Rhea, the Wisps and Rings
There is an oddity seen by NASA's Cassini and Voyager spacecrafts. They are wispy formations, and the reason for them may be caused by extensive tectonic activities. The two spacecraft were only doing a flyby, and did not get much information on the wisps, but the raw images show them as icy cliffs. Dione, the sister planet of Rhea, also has the wispy areas.
Rhea also may have three small rings around itself. This would be the first discovery of rings around a moon instead of a planet. These faint rings contain dust particles and debris.
Iapetus, or sometimes called Japetus, is the third-largest moon of Saturn, and eleventh in the solar system, discovered by Giovanni Domenico Cassini in 1671. Iapetus has a light hemisphere called Roncevaux Terraand, and a dark hemisphere called Cassini Regio, which gives it almost a ying-yang look. Iapetus also has a ridge near its equator that runs halfway around the moon. Iapetus is tidally locked, keeping the same side towards Saturn.
Iapetus is probably around 80% ice and 20% rocky material and has no atmosphere. It has somewhat flattened poles and a bulge in the middle, with the equatorial ridgeline like a half belt around the middle, distorting it's shape even further (picture a walnut). Iapetus has very large impact craters with steep rims.
Scientists are baffled by the ridge that follows along the equator almost perfectly. Some speculate it was formed when the moon was still young, rotating more rapidly but still tidally locked, bulging from the middle towards the pull of Saturn, and some think it may be solidified icy material that bulged from underground. It was once thought that Iapetus had rings that perhaps collapsed and gathered in this ridge on the surface, but this theory is the least likely since there seems to be some kind of tectonic faults beneath the surface of the moon, which would more likely support the first two theories.
NASA scientists now believe that the material in the dark hemisphere may be residue left over from the evaporation of water ice. The light hemisphere is believed to be caused by precipitation of this water ice. Scientists also believe that initially, this light-dark contrast began with debris being blown off other moons and collecting on half the surface of Iapetus, beginning the contrast which is being sustained by the contrast in temperatures in the moon.
Another way this material may have formed is from methane or ammonia ice erupting from it's interior, since there is a greater concentration on the crater floors.
Iapetus is said to greatly resemble the Death Star from "Star Wars" films!
Dione, the fourth largest moon of Saturn, was discovered by Giovanni Cassini in 1684. Except for being smaller and denser, Dione is similar to Rhea, with both having the wispy linea, known to be icy cliffs. Dione is believed to be composed of mostly water ice with a dense rocky core. Dione's surface is covered with those wispy linea, chasms and impact craters. Dione also has huge fractures on its trailing hemisphere (opposite side of the moon's orbit).
Dione and the Backside Craters
Dione is covered with small craters, large craters, and huge fractures. These areas seem to be somewhat regional, with part of her surface having a lesser number of small impact craters, and another region having a greater number of large impact craters. The smaller ones in the "plains" region are only about 19 miles (30km) in diameter, while the large ones average nearly 62 miles (99km) in diameter! What is odd is that the heavily cratered region is on the trailing hemisphere, which means on the backside of the moon opposite its orbital direction. Scientists believe that earlier in the life of Dione, the moon was tidally locked to Saturn opposite what it is today. Dione is a small enough moon that a large impact could have spun the moon to her current orientation, thus explaining how the backside (remember, the trailing hemisphere) got so heavily cratered.
Hey, Dione, wanna go for a spin?
Tethys was discovered by Giovanni Cassini in 1684, and is a lot like Dione and Rhea. It is made up of mostly water-ice and like Dione and Rhea, it is also heavily cratered. Tethys is extremely reflective at visual wavelengths. One of the most reflective in the solar system.
Tethys has two different regions, a heavily cratered region and a region that consists of a lightly cratered belt that extends across the moon.
Odysseus and the Ithaca Chasma are two amazing features on the Tethys moon.
The Ithaca Chasma is a large canyon. So large that it runs about 1242 miles(2000km)long. That means this canyon runs across 2/3 of the moon. Now that's humongous!
Odysseus is a crater, a huge impact crater that covers about 2/5 of the surface of Tethys. It is about 248 miles (400km) in diameter! This crater is rather flat, so it doesn't have the high peaks and a deep valley as with some craters. Scientists speculate that Tethys had to have been warm and malleable at the time of the Odysseus impact, otherwise, it would have blown the moon to pieces. You see, sometimes being a little softy is not so bad!
Enceladus (enhanced color) Credit:NASA
Enceladus was discovered in 1789 by William Herschel, and is the sixth largest moon of Saturn. While it is the sixth largest moon of Saturn, it is small enough to fit inside the state of Arizona (not that anyone has actually tried it). Enceladus is still geologically active, and has many different types of terrain features. Analysis have shown that the moon erupts plumes of liquid water and other chemicals from it's interior, and therefore, may be a perfect place for the study of astrobiology. It has been argued that materials from Enceladus may be the source for the E-ring of Saturn. This moon reflects almost 100% of the sunlight that reaches it, therefore it is extremely cold. By cold, I mean about 330 degrees (-201Â°C) below zero!
Enceladus the Tiger
Near the south pole of Enceladus, there are these Y and V shaped ridges and valleys loosely called "tiger stripes" by scientists. There are two theories for the cause of these "tiger stripes".
One theory is that Enceladus migrated towards Saturn increasing the rotation of the moon causing the rotation axis to flatten. This flattening would then cause the surface near the poles to push outward and buckle, forming the various ridges.
Another theory is that warm material rose from the interior to the surface leading to a shift in polar terrain from its current area to a new region. Enceladus would then adjust to match the new polar orientation. All of this activity would cause surface tension and the formation of this ridged terrain.