Saturn



Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus (the Titan father of Zeus), the Babylonian Ninurta and the Hindu Shani. Saturn's symbol represents the Roman god's sickle.

Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant. Together, these four planets are sometimes referred to as the Jovian, meaning "Jupiter-like", planets. Saturn has an average radius about 9 times larger than the Earth's. While only 1/8 the average density of Earth, due to its larger volume, Saturn's mass is just over 95 times greater than Earth's.

Because of Saturn's large mass and resulting gravitation, the conditions produced on Saturn are extreme if compared to Earth. The interior of Saturn is probably composed of a core of iron, nickel, silicon and oxygen compounds, surrounded by a deep layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium and finally, an outer gaseous layer.

Electrical current within the metallic-hydrogen layer is thought to give rise to Saturn's planetary magnetic field, which is slightly weaker than Earth's magnetic field and approximately one-twentieth the strength of the field around Jupiter.

The outer atmosphere is generally bland in appearance, although long-lived features can appear. Wind speeds on Saturn can reach 1,800 km/h, significantly faster than those on Jupiter.

Saturn has nine rings, consisting mostly of ice particles with a smaller amount of rocky debris and dust. Sixty-two known moons orbit the planet; fifty-three are officially named. This is not counting hundreds of "moonlets" within the rings. Titan, Saturn's largest and the Solar System's second largest moon (after Jupiter's Ganymede), is larger than the planet Mercury and is the only moon in the Solar System to possess a significant atmosphere.





   Eerie-sounding Radio Emissions From Saturn NASA - July 27, 2005




Music of the Rings



Cassini discovers music of the rings
New Scientist - November 9, 2004

Saturn's magnificent ring system - a huge disc resembling an old gramophone record - turns out to share another property with the LP: it constantly emits a melodic series of musical notes. The surprising discovery was made by radio and plasma wave detectors on board the Cassini spacecraft as it passed over Saturn's rings during its arrival at the planet in July. The tones are emitted as radio waves. Don Gurnett of the University of Iowa says his team reduced their frequencies by a factor of five to bring them into the range of human hearing. Gurnett says he was "completely astonished" when he heard the musical notes.

The tones are short, typically lasting between one and three seconds, and unlike the ethereal sliding tones associated with other cosmic processes, every one is quite distinct. The evidence suggests that each tone is produced by the impact of a meteoroid on the icy chunks that make up the rings.

Each hit, Gurnett says, creates a pulse of energy that is focused along the surface of a cone from the point of impact. By estimating the energy involved, he calculates that the impacting objects are about 1 centimetre across - although he cautions that his estimate could be out by as much as a factor of 10. Google Images




A Rough Comparison of the Sizes of Saturn and Earth

Due to a combination of its lower density, rapid rotation and fluid state, Saturn is an oblate spheroid; that is, it is flattened at the poles and bulges at the equator. Its equatorial and polar radii differ by almost 10% - 60,268 km versus 54,364 km.

The other gas planets are also oblate, but to a lesser extent. Saturn is the only planet of the Solar System that is less dense than water. Although Saturn's core is considerably denser than water, the average specific density of the planet is 0.69 g/cm3 due to the gaseous atmosphere. Saturn is only 95 Earth masses, compared to Jupiter, which is 318 times the mass of the Earth but only about 20% larger than Saturn.




Observing Saturn


Saturn's rings require at least a 15 mm diameter telescope to resolve and thus were not known to exist until Galileo first saw them in 1610. He thought of them as two moons on Saturn's sides. It was not until Christian Huygens used greater telescopic magnification that this notion was refuted. Huygens also discovered Saturn's moon Titan.

Some time later, Giovanni Domenico Cassini discovered four other moons: Iapetus, Rhea, Tethys and Dione.

In 1675, Cassini also discovered the gap now known as the Cassini Division.

No further discoveries of significance were made until 1789 when William Herschel discovered two further moons, Mimas and Enceladus. The irregularly shaped satellite Hyperion, which has a resonance with Titan, was discovered in 1848 by a British team.

In 1899 William Henry Pickering discovered Phoebe, a highly irregular satellite that does not rotate synchronously with Saturn as the larger moons do. Phoebe was the first such satellite found and it takes more than a year to orbit Saturn in a retrograde orbit.

During the early 20th century, research on Titan led to the confirmation in 1944 that it had a thick atmosphere - a feature unique among the solar system's moons.

Pioneer 11 Flyby

Saturn was first visited by Pioneer 11 in September 1979. It flew within 20,000 km of the planet's cloud tops. Low resolution images were acquired of the planet and a few of its moons; the resolution of the images was not good enough to discern surface features. The spacecraft also studied the rings; among the discoveries were the thin F-ring and the fact that dark gaps in the rings are bright when viewed towards the Sun, in other words, they are not empty of material. Pioneer 11 also measured the temperature of Titan. The Pioneer images of Saturn were significantly dimmer as the planet and its moons only receive 14.90 (Solar Irradiance) where Jupiter gets around 400. Camera technology would be improved in subsequent missions to the planet.

Voyager Flybys

In November 1980, the Voyager 1 probe visited the Saturn system. It sent back the first high-resolution images of the planet, its rings and satellites. Surface features of various moons were seen for the first time. Voyager 1 performed a close flyby of Titan, greatly increasing our knowledge of the atmosphere of the moon. It also proved that Titan's atmosphere is impenetrable in visible wavelengths; so, no surface details were seen. The flyby also changed the spacecraft's trajectory out from the plane of the solar system.

Almost a year later, in August 1981, Voyager 2 continued the study of the Saturn system. More close-up images of Saturn's moons were acquired, as well as evidence of changes in the atmosphere and the rings. Unfortunately, during the flyby, the probe's turnable camera platform stuck for a couple of days and some planned imaging was lost. Saturn's gravity was used to direct the spacecraft's trajectory towards Uranus.

The probes discovered and confirmed several new satellites orbiting near or within the planet's rings. They also discovered the small Maxwell Gap (a gap within the C Ring) and Keeler gap (a 42 km wide gap in the A Ring).

Cassini-Huygens Spacecraft

On July 1, 2004, the Cassini-Huygens space probe performed the SOI (Saturn Orbit Insertion) maneuver and entered into orbit around Saturn. Before the SOI, Cassini had already studied the system extensively. In June 2004, it had conducted a close flyby of Phoebe, sending back high-resolution images and data.

Cassini's flyby of Saturn's largest moon, Titan, has captured radar images of large lakes and their coastlines with numerous islands and mountains. The orbiter completed two Titan flybys before releasing the Huygens probe on December 25, 2004. Huygens descended onto the surface of Titan on January 14, 2005, sending a flood of data during the atmospheric descent and after the landing.

During 2005, Cassini conducted multiple flybys of Titan and icy satellites. Cassini's last Titan flyby started on March 23, 2008.

Since early 2005, scientists have been tracking lightning on Saturn. The power of the lightning is approximately 1000 times that of lightning on Earth.

In 2006, NASA reported that the Cassini probe found evidence of liquid water reservoirs that erupt in geysers on Saturn's moon Enceladus. Images had also shown particles of water in its liquid state emitted by icy jets and towering plumes.

Cassini probe photographs have led to other significant discoveries. They have revealed a previously undiscovered planetary ring, outside the brighter main rings of Saturn and inside the G and E rings. The source of this ring is believed to be the crashing of a meteoroid off two of the moons of Saturn.

In July 2006, Cassini images provided evidence of hydrocarbon lakes near Titan's north pole, the presence of which were confirmed in January 2007. In March 2007, additional images near Titan's north pole discovered hydrocarbon "seas", the largest of which is almost the size of the Caspian Sea.

In October 2006, the probe detected a 8,000 km diameter hurricane with an eyewall at Saturn's South Pole.

From 2004 to November 2, 2009, the probe discovered and confirmed 8 new satellites. Its primary mission ended in 2008 when the spacecraft had completed 74 orbits around the planet. The probe's mission was extended to September 2010 and then extended again to 2017, to study a full period of Saturn's seasons.




Exploration of Saturn


Saturn was first visited by Pioneer 11 in 1979. It flew within 20,000 km the planet's cloud-tops. Low-resolution images were acquired of the planet and few of its moons. Resolution was not good enough to discern surface features, however. The spacecraft also studied the rings; among the discoveries were the thin F-ring and the fact that dark gaps in the rings are bright when viewed towards the Sun, or in other words, they are not empty of material. It also measured the temperature of Titan.

In November, 1980, Voyager 1 probe visited the Saturn system. It sent back the first high-resolution images of the planet, rings, and the satellites. Surface features of various moons were seen for the first time. Voyager 1 performed a close flyby of Titan greatly increasing our knowledge of the atmosphere of the moon. However, it also proved that Titan's atmosphere is impenetrable in visible wavelengths, so no surface details were seen. The flyby also changed spacecraft's trajectory out from the plane of the solar system.

Almost a year later, in August, 1981, Voyager 2 continued the study of the Saturn system. More close-up images of Saturn's moons were acquired, as well as evidence of changes in the atmosphere and the rings. Unfortunately, during the flyby, the probe's camera stuck and some planned imaging was lost. Saturn's gravity was used to direct the spacecraft's trajectory towards Uranus.

The probes discovered and confirmed several new satellites orbiting near or within the planet's rings. They also discovered the small Maxwell and Keeler gaps.

On July 1, 2004 the Cassini-Huygens spacecraft performed the SOI (Saturn Orbit Insertion) maneuver and entered into orbit around Saturn. Before the SOI Cassini had already studied the system extensively. In June, 2004, it had conducted a close flyby of Phoebe sending back high-resolution images and data. The orbiter completed two Titan flybys before releasing the Huygens probe on December 25, 2004.

This image is an artist's impression of the descent and landing sequence followed by the European Space Agency's Huygens probe to Saturn's moon Titan.

Huygens descended onto the surface of Titan on January 14, 2005 sending flood of data during the atmospheric descent and after the landing. As of 2005, Cassini is conducting multiple flybys of Titan and icy satellites. The primary mission ends in 2008 when the spacecraft has completed 74 orbits around the planet.




Physical Characteristics

Saturn's shape is visibly flattened at the poles and bulging at the equator (an oblate spheroid); its equatorial and polar diameters vary by almost 10% (120,536 km vs. 108,728 km). This is the result of its rapid rotation and fluid state. The other gas planets are also oblate, but to a lesser degree. Saturn is also the only one of the Solar System's planets less dense than water, with an average specific density of 0.69. This is only an average value, however; Saturn's upper atmosphere is less dense and its core is considerably more dense than water.

Though there is no direct information about Saturn's internal structure, it is thought that its interior is similar to that of Jupiter, having a small rocky core surrounded mostly by hydrogen and helium. The rocky core is similar in composition to the Earth, but more dense. This is surrounded by a thicker liquid metallic hydrogen layer, followed by a liquid hydrogen/helium layer and a gaseous atmosphere in the outermost 1000 km. Traces of various volatiles are also present. The core region is estimated to be about 9–22 times the mass of the Earth.

Saturn has a very hot interior, reaching 11,700 °C at the core and it radiates 2.5 times more energy into space than it receives from the Sun. Most of this extra energy is generated by the Kelvin-Helmholtz mechanism (slow gravitational compression), but this alone may not be sufficient to explain Saturn's heat production. It is proposed that an additional mechanism might be at play whereby Saturn generates some of its heat through the "raining out" of droplets of helium deep in its interior, thus releasing heat by friction as they fall down through the lighter hydrogen.

Atmosphere

The outer atmosphere of Saturn consists of 96.3% molecular hydrogen and 3.25% helium. Trace amounts of ammonia, acetylene, ethane, phosphine and methane have also been detected. The upper clouds on Saturn are composed of ammonia crystals, while the lower level clouds appear to be composed of either ammonium hydrosulfide (NH4SH) or water. The atmosphere of Saturn is significantly deficient in helium relative to the abundance of the elements in the Sun.

The quantity of elements heavier than helium are not known precisely, but the proportions are assumed to match the primordial abundances from the formation of the Solar System. The total mass of these elements is estimated to be 19–31 times the mass of the Earth, with a significant fraction located in Saturn's core region.

Cloud Layers

Saturn's atmosphere exhibits a banded pattern similar to Jupiter's (the nomenclature is the same), but Saturn's bands are much fainter and are also much wider near the equator. At depth, extending for 10 km and with a temperature of -23°C, is a layer made up of water ice. Above this layer is probably a layer of ammonium hydrosulfide ice, which extends for another 50 km and is approximately -93°C. Eighty kilometers above that layer are ammonia ice clouds, where the temperatures are roughly -153°C. Near the top of the atmosphere, extending for some 200 km to 270 km above the visible ammonia clouds, are gaseous hydrogen and helium.

Saturn's winds are among the Solar System's fastest. Voyager data indicate peak easterly winds of 500 m/s (1800 km/h). Saturn's finer cloud patterns were not observed until the Voyager flybys. Since then, Earth-based telescopy has improved to the point where regular observations can be made.

Saturn's usually bland atmosphere occasionally exhibits long-lived ovals and other features common on Jupiter. In 1990 the Hubble Space Telescope observed an enormous white cloud near Saturn's equator which was not present during the Voyager encounters and in 1994, another smaller storm was observed.

The 1990 storm was an example of a Great White Spot, a unique but short-lived phenomenon which occurs once every Saturnian year, roughly every 30 Earth years, around the time of the northern hemisphere's summer solstice.

Previous Great White Spots were observed in 1876, 1903, 1933 and 1960, with the 1933 storm being the most famous. If the periodicity is maintained, another storm will occur in about 2020.

In recent images from the Cassini spacecraft, Saturn's northern hemisphere appears a bright blue, similar to Uranus, as can be seen in the image below. This blue color cannot currently be observed from Earth, because Saturn's rings are currently blocking its northern hemisphere. The color is most likely caused by Rayleigh scattering.

Infrared imaging has shown that Saturn's south pole has a warm polar vortex, the only example of such a phenomenon known to date in the Solar System. Whereas temperatures on Saturn are normally -185°C, temperatures on the vortex often reach as high as -122°C, believed to be the warmest spot on Saturn.




The Rings of Saturn


An ultraviolet photo of Saturn's rings

Saturn is probably best known for its planetary rings, which make it one of the most visually remarkable objects in the solar system. See rings of Saturn for a list of the planet's rings.

The rings were first observed by Galileo Galilei in 1610 with his telescope, but he clearly did not know what to make of them. He wrote to the Grand Duke of Tuscany that "Saturn is not alone but is composed of three, which almost touch one another and never move nor change with respect to one another. They are arranged in a line parallel to the zodiac, and the middle one [Saturn itself] is about three times the size of the lateral ones [the edges of the rings]." He also described Saturn as having "ears." In 1612 the plane of the rings was oriented directly at the Earth and the rings appeared to vanish, and then in 1613 they reappeared again, further confusing Galileo.

The riddle of the rings was not solved until 1655 by Christiaan Huygens, using a telescope much more powerful than the ones available to Galileo in his time.

In 1675 Giovanni Domenico Cassini determined that Saturn's ring was actually composed of multiple smaller rings with gaps between them; the largest of these gaps was later named the Cassini Division.

The rings can be viewed using a quite modest modern telescope or with a good pair of binoculars. They extend from 6,630 km to 120,700 km above Saturn's equator, and are composed of silica rock, iron oxide, and ice particles ranging in size from specks of dust to the size of a small automobile. There are two main theories regarding the origin of Saturn's rings. One theory, originally proposed by Edouard Roche in the 19th century, is that the rings were once a moon of Saturn whose orbit decayed until it came close enough to be ripped apart by tidal forces. A variation of this theory is that the moon disintegrated after being struck by a large comet or asteroid. The second theory is that the rings were never part of a moon, but are instead left over from the original nebular material that Saturn formed out of. This theory is not widely accepted today, since Saturn's rings are thought to be unstable over periods of millions of years and therefore of relatively recent origin.

While the largest gaps in the rings, such as the Cassini division and Encke division, could be seen from Earth, the Voyagers discovered the rings to have an intricate structure of thousands of thin gaps and ringlets. This structure is thought to arise from the gravitational pull of Saturn's many moons in several different ways.

Some gaps are cleared out by the passage of tiny moonlets such as Pan, many more of which may yet be undiscovered, and some ringlets seem to be maintained by the gravitational effects of small shepherd satellites such as Prometheus and Pandora. Other gaps arise from resonances between the orbital period of particles in the gap and that of a more massive moon further out; Mimas maintains the Cassini division in this manner. Still more structure in the rings actually consists of spiral waves raised by the moons' periodic gravitational perturbations.

Data from the Cassini space probe indicates that the rings of Saturn possess their own atmosphere, independent of that of the planet itself. The atmosphere is composed of molecular oxygen gas (O2) and is thought to be a product of the disintegration of water ice from the rings into its components, oxygen and hydrogen.

The side of Saturn's rings that is lit by the Sun looks very different to the backlit side, which is darker overall and appears almost black in the thick B ring. From Earth, we cannot appreciate this because the Earth cannot view Saturn from an angle that displays the backlit side of the rings, and our only views of it are from spacecraft. In 2004, the Cassini spacecraft revealed the first views of the backlit side in 25 years.

Until 1980, the structure of the rings of Saturn was explained exclusively as the action of gravitational forces. The Voyager spacecraft found dark radial features in the B ring, called spokes, which could not be explained in this manner, as their persistence and rotation around the rings were not consistent with orbital mechanics. It is assumed that they are connected to electromagnetic interactions, as they rotate almost synchronously with the magnetosphere of Saturn. However, the precise mechanism behind the spokes is still unknown.

As of February 2005, the Cassini spacecraft has not observed any spokes in the rings, despite possessing imaging equipment of higher quality than the Voyagers'. It is possible that the spokes appear and disappear seasonally.


Age of Saturn's Rings Revealed   Live Science - December 13, 2013
Saturn's iconic rings likely formed about 4.4 billion years ago, shortly after the planet itself took shape, a new study suggests. The origin of Saturn's ring system has long been the subject of debate, with some researchers arguing that it's a relatively young structure and others holding that it coalesced long ago, at roughly the same time as the gas giant's many satellites.




Orbit and Rotation

The average distance between Saturn and the Sun is over 1 400 000 000 km (9 AU). With an average orbital speed of 9.69 km/s, it takes Saturn 10,759 Earth days (or about 21 1/2 years), to finish one revolution around the Sun.

The elliptical orbit of Saturn is inclined 2.48° relative to the orbital plane of the Earth. Because of an eccentricity of 0.056, the distance between Saturn and the Sun varies by approximately 155 000 000 km between perihelion and aphelion, which are the nearest and most distant points of the planet along its orbital path, respectively.

The visible features on Saturn rotate at different rates depending on latitude and multiple rotation periods have been assigned to various regions (as in Jupiter's case): System I has a period of 10 h 14 min 00 s (844.3°/d) and encompasses the Equatorial Zone, which extends from the northern edge of the South Equatorial Belt to the southern edge of the North Equatorial Belt. All other Saturnian latitudes have been assigned a rotation period of 10 h 39 min 24 s (810.76°/d), which is System II. System III, based on radio emissions from the planet in the period of the Voyager flybys, has a period of 10 h 39 min 22.4 s (810.8°/d); because it is very close to System II, it has largely superseded it.

A precise value for the rotation period of the interior remains elusive. While approaching Saturn in 2004, the Cassini spacecraft found that the radio rotation period of Saturn had increased appreciably, to approximately 10 h 45 m 45 s. The cause of the change is unknown - it was thought to be due to a movement of the radio source to a different latitude inside Saturn, with a different rotational period, rather than because of a change in Saturn's rotation.

Later, in March 2007, it was found that the rotation of the radio emissions did not trace the rotation of the planet, but rather is produced by convection of the plasma disc, which is dependent also on other factors besides the planet's rotation. It was reported that the variance in measured rotation periods may be caused by geyser activity on Saturn's moon Enceladus. The water vapor emitted into Saturn's orbit by this activity becomes charged and "weighs down" Saturn's magnetic field, slowing its rotation slightly relative to the rotation of the planet. At the time it was stated that there is no currently known method of determining the rotation rate of Saturn's core.

The latest estimate of Saturn's rotation based on a compilation of various measurements from the Cassini, Voyager and Pioneer probes was reported in September 2007 is 10 hours, 32 minutes, 35 seconds.




Saturn's Satellites (Moons)



The moons of Saturn are numerous and diverse, ranging from tiny moonlets less than 1 kilometre across, to the enormous Titan, which is larger than the planet Mercury. Saturn has sixty-two moons with confirmed orbits, fifty-three of which have names, and only thirteen of which have diameters larger than 50 kilometres.

Before the advent of telescopic photography, eight moons of Saturn were discovered by direct observation using optical telescopes. Saturn's largest moon, Titan, was discovered in 1655 by Christiaan Huygens using a 57-millimeter (2.2 in) objective lens on a refracting telescope of his own design. Tethys, Dione, Rhea and Iapetus (the "Sidera Lodoicea") were discovered in 1671-1672 by Giovanni Domenico Cassini. Mimas and Enceladus were discovered in 1789 by William Herschel. Hyperion was discovered in 1848 by W.C. Bond, G.P. Bond and William Lassell.

The use of long-exposure photographic plates made possible the discovery of additional moons. The first to be discovered in this manner, Phoebe, was found in 1899 by W.H. Pickering.

In 1966 the tenth satellite of Saturn was discovered by Audouin Dollfus, when the rings were observed edge-on near an equinox. It was later named Janus. A few years later it was realized that all observations of 1966 could only be explained if another satellite had been present and that it had an orbit similar to that of Janus. This object is now known as Epimetheus, the eleventh moon of Saturn. It shares the same orbit with Janus - the only known example of co-orbitals in the Solar System.

In 1980 three additional Saturnian moons were discovered from the ground and later confirmed by the Voyager probes. They are trojan moons of Dione (Helene) and Tethys (Telesto and Calypso).

Saturn has seven moons that are large enough to become spherical, and dense rings with complex orbital motions of their own. Particularly notable among Saturn's moons are Titan, the second largest moon in the Solar System, with a nitrogen-rich Earth-like atmosphere and a landscape including hydrocarbon lakes and dry river networks, and Enceladus, which emits jets of gas and dust and may harbor liquid water under its south pole region.

Twenty-four of Saturn's moons are regular satellites; they have prograde orbits not greatly inclined to Saturn's equatorial plane. They include the seven major satellites, four small moons which exist in a Trojan orbit with larger moons, two mutually co-orbital moons and two moons which act as shepherds of Saturn's F Ring.

Two other known regular satellites orbit within gaps in Saturn's rings. The relatively large Hyperion is locked in a resonance with Titan. The remaining regular moons orbit near the outer edge of the A Ring, within G Ring and between the major moons Mimas and Enceladus. The regular satellites are traditionally named after Titans and Titanesses or other figures associated with the mythological Saturn.

The remaining thirty-eight, all small except one, are irregular satellites, whose orbits are much farther from Saturn, have high inclinations, and are mixed between prograde and retrograde. These moons are probably captured minor planets, or debris from the breakup of such bodies after they were captured, creating collisional families.

The irregular satellites have been classified by their orbital characteristics into the Inuit, Norse, and Gallic groups, and their names are chosen from the corresponding mythologies. The largest of the irregular moons is Phoebe, the ninth moon of Saturn, discovered at the end of the 19th century.

The rings of Saturn are made up of objects ranging in size from microscopic to hundreds of meters, each of which is on its own orbit about the planet. Thus a precise number of Saturnian moons cannot be given, as there is no objective boundary between the countless small anonymous objects that form Saturn's ring system and the larger objects that have been named as moons.

At least 150 moonlets embedded in the rings have been detected by the disturbance they create in the surrounding ring material, though this is thought to be only a small sample of the total population of such objects.




Titan

Titan (or Saturn VI) is the largest moon of Saturn. It is the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found.

Titan is the sixth ellipsoidal moon from Saturn. Frequently described as a planet-like moon, Titan has a diameter 50% larger than the Moon and is 80% more massive. It is the second-largest moon in the Solar System, after Jupiter's moon Ganymede, and is larger by volume than the smallest planet, Mercury, although only 40% as massive. Discovered in 1655 by the Dutch astronomer Christiaan Huygens, Titan was the first known moon of Saturn, and the fifth known satellite of another planet.

Titan is primarily composed of water ice and rocky material. Much as with Venus prior to the Space Age, the dense, opaque atmosphere prevented understanding of Titan's surface until new information accumulated with the arrival of the Cassini–Huygens mission in 2004, including the discovery of liquid hydrocarbon lakes in Titan's polar regions. The geologically young surface is generally smooth, with few known impact craters, although mountains and several possible cryovolcanoes have been found.

The atmosphere of Titan is largely composed of nitrogen; minor components lead to the formation of methane and ethane clouds and nitrogen-rich organic smog. The climate - including wind and rain - creates surface features similar to those of Earth, such as dunes, rivers, lakes, seas (probably of liquid methane and ethane), and deltas, and is dominated by seasonal weather patterns as on Earth. With its liquids (both surface and subsurface) and robust nitrogen atmosphere, Titan's methane cycle is viewed as an analogy to Earth's water cycle, although at a much lower temperature.

Titan was discovered on March 25, 1655, by the Dutch astronomer Christiaan Huygens. Huygens was inspired by Galileo's discovery of Jupiter's four largest moons in 1610 and his improvements in telescope technology. Christiaan, with the help of his brother Constantijn Huygens, Jr., began building telescopes around 1650. Christiaan Huygens discovered this first observed moon orbiting Saturn with the first telescope they built.


Titan in the News




Iapetus



Iapetus is the third-largest natural satellite of Saturn, and eleventh-largest in the Solar System Iapetus has a radius of about 42%, a density of about 32.5%, and a mass of about 2.5% of that of the Moon. Iapetus is best known for its dramatic 'two-tone' coloration, but discoveries by the Cassini mission in 2007 have revealed several other unusual physical characteristics, such as an equatorial ridge that runs about halfway around Iapetus.

Iapetus was discovered by Giovanni Domenico Cassini, an Italian–French astronomer, in October 1671. He had discovered it on the western side of Saturn and tried viewing it on the eastern side some months later, but was unsuccessful. The pattern continued the following year as he was able to observe it on the western side, but not the eastern side. Cassini finally observed Iapetus on the eastern side in 1705 with the help of an improved telescope, finding it two magnitudes dimmer on that side.

Cassini correctly surmised that Iapetus has a bright hemisphere and a dark hemisphere, and that it is tidally locked, always keeping the same face towards Saturn. This means that the bright hemisphere is visible from Earth when Iapetus is on the western side of Saturn, and that the dark hemisphere is visible when Iapetus is on the eastern side. The dark hemisphere was later named Cassini Regio in his honor.


2-Tone Saturn Moon Caught in Hi-Res National Geographic - October 9, 2007
The first ever high-resolution images of Saturn's moon Iapetus have helped unlock the mystery of the satellite's oddly mottled surface. The walnut-shaped moon has long been known to have dark areas and bright ones. But the detailed new images captured by NASA's Cassini orbiter in September and released yesterday reveal that Iapetus's two-tone color scheme is seen even at the crater level. That is, one side of a crater might be dark, while the other is white. Furthermore, the dark is very dark, while the white is frosty bright.

'Black and white moon' less grey BBC - October 9, 2007
Scientists think they are close to solving the mystery over why Saturnian moon Iapetus has a two-tone appearance. The satellite has a black surface facing in the direction it travels, and a white surface bringing up the rear. New data from the Cassini spacecraft seems to confirm Iapetus is picking up dusty material on its bow front. But, say the mission's scientists, this material is then being warmed by the Sun's rays, making it go even darker as it loses water vapor.

Saturn's Moon Iapetus Is The Yin-yang Of The Solar System Science Daily - September 18, 2007
Cassini scientists are poring through hundreds of images returned from the 10 September fly-by of Saturn's two-toned moon Iapetus. The pictures show the moon's yin and yang - a white hemisphere resembling snow, and the other as black as tar. Recently returned Images show a surface that is heavily cratered, along with the mountain ridge that runs along the moon's equator. Many of the close-up observations focused on studying the strange 20-km high mountain ridge that gives the moon a walnut-shaped appearance.


Iapetus in 3D - Equatorial Ridge NASA - September 15, 2007


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