The Planet Saturn

The farthest planet from the Sun that can be observed with the naked eye, the existence of Saturn has been known for thousands of years. And much like all celestial bodies that can be observed with the aid of instruments – i.e. Mercury, Venus, Mars, Jupiter and the Moon – it has played an important role in the mythology and astrological systems of many cultures.

Saturn is one of the four gas giants in our Solar System, also known as the Jovian planets, and the sixth planet from the Sun. It’s ring system, which is it famous for, is also the most observable – consisting of nine continuous main rings and three discontinuous arcs.

Saturn’s Size, Mass and Orbit:

With a polar radius of 54364±10 km and an equatorial radius of 60268±4 km, Saturn has a mean radius of 58232±6 km, which is approximately 9.13 Earth radii. At 5.6846×1026 kg, and a surface area, at 4.27×1010 km2, it is roughly 95.15 as massive as Earth and 83.703 times it’s size. However, since it is a gas giant, it has significantly greater volume – 8.2713×1014 km3, which is equivalent to 763.59 Earths.

The sixth most distant planet, Saturn orbits the Sun at an average distance of 9 AU (1.4 billion km; 869.9 million miles). Due to its slight eccentricity, the perihelion and aphelion distances are 9.022 (1,353.6 million km; 841.3 million mi) and 10.053 AU (1,513,325,783 km; 940.13 million mi), on average respectively.

Saturn Compared to Earth. Image credit: NASA/JPL
Saturn Compared to Earth. Image credit: NASA/JPL

With an average orbital speed of 9.69 km/s, it takes Saturn 10,759 Earth days to complete a single revolution of the Sun. In other words, a single Cronian year is the equivalent of about 29.5 Earth years. However, as with Jupiter, Saturn’s visible features rotate at different rates depending on latitude, and multiple rotation periods have been assigned to various regions.

The latest estimate of Saturn’s rotation as a whole are based on a compilation of various measurements from the Cassini, Voyager and Pioneer probes. Saturn’s rotation causes it to have the shape of an oblate spheroid; flattened at the poles but bulging at the equator.

Saturn’s Composition:

As a gas giant, Saturn is predominantly composed of hydrogen and helium gas. With a mean density of 0.687 g/cm3, Saturn is the only planet in the Solar System that is less dense than water; which means that it lacks a definite surface, but is believed to have a solid core. This is due to the fact that Saturn’s temperature, pressure, and density all rise steadily toward the core.

Standard planetary models suggest that the interior of Saturn is similar to that of Jupiter, having a small rocky core surrounded by hydrogen and helium with trace amounts of various volatiles. This core is similar in composition to the Earth, but more dense due to the presence of metallic hydrogen, which as a result of the extreme pressure.

Diagram of Saturn's interior. Credit: Kelvinsong/Wikipedia Commons
Diagram of Saturn’s interior. Credit: Kelvinsong/Wikipedia Commons

Saturn has a hot interior, reaching 11,700 °C at its core, and it radiates 2.5 times more energy into space than it receives from the Sun. This is due in part to the Kelvin-Helmholtz mechanism of slow gravitational compression, but may also be attributable to droplets of helium rising from deep in Saturn’s interior out to the lower-density hydrogen. As these droplets rise, the process releases heat by friction and leaves Saturn’s outer layers depleted of helium. These descending droplets may have accumulated into a helium shell surrounding the core.

In 2004, French astronomers Didier Saumon and Tristan Guillot estimated that the core must 9-22 times the mass of Earth, which corresponds to a diameter of about 25,000 km. This is surrounded by a thicker liquid metallic hydrogen layer, followed by a liquid layer of helium-saturated molecular hydrogen that gradually transitions to a gas with increasing altitude. The outermost layer spans 1,000 km and consists of gas.

Saturn’s Atmosphere:

The outer atmosphere of Saturn contains 96.3% molecular hydrogen and 3.25% helium by volume. The gas giant is also known to contain heavier elements, though the proportions of these relative to hydrogen and helium is not known. It is assumed that they would match the primordial abundance from the formation of the Solar System.

Trace amounts of ammonia, acetylene, ethane, propane, phosphine and methane have been also detected in Saturn’s atmosphere. The upper clouds are composed of ammonia crystals, while the lower level clouds appear to consist of either ammonium hydrosulfide (NH4SH) or water. Ultraviolet radiation from the Sun causes methane photolysis in the upper atmosphere, leading to a series of hydrocarbon chemical reactions with the resulting products being carried downward by eddies and diffusion.

NASA's Cassini spacecraft captures a composite near-true-color view of the huge storm churning through the atmosphere in Saturn's northern hemisphere. Image credit: NASA/JPL-Caltech/SSI
NASA’s Cassini spacecraft captures a composite near-true-color view of the huge storm churning through the atmosphere in Saturn’s northern hemisphere. Image credit: NASA/JPL-Caltech/SSI

Saturn’s atmosphere exhibits a banded pattern similar to Jupiter’s, but Saturn’s bands are much fainter and wider near the equator. As with Jupiter’s cloud layers, they are divided into the upper and lower layers, which vary in composition based on depth and pressure. In the upper cloud layers, with temperatures in range of 100–160 K and pressures between 0.5–2 bar, the clouds consist of ammonia ice.

Water ice clouds begin at a level where the pressure is about 2.5 bar and extend down to 9.5 bar, where temperatures range from 185–270 K. Intermixed in this layer is a band of ammonium hydrosulfide ice, lying in the pressure range 3–6 bar with temperatures of 290–235 K. Finally, the lower layers, where pressures are between 10–20 bar and temperatures are 270–330 K, contains a region of water droplets with ammonia in an aqueous solution.

On occasion, Saturn’s atmosphere exhibits long-lived ovals, similar to what is commonly observed on Jupiter. Whereas Jupiter has the Great Red Spot, Saturn periodically has what’s known as the Great White Spot (aka. Great White Oval). This unique but short-lived phenomenon occurs once every Saturnian year, roughly every 30 Earth years, around the time of the northern hemisphere’s summer solstice.

These spots can be several thousands of kilometers wide, and have been observed in 1876, 1903, 1933, 1960, and 1990. Since 2010, a large band of white clouds called the Northern Electrostatic Disturbance have been observed enveloping Saturn, which was spotted by the Cassini space probe. If the periodic nature of these storms is maintained, another one will occur in about 2020.

 The huge storm churning through the atmosphere in Saturn's northern hemisphere overtakes itself as it encircles the planet in this true-color view from NASA’s Cassini spacecraft. Image credit: NASA/JPL-Caltech/SSI
The huge storm churning through the atmosphere in Saturn’s northern hemisphere overtakes itself as it encircles the planet in this true-color view from NASA’s Cassini spacecraft. Image credit: NASA/JPL-Caltech/SSI

The winds on Saturn are the second fastest among the Solar System’s planets, after Neptune’s. Voyager data indicate peak easterly winds of 500 m/s (1800 km/h). Saturn’s northern and southern poles have also shown evidence of stormy weather. At the north pole, this takes the form of a hexagonal wave pattern, whereas the south shows evidence of a massive jet stream.

The persisting hexagonal wave pattern around the north pole was first noted in the Voyager images. The sides of the hexagon are each about 13,800 km (8,600 mi) long (which is longer than the diameter of the Earth) and the structure rotates with a period of 10h 39m 24s, which is assumed to be equal to the period of rotation of Saturn’s interior.

The south pole vortex, meanwhile, was first observed using the Hubble Space Telescope. These images indicated the presence of a jet stream, but not a hexagonal standing wave. These storms are estimated to be generating winds of 550 km/h, are comparable in size to Earth, and believed to have been going on for billions of years. In 2006, the Cassini space probe observed a hurricane-like storm that had a clearly defined eye. Such storms had not been observed on any planet other than Earth – even on Jupiter.

Saturn’s Moons:

Saturn has at least 150 moons and moonlets, but only 53 of these moons have been given official names. Of these moons, 34 are less than 10 km in diameter and another 14 are between 10 and 50 km in diameter. However, some of its inner and outer moons are rather large, ranging from 250 to over 5000 km.

Images of several moons of Saturn. From left to right: Mimas, Enceladus, Tethys, Dione, Rhea; Titan in the background; Iapetus (top) and irregularly shaped Hyperion (bottom). Some small moons are also shown. All to scale. Credit: NASA/JPL/Space Science Institute
Moons of Saturn (from left to right): Mimas, Enceladus, Tethys, Dione, Rhea, Titan in the background; Iapetus (top) and irregularly shaped Hyperion (bottom). Credit: NASA/JPL/Space Science Institute

Traditionally, most of Saturn’s moons have been named after the Titans of Greek mythology, and are grouped based on their size, orbits, and proximity to Saturn. The innermost moons and regular moons all have small orbital inclinations and eccentricities and prograde orbits. Meanwhile, the irregular moons in the outermost regions have orbital radii of millions of kilometers, orbital periods lasting several years, and move in retrograde orbits.

The Inner Large Moons, which orbit within the E Ring (see below), includes the larger satellites Mimas, Enceladus, Tethys, and Dione. These moons are all composed primarily of water ice, and are believed to be differentiated into a rocky core and an icy mantle and crust. With a diameter of 396 km and a mass of 0.4×1020 kg, Mimas is the smallest and least massive of these moons. It is ovoid in shape and orbits Saturn at a distance of 185,539 km with an orbital period of 0.9 days.

Enceladus, meanwhile, has a diameter of 504 km, a mass of 1.1×1020 km and is spherical in shape. It orbits Saturn at a distance of 237,948 km and takes 1.4 days to complete a single orbit. Though it is one of the smaller spherical moons, it is the only Cronian moon that is endogenously active – and one of the smallest known bodies in the Solar System that is geologically active. This results in features like the famous “tiger stripes” – a series of continuous, ridged, slightly curved and roughly parallel faults within the moon’s southern polar latitudes.

Large geysers have also been observed in the southern polar region that periodically release plumes of water ice, gas and dust which replenish Saturn’s E ring. These jets are one of several indications that Enceladus has liquid water beneath it’s icy crust, where geothermal processes release enough heat to maintain a warm water ocean closer to its core. With a geometrical albedo of more than 140%, Enceladus is one of the brightest known objects in the Solar System.

Artist's rendering of possible hydrothermal activity that may be taking place on and under the seafloor of Enceladus. Image Credit: NASA/JPL
Artist’s rendering of possible hydrothermal activity that may be taking place on and under the seafloor of Enceladus. Image Credit: NASA/JPL

At 1066 km in diameter, Tethys is the second-largest of Saturn’s inner moons and the 16th-largest moon in the Solar System. The majority of its surface is made up of heavily cratered and hilly terrain and a smaller and smoother plains region. Its most prominent features are the large impact crater of Odysseus, which measures 400 km in diameter, and a vast canyon system named Ithaca Chasma – which is concentric with Odysseus and measures 100 km wide, 3 to 5 km deep and 2,000 km long.

With a diameter and mass of 1,123 km and 11×1020 kg, Dione is the largest inner moon of Saturn. The majority of Dione’s surface is heavily cratered old terrain, with craters that measure up to 250 km in diameter. However, the moon is also covered with an extensive network of troughs and lineaments which indicate that in the past it had global tectonic activity.

The Large Outer Moons, which orbit outside of the Saturn’s E Ring, are similar in composition to the Inner Moons – i.e. composed primarily of water ice and rock. Of these, Rhea is the second largest – measuring 1,527 km in diameter and 23 × 1020 kg in mass – and the ninth largest moon of the Solar System. With an orbital radius of 527,108 km, it is the fifth-most distant of the larger moons, and takes 4.5 days to complete an orbit.

Like other Cronian satellites, Rhea has a rather heavily cratered surface, and a few large fractures on its trailing hemisphere. Rhea also has two very large impact basins on its anti-Saturnian hemisphere – the Tirawa crater (similar to Odysseus on Tethys) and an as-yet unnamed crater – that measure 400 and 500 km across, respectively.

A composite image of Titan's atmosphere, created using blue, green and red spectral filters to create an enhanced-color view. Image Credit: NASA/JPL/Space Science Institute
A composite image of Titan’s atmosphere, created using blue, green and red spectral filters to create an enhanced-color view. Image Credit: NASA/JPL/Space Science Institute

At 5150 km in diameter, and 1,350×1020 kg in mass, Titan is Saturn’s largest moon and comprises more than 96% of the mass in orbit around the planet. Titan is also the only large moon to have its own atmosphere, which is cold, dense, and composed primarily of nitrogen with a small fraction of methane. Scientists have also noted the presence of polycyclic aromatic hydrocarbons in the upper atmosphere, as well as methane ice crystals.

The surface of Titan, which is difficult to observe due to persistent atmospheric haze, shows only a few impact craters, evidence of cryo-volcanoes, and longitudinal dune fields that were apparently shaped by tidal winds. Titan is also the only body in the Solar System beside Earth with bodies of liquid on its surface, in the form of methane–ethane lakes in Titan’s north and south polar regions.

With an orbital distance of 1,221,870 km, it is the second-farthest large moon from Saturn, and completes a single orbit every 16 days. Like Europa and Ganymede, it is believed that Titan has a subsurface ocean made of water mixed with ammonia, which can erupt to the surface of the moon and lead to cryovolcanism.

Hyperion is Titan’s immediate neighbor. At an average diameter of about 270 km, it is smaller and lighter than Mimas. It is also irregularly shaped and quite odd in composition. Essentially, the moon is an ovoid, tan-colored body with an extremely porous surface (which resembles a sponge).  The surface of Hyperion is covered with numerous impact craters, most of which are 2 to 10 km in diameter. It also has a highly unpredictable rotation, with no well-defined poles or equator.

The two sides of Iapetus. Credit: NASA/JPL
The two sides of Iapetus, which is known as “Saturn’s yin yang moon” because of the contrast in its color composition. Credit: NASA/JPL

At 1,470 km in diameter and 18×1020 kg in mass, Iapetus is the third-largest of Saturn’s large moons. And at a distance of 3,560,820 km from Saturn, it is the most distant of the large moons, and takes 79 days to complete a single orbit. Due to its unusual color and composition – its leading hemisphere is dark and black whereas its trailing hemisphere is much brighter – it is often called the “yin and yang” of Saturn’s moons.

Beyond these larger moons are Saturn’s Irregular Moons. These satellites are small, have large-radii, are inclined, have mostly retrograde orbits, and are believed to have been acquired by Saturn’s gravity. These moons are made up of three basic groups – the Inuit Group, the Gallic Group, and the Norse Group.

The Inuit Group consists of five irregular moons that are all named from Inuit mythology – Ijiraq, Kiviuq, Paaliaq, Siarnaq, and Tarqeq. All have prograde orbits that range from 11.1 to 17.9 million km, and from 7 to 40 km in diameter. They are all similar in appearance (reddish in hue) and have orbital inclinations of between 45 and 50°.

The Gallic group are a group of four prograde outer moons named for characters in Gallic mythology -Albiorix, Bebhionn, Erriapus, and Tarvos. Here too, the moons are similar in appearance and have orbits that range from 16 to 19 million km. Their inclinations are in the 35°-40° range, their eccentricities around 0.53, and they range in size from 6 to 32 km.

Saturns rings and moons Credit: NASA
Saturns rings and moons, shown to scale. Credit: NASA

Last, there is the Norse group, which consists of 29 retrograde outer moons that take their names from Norse mythology. These satellites range in size from 6 to 18 km, their distances from 12 and 24 million km, their inclinations between 136° and 175°, and their eccentricities between 0.13 and 0.77. This group is also sometimes referred to as the Phoebe group, due to the presence of a single larger moon in the group – which measures 240 km in diameter. The second largest, Ymir, measures 18 km across.

Within the Inner and Outer Large Moons, there are also those belonging to Alkyonide group. These moons – Methone, Anthe, and Pallene – are named after the Alkyonides of Greek mythology, are located between the orbits of Mimas and Enceladus, and are among the smallest moons around Saturn.

Some of the larger moons even have moons of their own, which are known as Trojan moons. For instance, Tethys has two trojans – Telesto and Calypso, while Dione has Helene and Polydeuces.

Saturn’s Ring System:

Saturn’s rings are believed to be very old, perhaps even dating back to the formation of Saturn itself. There are two main theories as to how these rings formed, each of which have variations. One theory 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.

In version of this theory, the moon was struck by a large comet or asteroid – possible during the Late Heavy Bombardment – that pushed it beneath the Roche Limit. The second theory is that the rings were never part of a moon, but are instead left over from the original nebular material from which Saturn formed billions of years ago.

The structure is subdivided into seven smaller ring sets, each of which has a division (or gap) between it and its neighbor. The A and B Rings are the densest part of the Cronian ring system and are 14,600 and 25,500 km in diameter, respectively. They extend to a distance of 92,000 – 117,580 km (B Ring) and 122,170 – 136,775 km (A Ring) from Saturn’s center, and are separated by the 4,700 km wide Cassini Division.

Saturn's rings. Credit: NASA/JPL/Space Science Institute.
Saturn’s rings. Credit: NASA/JPL/Space Science Institute.

The C Ring, which is separated from the B Ring by the 64 km Maxwell Gap, is approximately 17,500 km in width and extends 74,658 – 92,000 from Saturn’s center. Together with the A and B Rings, they comprise the main rings, which are denser and contain larger particles than the “dusty rings”.

These tenuous rings are called “dusty” due to the small particles that make them up. They include the D Ring, a 7,500 km ring that extends inward to Saturn’s cloud tops (66,900 – 74,510 km from Saturn’s center) and is separated from the C Ring by the 150 km Colombo Gap. On the other end of the system, the G and E Rings are located, which are also “dusty” in composition.

The G Ring is 9000 km in width and extends 166,000 – 175,000 km from Saturn’s center. The E Ring, meanwhile, is the largest single ring section, measuring 300,000 km in width and extending 166,000 to 480,000 km from Saturn’s center. It is here where the majority of Saturn’s moons are located (see above).

The narrow F Ring, which sits on the outer edge of the A Ring, is more difficult to categorize. While some parts of it are very dense, it also contains a great deal of dust-size particles. For this reason, estimates on its width range from 30 to 500 km, and it extends roughly 140,180 km from Saturn’s center.

History of Observing Saturn:

Because it is visible to the naked eye in the night sky, human beings have been observing Saturn for thousands of years. In ancient times, it was considered the most distant of five known the planets, and thus was accorded special meaning in various mythologies. The earliest recorded observations come from the Babylonians, where astronomers systematically observed and recorded its movements through the zodiac.

From the stone plate of the 3rd—4th centuries CE, found in Rome.
Roman astrological calendar, from the stone plate of the 3rd—4th centuries CE, Rome. Credit: Museo della civiltà romana

To the ancient Greeks, this outermost planet was named Cronus (Kronos), after the Greek god of agriculture and youngest of the Titans. The Greek scientist Ptolemy made calculations of Saturn’s orbit based on observations of the planet while it was in opposition.The Romans followed in this tradition, identifying it with their equivalent of Cronos (named Saturnus).

In ancient Hebrew, Saturn is called ‘Shabbathai’, whereas in Ottoman Turkish, Urdu and Malay, its name is ‘Zuhal’, which derived is from the original Arabic. In Hindu astrology, there are nine astrological objects known as Navagrahas. Saturn, which is one of them, is known as “Shani”, who judges everyone based on the good and bad deeds performed in life. In ancient China and Japan, the planet was designated as the “earth star” – based on the Five Elements of earth, air, wind, water and fire.

However, the planet was not directly observed until 1610, when Galileo Galilee first discerned the presence of rings. At the time, he mistook them for two moons that were located on either side. It was not until Christiaan Huygens used a telescope with greater magnification that this was corrected. Huygens also discovered Saturn’s moon Titan, and Giovanni Domenico Cassini later discovered the moons of Iapetus, Rhea, Tethys and Dione.

No further discoveries of significance were made again until the 181th and 19th centuries. The first occurred in 1789 when William Herschel discovered the two distant moons of Mimas and Enceladus, and then in 1848 when a British team discovered the irregularly-shaped moon of Hyperion.

Robert Hooke noted the shadows (a and b) cast by both the globe and the rings on each other in this drawing of Saturn in 1666. Robert Hooke - Philosophical Transactions (Royal Society publication)
Drawing of Saturn by Robert Hook, taken from Philosophical Transactions (1666). Credit: Wikipedia Commons

In 1899 William Henry Pickering discovered Phoebe, noting that it had a highly irregular orbit that did not rotate synchronously with Saturn as the larger moons do. This was the first time any satellite had been found to move about a planet in retrograde orbit. And by 1944, research conducted throughout the early 20th century confirmed that Titan has a thick atmosphere – a feature unique among the Solar System’s moons.

Exploration of Saturn:

By the late 20th century, unmanned spacecraft began to conduct flybys of Saturn, gathering information on its composition, atmosphere, ring structure, and moons. The first flyby was conducted by NASA using the Pioneer 11 robotic space probe, which passed Saturn at a distance of 20,000 km in September of 1979.

Images were taken of the planet and a few of its moons, although their resolution was too low to discern surface detail. The spacecraft also studied Saturn’s rings, revealing the thin F Ring and the fact that dark gaps in the rings are bright when facing towards the Sun, meaning that they contain fine light-scattering material. In addition, Pioneer 11 measured the temperature of Titan.

The next flyby took place in November of 1980 when the Voyager 1 space probe passed through the Saturn system.  It sent back the first high-resolution images of the planet, its rings and satellites – which included features of various moons that had never before been seen.

These six narrow-angle color images were made from the first ever 'portrait' of the solar system taken by Voyager 1, which was more than 4 billion miles from Earth and about 32 degrees above the ecliptic. The spacecraft acquired a total of 60 frames for a mosaic of the solar system which shows six of the planets. Mercury is too close to the sun to be seen. Mars was not detectable by the Voyager cameras due to scattered sunlight in the optics, and Pluto was not included in the mosaic because of its small size and distance from the sun. These blown-up images, left to right and top to bottom are Venus, Earth, Jupiter, and Saturn, Uranus, Neptune. The background features in the images are artifacts resulting from the magnification. The images were taken through three color filters -- violet, blue and green -- and recombined to produce the color images. Jupiter and Saturn were resolved by the camera but Uranus and Neptune appear larger than they really are because of image smear due to spacecraft motion during the long (15 second) exposure times. Earth appears to be in a band of light because it coincidentally lies right in the center of the scattered light rays resulting from taking the image so close to the sun. Earth was a crescent only 0.12 pixels in size. Venus was 0.11 pixel in diameter. The planetary images were taken with the narrow-angle camera (1500 mm focal length). Credit: NASA/JPL
These six narrow-angle color images were made from the first ever ‘portrait’ of the solar system taken by Voyager 1 in November 1980. Credit: NASA/JPL

In August 1981, Voyager 2 conducted its flyby and gathered more close-up images of Saturn’s moons, as well as evidence of changes in the atmosphere and the rings. The probes discovered and confirmed several new satellites orbiting near or within the planet’s rings, as well as the small Maxwell Gap and Keeler gap (a 42 km wide gap in the A Ring).

In June of 2004, the Cassini–Huygens space probe entered the Saturn system and conducted a close flyby of Phoebe, sending back high-resolution images and data. By July 1st, 2004, the probe entered orbit around Saturn, and by December, it had completed two flybys of Titan before releasing the Huygens probe. This lander reached the surface and began transmitting data on the atmospheric and surface by by Jan. 14th, 2005. Cassini has since conducted multiple flybys of Titan and other icy satellites.

In 2006, NASA reported that Cassini had found evidence of liquid water reservoirs that erupt in geysers on Saturn’s moon Enceladus. Over 100 geysers have since been identified, which are concentrated around the southern polar region. In May 2011, NASA scientists at an Enceladus Focus Group Conference reported that Enceladus’ interior ocean may be the most likely candidate in the search for extra-terrestrial life.

In addition, Cassini photographs have revealed a previously undiscovered planetary ring, eight new satellites, and evidence of hydrocarbon lakes and seas near Titan’s north pole. The probe was also responsible for sending back high-resolution images of the intense storm activity at Saturn’s northern and southern poles.

Cassini’s primary mission ended in 2008, but the probe’s mission has been extended twice since then – first to September 2010 and again to 2017. In the coming years, NASA hopes to use the probe to study a full period of Saturn’s seasons.

Cassini-Huygens Mission
Artist Illustration of the Cassini space probe to Saturn and Titan, a joint NASA, ESA mission. Credit: NASA/JPL

From being a very important part of the astrological systems of many cultures to becoming the subject of ongoing scientific fascination, Saturn continues to occupy a special place in our hearts and minds. Whether it’s Saturn’s fantastically large and beautiful ring system, its many many moons, its tempestuous weather, or its curious composition, this gas giant continues to fascinate and inspire.

In the coming years and decades, additional robotic explorer missions will likely to be sent to investigate Saturn, its rings and its system of moons in greater detail. What we find may constitute some of the most groundbreaking discoveries of all time, and will likely teach us more about the history of our Solar System.

Universe Today has articles on the density of Saturn, the Orbit of Saturn, and Interesting Facts about Saturn.

If you want to learn more about Saturn’s rings and moons, check out Where Did Saturn’s Rings Come From? and How Many Moons Does Saturn Have?

For more information, check out Saturn and all about Saturn, and NASA’s Solar System Exploration page on Saturn.

Astronomy Cast has an episode on the subject – Episode 59: Saturn.

What’s This Ridge on Iapetus?

The strangest feature on Iapetus is the equatorial ridge. What could possibly create a feature like this?

To paraphrase the British geneticist J.B.S Haldane, “in my suspicion, the Universe is not only stranger than we suppose, it’s stranger than we can suppose.” The context was life and evolution, but he might as well been talking about Saturn’s moons. Those teeny worlds are some of the strangest places we’ve ever seen.

Titan is a massive moon with an atmosphere thicker than Earth’s. If it wasn’t for the bone crippling cold and unbreathable atmosphere, you could wear a pair of wings and fly around in the Titanic skies.

There’s Enceladus, an icy moon that blasts water out into space through geysers at its southern pole. But the Saturnian moon that fascinates me the most has got to be Iapetus, also known as Saturn’s yin-yang moon.

Here’s a photo captured by Cassini. Check out the bizarre surface features, where half of the moon is icy white and the other is brownish red. Astronomers believe this strange coloration comes from the ice on the warmer side sublimating away, leaving this darker material beneath.

Sure that’s a bit odd, but the strangest feature on Iapetus is the equatorial ridge. This feature measures 1,300 km long and it makes the moon look like a space walnut. Because of the heavy cratering on the ridge, astronomers know that it’s ancient, nearly as old as the moon itself. At 13 kms high, it’s tall enough to keep out the most persnickety white walker or wildling mammoth & giant battalion.

What could possibly create a feature like this?

Astronomers are of a few camps. The first think it formed through convective activity early on in the moon’s history. Saturn pulls Iapetus with its tremendous gravity, and the moon undergoes massive tidal forces. This generates heat in the moon’s interior, and it might have caused it to blob out at the equator.

A ridge that follows the equator of Saturn's moon Iapetus gives it the appearance of a giant walnut. This image was taken by the Cassini spacecraft. Credit: NASA/JPL/SSI
A ridge that follows the equator of Saturn’s moon Iapetus gives it the appearance of a giant walnut. This image was taken by the Cassini spacecraft. Credit: NASA/JPL/SSI

A second idea is that Iapetus consumed one of Saturn’s rings, billions of years ago. The moon might have slowly wandered through the ring plane, and accreted all the ring material, like snow piling up in front of a plow.

A third is that Iapetus was smashed into by a massive asteroid billions of years ago. This impact caused the moon to fly apart, but then mutual gravity pulled it back together. The force of this recombination squeezed out material at the equator, which then solidified in place.

Alternately, it might be a walnut from a Galactus family Christmas stocking. So which is it?

It turns out that Saturn has two more moons in its system with similar equatorial ridges. Its moon Atlas is just 15 km across, but it’s dominated by an equatorial ridge. It looks like a UFO, and Pan has a similar feature.

Saturn's relatively thin main rings are about 250,000 km (156,000 miles) in diameter. (Image: NASA/JPL-Caltech/SSI/J. Major)
Saturn’s relatively thin main rings are about 250,000 km (156,000 miles) in diameter. (Image: NASA/JPL-Caltech/SSI/J. Major)

Astronomers know that both of these created their ridges by pulling material out of the rings and piling it up on their surface. This is the mechanism that seems to match what’s going on with Iapetus.

One mystery, is how distantly Iapetus orbits Saturn. There’s no ring that far out, so where did it get the material to consume? Is it possible that Iapetus drifted outward, or had a ring system of its own?

You want puzzles? Iapetus is one of the strangest places in the Solar System, and it would be my candidate for a future orbiter or lander. Let’s explore it closer.

What’s your favorite bizarre object in the Solar System? Tell us in the comments below.

If Earth Had Saturn’s Rings, This is What it Would Look Like

We Earthlings love to dream, conjure and extrapolate. If you pose a question such as, “What if Earth had Saturn’s rings?” with all the resources available these days someone will not only answer the question but create some beautiful graphics to depict it! Yesterday, we saw this amazing graphic posted on reddit of a to-scale depiction of how Earth would look like with Saturn’s rings, and thanks to those who helped find the original source, the original image was created by John Brady at Astronomy Central. (We apologize… we originally credited the wrong person).

Of course, “What if Earth had Saturn’s rings?” is not a new question. In fact we’ve discussed it previously on Universe Today, and in 2013, illustrator and author Ron Miller put together some incredible visualizations of what Earth’s skies would look like with Saturn’s rings.

Also, last year someone on imgur put together a wonderful set of images of Earth with Saturn’s rings, as it would look from Earth’s Moon:

This video depicts rings around Earth, but the scale of the rings are not the size of Saturn’s:

Phil Plait discussed the problems that might arise for us if there were Earthrings, such as the rings would change the amount of sunlight reaching the Earth, and our view of the night sky would be hampered. And then Earth could potentially be shredded by ring debris.

Oh well, we can dream, can’t we?

See more great size comparisons of things in our Solar System and Universe at John Brady’s post on Astronomy Central, including a look at how many Earth’s would span across Saturn’s rings.

Saturn’s Ring Shows A Twist In Cassini’s Glimpse Of Planet

What’s up with this distortion? This picture from the Cassini spacecraft shows some kind of twist happening in the F ring of Saturn. Scientists in fact have seen other strange shapes in this delicate ring, indicating that something is disturbing it from time to time.

“Saturn’s F ring often appears to do things other rings don’t. In this Cassini spacecraft image, a strand of ring appears to separate from the core of the ring as if pulled apart by mysterious forces. Some ring scientists believe that this feature may be due to repeated collisions between the F ring and a single small object,” NASA stated this month.

There’s a debate in the scientific community about where the rings arose in the first place. “It’s been going back and forth for ages and it still goes back and forth. Are they old, or have they been there a long period of time? Are they new? I don’t know what to think, to be quite honest. I’m not being wishy-washy, I just don’t know what to think anymore,” Kevin Grazier, a planetary scientist with the Cassini mission for over 15 years, told Universe Today in December.

While this picture dates from October, you can check out Cassini images as they come in to NASA’s raw image database. Even in unprocessed form, the planet and its rings look beautiful — as you can clearly see in samples below.

The bulk of Saturn looms to the side of this shot of Saturn's rings taken in February 2014 by the Cassini spacecraft. Credit: NASA/JPL/Space Science Institute
The bulk of Saturn looms to the side of this shot of Saturn’s rings taken in February 2014 by the Cassini spacecraft. Credit: NASA/JPL/Space Science Institute
The variety of Saturn's rings is visible in this raw shot from the Cassini spacecraft taken in February 2014. Credit: NASA/JPL/Space Science Institute
The variety of Saturn’s rings is visible in this raw shot from the Cassini spacecraft taken in February 2014. Credit: NASA/JPL/Space Science Institute
Although Saturn's rings look solid and substantial in images such as this, they are made up of many tiny, icy objects collecting as thin as 6.2 miles (10 kilometers) deep.  Image taken by the Cassini spacecraft in February 2014. Credit: NASA/JPL/Space Science Institute
Although Saturn’s rings look solid and substantial in images such as this, they are made up of many tiny, icy objects collecting as thin as 6.2 miles (10 kilometers) deep. Image taken by the Cassini spacecraft in February 2014. Credit: NASA/JPL/Space Science Institute

Cassini Watches as Meteors Hit Saturn’s Rings

From tell-tale evidence, we know that Earth, our Moon and other bodies in our Solar System are constantly barraged with both small meteoroids and larger asteroids or comets. And sometimes – like in the case of seeing meteors fling across our sky, or flashes on the Moon or Jupiter getting hit by Comet Shoemaker-Levy 9 — we even get to watch as it happens. Now, for the first time the Cassini spacecraft has provided direct evidence of small meteoroids crashing into Saturn’s rings.

Researchers say that studying the impact rate of meteoroids from outside the Saturnian system helps scientists understand how different planet systems in our solar system formed.

Saturn’s rings act as very effective detectors of many kinds of surrounding phenomena, including the interior structure of the planet and the orbits of its moons. For example, a subtle but extensive corrugation that ripples 12,000 miles (19,000 kilometers) across the innermost rings tells of a very large meteoroid impact in 1983.

“These new results imply the current-day impact rates for small particles at Saturn are about the same as those at Earth — two very different neighborhoods in our solar system — and this is exciting to see,” said Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “It took Saturn’s rings acting like a giant meteoroid detector — 100 times the surface area of the Earth — and Cassini’s long-term tour of the Saturn system to address this question.”

The Saturnian equinox in summer 2009 was an especially good time to see the debris left by meteoroid impacts. The very shallow sun angle on the rings caused the clouds of debris to look bright against the darkened rings in pictures from Cassini’s imaging science subsystem.

This animation depicts the shearing of an initially circular cloud of debris as a result of the particles in the cloud having differing orbital speeds around Saturn. Image credit: NASA/Cornell

“We knew these little impacts were constantly occurring, but we didn’t know how big or how frequent they might be, and we didn’t necessarily expect them to take the form of spectacular shearing clouds,” said Matt Tiscareno, lead author of the paper and a Cassini participating scientist at Cornell University in Ithaca, N.Y. “The sunlight shining edge-on to the rings at the Saturnian equinox acted like an anti-cloaking device, so these usually invisible features became plain to see.”

Tiscareno and his colleagues now think meteoroids of this size probably break up on a first encounter with the rings, creating smaller, slower pieces that then enter into orbit around Saturn. The impact into the rings of these secondary meteoroid bits kicks up the clouds. The tiny particles forming these clouds have a range of orbital speeds around Saturn. The clouds they form soon are pulled into diagonal, extended bright streaks.

“Saturn’s rings are unusually bright and clean, leading some to suggest that the rings are actually much younger than Saturn,” said Jeff Cuzzi, a co-author of the paper and a Cassini interdisciplinary scientist specializing in planetary rings and dust at NASA’s Ames Research Center in Moffett Field, Calif. “To assess this dramatic claim, we must know more about the rate at which outside material is bombarding the rings. This latest analysis helps fill in that story with detection of impactors of a size that we weren’t previously able to detect directly.”

Source: JPL

Rain is Falling from Saturn’s Rings

Astronomers have known for years there was water in Saturn’s upper atmosphere, but they weren’t sure exactly where it was coming from. New observations have found water is raining down on Saturn, and it is coming from the planet’s rings.

“Saturn is the first planet to show significant interaction between its atmosphere and ring system,” said James O’Donoghue, a postgraduate researcher at the University of Leicester and author of a new paper published in the journal Nature. “The main effect of ring rain is that it acts to ‘quench’ the ionosphere of Saturn, severely reducing the electron densities in regions in which it falls.”

Using the Keck Observatory, O’Donoghue and a team of researchers found charged water particles falling from the planet’s rings into Saturn’s atmosphere. They also found the extent of the ring-rain is far greater, and falls across larger areas of the planet, than previously thought. The work reveals the rain influences the composition and temperature structure of parts of Saturn’s upper atmosphere.

O’Donoghue said the ring’s effect on electron densities is important because it explains why, for many decades, observations have shown electron densities to be unusually low at some latitudes at Saturn.

“It turns out a major driver of Saturn’s ionospheric environment and climate across vast reaches of the planet are ring particles located 120,000 miles [200,000 kilometers] overhead,” said Kevin Baines, a co-author on the paper, from the Jet Propulsion Laboratory. “The ring particles affect which species of particles are in this part of the atmospheric temperature.”

In the early 1980s, images from NASA’s Voyager spacecraft showed two to three dark bands on Saturn and scientists theorized that water could have been showering down into those bands from the rings. Then astronomers using ESA’s Infrared Observatory discovered the presence of trace amounts of water in Saturn’s atmosphere back in 1997, but couldn’t really find an explanation for why it was there and how it got there.

Then in 2011 observations with the Herschel space observatory determined water ice from geysers on Enceladus formed a giant ring of water vapor around Saturn.

But the bands seen by Voyager were not seen again until 2011 as well, when the team observed the planet with Keck Observatory’s NIRSPEC, a near-infrared spectrograph that combines broad wavelength coverage with high spectral resolution, allowing the observers to clearly see subtle emissions from the bright parts of Saturn.

The ring rain’s effect occurs in Saturn’s ionosphere (Earth has a similar ionosphere), where charged particles are produced when the otherwise neutral atmosphere is exposed to a flow of energetic particles or solar radiation. When the scientists tracked the pattern of emissions of a particular hydrogen molecule consisting of three hydrogen atoms (rather than the usual two), they expected to see a uniform planet-wide infrared glow.

What they observed instead was a series of light and dark bands with a pattern mimicking the planet’s rings. Saturn’s magnetic field “maps” the water-rich rings and the water-free gaps between rings onto the planet’s atmosphere.

They surmised that charged water particles from the planet’s rings were being drawn towards the planet by Saturn’s magnetic field and neutralizing the glowing triatomic hydrogen ions. This leaves large “shadows” in what would otherwise be a planet-wide infrared glow. These shadows cover 30 to 43 percent of the planet’s upper atmosphere surface from around 25 to 55 degrees latitude. This is a significantly larger area than suggested by the Voyager images.

Both Earth and Jupiter have a very uniformly glowing equatorial region. Scientists expected this pattern at Saturn, too, but they instead saw dramatic differences at different latitudes.

“Where Jupiter is glowing evenly across its equatorial regions, Saturn has dark bands where the water is falling in, darkening the ionosphere,” said Tom Stallard, one of the paper’s co-authors at Leicester. “We’re now also trying to investigate these features with an instrument on NASA’s Cassini spacecraft. If we’re successful, Cassini may allow us to view in more detail the way that water is removing ionized particles, such as any changes in the altitude or effects that come with the time of day.”

Sources: Keck Observatory
, Nature.

Outer Space – Mind Blowing Video from Jupiter and Saturn

Video Caption: This mesmerizing video unveils incredibly amazing sequences around Jupiter and Saturn from NASA’s Cassini and Voyager missions set to stirring music by “The Cinematic Orchestra -That Home (Instrumental)”. Credit: Sander van den Berg

Don’t hesitate 1 moment ! Look and listen to this mind blowing video of the Jupiter and Saturnian systems.

If you love the wonders of the hitherto unknown Universe unveiled before your eyes – and long to explore – feast your eyes on this short new video right now titled simply; “Outer Space”. Continue reading “Outer Space – Mind Blowing Video from Jupiter and Saturn”

Cassini’s Majestic Saturn Moon Quintet

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Check out this gorgeous new portrait of a Saturnian moon quintet taken by Earths’ emissary – NASA’s Cassini Orbiter. The moons are majestically poised along a backdrop of Saturn’s rings, fit for an artist’s canvas.

Janus, Pandora, Enceladus, Mimas and Rhea are nearly lined up (from left to right) in this view acquired by Cassini at a distance of approximately 684,000 miles (1.1 million kilometers) from Rhea and 1.1 million miles (1.8 million kilometers) from Enceladus.

The newly released image was taken by Cassini’s narrow angle camera on July 29, 2011. Image scale is about 4 miles (7 kilometers) per pixel on Rhea and 7 miles (11 kilometers) per pixel on Enceladus.

Cassini will stage a close flyby of Enceledus – Satarn’s geyser spewing moon – in about two weeks, swooping within 99 km

Moon Facts from JPL:
Janus (179 kilometers, or 111 miles across) is on the far left. Pandora (81 kilometers, or 50 miles across) orbits between the A ring and the thin F ring near the middle of the image. Brightly reflective Enceladus (504 kilometers, or 313 miles across) appears above the center of the image. Saturn’s second largest moon, Rhea (1,528 kilometers, or 949 miles across), is bisected by the right edge of the image. The smaller moon Mimas (396 kilometers, or 246 miles across) can be seen beyond Rhea also on the right side of the image.

This view looks toward the northern, sunlit side of the rings from just above the ring plane. Rhea is closest to Cassini here. The rings are beyond Rhea and Mimas. Enceladus is beyond the rings.

The simple graphic below shows dozens of Saturn’s moons – not to scale. So far 62 have been discovered and 53 have been officially named.

Saturn’s moons. Click on link below to learn more about each moon. Credit: NASA/JPL

Learn more about Saturn’s moons at this link

List of Saturn’s officially named moons:
Aegaeon, Aegir, Albiorix, Anthe, Atlas, Bebhionn, Bergelmir, Bestla, Calypso, Daphnis, Dione, Enceladus, Epimetheus, Erriapus, Farbauti, Fenrir, Fornjot, Greip, Hati, Helene, Hyperion, Hyrrokkin, Iapetus, Ijiraq, Janus, Jarnsaxa, Kari, Kiviuq, Loge, Methone, Mimas, Mundilfari, Narvi, Paaliaq, Pallene, Pan, Pandora, Phoebe, Polydeuces, Prometheus, Rhea, Siarnaq, Skadi, Skoll, Surtur, Suttung, Tarqeq, Tarvos, Telesto, Tethys, Thrym, Titan and Ymir.

Why Does Saturn Have Rings

Why Does Saturn Have Rings

Saturn has fascinated amateurs and professionals alike for centuries. As quickly as the planet’s ring system was discovered the popular question became ‘why does Saturn have rings?’ usually followed by ‘what are Saturn’s rings made of?’. Well, here are the answers to both questions.

The simplest answer as to why Saturn has rings and what they are made of is that the planet has accumulated a great deal of dust, particles, and ice at varying distances from its surface. These items are most likely trapped by gravity. The rings appear because of the wavelengths of light reflected by these rings of debris.

Some scientists speculate that Saturn may be too big. Its gravitational pull is so strong that it has been able to snatch debris from space. Some of which is as large as an entire building. That pull is why it has at least 62 moons. Those moons contribute dust to the rings as well as absorb dust from the rings.

A common theory as to how all of the material initially accumulated in Saturn’s rings is a series of asteroid impacts. Not with the planet, but with the moons around it. After the impact the remnants of the asteroids and the debris from the moons could not escape the gravitational pull of the planet.

One other theory holds that the rings of Saturn formed as other moons broke apart in ancient times. Additionally, this theory states that some of the material could be from earlier, during the formation of the solar system, and Saturn could not accrete the material while it was forming and it has been in orbit ever since.

No matter which theory you believe, the rings of Saturn are spectacular. After researching Saturn’s rings a little more, be sure to investigate the ring systems around Neptune, Uranus, and Jupiter. Each system is fainter than Saturn’s, but still interesting.

We have written many articles about Saturn for Universe Today. Here’s an article about the color of Saturn, and here are some pictures of Saturn.

If you’d like more info on Saturn, check out Hubblesite’s News Releases about Saturn. And here’s a link to the homepage of NASA’s Cassini spacecraft, which is orbiting Saturn.

We’ve also recorded an episode of Astronomy Cast all about Saturn. Listen here, Episode 59: Saturn.

Reference:
NASA

Saturn’s Rings Formed from Large Moon’s Destruction

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The formation of Saturn’s rings has been one of the classical if not eternal questions in astronomy. But one researcher has provided a provocative new theory to answer that question. Robin Canup from the Southwest Research Institute has uncovered evidence that the rings came from a large, Titan-sized moon that was destroyed as it spiraled into a young Saturn.

Over the years, different theories have evolved on how the rings formed around Saturn. The two leading theories involve a small moon that was shattered by meteor impacts, or the tidal disruption of a comet coming too close to Saturn.

But Saturn’s main rings are about 90% water ice by mass, and because bombardment of the rings by micrometeoroids increases their rock content over time, Canup said the rings’ current composition implies that they were essentially pure ice when they formed.

However, disruption of a small moon would generally lead to a mixed rock-ice ring, while tidal disruptions of comets would occur much more often at Jupiter, Uranus and Neptune than at Saturn.

New insights into the nature of Saturn’s rings are revealed in this panoramic mosaic of 15 images taken during the planet’s August 2009 equinox. Image credit: NASA/JPL/SSI

Additionally, neither of these theories would explain Saturn’s inner moons, which have low enough densities that they too must be comprised of nearly pure ice.

Canup’s new alternative theory is that Titan-sized moon with a rocky core and an icy mantle spiraled into Saturn early in solar system history. Tidal forces ripped off part of the icy mantle, distributing it into what would become the rings. But the rocky core was made of more durable material that held together until it hit Saturn’s surface. “The end result is a pure ice ring,” Canup said in an article in Nature.

Over time the ring spreads out and its mass decreases, and icy moons are created. Due to changes in the evolving Saturn system, these “spawned” moons then spiraled outward rather than inward. In this way, ice rings and ice-enhanced inner moons originate as a primordial byproduct of the same process that produces Saturn’s regular satellite system, making the whole process simpler than if there were several events.

Canup studies formation events with detailed computer simulations, including studying how our own Moon formed.

She presented her findings at the American Astronomical Society’s Division for Planetary Science meeting this week, in Pasadena, California, and her presentation was detailed in an article in Nature.

Sources: Canup’s abstract, Nature