Reigning majestically in the cold, dark outer kingdom of our Solar System, a quartet of enormous gaseous planets circle our Star. Saturn and Jupiter are our Solar System’s gas giant duo, and both are shrouded by deep, dense envelopes of gas. Saturn is the smaller of the two gas-giants, but it is larger than the two other, more distant planetary denizens of our Solar System’s outer limits–Uranus and Neptune–which are classified as ice-giants because they contain larger cores shrouded under thinner gaseous envelopes of blanketing gas. Saturn is a beautiful, distant world, famous for its enchanting, bewitching, and lovely system of rings, that are mostly composed of a dancing multitude of tiny, twirling icy fragments. The rings of Saturn are the most extensive planetary ring system of any planet in our Solar System, and in September 2015, a team of astronomers reported their study suggesting that the icy bits tumbling around in one section of Saturn’s rings are denser than elsewhere, and that this is possibly due to solid, icy cores. This finding could mean that this particular ring is considerably more youthful than the others.
Back in August 2009, a distant sunset on Saturn’s mesmerizing rings was carefully observed by astronomers who were part of NASA’s Cassini mission. It was the equinox–one of two periods of the Saturnian year when our Star shines brightly on the planet’s immense and majestic system of gossamer rings edge-on. The event provided a valuable opportunity for the orbiting Cassini spacecraft to observe brief alterations in the rings that could reveal important clues about their intriguingly mysterious nature.
NASA’s Cassini spacecraft entered orbit around Saturn on July 1, 2004, and started to take some very revealing images of this beautiful planet, its rings, and its myriad moons. Although Saturn appears to be a calm, peaceful planet when it is observed from a great distance, the up close and personal observations derived from the Cassini probe showed how deceptive distant appearances can be. In fact, Cassini successfully managed to image the Great Springtime Storm that shook up Saturn in early 2011. The whirling, swirling tempest was reported by NASA on October 25, 2012, and this furious storm displayed an enormous cloud cover as big as our entire planet!
Saturn is tilted on its axis–just like our own planet. Over the long passage of its 29-year-long orbit, our Sun’s brilliant and illuminating rays of light travel from north to south over the gas-giant and its rings, and back again. The changing sunlight causes the temperature of the rings–which are composed of trillions of glittering, frozen bits of somersaulting ice–to vary from one season to the next. During the equinox, which lasts for only a few days, strange and weird shadows and wavy structures appeared and, as they lingered in the distant twilight of this faraway world, the rings began to cool.
In a study published in the planetary science journal Icarus, the team of Cassini scientists reported that one section of the rings seems to have warmed up during the Saturnian equinox. This somewhat toasty temperature provided a one-of-a-kind peek through a window of opportunity into the secretive interior structure of ring particles not normally available to curious astronomers.
Rings And Icy Things
In 1610, the great Italian astronomer Galileo Galilei turned his primitive telescope to the starlit sky, and became the very first person to see the rings of Saturn. Although reflection from the rings increases Saturn’s brightness, they cannot be seen from Earth with the unaided human eye, and Galileo was unable to see them well enough to discern their true nature. Galileo wrote to the Duke of Tuscany that “[T]he planet 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]”. Galileo went on to describe Saturn as possessing “ears”. In 1612, the plane of the rings was oriented directly at our planet and the rings seemed to vanish. The bewildered Galileo wondered, “has Saturn swallowed its children?”, referring to an ancient Greek and Roman myth in which Saturn (Greek, Cronus) devoured his own children to prevent them from overthrowing him. But, then, the mysterious structure reappeared in 1613, further confusing Galileo.
In 1655, the Dutch mathematician and astronomer, Christiaan Huygens, became the first person to describe this mysterious structure as a disk encircling Saturn, and he did this using a defracting telescope that he had designed himself. This early telescope was actually superior to what Galileo had used, and Huygens was able to observe Saturn. Huygens noted that “It [Saturn] is surrounded by a thin, flat, ring, nowhere touching, inclined to the ecliptic.” The British scientist Robert Hooke was also an early observer of the Saturnian rings.
In 1675, the Italian astronomer Giovanni Domenico Cassini was able to determine that Saturn’s ring was made up of multiple smaller rings with gaps between them, and the largest of these gaps was later named in his honor–the Cassini Division. This division is a 4,800 kilometer wide gap between the A ring and the B ring
In 1787, The French scientist Pierre-Simon Laplace proposed that the rings were made up of a large number of solid ringlets, and in 1659, the British astronomer James Clerk Maxwell calculated that the rings could not possibly be solid because, if they were, they would become unstable and fall apart. He then suggested that Saturn’s rings must be composed of a multitude of tiny particles–all independently circling Saturn.
The rings form a very thin, wide, and ethereal expanse that is about 250,000 kilometers across–but less than tens of hundreds of meters thick. Historically, the age and origin of Saturn’s rings have been difficult for astronomers to determine, some saying they are very young structures, and others saying they are actually primordial structures–as old as our 4.56 billion year old Solar System.
The icy fragments that compose Saturn’s magnificent system of rings range in size from frigid smoke-sized particles to boulders as large as some skyscrapers. These frozen, tiny, swirling objects jitter-bug around together in a distant dance around their planet, interacting with one another, and twirling around together. The icy ring fragments are also influenced by their planet’s magnetosophere. The magnetosophere is the region of a planet’s magnetic influence, and these tiny, frozen objects are also under the influence of the larger of the 62 known moons of Saturn.
Saturn’s rings are named alphabetically according to the order they were detected. The main rings are designated C, B, and A–with A being the outermost, C being the innermost, and B situated between the two. There are several fainter rings that were discovered more recently.The D ring is the one closest to its planet, and it is extremely faint. The slender F ring is located just outside of the A ring, and beyond that are a duo of much fainter rings dubbed G and E. The rings display a great deal of structure on all scales, and some are influenced by perturbations caused by Saturn’s moons. However, much still remains to be explained.
Data derived from the Cassini space probe indicate that the Saturnian rings possess their own atmosphere independent of that of their planet. The atmosphere is composed of molecular oxygen gas that forms when ultraviolet light from our Star interacts with the water ice of the rings. Chemical reactions that occur between water molecule fragments and further ultraviolet interactions form, and then toss out, oxygen gas–among other things. This ring atmosphere, despite being very thin, was spotted from Earth by the Hubble Space Telescope.The rings themselves possess a total mass that amounts to only a small fraction of the total mass of Saturn, and is just a bit less than the icy, mid-sized Saturnian moon Mimas.
Saturn’s Strange Young Ring
“For the most part, we can’t learn much about what Saturn’s ring particles are like deeper than 1 millimeter below the surface. But the fact that one part of the rings didn’t cool as expected allowed us to model what they might be like on the inside,” explained Dr. Ryuji Morishima in a September 2, 2015 NASA Jet Propulsion Laboratory (JPL) Press Release. Dr. Morishima of the JPL in Pasadena, California, led the study.
The astronomers carefully scrutinized data gathered by Cassini’s Composite Infrared Spectrometer during the year around equinox. The instrument obtained valuable information about the temperature of the rings as they cooled. The scientists then compared the temperature data with supercomputer models that had been designed to describe the properties of the ring particles on an individual scale.
What the scientists found was a puzzle. For most of the enormous expanse of Saturn’s rings, the computer models correctly predicted just how the rings would cool off as they descended into the cold, mysterious darkness. However, one rather large segment–the outermost of the large, main rings, dubbed the A ring–was considerably more balmy than models predicted. The temperature spike was particularly intense in the middle of the A ring.
In order to address the bewitching, bewildering, and bothersome mystery, Dr. Morishima and his team conducted a detailed study of just how ring particles with varying structures would warm up and then cool down during the passing of Saturnian seasons. Earlier studies based on data derived from Cassini have revealed that Saturn’s icy ring particles have fluffy exteriors, that are like fresh snow. This outer, fluffy, snowy coating–termed regolith–forms over the passage of time, as tiny impacts smash the surface of each frozen, icy particle. The team’s analysis indicates that the best explanation for the A ring’s strange equinox temperatures is for the ring to be made up primarily of particles roughly 3 feet wide composed mostly of solid ice, with only a very thin coating of snowy regolith.
“A high concentration of dense, solid ice chunks in this one region of Saturn’s rings is unexpected. Ring particles usually spread out and become evenly distributed on a timescale of about 100 million years,” Dr. Morishima noted in the September 2, 2015 JPL Press Release.
The piling up of dense ring particles in one region suggests that some process either moved the particles there in the recent geologic past or the particles are being confined there, for some undetermined reason. The astronomers suggest some possibilities to explain how this accumulation happened. A moon may have once danced around at that location within approximately the past hundred million years, but it was doomed to destruction–perhaps it was the luckless victim of a giant, catastrophic smash-up with another object. If this actually occurred, debris from the impact might not have had sufficient time to spread evenly throughout the ring. Alternatively, the astronomers suggest that tiny, rubble-pile moonlets–whose component fragments are only loosely held together by gravity to form what looks like a single object–could be carrying the frozen, dense particles as they wander around within the ring. The moonlets could disperse the icy fragments in the middle A ring as they disintegrate there under the merciless gravitational influence of Saturn and its larger moons.
Cassini project scientist, Dr. Linda Spilker of the JPL, and a co-author of the study, told the press on September 2, 2015 that “This particular result is fascinating because it suggests that the middle of Saturn’s A ring may be much younger than the rest of the rings. Other parts of the rings may be as old as Saturn itself.
During its last close orbits around Saturn, Cassini will measure the mass of the gas-giant’s rings for the first time, using gravity science. Astronomers will then use the mass of the rings to place constraints on their true age.