Hello and welcome to “Settle the Stars”.
Episode 13, “Saturn: Put a Ring On It”.
Hey folks, this is Alexander Winn.
This week we continue our journey across the solar system and visit what will definitely be one of the top tourist destinations in the distant future: Saturn. This gas giant may not hold as many records as its giant neighbor Jupiter, but the dazzling ring system that the planet is known for make it a fan favorite. And today, we get to visit!
Even before telescopes could reveal the beauty of its rings, ancient observers held the yellowish light in the sky in high esteem. Ancient Babylonians recorded its movements meticulously, and in ancient Greece the planet was known by the name Phainon, later to be called the “star of Saturn” by the Romans. Saturn was the Roman god of agriculture and wealth, appropriate given the planet’s tendency to grow and fade in brightness over the years – much like fortune can. This cycle of gradual brightening and darkening is caused by Saturn’s rings. As the planet appears to rotate in relation to Earth, the axial tilt of the planet causes the rings to be seen more from above, below or from the side, at which point they’re almost invisible. That reflects different amounts of light, altering the planet’s appearance in the sky.
The astrological symbol used for Saturn is a stylized sickle representing the god’s agricultural association, and everyone’s favorite day of the week – Saturday – still bears the Roman name for the planet assigned around the 2nd century AD.
To the Hindu astrologers, the planet was known as “Shani,” the judge of everyone based on their behavior and deeds in life. Ancient Hebrews called Saturn “Shabbathai,” attributed to the angel Cassiel and governed by the beneficial spirit Agyal and darker spirit Zazl.
Detailed observation of the planet couldn’t take place without the aid of a telescope and while Galileo’s were powerful enough to see Jupiter’s moons, he mistakenly believed that Saturn’s rings were actually moons alongside the planet. It was Dutch polymath Christiaan Huygens and his improved telescope who first recognized and recorded the rings in 1659.
Over the next several centuries the moons began to be uncovered which we’ll get into more detail during the next episode, and the first low-resolution images were obtained at a distance of only 20,000 kilometers by Pioneer 11 as it flew by. Pioneer also studied the rings more closely, identifying an additional thin ring as well as making note that many of the “gaps” noticed earlier were not quite empty after all.
In 1980 Voyager 1 completed a flyby as it passed through, accomplishing the first high-resolution photography of the planetary features as well as images of many of the moons. Scientists in charge of the mission decided to sacrifice part of Voyager 1’s mission in order to alter course for a closer look at the moon Titan. They learned that Titan’s thick atmosphere is impenetrable to visible wavelengths of light and were unfortunately unable to obtain images of the landscape through the quick clouds, and as a result of the mission modification Voyager 1 was unable to visit Uranus, Neptune or Pluto.
The following year in 1981 Voyager 2 passed by Saturn to obtain more high-resolution images of the rings to track any changes since Voyager 1’s visit, and it also scanned the atmosphere of the planet with radar to measure temperature and density. Voyager 2’s innovative spinning module was unfortunately stuck for part of the flyby and was unable to take more planned photos, and as it passed by the spacecraft was able to use Saturn’s gravity to swing onward to Uranus.
Altogether the two spacecraft were able to identify several new satellites interacting with the ring system and observed previously unknown gaps within the rings.
In 2004, NASA spacecraft Cassini-Huygens entered orbit around Saturn for an extended mission to study the moons and rings of Saturn. This was an ambitious plan to extensively study the moons and atmosphere of Saturn, and featured a detachable probe named for Huygens that would fall into the atmosphere of Titan to collect valuable data.
Photographs from the orbital Cassini module captured a previously undiscovered ring, and fascinating images of the atmosphere in unprecedented detail. As a final maneuver, Cassini accomplished a series of impressive passes through the gaps between Saturn and its ring system before completing the mission by entering the atmosphere.
The accumulation of telescopic images over the centuries as well as more recent sensory readings and photographs from these spacecraft have provided us a wealth of information about the planet. Many of its most interesting features were complete surprises to learn, and there are still many mysteries left to solve. Let’s dive in.
The second largest planet behind Jupiter, Saturn orbits the sun about nine times the distance that Earth does. Even clipping through the solar system at 9.7 km/s, it takes Saturn about 29 ½ years to complete one full trip around the Sun. As with most Gas Giants, it’s difficult to assign a length of day for Saturn, as the swirling atmosphere travels at different rotational speeds depending on where you look.
Saturn’s atmosphere primarily consists of hydrogen and helium, the same main components as the Sun and Jupiter, suggesting it formed along with the others in the early solar system from the same nebular dust before settling into its current orbit. Interestingly, Saturn’s density is less than that of normal water: if you had a big enough bathtub, Saturn would literally float in it!
Despite being around the same size as Jupiter, Saturn is much less dense with only about a third the mass of its larger neighbor. Together Jupiter and Saturn account for over 90% of the total planetary mass in the solar system, and as a result have likely done much over the 4 or 5 billion years they’ve been around to stabilize and maintain the orbits of the other planets where they are.
A particular orbital oddity about Saturn is that it does not appear to have any trojan asteroids at all. As we learned last week, Jupiter – admittedly much more massive – shepherds an estimated two million small asteroids along its orbit path ahead and behind it as it travels around the Sun. Mars, Neptune and even Earth have been found with small trojan asteroids, but somehow Saturn has none to be found. Fortunately for travelers on approach like us, fewer asteroids to dodge is pretty much always a positive.
Approaching from inside Saturn’s orbit, we’ll enjoy a full-lit front view of the gas giant as we get closer. The rich yellow-beige of the atmosphere shines brightly, and from here the icy rings twinkle brilliantly.
The ring system is impressive, and hugs the planet closely, well within the orbits of most of the larger moons. Striking banding patterns make it look as though there are dozens of individual rings nestled tightly together – which there are – but for classification purposes the bands have been grouped into fourteen rings. On today’s trip we’ll focus only on the inner rings named alphabetically A through F – that is, the rings that are commonly shown as visible bands around the planet. There are some larger rings farther out, but we’ll explore those in more depth in next week’s episode. We’ll imagine a close approach above these rings for a great view as we get closer.
Passing below us first we’ll see the faint F ring, outermost of the discrete rings and discovered first in 1979 by the Pioneer 11 mission. While relatively thin at only a few hundred kilometers wide compared to the other inner rings, the one has a very interesting visual feature – there’s actually a wispy spiral strand coiled around the main ring. This ring was likely formed when the moons Prometheus and Pandora collided sometime in the past. Now the two moons march just alongside the ring, Prometheus inside and Pandora outside. As Prometheus passes by, you might notice a “ripple” effect trailing it within the ring. These ripples are waves caused by the gravity of the moon when it gets near, pulling the smaller coil of material closer and closer to it.
Leaving the F ring behind, we’ll move across what might seem like a 2,600 kilometer band of empty space at first but there is actually a sheet of dust spanning the gap. This region is called the “Roche Division” after French physicist Édouard Roche and it separates the F and A rings. This shouldn’t be confused with the “Roche Limit” which is a term for the distance at which a large object becomes too close to a planet and will be torn apart by tidal gravitation forces. It just so happens that the Roche Division is coincidentally near to the Roche limit of Saturn, which is why these inner rings have not coalesced into a new moon.
The A ring will appear next for us, a comparably massive 14,600 kilometer wide ring. The boundaries of the ring are very sharp, and the ring is about 10 to 30 meters thick. Before we get far we’ll encounter the Keeler Gap, discovered by Voyager. The gap is only 42-kilometers wide and carved out by the small moon Daphnis. As we pass above the moon we’ll see great waves rising from the ring to a height of about 1.5 kilometers.
We’ll witness more of the A ring pass before we reach the next landmark (or… ring-mark?), the Enke Gap. Much larger than the Keeler, the Enke gap spans 325 kilometers and forms a path for the moon Pan. We can see three small intertwined ringlets within the gap from here, and spiral density waves within the ring on either side of the gap due to Pan’s gravitation and that of some small moons outside of the rings.
As the remaining A ring passes below us, you might notice small propeller-shaped wave patterns in the ring. These are caused by small moonlets passing near the ring, and it’s estimated that there could be thousands of them in the A ring.
The boundary between the A and B rings are a darker portion called the Cassini Division after their discoverer Giovanni Cassini in 1675. He can be forgiven for believing the space is empty, but Voyager revealed that the 4,800-kilometer-wide band is actually filled with many small ringlets of darker material similar to that found in the C ring.
The largest and brightest ring, B ring, now shines ahead of us, only 5 to 15 meters thick but more densely packed with particles from the size of dust up to the size of a house. The optical depth is greater than 5 in some parts of the ring, meaning up to 90% of the light shone from the Sun is blocked. The outer edge of the ring contains strange structures, they look like great plumes or ridges jutting out perpendicular to the ring, sometimes as high as 2.5 kilometers from the ring itself. These structures are mysterious, but likely caused by unseen moonlets.
Continuing inward, we reach the interface between B and C rings, called the Colombo Gap. The gap contains a small ring called the Titan ringlet named for the moon, which is actually located far out beyond the rings. It’s named for Titan due to an orbital resonance with the planet, which has slightly elongated the ring into an elliptical shape, rather than circular. The shape of the ring moves with the moon, so that the longer part of the ringlet always points toward the moon.
The C ring is composed of darker material, which is also sparser than the material in the B ring, resulting in a transparent, dimmer ring than the brilliant one we just passed. It’s also much thinner than the B ring at only 17,500 kilometers.
Marking the boundary of the C ring is another gap, called the Maxwell Gap which also contains a single ringlet by the same name. There is a wave-like structure in this ringlet hinting at the influence of a moon, but as of today no small moon has yet been discovered to explain the waves.
Now we arrive at the innermost ring, D ring. There are wave patterns observed within D ring as well, but scientists noticed there is no identifiable cause, and the waves appear to be dissipating over time. This suggests that the waves were caused by disruptions from passing debris falling to the planet. Similar patters observed in Jupiter’s ring caused by material from comet Shoemaker-Levy 9 in 1994 support this theory. In 1980 Voyager 1 identified three ringlets within ring D, and by the time Cassini arrived 25 years later the innermost had actually moved 200 kilometers closer to the planet. It’s an example of the dynamic nature of the ring system, and a reminder that as material is lost either to ejection or falling into Saturn, the rings aren’t permanent.
The origin and fate of the ring system are still open questions. Some scientists believe the rings are very young, pointing to the fact that the ice particles still appear relatively fresh when it is known they should darken over time as more dust and debris accumulate. The theory proposed by Édouard Roche in the 1800’s is that the rings were once part of a moon named Veritas. This moon would have been destroyed, either torn apart by tidal forces at the – you guessed it – Roche Limit, or obliterated by a collision with another large object. The debris then eventually migrated into the current arrangement.
Cassini spacecraft data support this theory that they are younger, between 10 and 100 million years old, and by measuring and extrapolating depletion rates within these theoretical parameters it is thought the rings could completely disappear within the next 300 million years.
A competing theory is that the debris are actually remnants of a small planet that was torn apart by Saturn much earlier in its history, while still surrounded by a gaseous nebula. The planet’s heavier core would have been devoured by Saturn, leaving the stripped outer layers which could explain why there is so little rocky material currently in the rings.
There’s also a theory that the rings are accumulated from micrometeoroids over time, which would need a much longer timeframe to accumulate – more like a billion years.
The other mystery is how much longer the rings will last. One method scientists have tried to estimate this is by estimating the rate at which material is measured to be falling into Saturn. Charged ice material can be pulled along magnetic field lines by gravity in a process called “ring rain,” which the Keck Observatory in Hawaii calculated to occur at between 432 to 2,870 kilograms of matter per second. Added to good ol’ fashioned gravity pulling charge-neutral material to the planet, which the Cassini mission measured to be between 4,800 to 44,000 kilograms per second, scientists believe the rings will be completely gone in less than 300 million years, maybe as little as 100 million years.
So as you can see, there’s a lot to still find out when it comes to the rings – but we still have a journey to complete. Looming ahead of us, filling our view is the massive atmosphere of Saturn. Before we dive in, we’ll take a loop around for a look at the poles.
Heading north first, we’ll notice the aurora as we approach, a product of the magnetic field surrounding the planet. Slightly weaker than Earth’s – despite being more than 90 times as massive – the magnetosphere deflects solar wind particles from the sun and extends a modest one million kilometers behind the planet.
The north pole is consumed by a massive vortex with a curious hexagonal shape first observed by Voyager. Each side of the hexagon is longer than Earth at about 14,500 kilometers long and rotates once about every 10 hours and 40 minutes, apparently the same rate of rotation for the interior of the planet. The hexagonal pattern is an unsolved mystery, but most scientists believe it is caused by a standing wave pattern, supported by other polygonal shape patterns achievable in rotating fluids within a laboratory setting.
Zipping on around to the south pole (a trip which would have taken the Cassini spacecraft over three days to complete, by the way – aren’t virtual explorations great?) we find another vortex spinning, curiously this time in a circular shape.
The massive hurricane-like storm observed by the Cassini craft sits right at the pole, the size of Earth with a clearly defined eyewall and winds churning at 550 kilometers per hour. Eyewalls have not yet been observed in any storms outside of Earth – not even Jupiter’s Great Red Spot – which makes this a unique find.
Returning to the equatorial region, we can see banding patterns similar to Jupiter, although more faintly defined. They’re named according to similar classifications as Jupiter as well, but don’t let their faint blending fool you – the winds on Saturn are much faster than Jupiter. Large, persistent storms similar to the Great Red Spot of Jupiter are possible on Saturn, but even the standard winds are faster, blasting up to 1,800 kilometers per hour at the equator.
So buckle up as we head down.
First we’ll encounter the upper cloud layers which are mostly made up of ammonia ice, starting our descent at a temperature as low as 100 Kelvin or negative 280 degrees Fahrenheit. The pressure here is roughly comparable to what we experience on the surface of Earth.
As we descend and pressure begins to build, the clouds become water ice, transitioning to a band of ammonium hydrosulfide ice where temperatures rise to about 200 K or negative 100 degrees Fahrenheit. Eventually we reach an aqueous layer of water droplets with ammonia. Here pressure can reach about twenty times what we experience on Earth, with temperatures up to 330 Kelvin or 134 degrees Fahrenheit.
We’ll continue our fall until we reach the base of the atmosphere which begins to transition into a layer of liquid helium-saturated molecular hydrogen and eventually a metallic hydrogen inner layer. The interior of Saturn is hot – hotter than scientists expected to find. The planet radiates two and a half times more energy than it receives from the Sun. Scientists theorize the heat is generated by “raining out” of helium droplets into the less dense hydrogen liquid which causes warming friction and leaves the outer layers depleted of helium. Whatever the cause is determined to be, temperatures here in the interior can reach up to almost 12,000 K or 21,000 degrees Fahrenheit.
Finally we reach Saturn’s core, which unlike Jupiter’s is theorized to be completely solid. Scientists have estimated the core to be about 25,000 kilometers wide with as much as 9 to 22 times as much mass as planet Earth crunched in there. And here concludes our journey. It would be nice to think we might find the remnants of the Cassini orbiter, but would have certainly been vaporized long before reaching the core.
We’ve come a long way on our journey today. We witnessed the dancing and rippling waves of the rings of Saturn before taking a dive into the turbulent and hostile gas giant, and we learned more about the important and inspiring work of the scientists who have made today’s journey possible. In the next episode we’ll tour the many moons of Saturn and learn what lessons they teach us about our solar system and the universe at large – and about what mysteries they still hold for future scientists to discover.
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Thank you all for listening, and as always, happy terraforming.
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