A large part of what we’ve discovered so far about Saturn’s rings have all turned out to be counter-intuitive and ephemeral, and future data is going to be even more so.
Our solar system has a glittering, mesmerising crown jewel that has captivated scientists and public alike for centuries – Saturn. As we all know, what sets Saturn apart from other planets is its dazzling halo of rings. While the planet makes for a wonderful photographic and an even more rewarding research subject, the very formation of these rings have eluded us since their discovery. The Cassini Orbiter, named for the 17th century Italian astronomer Giovanni Cassini and currently swinging around Saturn, threw up yet another mystery earlier this month to add to the stack of puzzles the rings have piled up about themselves over the years.
Saturn’s rings are majestic for a reason: they’re not clearly discernible or fitted into plain categories. In fact, they’re not even exact concentric rings, one within the other. The ring system is complex and vast, spread out over one plane, often intertwined and braided. There are seven main rings and several other interspersed fainter rings. In classic, confusing, lackadaisical fashion, astronomers thought it would be a sensible idea to name the main rings in the order in which they were discovered. So going outward from Saturn, the main rings are named D, C, B, A, F, E, and G in that order.
The rings are composed of dust and ices that are nearly 99% pure water ice. (When astronomers talk about ice, it’s not always water ice. Other hydrocarbons and gases also freeze into a solid state out in space.) Each ring spans anywhere from 30 km to 7,500 km in diameter. They’re also not uniformly squashed on the equatorial plane like a smooth sheet. Different rings are of different thicknesses – from 10 m to 10 km thick. Furthermore, the individual ‘particles’ that comprise the rings are anywhere from a few sub-microns to 3km in size. The rings are not all identical in colour and density either: the A ring is very translucent as it has a lot of scattered particles and dust while the B ring is almost opaque.
And there’s more. The rings aren’t contiguous. It’s not a full sheet of material stretching all the way till the end. There are large divisions in the rings, and gaps of empty space between them, called… well, divisions and gaps. Each one is named after an astronomer in a completely non-sequential manner. And together with divisions, gaps, ices and rocks, the rings start at 75,000 km from the equator of the planet and extend all the way up to the 480,000 km mark. To compare, the radius of Saturn is just 58,232 km.
We didn’t know a lot about Saturn’s rings till Cassini got into orbit around Saturn in 2004. And as it turns out, we’ve even more questions now. The first, when you consider that Jupiter, Uranus and Neptune also have rings, is why Saturn’s are so large and variegated. Followed by when did the rings form? What are they made of? How do they stay in place? And so forth. Perhaps the most significant of these is how the rings formed in the first place.
Let’s step back in time to when the Solar System was being formed. The entire solar neighbourhood was a dense and roiling cloud of gas and dust. After the Sun took shape followed by some of the larger planets, there was still a lot of dust and residual material left over from this protoplanetary disk. Saturn, for some reason, held on to some of the swirling debris and kept the rings in place. Or so some people like to think.
That makes the rings very old, nearly as old as Saturn itself. But there’s major opposition to this idea. If the rings are that old, surely they would’ve been swallowed by Saturn? The massive planet would have slowly attracted the particles closest to it closer and closer over time, causing them to burn up in its atmosphere. But the rings are still here, which means they likely formed recently, on the order of a few hundred millions of years ago.
There’s a demarcation between the gravitational influence of a body and regions beyond this effect. Around every body with gravity is the hypothetical sphere called the Roche limit. Beyond this sphere, moons can exist; within this sphere, the gravitational influence of the body is so great it breaks up anything that comes closer. So if moons or objects fall within Saturn’s Roche limit, they would break up and, possibly, form a ring. It’s plausible that about a hundred million years ago, a comet came too close to Saturn, fell within the Roche limit and promptly disintegrated under its massive gravitational influence, forming some of the rings. It could even have been a large moon or multiple moons that broke and scattered debris everywhere. This is one explanation for why Cassini found evidence that many of the rings had new material.
There’s another explanation for the rings persisting to exist: replenishment. It’s possible that the rings continue to be here today because, as some of the material disappears by sublimating into space or burning up in Saturn’s atmosphere, they are constantly refilled with more material. And Cassini found just the evidence for this on the wispy E ring.
The E ring contains micron-sized particles of water ice and dust. Ice particles that size don’t really last through the ages: they sublimate and disappear in tendrils of vapour. But Cassini noticed that nearly all of the E ring material was new, prompting scientists to think about how the ring is being replenished. And the source for the E ring quickly turned out to be one of Saturn’s icy moons.
Enceladus, the moon, is covered with a thick crust of ice that acts as the roof of a global liquid water ocean, and orbits very close to the E ring. Owing to heat generated in its core due to Saturn’s gravitational pulling of its insides, the moon exhibits geothermal activity. It periodically ejects stuff from within its crust in the form of icy plumes. Just like how volcanoes on Earth spray out lava and molten rock, these volcanoes –called cryovolcanoes – throw up ice, salts and dust into Enceladus’s atmosphere. Sometimes the plumes eject material with enough energy for the ejecta to leave Enceladus’s low gravity and sputter into space. This material was actually observed entering and feeding the E ring.
Despite this finding, astronomers still don’t fully understand how other rings function as they all don’t have icy moons that feed them. They especially don’t know when the rings were formed and how. And as Cassini discovered more details about the rings and sent home comprehensive images, the mysteries deepened. Latest in the puzzle is an issue concerning the opacity of the B ring.
This ring is the brightest and the most opaque as well. If you shone a light over the B ring’s plane, it wouldn’t be visible below the ring. Contrast this with the nearly translucent A ring: if you shone enough light through the A ring from the top, a person below the ring would be actually able to see it. This intuitively means that the B ring is dense and chock full of particles. And so, that’s what we assumed all these years. Turns out we were quite wrong.
Despite the ring’s opacity, Cassini data shows that the ring isn’t as densely packed as previously assumed. NASA uses an easy-to-understand analogy to explain this: muddy water is more opaque than clear water because it has more mud particles in it. Picture a sheet of butter paper. If you shine a flashlight through it, you’ll be able to see the light clearly on the other side. Repeat the same thing with a good quality bond paper. The amount of visible light goes down but you’d still be able to make out a bright source of light on the other side. Now repeat the experiment with thick cardboard. You wouldn’t be able to see the light. That’s because the cardboard is thicker, denser – and increased density means increased matter concentrated in an area. But the B ring, analogous to the cardboard, turns out to have similar density to the butter paper!
What’s more, the density, or amount of material, and mass remain nearly constant throughout B ring while its opacity varies quite a bit as you move over one end of the B ring to the other. Phil Nicholson of Cornell University has another apt analogy for this: “Appearances can be deceiving,” Nicholson stated in a NASA press release. “A good analogy is how a foggy meadow is much more opaque than a swimming pool even though the pool is denser and contains a lot more water.”
An important clue to answering this perplexing opacity problem, as well as other questions the rings raise, would be finding out the mass of Saturn’s rings. It’s impossible to measure just the mass of the rings because crafts orbiting a body measure its mass using gravity. Gravity is a function of mass: the more the mass of a body, the higher its gravity. Thus, when you observe two bodies interacting with each other gravitationally and calculate how much each of them is tugging at the other, you can deduce their mass.
We can accurately measure the Sun’s mass by observing Earth’s motion around it. We accurately measured Pluto’s mass by observing its effects on its moon, Charon. And in the case of multiple moons or smaller bodies like asteroids, we can use spacecraft. Scientists measure the amount of tug a body exerts on the craft by measuring the rate at which it falls into the body. This technique was used to confirm the mass of Pluto, calculate the mass range of the asteroid Ida, and will be employed to determine more details about Jupiter’s core when the spacecraft Juno reaches the planet later this year.
Cassini flew around Saturn and its rings, providing us with the mass of Saturn and its rings combined. The rings have enough material to have a non-negligible change in the mass of the Saturnian system. But Saturn’s mass itself will be calculated the same way we calculate the weight of a kitten on a human scale: we stand alone and measure our weight, then we stand with the kitten and measure the combined weight, and finally we subtract the former from the latter. In 2017, Cassini will fly between Saturn and its rings to measure Saturn’s mass, and which will then be subtracted from the total mass of the system.
So, how were Saturn’s rings formed? We don’t know. How old are they? We don’t know. Why are non-dense regions so opaque? We don’t know. How much do they weigh? We don’t know. So really, we’re hoping all the data Cassini will throw at us, especially during its Grand Finale descent into Saturn’s atmosphere. Toward the end of 2016, the orbiter will start a completely new set of manoeuvres and orbits, getting closer to Saturn and beaming intricate details. The end of its 20-year mission will be marked by the orbiter finally plunging into the atmosphere of Saturn, taking care to stay far away from water-ice-occupied moons like Titan and Enceladus, and rapidly burn up in Saturn’s upper layers.
We’re hoping that the last few days of data will be able to explain at least some things we don’t have answers for today. A large part of what we’ve discovered so far about the rings have all turned out to be counter-intuitive and ephemeral, and future data is going to be even more so. It seems Saturn, the Lord of the Rings, turned out also to be very much like the Necromancer: a master of illusion and deception.
Sandhya Ramesh is a science writer focusing on astronomy and earth science.