# Why is Saturn the only large planet without any trojans?

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Every planet apart from Mercury and Saturn has trojans in its L4 and/or L5 points.

Mercury is easy enough to explain: It is small, has an eccentric orbit that precesses, and any trojans it has would be heavily perturbed by the Sun's gravity.

However Saturn doesn't have any of those complications. The answer given to this question is that Jupiter's gravity perturbs any trojans at Saturn's L4 and L5 points.

But if Mars, which is much less massive than Saturn and comes closer to Jupiter, can have multiple trojans then there's no reason why Saturn shouldn't.

They are pulled out of stable orbits by Jupiter.

The details are in https://ui.adsabs.harvard.edu/abs/2012jsrs.conf… 225B/abstract

Full text https://syrte.obspm.fr/jsr/journees2011/pdf/baudisch.pdf

The planet Jupiter is solely responsible for the hole of instability for short time integrations ($$T < 10^{7}$$) compared to the age of the planetary system. On the long term scale this planet also destabilizes the whole region around the Saturnian libration points. If we find in the future Trojans of Saturn, these Trojans could only be captured asteroids, in orbits in the 1:1 MMR for a short time

The answer is complex but likely relates to the properties of (near-)resonances in the Solar System, which can stabilise or destabilise orbits at quite a long range.

As noted in James K's answer, perturbations from Jupiter are the main reason for the absence of Saturnian Trojans. While the exact process is complex, one major factor appears to be Laplace's "Great Inequality": the near-5:2 resonance between Jupiter and Saturn. As remarked in de la Barre et al. (1996) (who use the term "Bruin" to describe Saturnian Trojans), this relationship is a major factor in the dynamics:

We numerically integrated various Bruin orbits using different Solar System models to develop a Hamiltonian perturbation theory for low-inclination Bruin orbits. Although only at the beginning stages of development, the theory already identifies three separatrices of Bruin motion due in part to the Great Inequality (GI) between Jupiter and Saturn. These GI separatrices are a major contributor to the unstable region near Saturn's L4 and L5 points.

In contrast, the Martian Trojan points can support orbits with lifetimes comparable to the age of the Solar System. Mars and Jupiter are not in a Great Inequality-like relationship, which may explain why the influence of Jupiter isn't as destructive.

## Exploring planet Saturn and its elegant rings

“The Saturn system is a rich planetary system. It offers mystery, scientific insight, and obviously splendor beyond compare, and the investigation of this system has enormous cosmic reach… just studying the rings alone, we stand to learn a lot about the discs of stars and gas that we call the spiral galaxies”

-Carolyn Porco

A survey was taken by ranker to rank the planets in the solar system. It was clear how much people loved Saturn as Saturn ranked to on position two after Earth (of course we won’t stop loving our home planet right?).

Saturn is for sure the most dazzling planet in the solar system. Its bright yellowish-brown surface and the most extensive ring system contributes to its distinctive nature.

• 2010 TK 7 : A 300 metre diameter asteroid, discovered using the Wide-field Infrared Survey Explorer (WISE) satellite, is the only confirmed Earth trojan as of 2017. [1][2][3]

No known objects are currently thought to be L5 trojans of Earth. A search was conducted in 1994 covering 0.35° 2 of sky under poor observing conditions [4] which failed to detect any objects "The limiting sensitivity of this search was magnitude

22.8, corresponding to C-type asteroids

350m in diameter or S-type asteroids

• 2020 XL 5 : In 2021 it was discovered that asteroid 2020 XL5 appears to be librating around L4, making it another Earth Trojan if confirmed. Subsequent analysis confirmed modeling stability for at least several thousand years into the future based on existing orbital parameters. [5][6] This would make 2020 XL5 more stable than 2010 TK7 , which is potentially unstable of time scales of less than 2,000 years. [7]

2010 TK 7 was discovered using the Wide-field Infrared Survey Explorer (WISE) satellite, on January 25, 2010.

In February 2017, the OSIRIS-REx spacecraft performed a search from within the L4 region on its way to asteroid Bennu. [8] No additional Earth trojans were discovered. [9]

In April 2017, the Hayabusa2 spacecraft searched the L5 region while proceeding to asteroid Ryugu, [10] but did not find any asteroids there. [11]

The orbits of any Earth trojans could make them less energetically costly to reach than the Moon, even though they will be hundreds of times more distant. Such asteroids could one day be useful as sources of elements that are rare near Earth's surface. On Earth, siderophiles such as iridium are difficult to find, having largely sunk to the core of the planet shortly after its formation. A small asteroid could be a rich source of such elements even if its overall composition is similar to Earth's because of their small size, such bodies would lose heat much more rapidly than a planet once they had formed, and so would not have melted, a prerequisite for differentiation (even if they differentiated, the core would still be within reach). Their weak gravitational fields also would have inhibited significant separation of denser and lighter material a mass the size of 2010 TK7 would exert a surface gravitational force of less than 0.00005 times that of Earth (although the asteroid's rotation could cause separation).

A hypothetical planet-sized Earth trojan the size of Mars, given the name Theia, is thought by proponents of the giant-impact hypothesis to be the origin of the Moon. The hypothesis states that the Moon formed after Earth and Theia collided, [12] showering material from the two planets into space. This material eventually accreted around Earth and into a single orbiting body, the Moon.

At the same time, material from Theia mixed and combined with Earth's mantle and core. Supporters of the giant-impact hypothesis theorise that Earth's large core in relation to its overall volume is as a result of this combination.

Astronomy continues to retain interest in the subject. A publication [13] describes these reasons thus:

The survival to the present day of an ancient [Earth Trojan] population is reasonably assured provided Earth's orbit itself was not strongly perturbed since its formation. It is therefore pertinent to consider that modern theoretical models of planet formation find strongly chaotic orbital evolution during the final stages of assembly of the terrestrial planets and the Earth–Moon system. Such chaotic evolution may at first sight appear unfavorable to the survival of a primordial population of [Earth Trojans]. However, during and after the chaotic assembly of the terrestrial planets, it is likely that a residual planetesimal population, of a few percent of Earth's mass, was present and helped to damp the orbital eccentricities and inclinations of the terrestrial planets to their observed low values, as well as to provide the so-called "late veneer" of accreting planetesimals to account for the abundance patterns of the highly siderophile elements in Earth's mantle. Such a residual planetesimal population would also naturally lead to a small fraction trapped in the Earth's Trojan zones as Earth's orbit circularized. In addition to potentially hosting an ancient, long-term stable population of asteroids, Earth's Trojan regions also provide transient traps for NEOs that originate from more distal reservoirs of small bodies in the solar system like the main asteroid belt.

Several other small objects have been found on an orbital path associated with Earth. Although these objects are in 1:1 orbital resonance, they are not Earth trojans, because they do not librate around a definite Sun–Earth Lagrangian point, either L4 or L5.

Earth has another noted companion, asteroid 3753 Cruithne. About 5 km across, it has a peculiar type of orbital resonance called an overlapping horseshoe, and is probably only a temporary liaison. [14]

469219 Kamoʻoalewa, an asteroid discovered on 27 April 2016, is possibly the most stable quasi-satellite of Earth. [15]

## Scientists model Saturn's interior, explain planet's unique magnetic field

New Johns Hopkins University simulations offer an intriguing look into Saturn's interior, suggesting that a thick layer of helium rain influences the planet's magnetic field.

The models, published this week in AGU Advances, also indicate that Saturn's interior may feature higher temperatures at the equatorial region, with lower temperatures at the high latitudes at the top of the helium rain layer.

It is notoriously difficult to study the interior structures of large gaseous planets, and the findings advance the effort to map Saturn's hidden regions.

"By studying how Saturn formed and how it evolved over time, we can learn a lot about the formation of other planets similar to Saturn within our own solar system, as well as beyond it," said co-author Sabine Stanley, a Johns Hopkins planetary physicist.

Saturn stands out among the planets in our solar system because its magnetic field appears to be almost perfectly symmetrical around the rotation axis. Detailed measurements of the magnetic field gleaned from the last orbits of NASA's Cassini mission provide an opportunity to better understand the planet's deep interior, where the magnetic field is generated, said lead author Chi Yan, a Johns Hopkins PhD candidate.

By feeding data gathered by the Cassini mission into powerful computer simulations similar to those used to study weather and climate, Yan and Stanley explored what ingredients are necessary to produce the dynamo -- the electromagnetic conversion mechanism -- that could account for Saturn's magnetic field.

"One thing we discovered was how sensitive the model was to very specific things like temperature," said Stanley, who is also a Bloomberg Distinguished Professor at Johns Hopkins in the Department of Earth & Planetary Sciences and the Space Exploration Sector of the Applied Physics Lab. "And that means we have a really interesting probe of Saturn's deep interior as far as 20,000 kilometers down. It's a kind of X-ray vision."

Strikingly, Yan and Stanley's simulations suggest that a slight degree of non-axisymmetry could actually exist near Saturn's north and south poles.

"Even though the observations we have from Saturn look perfectly symmetrical, in our computer simulations we can fully interrogate the field," said Stanley.

Direct observation at the poles would be necessary to confirm it, but the finding could have implications for understanding another problem that has vexed scientists for decades: how to measure the rate at which Saturn rotates, or, in other words, the length of a day on the planet.

This project was conducted using computational resources at the Maryland Advanced Research Computing Center (MARCC).

### Is Saturn a failed star?

A common question about both Saturn and Jupiter is whether they are failed stars, or a failed Sun. This is because they’re largely made up of the same properties as stars, hydrogen and helium. The issue with both of these planets is that they don’t have enough mass to actually create the temperature required to shine like a star does. So, some people refer to Saturn as a failed star, but it doesn’t have anywhere near the mass to be considered anything near a star, to be honest.

### Is Saturn losing its rings?

Astronomers think that in the next 100 million years, Saturn will have lost some, if not all of it’s rings. This is because eventually, the rings will be pulled into Saturn by it’s gravity, causing them to disappear.

### Who discovered Saturn?

Saturn has been known to humans for thousands of years, dating back to the Babylonians and Ancient Greece – we can find writings about Saturn in different cultures, including ancient Chinese culture. However, when we’re talking about Saturn being seen properly through a telescope, we generally look to Galileo Galilei in the 1600s as the first person to really see Saturn.

### Does Saturn rain diamonds?

With lightning on Saturn is more than 10,000x the power of the lightning on Earth this lightning transforms methane gas into large clouds of soot. The pressure applied to this soot is so much that the soot literally transforms into diamonds. However, these diamonds won’t last forever, and they’ll eventually become liquified.

### Does Saturn have water?

In our search of the solar system for water, Saturn is one that many people ask about, even though it’s more than a billion kilometres away from Earth. Saturn does actually have trace amounts of water amongst it’s hydrogen and helium body, as well as iced water across it’s rings.

## Saturn’s large moon Titan is drifting away 100 times faster than anyone knew

Titan orbiting Saturn, as seen by the Cassini spacecraft on May 12, 2012. Image via NASA/ JPL-Caltech/ Space Science Institute.

Researchers knew Saturn’s moon, Titan, was moving away from its planet, just as Earth’s moon gradually orbits farther from Earth. But – while Titan’s outward drift is still extremely slow – this large moon is now known to be moving away from Saturn 100 times faster than previously thought. That’s according to a June 2020 announcement by scientists in the U.S., France and Italy.

The peer-reviewed findings were published on June 8 in the journal Nature Astronomy.

The rate of Titan’s movement away from Saturn had been thought to be well understood, but as often happens in science, a new discovery has upended that idea. The discovery was made via a new analysis of data from the Cassini spacecraft, which orbited Saturn from 2004 to 2017. The data show Titan moving outward at about 4 inches (11 centimeters) per year.

That might not sound like a lot, but it’s significantly faster than the rate at which our moon drifts away from Earth. Our moon is drifting outward at only about 1.5 inches (3.8 centimeters) each year.

Titan’s surface as seen in infrared via Cassini on November 13, 2015. Titan is completely surrounded by thick haze, but Cassini’s radar penetrated the haze to reveal surface details. Image via NASA/ JPL/ University of Arizona/ University of Idaho.

The researchers reached this conclusion after studying images of stars sent back by Cassini. They mapped the background stars and tracked the position of Titan among them. Those images were then compared to a completely separate dataset, Cassini’s radio science data. Cassini sent radio waves back to Earth during 10 close flybys that the spacecraft conducted of Titan between 2006 and 2016. By examining how the frequency of the radio signals was affected by interactions with its environment in space, the researchers could estimate how Titan’s orbit evolved and changed over the past few billion years. Study coauthor Paolo Tortora, of Italy’s University of Bologna, explained in a statement:

By using two completely different datasets, we obtained results that are in full agreement, and also in agreement with Jim Fuller’s theory, which predicted a much faster migration of Titan.

Why do moons move away from their planets, anyway?

Titan is one of the outer moons of Saturn, orbiting well beyond the main rings and out past Rhea. Image via NASA/ JPL/ Wikipedia.

As a moon orbits a planet, its gravity tugs a bit on the planet – a process called tidal friction – creating strain and a resulting very slight bulge on the planet. On Earth, this bulge happens most noticably in our oceans – called a tidal bulge – and causes the cycle of high and low tides, but planets without oceans can bulge, too. This cyclic process of bulging and then subsequent relaxing creates a lot of energy over a long period of time. Tidal friction, however, also prevents the tidal bulge on Earth from remaining directly beneath the moon instead it is carried along with the rotation of the Earth. The energy, created by mutual attraction between the moon and the material in the bulge, accelerates the moon slightly in its orbit. This causes the moon to drift a tiny bit farther away from Earth over time.

The new result also has implications for the age of the entire Saturn system. It’s still uncertain just how old Saturn’s rings are, as well as the planet’s moons. Right now, Titan is 759,000 miles (1.2 million kilometers) from Saturn if the new measurement of the moon’s drift rate is correct, it means that Titan must have once been closer to Saturn than previously understood, and that Titan has migrated to its current position far out from the planet. In other words, Titan must have gone from being an inner moon to being an outer moon.

The new result further implies that the entire Saturn system of moons expanded faster than thought. Valery Lainey, lead author of the new study, formerly a scientist at the Jet Propulsion Laboratory (JPL) and now at the Paris Observatory at PSL University, stated:

This result brings an important new piece of the puzzle for the highly debated question of the age of the Saturn system and how its moons formed.

Artist’s concept of Cassini near Titan. Image via Kevin Gill/ Flickr/ Smithsonian Magazine.

Plus, the findings offer validation for a theory about how planets affect the orbits of their moons. Previous theories have stated that moons farther out from a planet migrate more slowly than moons closer in. The assumption was that the planet’s gravity would have a greater affect on moons that were closer, which sounds logical. But that view came into dispute four years ago, thanks to theoretical astrophysicist Jim Fuller at Caltech. He predicted that outer moons and inner moons should actually migrate at similar rates, because outer moons have a different orbit pattern linked to the “wobble” of a planet. That wobble can sling inner moons outward. Fuller said:

The new measurements imply that these kind of planet-moon interactions can be more prominent than prior expectations and that they can apply to many systems, such as other planetary moon systems, exoplanets – those outside our solar system – and even binary star systems, where stars orbit each other.

Valery Lainey at JPL and the Paris Observatory, lead author of the new study. Image via ResearchGate.

Cassini orbited Saturn for more than 13 years, collecting vast amounts of data and taking thousands of images. The mission ended in September 2017 after the spacecraft finally ran out of fuel. By design, Cassini plummeted into Saturn’s deep, tumultuous clouds to burn up so as to not risk contaminating any of the moons, especially Enceladus or Titan, with any stray microbes from Earth that may have still been onboard. Cassini revolutionized our knowledge about the Saturn system, and as this study shows, there is still much to learn.

Bottom line: New study shows that Titan is moving away from Saturn 100 times faster than first thought.

## Saturn's moons explain the planet's tilt and why it's increasing

Researchers at the Paris Observatory's Institute of Celestial Mechanics and Ephemeris Calculation have found that the unusual tilt of Saturn's axis is due to the periodic gravitational pull of its moons over the last billion years.

Everyone who has taken basic geography knows that the Earth is tilted on its axis, but so are the other planets and other bodies in the solar system. The degree of these tilts vary so dramatically that, at first glance, this seems to be random. The tilt of a planet "just is."

However, this turns out not to be the case. Saturn has a tilt in relation to its orbit of 26.73°, but this is not the result of chance. In fact, according to a pair of scientists from CNRS and the Sorbonne University working at the Paris Observatory, it's due to a complex ballet of gravitational forces.

The mechanism is something called orbital resonance. As bodies revolve around the Sun and each other, they tug at one another periodically. It's a very small tug, but over time, this can have a very large cumulative effect. The same principle is at work when you push a child in a swing. The push can be very small, but a swing, and an orbit, has a natural frequency at which it naturally vibrates, so each small push adds up to a big arc.

Animation showing the migration of Saturn's tilt change and the migration of its moons

This concept of orbital resonance explains a lot about how orbiting bodies interact. In the case of Saturn, the research team found that the planet's tilt wasn't the result of an interaction with the gas giant Neptune four billion years ago, as previously thought, but by the pull of Saturn's moons, especially its largest satellite, Titan.

Observations have determined the Titan and the other Saturnian moons are migrating. Over time, they are moving away from the planet, much as the Moon is gradually moving away from the Earth. However, the rate of the Saturnian moons' migration is faster than previously thought, resulting in a greater tilt of the planet.

According to the research, Saturn had only a slight tilt for its first three billion years of existence. Then, about a billion years ago, the moons' pull set up the orbital resonance that quickly, in cosmic terms, increased the tilt, which is still ongoing. It's estimated that over the next billions of years, the tilt will become more pronounced.

Burkhard Militzer is an associate professor of Earth and Planetary Science and of Astronomy at the University of California, Berkeley in Berkeley, California.

JOHN DANKOSKY: This is Science Friday. I’m John Dankosky. Ira Flatow is away.

Later in the hour, we’re talking about conflicts of interest and researchers who aren’t disclosing them to journals. If you’re a medical professional, we’d like to hear from you. How do you approach conflicts of interest? Give us a call. Our number’s 844-724-8255. That’s 844-SCI-TALK. Or you can always tweet us @scifri.

But first, in our solar system, there’s one planet that is immediately identifiable, Saturn, with its iconic rings. Saturn just wouldn’t be Saturn without that cosmic halo, but those rings weren’t always there. The planet didn’t put a ring on it until relatively recently. They’re estimated to have formed 10 to 100 million years ago, billions of years after the planet formed. Researchers were able to give an age to the rings using data from the final dive of the Cassini mission. Their results were published in the journal Science.

My next guest is here to fill us in. Burkhard Militzer is an author on that study. He’s a professor of Earth and Planetary Science and of Astronomy at the University of California at Berkeley. Welcome to Science Friday. Thanks for being here.

BURKHARD MILITZER: Hello. It’s great to be on the show.

JOHN DANKOSKY: Saturn has four groups of rings around it. Let’s start there. What are these rings made of?

BURKHARD MILITZER: So we can judge them from the color. We’ve never actually taken a sample of it, so we have to tell from remote observations what they’re made of. And from the color, we can tell what the ratio between ice and the rocky component is. They’re mostly ice, so we also think they started out as almost purely icy objects.

JOHN DANKOSKY: Started out as mostly ice, and they’ve gotten more rocky over time?

BURKHARD MILITZER: That’s exactly right. So we can estimate how many meteorites hit these rings over time, and they progressively come slightly more darker every millions of years or so. So there’s an influx of meteorites. The same way that meteorites hit the Earth, and we know how many are hitting the Earth every year, we know also how many hit Saturn every year, but nothing so often.

JOHN DANKOSKY: So what are some ideas about how these rings were formed? What are the theories that are out there?

BURKHARD MILITZER: So to be honest, the most fundamental question was actually– originally, we thought, and I thought, that the rings are as old as the planet. And that is now what we are now refuting. Now, we think the rings formed very recently and are left to come up with another sort of mechanism.

So the two ideas concerning what could have happened involve something totally drastic. The rings are billions of icy particles, so you have to have some drastic mechanism to get them there in place. And what really happened is some bigger object got blown into pieces.

And the two scenarios– we’re not sure which one’s right– but one is that there was a big comet from the Kuiper belt that somehow got gathered, and it got in the gravity field of Saturn. And it got closer and closer, and that got disrupted by Saturn. And then you have so many ring particles left. So if it comes from the Kuiper belt, that is one of the driving arguments, but some bigger object got disrupted by Saturn’s gravity.

JOHN DANKOSKY: So most planets don’t have rings. What’s special about Saturn that has allowed the planet to form these rings around it and maintain them over these years?

BURKHARD MILITZER: So if you look very, very carefully, you’ll find faint rings around all of the giant planets. So Jupiter has one, and Uranus and Neptune have very faint rings as well, but Saturn has one that is so prominent. That makes it so special.

So there’s quite a lot of mass, a lot of particles. And the way of maintaining them is not that difficult because the ring particles follow the same trajectories like a moon, so they’re relatively stable over time. But the question is, how do you get so many particles at just an exquisite location, because if you are slightly out of the orbit with the other ring particles, you’re bound to hit the other ring particles at some point when you go around Saturn, and then you get kicked out of the orbit.

So you have to have many particles all perfectly aligned. And we think we have many, many more particles, and those who are not in line just get kicked out of the orbits around Saturn. It’s just that many of them will find a line, and they are still there today.

JOHN DANKOSKY: Well, what else is special about Saturn though? I understand it spins very fast for such a large planet, so the gravity around Saturn must be interesting. Does that have anything to do with the rings?

BURKHARD MILITZER: Well, that’s– exactly. What the Cassini spacecraft did in its last 25 orbits, before it actually burned up in the atmosphere of Saturn, it measured the gravity field. And I was on the team anticipating these measurements, and what– I’m a theorist– so what we do is, before our experimental colleagues make the observations, we give them some numbers, what they should find.

So we did lots of calculations in the year before these measurements were made, and we told our Italian colleague that this– we mapped the gravity field– told him this number should be between minus 9 and minus 8.5, and we were sure it would be in between these numbers. And then they measured minus 14.

So at that point, we were totally puzzled because Saturn’s gravity field was totally different from all the models we constructed before, and the reason was that Saturn has very deep winds. So we left them out of the models. Now, we know that the winds that you see from the clouds that we’ve seen before, they’re not just very thin surface clouds like we see on our planet.

They were super deep, and they go about 9,000, or possibly more, kilometers into the planet. But the winds are moving slightly faster than the rest of Saturn, and that changes the gravity field. They’re so massive, and that’s what the spacecraft picked up. That was the one remarkable measurement, and the other home was, it measured the mass of the rings. And that was equally important.

JOHN DANKOSKY: Yeah. And tell us more about that, about the mass of the rings.

BURKHARD MILITZER: So this was the first time that the mass of the rings was determined from gravity. And you needed to go in between the rings so that the rings are pulling you out while the massive planet pulls you the other way. Then you can really nicely detect the signal from the rings.

And they are not that heavy. They are about– I don’t know– one four thousandth of the mass of our moon. But now we have this number very accurately, and we can really say the rings are not massive.

And from that mass measurement, and the color of rings that tell us the rock to ice ratio, and the information regarding how many rocky components are added to the rings every year, we can extrapolate back in time, and we can say when the rings have formed. And that’s not 4.5 billion years ago. This is almost yesterday. This is like 100 million years ago, so there must have been something really drastic happening around Saturn relatively recent that produced the rings.

JOHN DANKOSKY: Does that suggest to you, though, that these rings are somewhat ephemeral, that as quickly as they were formed not too terribly long ago, that they could disappear again someday relatively soon?

BURKHARD MILITZER: Well, I think not tomorrow. I think it’s good. If you want to have a look at Saturn, you can go tomorrow, but next week it will still be there, and when you retire, you’re safe.

But they do degrade. So the ring particles– you think the rings are all perfectly stable, but as we get better and better spacecraft data, we see how dynamic they are. And the best analogy is actually the Earth’s ocean. If you look from far away, it’s just a smooth surface, but if you’re actually on it, you see the waves and sometimes the bigger waves.

And similarly, that happens in the Saturnian ring system. You lose particles because they drift in and they’re swallowed up by Saturn. And sometimes, there’s a moon embedded in the rings, and then when that comes by, it leaves a big wake in the ring particles nearby.

So there’s some loss. So over, we think now, 100 million years, maybe they’re gonna be very, very faint or almost gone in 100 million years. We think they were more massive when they started out, and they’re losing mass over time gradually.

JOHN DANKOSKY: I have to ask you quickly– because this is based on this amazing trip of Cassini, that crashes into Saturn and gives you all this information on the way down– if you could just describe the path that Cassini took as it was gathering this data for you, flying between the rings and Saturn.

BURKHARD MILITZER: So the Cassini spacecraft has been in orbit around Saturn [INAUDIBLE] for 13 years. And they flew by the moons. And they made very intricate orbit calculations for everything, but they saved the best for last. And if you look at these orbits– the rings, they look big– but if you’re the Cassini spacecraft, you orbit was far outside the ring system, so they had to brake a little bit to do the dive on the inside.

And that is– it can all be done and calculated, and you can do it with high precision, but you’re never quite sure that you wouldn’t run into one of those errant ring particles that just happened to drift in your trajectory. Nothing happened, but there was– the risk was not zero. So they calculated it and they went inside the rings, which meant they had to come really close to the surface, go through the gap of the innermost ring and the surface of the planet, and then go back out.

And the orbits are very elliptical. They have the size of 15 times the diameter of Saturn. So they only come for a short moment in this small gap– relatively small– and then they go back out. They’re very elliptical and come back and [INAUDIBLE]

They measured the gravity field, which made these two observations possible, the deep winds and the mass of the rings. And then they came closer, and they got a little bit of a snippet from the atmosphere. They looked at what molecules there are– there was hydrogen and helium– and what their ratio is, and so on.

JOHN DANKOSKY: So what does all this ring data tell us about a picture of how Saturn was formed and how, honestly, the entire solar system was formed? What can you learn from this?

BURKHARD MILITZER: So we get the– first of all, the fact that the rings are so young, it tells us our solar system, even today, is not perfectly stable as we think it is. There’s still collisions. So 100 million years ago, something happened there. We know there was a big impact 65 million years ago on our planet, so in the million year timescale, there’s still sizable impacts or drastic events. That’s the first thing.

So the other thing, which is a little indirect, that’s why we actually fly these missions. We want to understand how the solar system formed. And how many rocks and how much ice was there available, and how were they distributed?

And the rocks and the ice, these particles collided, and they made the four rocky planets in the inner part of the solar system. And there was more ice, and it made the giant planets in the outer part of the solar system. We want to understand this better. We have an idea of what happened, but the details are really murky.

JOHN DANKOSKY: The details are murky, and hopefully, we’ll find out more details over time, but this is fascinating. Burkhard Militzer is a professor of Earth and Planetary Science and of Astronomy at the University of California at Berkeley. Thank you so much for joining us. I really appreciate it.

## Saturn and Its Rings by Andrew Fraknoi

Perhaps the most imposing sight you can see through a telescope is the planet Saturn, with its magnificent system of rings. Many dedicated amateur astronomers say they were turned on to stargazing by seeing Saturn through a telescope. And Saturn is impressive in more ways than just its looks.

One of the giant planets in the outer solar system, Saturn contains enough material to build 95 Earths. Its diameter is 75,000 miles&mdashlarge enough to fit more than nine Earths across it. Yet for all its bulk, Saturn is a lightweight planet. Made mostly of the simplest gases in the universe, its average density is less than that of water&mdashso if you had a bathtub big enough, Saturn would float in it.

The giant planet takes only about 10 hours to spin once&mdasha day less than half of our much smaller Earth's. This rapid spin plus heat rising from its interior create powerful and complex weather systems in Saturn's atmosphere. The storms get particularly intense every 30 years&mdashone Saturnian year&mdashwhen summer comes to the ringed planet's northern hemisphere. In 1990, the largest of the season's storms could be seen even with modest telescopes. It was amateur astronomers who first alerted professional scientists that a huge storm was brewing. It eventually spread to encircle the Saturn globe.

These days, a sophisticated spacecraft called Cassini is conducting close-up observations of the Saturn system, orbiting the planet and recording images and data about the atmosphere, the rings, and Saturn's moons. You can see some of the pictures it is sending back at: saturn.jpl.nasa.gov/multimedia/images/index.cfm

Saturn has a swarm of moons that interact with the rings. As of mid-2007, 59 moons had been identified, most of them relatively small. One moon, however, called Titan, is the second largest in the solar system, and bigger than Mercury, Eris, and Pluto. It has a smoggy atmosphere and rivers of liquid swamp gas (methane) appear to flow on its cold surface. Bright and rather ruddy looking, Titan can be readily discerned out beyond the rings it looks like a star in smaller telescopes, while larger ones show it is a disc.

Saturn's rings are made up of billions of icy pieces, organized by their mutual gravity and the interfering gravity of nearby moons into thousands of strands and ringlets. The ring particles range in size from smoke particles to the bulk of a small truck. Occasionally, an especially strong interaction between a moon and the ring particles will produce a noticeable gap in the rings some of the gaps appear in the pictures on this page. When the edge of a ring or gap is especially sharp, there is a good chance that a shepherd moon is responsible. Just as shepherds and shepherd dogs keep flocks of sheep from straying, shepherd moons keep ring particles from moving away from the orbit they mark out, leaving a clear ring edge in their wake.

As described on Seeing in the Dark, in the mid-1970's, Stephen O'Meara, a young amateur astronomer with phenomenal observing skills, noticed dark radial features on the rings of Saturn that reminded him of spokes in a bicycle wheel. Although they had been spotted by a few earlier observers, the consensus was that they had to be optical illusions. Scientists reasoned as follows: Every chunk of ice in the rings orbits Saturn at its own rate, with inner bodies&mdashwhich, being closer to Saturn, experience a stronger force of gravity&mdashmoving faster than the outer ones. If you flew a spaceship over the rings and painted a radial line across them, the inner part of the line would move ahead while the outer part lagged behind, and the line would quickly disappear. So O'Meara was unable to get his drawings of the spokes published. Nobody took them seriously.

Then, in 1979, the Voyager 1 spacecraft flew by Saturn and took close-up pictures of its rings with unprecedented detail. Clearly visible on some of the pictures were long straight spokes across the rings, much like the ones O'Meara had sketched.

No one fully understands the reason these spokes exist, but the fact that they do NOT drift apart means they must be connected with the spin of Saturn and not with the motion of the many ring chunks. One theory, mentioned in the film, is that Saturn's magnetic field captures any small pieces of icy dust that have an electric charge and levitates them above the rings while stringing them out into straight marks that trace lines in the magnetic field. Magnetically lifted above the main rings, such particles respond to the rotation rate of Saturn's magnetic field rather than of the rings themselves.

The evidence suggests that spokes appear mainly during certain seasons of the long Saturn year, perhaps in response to the changing angle at which sunlight hits the rings. This may explain why the Cassini space probe, unlike Voyager, found only a few, dim spokes. The facts are hard to pin down, but what is clear is that the patient observations of a young amateur astronomer helped generate an intriguing new branch of Saturn ring science, and that the Lord of the Rings is very likely nowhere near finished spinning puzzles for Earth's observers.

## Curiosities: Why do some planets have rings?

Planetary ring systems are complicated, notes UW Space Place Director Jim Lattis, and they are more common than once believed.

For ages, Saturn was thought to be the only planet in our solar system with a ring system. But in recent years ring systems have been discovered around Jupiter, Uranus and Neptune as well.

&ldquoThere are various theories about planetary rings, like the fantastic rings around Saturn, but we cannot say for sure how they are formed,&rdquo explains Lattis.

One theory is that the rings formed at the same time as the planet and its major moons. In this case, if material is close to the planet, the planet&rsquos gravitational pull is too strong to coalesce into a moon and the particles that would otherwise form a moon spread out in orbit around the planet as a ring.

Another idea is that a close call by a moon or comet results in the planet’s gravitational tidal force breaking up those bodies, the debris of which then becomes a ring system.

Although astronomers can’t say for certain what causes planetary rings, Lattis says that the Cassini spacecraft now in orbit around Saturn is beginning to provide tantalizing new clues to the forces that govern the physics of planetary rings.

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