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Frequently Asked Questions - Saturn

Frequently Asked Questions - Saturn


Who discovered Saturn?

Saturn is easily seen with the naked eye, so people must have viewed it since prehistoric times. Galileo was first to see its rings, but his telescope wasn't powerful enough to show them clearly, and he initially thought he was observing a triple planet. Dutch astronomer Christiaan Huygens was first to identify a ring around Saturn.

Where can I find Galileo's sketches of Saturn?

Here are two web sites where you can see Galileo's sketches and read about the history of Saturn observation:

http://es.rice.edu/ES/humsoc/Galileo/Things/saturn.html

http://www2.jpl.nasa.gov/saturn/back.html

Where can I find technical information about Saturn?

For a fact sheet on Saturn, visit http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturnfact.html.

For Saturn's moons, visit http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturniansatfact.html.

For its rings, visit http://nssdc.gsfc.nasa.gov/planetary/factsheet/satringfact.html.

What would happen if you tried to land on Saturn?

Saturn has no solid surface, so you'd sink down into it. The deeper you sank, the more heat and pressure you'd experience. Eventually, not even a spaceship built like the strongest submarine would be able to withstand the pressure, and you'd be crushed and roasted.

How does Saturn's size compare with Jupiter?

It depends on whether you're talking about volume or mass. Saturn has about 84% the diameter of Jupiter, but only about 30% as much mass. Such a large volume with so little mass makes Saturn the least dense of all the planets, and the only one that would float if it were possible to place it into a tub of water.

What are Saturn's rings made of? How many rings does Saturn have? How big are the ring particles?

Saturn's rings are an optical illusion on a cosmic scale. Far from solid, they're actually a blizzard of water-ice particles mixed with dust and rock fragments. Most are from around 1 cm to 5 meters, but they range from smoke-size to as big as a house. A few may reach kilometer-size. Each "particle" orbits Saturn independently, a moon unto itself. They race around the planet in distinct tracks, herded by the gravity of Saturn's "shepherd" moons, and perhaps by other influences yet to be discovered. In this sense, there are actually thousands of ringlets.

How thick are the main rings?

Based on stellar occultation measurements of ring edges, it is looking like the main rings (A, B and C) are on the order of 10 meters (33 feet) thick, or less. One exception are the bending waves, created by Mimas, whose orbit is slightly inclined with respect to the rings. Bending waves are vertical waves, and their height might reach 1 kilometer (.6 miles) or so above and below the rings.

However, seen edge-on, Cassini would be primarily seeing the F ring, not the main rings. The F ring has a slight inclination, and it can give an apparent thickness that is much different from the thickness of the main rings.

What exactly is a Saturn ring plane crossing? How often does one occur? Why is it important? When is the next ring place crossing?

Saturn's rings are tilted with respect to the Sun, and as the planet orbits from one side of the Sun to the other, the angle at which the Sun "sees" the rings changes. Twice during each orbit, as the Sun crosses from one side of the ring plane to the other (similar to our equinoxes here on Earth), the rings appear edge-on to the Sun. It takes Saturn nearly 30 Earth-years to complete one revolution around the Sun, so a ring-plane crossing occurs about every 15 years.

Earth is very close to the Sun compared with Saturn, so what we see is pretty close to what the Sun "sees." However, because the plane in which we orbit the Sun is at an angle to the plane of Saturn's rings, we actually cross the ring plane either once or three times during any 15-year half-orbit of Saturn.

The rings are so thin that when we see them edge-on as we cross the ring plane, they seem to disappear when viewed through a small telescope. (That happened to Galileo after he discovered the rings, causing him a great deal of confusion.) During these events, the glare from the rings is considerably reduced, and faint objects near Saturn are easier to see. Months before and after the crossings, unique observations of Saturn, its rings and moons can be made from Earth that are available at no other time. This is the best time to discover a new moon, to observe Saturn's cloud tops, and to observe the tenuous E and G rings.

The next two ring plane crossings will occur on September 4, 2009 and March 23, 2025, but they will be difficult to observe since Earth and Saturn will be on opposite sides of the Sun. The next opportunity to view a ring plane crossing from the same side of the Sun will be in 2038.

Do any other planets have rings?

Yes, Jupiter, Uranus and Neptune all have rings. None of those rings are as spectacular as Saturn's, but it appears to be a common phenomenon for large planets.

Where do planetary rings come from? How did rings form around Saturn?

That's one of the things we're hoping Cassini will help to answer. There are currently three theories of how Saturn got its rings:

  1. The rings may be remnants of the material that formed Saturn's moons, but which were prevented from coalescing into a moon because they were inside the Roche limit .
  2. A medium-size moon might have strayed inside the Roche limit, and been pulled to pieces by tidal forces.
  3. A moon might have been shattered by meteor impacts, and its debris might have moved to within the Roche limit, where it was unable to reunite into a large body.

What is the Roche limit?

The closer you are to a planet, the stronger is its gravitational pull on you. For a large moon, this means that the side closest to the planet is being pulled substantially more forcefully than the side facing away from the planet. Within a certain distance from the planet, that difference can be enough to pull the moon apart. The Roche limit is the minimum distance that a moon (or other large object) can be from a planet without being torn to bits. (For smaller objects, the difference in gravitational pull from one side to the other isn't enough to pull it apart.) If the planet and the orbiting body have the same density, that distance is about 2.5 times the radius of the planet.

What is the cause of the "spokes" seen on the rings?

The mysterious dark "spokes" that radiate across the B-ring are thought to consist of tiny charged particles that have become trapped, or "frozen," within the lines of Saturn's magnetic field. They can develop quite rapidly and then slowly fade. Voyager spotted one that grew over 6,000 km (3,700 miles) in just five minutes.

What are shepherd moons?

Shepherd moons are small moons that appear to be responsible for defining the boundaries of some -- and maybe most -- of Saturn's rings. Their gravitational fields keep the ring particles from straying, much like a shepherd keeps sheep from wandering away from the flock.

One startling example is the F-ring, which is tended by two tiny moons, Prometheus and Pandora. They seem to be responsible not only for maintaining the slender ring, but for its apparent braids and kinks. Cassini sent us a picture of these moons and their ring: http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=817

Why do Saturn's rings all lie in the same plane? Why are planetary rings always found in their equatorial planes and not sometimes crossing their poles?

Saturn, its rings, and most of its moons all probably formed from a spinning disk of gas and dust within the original solar nebula. As it collapsed toward its center, it would have formed a central sphere (which became Saturn), leaving a disk of material orbiting its equator. That material probably coalesced into many or most of Saturn's moons. The ring particles may be material that was left over from this process, or they may be the remnants of a moon that was shattered by collision or by tidal forces (see Roche limit). In either case, the ring particles would have kept the angular momentum of the original disk, and continued orbiting Saturn in its equatorial plane.

If Saturn captures particles coming in from other directions (along the polar plane, for example), they will tend to be pulled toward the equatorial plane, too. Saturn's rapid rotation creates a centrifugal effect that produces a bulge around its equator. With more mass around its equator than at its poles, Saturn's gravity is stronger around its middle, so incoming particles would tend to be drawn there. Once in the area, they'd be likely to collide with some of the many other particles orbiting in that plane, which would rob them of their initial momentum and encourage them to join the throng moving along the equatorial plane.

What is the mean distance between the surface of Saturn and the first ring? What is the mean outer diameter of the Saturn ring system?

The mean distance from Saturn to the outer edge of the F ring is 2.33 Saturn radii, or 140,270 km (87,160 miles). The mean distance between the surface of Saturn and the first ring (we use the inner B ring) is 1.52 Saturn radii or 92,000 km (about 57,000 miles).

To make your own model of Saturn and its rings, cut a 1.5-inch Styrofoam ball in half, and glue each half to opposite sides of a CD. That gives you pretty accurate proportions. Two differences: Saturn's rings don't actually touch the planet, and the CD is too thick. To get that proportion right, you'd need a single sheet of paper about 3 km (2 miles) in diameter and a much larger Styrofoam ball.

For a gallery of stunning images of Saturn and its rings, go to http://saturn.jpl.nasa.gov/photos/?category=5

In describing the particles in Saturn's rings as forming clumps and then dispersing, do they disperse at the same size as they were before they clumped, or is it more like an explosion, to be dramatic, into different sized pieces?

The authors of the papers on clumping and dispersal in Saturn's rings (see http://saturn.jpl.nasa.gov/news/cassiniscienceleague/science20120112/) do not specifically address fragment sizes during dispersal. It is probably safe, though, to use terrestrial analogs to answer your questions.

First, the dispersal process is not explosive. A dispersal would be explosive if there was a hyper-velocity collision. Such an event is more likely due to an external meteoroid/asteroid collision with the clump in the rings rather than collisions of particles or clumps internal in the rings, which are moving at similar speeds around Saturn.

When it occurs, dispersal is most likely a gentle separation of the constituent fragments that had gently combined earlier. The separation is gentle because it is induced by the force of gravity, with some external source of gravity (another clump, a moon of Saturn, or even a planetary or solar tide) overcoming the strength of the gravity holding the clump together.

Earth's ocean tides are an example of a fragment (water) gravitationally bound to another fragment (Earth) which is being pulled away by the gravity of another body (the Moon). (That said, please don't push this analogy beyond this oversimplification as described here. Tides are more complex.)

Think about what happens when you wash a clod of dirt in water. The components in the clod, mineral fragments, roots, rocks, etc. separate out without changing their sizes or shapes. A snowball or crust of snow shatters into pieces the size of the snow that was originally collected. Unless chemical reactions occur (doubtful) or physical changes occur due to significant compression (possible in the cores of larger clumps), the unconsolidated components of a clump will generally disperse back into pieces of similar size that formed the clump.

If Saturn's atmosphere is composed mainly of flammable gases, why don't the lightning storms ignite the planet?

Although composed mainly of hydrogen, helium and methane, Saturn's atmosphere lacks enough oxygen to sustain a fire. Only in the presence of a rich supply of oxygen, such as that found on Earth, do gases like hydrogen and methane ignite when hit by lightning.