Planetary Rings All four jovian planets have ring systems girdling their equators. Many of the inner jovian moons orbit close to (or even within) the parent planet’s rings. Saturn’s rings are the best known and best observed, and we will describe them in detail.

Saturn’s Spectacular Ring System Features: Cassini Division - dark gap about 2/3 way out from inner edge. A ring - outside the Cassini Division. Inner rings - B and C rings. B ring is the brightest. C ring is almost translucent. Encke gap - smaller division located in the outer part of the A ring. Additional rings (D, E, and F) cannot be seen in image.

Saturn’s Spectacular Ring System Rings lie in Saturn’s equatorial plane. Since Saturn’s rotation axis is tilted, the appearance of the rings changes seasonally (depending on how the sunlight illuminates them). The rings are very thin - thickness is less than a few hundred meters even though they are more than 200,000 km in diameter.

Saturn’s Spectacular Ring System Rings are not solid objects. Composed of many small solid particles orbiting the planet, like many tiny moons. High reflectivity of rings suggests the particles are made of ice, with small rocky particles and dust mixed in. Particles range in size from fractions of a millimeter to tens of meters in diameter. Most particles are the size (and composition) of a large and dirty snowball.

The Roche Limit Consider the fate of a small moon orbiting close to a massive planet. As the moon gets closer, tidal forces increase, stretching the moon along the direction to the planet. Eventually, the moon is pulled apart by the planet’s gravity. The pieces of the satellite move on their own individual orbits around that planet, and eventually spread all the way around forming a ring. The Roche limit is the critical distance from the parent planet, inside of which a moon will be destroyed. For Saturn, the Roche limit lies 150,000 km from the planet’s center, just outside the outer edge of the A ring. All ring systems are found within the parent planet’s Roche limit. These arguments apply only to objects held together by self-gravity (e.g. this doesn’t apply to artificial satellites that are held together by interatomic forces).

Fine Structure Saturn’s main rings are composed of thousands of narrow ringlets. This structure varies with both time and position in the rings. Ringlets are formed when the mutual gravitational attraction of the ring particles creates waves of matter, regions of high and low density, that move in the plane of the rings like ripples on the surface of a pond. Narrower gaps in the rings (about 20) are swept clean by action of small moonlets embedded in them. The moonlets (10 or 20 km across) are larger than the particles and sweep up ring material as they go. Largest gap, the Cassini Division, has a different origin - the gravitational influence of the inner-most medium-sized moon Mimas. Its gravity has kicked out particles that once lied in the Division.

The Rings of Jupiter Jupiter’s ring system lies 50,000 km above the top cloud layer, inside the orbit of the innermost moon. Composed of dark fragments of rock and dust chipped off the inner moons by meteorites. A few tens of km thick.

The Rings of Uranus Uranus’s ring system consists of 11 thin rings and is too faint to detect directly. Dark, narrow, and widely spaced. Less than a few tens of meters thick. Shepherd satellites exist that keep their positions in place.

The Rings of Neptune Neptune is surrounded by 4 dark rings. 3 are quite narrow like Uranus’s, while 1 is quite broad and diffuse like Jupiter’s. Outermost ring is noticeably clumped in places. No connection yet established between the small inner satellites and the rings, although clumping may be caused by shepherd satellites.

The Formation of Planetary Rings Believe the rings are quite young (no more than 50 million years old) due to their dynamic behavior (waves, collisions, and interactions with moons). If they are young, perhaps they are replenished from time to time (fragments broken off of inner moons by impacts, or a moon recently torn apart by tidal forces or destroyed in an impact with another object). Total mass of Saturn’s rings is enough to make a satellite about 250 km in diameter. Neptune’s large moon Triton will likely be destroyed once it crosses the Roche limit and will form into a ring system (within the next 100 million years). By then Saturn’s ring system could have disappeared and Neptune would be known for the great ring system.

The Discovery of Pluto Observations of Uranus’s and Neptune’s orbits led to the conclusion that there must be another object out there (similar to what led to the discovery of Neptune). Percival Lowell attempted to find the 9th planet, but it was Clyde Tombaugh who found it while working at Lowell Observatory, 6 degrees away from where Lowell predicted it would be. Well, the supposed irregularities in Uranus’s and Neptune’s orbits don’t actually exist, and Pluto’s not massive enough to have caused them anyway.

The Discovery of Pluto At nearly 40 AU from the sun, Pluto is hard to distinguish from the background stars. Never visible to the naked eye (like Neptune). Only “planet” not studied at close range by NASA spacecraft (new mission should be there by 2015 though). Above image taken with the Hubble Space Telescope. Only surface features to be conclusively identified are the bright polar caps. Some of the dark regions may be craters or impact basins.

The Pluto-Charon System Pluto’s satellite, Charon, was discovered in 1978. Based on Charon’s orbit, Pluto’s mass has been determined to be about 0.0021 Earth masses, or 0.17 times the mass of Earth’s moon. Charon’s mass is about 0.12 that of Pluto, giving the Pluto-Charon system the largest satellite-to-planet mass ratio in the solar system. Pluto’s radius is 1150 km (1/5 Earth’s) and Charon’s is 600 km. Pluto and Charon are tidally locked as they orbit each other. Pluto is the 3rd planet in the solar system to have retrograde rotation. 2 candidate moons were found in 2005 using HST, each about 100 to 200 km in diameter.

Pluto’s Origin Mass, radius, and density are what we’d expect of an icy moon of a jovian planet. Similarities to Triton (Neptune’s large moon) stem from the fact that both probably formed in the Kuiper belt. Many Pluto-sized objects existed in the early system, Pluto is simply the largest surviving member of the class. Charon was captured by Pluto late in the formation process, following either a collision or near-miss between the two (reminiscent of the formation of Earth’s moon).

Is Pluto a Planet? To be discussed in lab, so let’s just recall that the IAU has deemed that Pluto no longer merits planet status.