Characteristics and Origins of the Solar System

Lecture 23

November 6, 2000

Saturn and its interesting properties

 

 

            This lecture will mainly deal with properties of the rings of Saturn and its moon Titan.

 

 

Planetary Rings

 

            Planetary rings are discussed in Section 11.4, p250 of your textbook.  The rings of Saturn are the most prominent of the four outer planets.  There is a picture in Figure 10.10 of your textbook. 

 

            Should state at outset that rings are not solid material.  They are composed of “billions of small particles or moonlets”.  Fact that they could not be solid was demonstrated by the famous James Clerk Maxwell in 1859.  An artist’s conception of what these things look like is shown in Figure 11.19. 

 

            Question for the august assembly: look at the picture of Saturn’s ring and think about what this has to do with the material presented in the lecture on gravity. 

 

            There are a number of interesting features about Saturn that have been  known for a long time. 

  1. There is a gap between the A ringand B ring, called “Cassini’s division”.
  2. The rings are unbelievably thin.  Although they are of the order of 70,000 kilometers wide, estimates of their thickness are between 20 and 200 meters.  Read the “scale model” description in your textbook. 
  3. Because of their thinness, the rings “disappear” to Earthling observers when they are edge-on.  (This is not the case right now). 

 

Figure 11.17 shows a map of Saturn’s rings and shows the similar ring systems for other planets. 

 

            The Voyager spacecraft showed that the rings of Saturn are divided up into countless ringlets.  >>>>> Picture of structure of Saturn’s ring.

 

            This is a continuation of “Cassini’s division” to smaller and smaller scales.  The ring of Saturn is broken up into rings and blank regions.  This is a beautiful illustration of the concept of resonance.  Let’s look at the major moons of Saturn (bigger than about 400 kilometers in diameter). 

Moon

Distance in Kilometers

Orbital Period in Days

Mimas

186000

0.94

Enceladus

238000

1.37

Tethys

295000

1.89

Dione

377000

2.74

Rhea

527000

4.52

Titan

1,222,000

15.95

 

 

 

 

 

            Cassini’s division is due to the 2:1 resonance with Mimas.  Looking at this table shows that some kind of resonance with one of these moons will be very common in Saturn’s ring. 

 

            There is an interesting feature of the origin of the rings.  The most likely explanation is that Saturn had a previous moon that was “herded” in too close and was tidally disrupted.  It has been only in the past ten years that it has been realized that this must have happened recently, astronomically speaking.  Calculations show that rings will dissipate in a few hundred million years, so the rings of Saturn were produced probably within the last 500 million years. 

 

 

Titan

 

            As you can see in the “family portrait” picture of Saturn, Saturn has lots of moons.  However, one of these, Titan, is far more interesting than the others, and is in fact one of the most interesting objects in the solar system. 

 

            The interesting aspect of Titan is that it has a dense atmosphere.  You cannot see the surface at visual wavelengths.  Look at the picture in Figure 11.11.  We are seeing the top of a methane atmosphere. 

 

            A particularly intriguing aspect of the atmosphere of Titan relates to the role of methane in its atmosphere. 

 

            The average temperature and pressure on Earth (273K and 1 atmosphere) are close to the triple point of water; physical condition where water can exist simultaneously in the solid, liquid, and vapor phases.  This fact is responsible for the nature of the Earth as it is, and is indispensable for the Earth to be habitable. 

 

            The surface temperature of Titan is 90K (compared with 77K for liquid nitrogen). 

At that temperature, and at a pressure of 1.6 atmospheres, conditions are near to the triple point of methane.  It is probable that there is methane meteorology on the surface of Titan, with methane rain, snow, lakes, and oceans. 

 

            Calculations indicate that methane raindrops on Titan would be large, about 1 centimeter in diameter.  However, because of the weak gravity on Titan, they would fall at about the speed of snowflakes on Earth!

 

            In addition to methane and ethane (C2H6) , the upper atmosphere of Titan has a rich photochemistry.  The following compounds have been identified: hydrogen cyanide, cyanogens, cyanoacetylene, and others.  Given this mixture, the book by J. Fix notes:

“This is very much the way we believe organic compounds fromed of Earth when it still had its original atmosphere”

 

Final point: Telescopic observations have been made in the last few years at infrared wavelengths where the methane has a spectral “window”.  These observations show brightness variations which show some sort of terrain.  The current speculation (recognized as such by the scientists doing the research) is that the bright areas are highlands where methane rainfall hs washed away the organic gunk that covers Titan.  The dark areas may be methane or ethane oceans. 

 

            This issue should be considerably illuminated after October, 2004, when the Cassini spacecraft begins to make a radar map of the surface, and the Huygens probe descends into the murky, organic muck and looks at what is there.