Exploration of the Solar System

Topic 13, Week 9

Saturn and its interesting properties

 

 

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

 

 

Rings

 

            In the last lecture,  I talked at length about the physical phenomenon of resonance.  Now let’s discuss the relevance for the ring of Saturn. 

 

Let’s look at detailed pictures of Saturn’s ring, taken by the Voyager spacecraft about 20 years ago. 

 

 

Notice, in addition to Cassini’s division,  a number of almost-artifical-looking “fibers” of ring material separated by empty space. 

 

 

This picture shows that Cassini’s division is only the most prominent of a huge number of gaps in the rings.  This was also illustrated in the picture from the Cassini spacecraft which was in the last lecture. 

 

            Let’s summarize some of the features about Saturn’s ring,  some of which have been  known for a long time,  and others of which were discovered in the “space age”.

  1. There is a gap between the A ring and B ring, called “Cassini’s division”.  This is visible in relatively small telescopes, of the sort that we use in our astronomy courses here at the University of Iowa (unfortunately, you have to be on-campus to use them!). 
  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). 
  4. The Voyager and Cassini spacecraft showed that the rings of Saturn are divided up into countless ringlets. 

 

 

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.  This is responsible for the fine structure of the ring into ring material and the absence of ring material. 

 

            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

 

            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.  It is so interesting that a major part of the Cassini mission,  the Huygens probe,  is designed specifically to land on Titan and radio back its findings.  

 

One interesting aspect of Titan is that it is the second largest satellite in the solar system (after Ganymede).  In fact,  it is larger in diameter than Mercury,  and not much smaller than Mars.  It is an easy object to see in an amateur astronomy telescope. 

 

            An even more  interesting aspect of Titan is that it has a dense atmosphere.  You cannot see the surface at visual wavelengths.  Look at the picture below, taken from the Voyager spacecraft.  We are seeing the top of an atmosphere.  The fact that the atmosphere is not transparent is due to flakes or droplets of material suspended in the atmosphere.

 

 

 

 

            Observations show the presence of the gases methane (CH4)  and ethane (C2H6)  in the atmosphere. 

 

Think about the following:  How could we know that?  Has someone taken their chemistry set out there?  What measurement or observation could tell us that the atmosphere of Titan contains methane and ethane?

 

Methane and ethane are apparently trace gases in the atmosphere of Titan.  The predominant gas is nitrogen (N2). 

 

From a combination of observation and theoretical modeling,  we can estimate the structure of the atmosphere of Titan.  This is shown in Figure 14-30 of the textbook. 

 

Note that the temperature at the surface of Titan is 90 – 94K,  not much above the temperature of liquid.  The temperature at places higher in the atmosphere is even colder. 

 

            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. 

 

            At the surface temperature of Titan  (90K),  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 notes:

“This is very much the way we believe organic compounds formed on 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  haze doesn’t make the atmosphere translucent,  and we can see to to the surface.  These observations show brightness variations which show some sort of terrain.  The picture below shows four infrared pictures of Titan at different orientations (so we can see the whole object).  There are clearly different regions of contrasting brightness. 

 

 

 

 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. 

 

 

 

Even though the Cassini spacecraft has only been in the Saturnian system for a few months,  it is already returning images of unprecedented quality,  such as the one below.  One again,  this picture is taken with near-infrared light which penetrates through the haze layer on Titan,  and reveals the landscape of this intriguing world. 

 

 

The above picture  was taken at a considerable distance from Titan,  when the Cassini spacecraft was entering the Saturnian system.  On October 26 will be the first flyby of Titan by the Cassini spacecraft, which will yield vastly more detailed pictures than that shown above. 

 

            The ultimate in studies of Titan will occur next January, 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. 

 

The illustrations below show an artist’s conception of the Huygens spacecraft parachuting to the surface,  and the lander on the surface of Titan.  The measurements made by this spacecraft will allow us to test a lot of the conjectures described in this lecture. 

 

 

On the surface…..

 

 

Final Remarks…..

 

The intriguing feature of Titan is that despite many alien aspects (distance from the Sun,  temperature,  being a huge moon rather than a planet)  its surface may be,  in many ways,  the most Earthlike scene in the solar system.  If we stood on the surface of Titan,  we would look about and see (in the dim light)  terrain with relief,  perhaps mountains.  The dense atmosphere would scatter the light and give an appearance suggestive of the Earth.  Finally, it is possible (Cassini/Huygens will tell us for sure)  that we would see liquid lakes and oceans,  rain,  and snow.  The rain and snow, however, instead of being water would be liquid hydrocarbons.  Finally,  as described in the book,  this scene might resemble that of the early Earth at the dawn of life.