Characteristics and Origins of the Solar System

Lecture 19

October 20, 2000

On to the Outer Planets!!!!!!

 

            Today we will start moving out in the solar system.  To the outer planets, called Jovian planets, and we begin with Jupiter and Saturn.

 

            First, let’s get oriented in the sky.  Be sure to go out at about 10PM and see them as the brightest objects in the eastern sky.  They are fairly close together on the sky. 

 

            In what follows, I will discuss some of the primary features of these two planets. I will say at the outset that we are in for a big change compared with the terrestrial planets.  The Jovian planets are really entirely different objects than Mercury, Venus, Earth, and Mars.  About the only thing in common is the fact that they all are much smaller than the Sun and orbit the Sun.  However, in a very real sense, the Jovian planets are like intermediate objects between the terrestrial planets and the stars.

 

  1. Where are they?  The semimajor axis of Jupiter’s orbit is 5.2 astronomical units, that of Saturn 9.5.  >>>>>>> Rough blackboard sketch of orbits. >>>>>> Galileo homepage and location of the outer planets.  The corresponding orbital periods are 11.9 years and 29.5 years.
  2. It’s cold out there.  The intensity of sunlight (technically speaking, the flux of sunlight in units of Watts/m2 ) falls off via the inverse square law. Saying it via and equation, we have (SE/SJ) = (rJ / rE )2.  For Jupiter (5.2)**2 = 27.04, for Saturn (9.5)**2 = 90.25.  Extreme cold dominates the physics of the outer solar system.
  3. The outer planets are giants.  Consider the relative characteristics of Jupiter, and Saturn.

 

Planet

Diameter

Relative to Earth

Earth

12756

1

Jupiter

142800

11.2

Saturn

120,540

9.45

 

 

            Question for the august assembly: how do we know that they are this large?  What kind of measurement could you do to convince yourself these numbers are true?

 

  1. The outer planets are giants (continued).  In terms of mass (relative to the Earth), we have Jupiter as 318 times the mass of the Earth, and Saturn is 95 times that of Earth.  Question for the august assembly: how could we know that?
  2. To give you a real sense of these sizes, and how the Jovian planets stack up relative to the Earth, look at p214, which  has the big fellas to scale.  Notice the great red spot on Jupiter.  Now look at Figure 10.15 on p229, which shows the Earth relative to the great red spot.
  3. We don’t see the surfaces of the Jovian planets.  As in the case of Venus, we see the tops of cloud layers.  Unlike the case of Venus, there isn’t any geology beneath the clouds because there isn’t a surface!
  4. The Jovian planets have neat  moons.  >>>>>>>> famous transparency. Much has been learned about the moons of Jupiter in the last few years.  In 2004 we will begin to learn a whole bunch about the moons of Saturn. We have the following highlights of moons in the outer solar system: Jupiter; the Galilean satellites of Io, Europa, Ganymede, and Callisto.  Each one is unique.  Saturn: Titan,  the only moon in the solar system with an atmosphere. 

 

Next time, on to some of the deeper issues about the Jovian planets.