Characteristics and Origins of the Solar System

Lecture 17

March 12, 2004

On to the Outer Planets!!!!!!

 

Beginning announcements:  (1)  Watch the skies while on Spring Break.  (2) Special lecture:  the director of the SETI (Search for Extraterrestrial Intelligence) Institute,  Dr. Jill Tarter,  will present a lecture on Tuesday,  March 23,  at 7:30 PM in LR 1 of VAN.  The description is http://www.physics.uiowa.edu/lecture_series/

 

            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.  The skies are absolutely perfect for discussing the material we are now dealing with in class.  If you go out at 7-8 PM,  you can find Saturn nearly directly overhead,  and Jupiter will be the brightest object in the eastern sky.  For Saturn,  it will help to use your SC1 chart,  since it is in the constellation of Gemini,  among bright stars. 

 

            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.    .  The corresponding orbital periods are 11.9 years and 29.5 years. Let’s see where they are in a present map of the solar system.

 

The present distances of Jupiter and Saturn are 4.45 astronomical units and 8.7 astronomical units, respectively.  

 

 

 

  1. 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 an 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.  As our discussion continues, we will discuss some of the physical properties of extreme cold. 
  2. 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).  Let’s take a look at them.

 

 

 

 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?

  1. To give you a real sense of these sizes, and how the Jovian planets stack up relative to the Earth, look at p268, which  has the big fellas to scale.  Notice the great red spot on Jupiter
  2. We don’t see the surfaces of the Jovian planets.  As in the case of Venus, we see the tops of cloud layers.   A picture of Jupiter is shown below.

 

 

 Unlike the case of Venus, there isn’t any geology beneath the clouds because there isn’t a surface!  We have measured the characteristics of the atmosphere of the Jovian atmosphere via the Galileo probe, which in 1995 parachuted into the atmosphere and descended until it was crushed   A picture of the “skyscape”  that Galileo saw is shown in the next figure;  be sure to look  at the picture at the beginning of Chapter 12. 

 

 

 

The atmosphere of Jupiter is turbulent,  restless place.  Convection  is occurring there,  with updrafts from the warmer interior,  and downdraft as gas that has cooled off by exposure to the cold of outer space sinks back down.  This convection is responsible for the   banded appearance of the atmosphere of Jupiter,  that is obvious even in a small telescope,  and which is shown in Figure 12.17 of the book. 

 

There is an even more intriguing feature of Jupiter.  There is a narrow range of altitude (or location)  in the atmosphere of Jupiter where you would be fairly comfortable if you had an oxygen mask.  Let’s look at Figure 12.18  of the book. 

 

 

 

Notice that at an altitude of 0 kilometers (relative to some reference level),  the temperature is about what it is in this room,  and the atmospheric pressure is a few atmospheres.  Note this picture (see also Figure 12.19)  also shows that the different clouds on Jupiter are formed of different kinds of ice. 

  1. The Jovian planets have neat  moons.  >>>>>>>> famous transparency. Much has been learned about the moons of Jupiter since 1995 when the Galileo spacecraft arrived.  

In just a few months,  we will begin to learn a great deal about Saturn and its moons.  On July 1,  the Cassini spacecraft completes its 7 year voyage to Saturn when it fires its rockets and settles into a permanent orbit around Saturn. 

http://www.jpl.nasa.gov/galileo/images/galprobe.gif

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.