Exploration of the Solar System

Week 8,  Topic 11

On to the Outer Planets!!!!!!

 

 

            Now 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 main Jovian planets, Jupiter and Saturn,  are not very convenient for observing.  We’ll see why in a moment.   If you look due east at about 1:30 AM,  you will see a reasonably bright object.  That is Saturn.  By early morning,  just before dawn twilight,  it will be high in the sky.  If you want to get some practice with your SC1 chart,  Saturn is between the constellations of Gemini and Cancer. 

 

The other two Jovian planets,  Uranus and Nepture,  are in ideal position for evening observation in the constellation of Capricornus.  Unfortunately,  they are not naked eye objects.  To see them, you need their coordinates,  a good set of star charts,  a pair of binoculars,  and clear skies. 

 

The Jovian planets are in the news now because of the Cassini spacecraft,  which is re-writing the textbooks on the planet Saturn.  We’ll hear more about it in the next lecture. 

 

            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 from the Earth are 6.4 astronomical units and 9.0 astronomical units, respectively.   The diagram below shows the present orientation of Jupiter, Saturn, and the Earth.  You can readily see why Jupiter is not obvious in the sky;  it is nearly exactly on the far side of the Sun,  so it is not up in the night sky.  This diagram also clearly illustrates how far out in the solar system the Jovian planets are relative to the Earth. 

 

 

 

  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

 

 

            At this point, ask yourself a question: 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.  Once again, ask yourself: how could we know that? What kind of physical principle could we apply to give us this information?

  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 the room where you are sitting,  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. Much has been learned about the moons of Jupiter since 1995 when the Galileo spacecraft arrived.  

 

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