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.
- 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.
- 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.
- 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?
- 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?
- 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.
- 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!
- 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.