Exploration of the Solar System

Week 8,  Topic 11 and Topic 12

The Interior Structure of Jupiter and Saturn; the Galilean satellites of Jupiter



            Last time we saw that the atmospheres of Jupiter and Saturn are primarily hydrogen and helium.  Furthermore, physical reasoning says that the whole body of these planets must be mainly hydrogen and helium.  But the question remains: what kind of hydrogen and helium?


            Remember that hydrogen and helium as you encounter them in high school chemistry class is as light, transparent gases.


Look at the demonstration video on your CD, or the on-line video clips,  about the properties of hydrogen.


            The Galileo probe descended into the atmosphere of Jupiter, and showed the pressure and temperature continuing to increase.  It lasted for 57 minutes,  and descended 200 kilometers,  while being blown 500 km sideways by the strong winds on Jupiter.  Eventually it failed due to  high temperature and pressure in the interior of Jupiter.  We don’t have direct measurements beyond the point where Galileo ceased to function, but we can use the laws of physics to tell us what conditions must be like.


Using a combination of our observations of Jupiter (and Saturn)  such as mass, radius,  and chemical composition,  and our knowledge of the laws of physics,  we have constructed the following models of Jupiter and Saturn,  which incorporate this information.  It is shown in Figure 12.21 




Like all metals, liquid metallic hydrogen can carry an electrical current.  Evidence that there are electrical currents flowing in the deep interior of Jupiter is provided by the strong magnetic field of Jupiter which establishes a vast magnetosphere of this planet. Professor Van Allen of the University of Iowa was one of the first explorers of the magnetosphere of Jupiter, as well as the discoverer of the magnetosphere of the Earth. 


An illustration of the magnetosphere of Jupiter, which has been repeatedly investigated by spacecraft,  most notably Galileo,  is shown in Figure 12.22




The Galilean Satellites of Jupiter


            One of the most interesting aspects of Jupiter is its system of satellites (moons).  It has 16 in all,  but the four Galilean satellites are the most interesting.  These are Io, Europa, Ganymede, and Callisto.  All are about the size of the Earth’s moon. 


  Look at a group picture of them at   http://www.solarviews.com/eng/homepage.htm

 You can see immediately that each one is distinctive in appearance.  Almost everything we know  about them comes from exploration in the “space age

A simulation of the Galileo spacecraft swinging by Europa is given in  http://www.jpl.nasa.gov/galileo/europa/e19anim.html



  Let’s look at their distances and orbital periods (around Jupiter) first.


Distance (km)

Period (days)

Density (g/cc)




















            There are a couple of things worth emphasizing at the start about the Galilean satellites. (1)  There is a great deal of excitement about them nowadays.  In fact, Europa is considered one of the main targets (along with Mars) of the search for life elsewhere in the solar system. (2) To state the conclusion at the start, the geology of the Galilean satellites is determined by tides from Jupiter.  The strong gravitational force of Jupiter causes tides which flex the satellites, causing internal heating.  The inner satellites are much more active objects than they would be if they were just out in space. 


            The Galileo orbiter has provided huge amounts of information on these objects.  What we know now is vastly more than was available five years ago.


Callisto is the furthest out .  It has a density of 1.86 grams/cubic cm.  This means it can’t be rock, and must be largely composed of ice.  At the temperature of Callisto (130K) ice is a mineral.  The surface of Callisto is heavily cratered, and you can see shatter-marks around some of the big craters (see Figure 11.3).  From the nearly continuous crater cover, we conclude that not much has happened on Callisto since the “good old days”. 


A surprise in the last year is that even though Callisto appears geologically dead,  it may have a buried ocean.  A magnetometer (instrument that measures magnetic fields) showed an anomaly when it passed near Callisto.  One explanation for this is if there were a shell of salt water (a good conductor of electricity) beneath the surface. 


Ganymede is the biggest Moon in the solar system.  The density of 1.94 grams per cubic centimeter means it has somewhat more rock, but must also be mainly ice.   Unlike Callisto, Ganymede shows geological diversity.  Although there are heavily cratered parts, there are also parts where craters are few and far between.  There are stress marks on the surface that some people interpret as evidence of tectonic forces on Ganymede.  There also seem to be features which might be the result of water gushing from the interior of the world.  This increased internal activity is due to the fact that Ganymede is closer to Jupiter, and the tides are stronger.


Europa has become one of the most interesting objects in the solar system.  It has been known since the Voyager flyby missions in the 1970’s that the moon is cased in a covering of water ice. This is responsible for its cue-ball like appearance in figure 11.1.

Its density is 2.87, which is nearly the density of rock.  Apparently Europa is like a terrestrial planet with an icy case. 


            The Galileo orbiter has made many close flybys of Europa, and in some of these passes it almost could have seen aliens on the surface (those pictures have been suppressed in a government coverup, however).  The evidence has tended to corroborate speculations that there might be a liquid ocean under the ice.

  1. There are long cracks in the ice and evidence that water has flowed up through them and on to the surface. (See figure 11.7)
  2. There are features which look like icebergs in the frozen arctic ocean on Earth.  This suggests that these blocks are moving around on top of a liquid ocean..
  3. Some of the ( rare) craters look like the projectile punched through to a liquid layer.
  4. Crater counts indicate that the surface that we now see is less than 10 million years old.  Melting and refreezing of the ice is one of the easiest ways to accomplish this.


Educated guesses are that the thickness of the ice is from a few kilometers to a few tens of kilometers in thickness.


            What makes Europa particularly exciting is recent discoveries on Earth as well as in space.  It has been found that life abounds around “black smoker” vents at the bottom of the ocean.  It is unclear if some of these species might have originally evolved down there.  If this could have happened on Earth, geothermal vents on a tidally-heated Europa might have served as home to horrible, monstrous alien forms of life. NASA has plans for a Europa orbiter to try and learn more about this planet.  In the future, we can practically count on robots landing and drilling through the ice.


Io  is the closest and the most vigorously tidally heated.  Because of this anomalous interior heating, the moon has vigorous volcanic activity.  The rate of these processes is so furious that the landscape changes on timescales of only a few years (see Figure 11.10).  Not surprisingly, there are no craters on visible on Io, although geological robots could doubtlessly find evidence of  ancient craters formed billions of years ago in the early days of the solar system.