Characteristics and Origins of the Solar System

Lecture 19

October 22, 2001

Jupiter and the Jovian Planets

 

            The textbook takes a good approach to the subject, which is to discuss the Jovian planets together, emphasize properties in common. 

 

From last time: the Jovian planets have neat moons.  >>>>> transparency with moons of  Jupiter.  These attributes include the Galilean satellites of Jupiter:  Io, Europa, Ganymede, and  Callisto, each one different.  Also, Titan, the largest moon of Saturn, which is the only moon in the solar system with an atmosphere. 

 

What are the Jovian planets made of?   Let’s start by reviewing the case of the Earth, which also holds for the other terrestrial planets. 

Element

Percent by Weight

Oxygen

47

Silicon

28

Aluminum

8

Iron

5

Sodium

3

Potassium

3

Magnesium

2

Calcium

4

 

 

 

 

            These numbers relate to the rocks on the Earth’s crust. It is believed that as you go deeper, the percentage of the “heavies” goes up.  Nonetheless, the Earth is primarily composed of the heavier elements. 

 

            For the Jovian planets we had no direct samples until the Galileo probe in 1995.  The only clue we had was of spectroscopy, the study of the intensity of reflected sunlight as a function of wavelength of  light.  Certain gases, like methane, absorb light of certain wavelengths and not others, so the presence of an “absorption feature” can be used to deduce the presence of that gas in the atmosphere of an object. 

 

>>>>>>> diagram of spectrum of a planet

 

            Observations carried out 70 – 80 years ago showed strong spectroscopic features of the gases ammonia and methane in the spectra of Jupiter and Saturn.  From this, astronomers concluded that most of the mass of Jupiter and Saturn must be in the form of hydrogen and helium, and two lightest elements.  

 

            There were a couple of reasons behind this.  First, if oxygen were more abundant, we would have gases characteristic of an oxidizing atmosphere, such as CO, carbon dioxide, NO, etc.  Instead we have NH3  and CH4.  This indicates that there is lots of hydrogen around to bind up all the other elements in hydrogenic molecules. 

 

>>>>>>> Question for the august assembly: can we make the same argument for the presence of helium?  Could we look for spectral lines of HeO for example? 

 

            The second reason was even cleverer, and shows the power of theoretical physics.  We know with great accuracy the mass, radius, and therefore density of Jupiter, Saturn, etc.  The densities of Jupiter and Saturn are 1.3 and 0.7 grams/cubic centimeter (remember the corresponding value for the Earth).  About 50 years ago, people realized that only an object made of light elements, H and He, held together by its own gravity, could have that mass and radius. 

 

            Quote from the book I used (publication date of 1966) “Because of the tremendous gravitational attraction of Jupiter for its constituent parts, it would most certainly be compressed to a far greater mean density unless it were composed almost entirely of hydrogen and helium”.  That book also gives spectroscopic evidence for this. 

 

These conclusions were borne out by direct samples by later spacecraft missions, most definitively the Galileo probe that entered the atmosphere of Jupiter in December, 1995. 

 

The result of all this is that Jupiter is about 75 hydrogen by mass, and 24 percent Helium by mass.  >>>>>> Question for the august assembly.  What does this mean about the relative abundance of atoms?  What fraction of atoms in Jupiter are hydrogen? 

 

            To grasp the significance of this, look ahead to Table 14.2 on p299 in the chapter on the Sun (we’ll get to this All in good time, my pretty!!!! As the protagonist of the Wizard of Oz liked to say) .  The Sun is 73.4 percent hydrogen by mass, and 25 percent Helium. 

 

            Jupiter, Saturn, and the Jovian planets have the same chemical recipe as the Sun.  It is the Earth and the terrestrial planets that have the anomalous composition.  Our interpretation is that the Jovian planets are composed of original samples of the gas cloud from which the solar system formed 4.5 billion years ago.  Some highly selective process was responsible for the generation of the terrestrial planets. 

 

            When the Galileo spacecraft entered the atmosphere of Jupiter, it was sampling “star-stuff”, relatively pristine material left over from the formation of the solar system. 

Take a look at Figure 10.3 for an artist’s conception of this event.

 

            The Clouds of Jupiter and Saturn.  As I mentioned in class last time, when we look at the Jovian planets, we are seeing the top of a cloud layer.  But what kind of clouds?  In the Earth’s atmosphere, the clouds are liquid or solid water.  But the outer layers of the atmospheres of the Jovian planets are too cold.  The clouds we see are predominantly ammonia ice clouds for Jupiter and Saturn.

 

>>>>>>>> Diagram showing profile of atmosphere of Jupiter.

  To form solid ammonia, the temperature needs to drop as low as 140 K (140 degrees Centigrade above absolute zero).  This corresponds to a temperature of 207 degrees below zero Fahrenheit. 

 

            In the case of Jupiter and Saturn, we believe there are three cloud layers at different depths (see Figure 10.11).  At the highest level, dominating the clouds we see, are ammonia ice crystals.  About 50 kilometers below (remember that we are talking about an object with a radius of 70000 kilometers) there should be a cloud layer of ammonium hydrosulfide (NH4HS), and below that at a distance of about 20 kilometers, would be water clouds.

 

            There are a number of other interesting facts to bring up about the atmosphere of Jupiter and the Jovian planets.

·      In the case of Uranus and Neptune, they are too cold even for ammonia ice, and the clouds are believed to be formed of methane ice, which forms at even lower temperatures.

·      A remaining mystery of Jupiter (and to a much lesser extent, Saturn) is the cause of its colors.  Ammonia ice is white, whereas the cloud layers are brown, dull red, etc.  It is believed that these are due to photochemistry that generates colored smog composed of complex organic molecules, but the details are fuzzy.

·      Another oddity, and in contradiction to the picture above, is the fact that the Galilio probe did not detect clouds at the level where the water clouds should have been, and in fact the atmosphere of Jupiter was drier than expected.

·      Although Jupiter is a pretty alien place, there is a thin layer in its atmosphere, probably only a few kilometers thick, where the temperature is about the same as in this room, and the atmospheric pressure is a couple of  atmospheres.  You could float in an open-gondola balloon (you would need your own oxygen supply, though)  and enjoy the spectacular view!