• Bulk Properties
• Interior Structure
• Magnetic Fields

The giant planets do not have any solid surfaces and are mostly atmosphere - this means that giant planet atmospheres is an important topic! We shall compare the different atmospheres of Jupiter, Saturn, Uranus and Neptune - thinking about aspects that they share in common (e.g. rotation, clouds, banded structures, etc.;) and considering their differences (e.g. colors, composition, seasons, etc.;).

• Rotation
• Colors
• Clouds
• Weather
• Seasons

Bulk Properties

The main properties of the giant planets are that they are LARGE, MASSIVE and of LOW DENSITY. The low density tells us that these planets cannot be made up mostly of rock and iron (like the terrestrial planets) but must be mostly gas. In fact, the most common gas is the lightest gas, hydrogen (which is accompanied by a few percent of helium). We can measure the composition of the atmospheres of the giant planets using spectroscopy (see Session on Light).

If we measure density - which is, you will remember, mass/volume - in units of grams per cubic centimeter (gm/cm³), then it is easy to compare with the density of water which has a density close to 1 gm/cm³. Something that has a bulk density less than 1 gm/cm³ will float in water. So, looking at the graphics above you will see that all of these giant planets have density not far from that of water and that, should one be able to find a HUGE bathtub, Saturn floats!. The point of making this comparison with the density of water is really to provide a sense of a planet's bulk density in "everyday" terms. Of course, you should not take this "bath tub" analogy too seriously - you cannot really put a bathtub in space, let alone find one of planetary scale. No planet would either sink or float since there would not be "up" or "down"..... It is a silly idea - but I think you get the picture.

(1) (a) Familiarize yourself with just how LARGE and MASSIVE the giant planets are by filling out the table below. (See Table 11.1 of the textbook) (b) HOW do we measure the density of planets? (c) Why is hydrogen the main composition of the giant planets? This table of cosmic abundances will give you a clue.

Interior Structure

The large MASS of the giant planets means that gravitational forces holds the planet together, holding in even the lightest gas, hydrogen. Just as a deep-sea diver experiences greater water pressure the deeper s/he dives, pressure (and density) increases with depth inside the gaseous giant planets. At low pressures hydrogen as a molecule of 2 hydrogen atoms (H₂). As hydrogen gas becomes compressed at higher pressures it becomes liquid. At even higher pressures the hydrogen molecules break apart and the protons and electrons of the hydrogen atoms are separated to make a "sea" of these electrically charged particles. With the protons and electrons free to carry charge, hydrogen becomes highly conducting, just like metals.

Look at Figure 11.2 of the text.

Inside both Jupiter and Saturn the pressures are great enough to make hydrogen metallic - though the volume of metallic hydrogen is smaller in Saturn (which is consistent with Saturn being less massive than Jupiter). At the very core of each of the giant planets it is believed that there is a small core of rock and iron - about 10 times the mass of the Earth. That's a pretty respectable terrestrial planet!

Figure 11.2 in the text shows how the temperature as well as pressure increases inside Jupiter. Why is Jupiter so hot inside? Well, recall from our studies of the terrestrial planets that (i) planets heat up when they are formed by material colliding together and that (ii) the larger the planet, the slower it cools down. Thus, the giant planets retain much of their primordial heat (they are like really giant potatoes!).

Jupiter and Saturn have a composition very similar to the Sun, but their masses are far too low to provide sufficient pressure needed for nuclear fusion. Calculations show that nuclear fusion is possible only with a mass at least 80 times that of Jupiter. Some people call Jupiter a "failed star". Some prefer to think of it as a very successful planet.

The pressures inside Uranus and Neptune are not sufficient to make hydrogen metallic. Instead, the temperatures are cooler so that hydrogen can bind with other elements to make compounds.

(4) (a) Look at the table of cosmic abundances and find the elements that are most abundant after hydrogen and helium - since we are looking for elements that will make compounds, ignore the "inert" elements. (b) Next, write down the chemical formulae for the hydrogen compounds: water, ammonia and methane - that are believed to be important in the interiors of Uranus and Neptune. (Make a guess!)

Magnetic Fields

In Session 12 we first thought about planetary magnetic fields in the context of Mercury (which does have a magnetic field) and the Moon (which does not). The process whereby a magnetic field is generated inside a planet (or stars, for that matter) is not well understood. It is known, however, that there are 3 necessary ingredients:

• A region containing an electrically conducting fluid (such as liquid iron)
• Rotation
• A source of energy, to stir up the conducting fluid

The result is a global magnetic field (as if there was a large bar magnet in the center) which extends well beyond the planet and can be measured by a passing spacecraft. It turns out that all of the planets (and their moons) probably have sufficient rotation so that the existance (or absence) of a magnetic field really tells us whether or not there is a substantial region of liquid conducting material. In the cases of Jupiter and Saturn the magnetic field is generated in the region of metallic hydrogen. The highly conducting metallic hydrogen occupies a large volume inside Jupiter and, presumably, explains Jupiter's strong magnetic field. Saturn's field is weaker, but still much stronger than the Earth's magnetic field. In Uranus and Neptune the liquid "oceans" of wather, ammonia and methane are probably as electrically conducting as our ocean and provide the medium for the generation of their magnetic fields.

Figure 11.14 shows a comparison of the magnetic fields of Earth, Jupiter, Saturn, Uranus and Neptune. It shows the magnetic fields as bar magnets, tilted and displaced from the center of the planet. A planet's magnetosphere is the "sphere of influence" of the planet's magnetic field. Surrounding the planets' magnetospheres is the solar wind - a wind of charged particles that carry the Sun's magnetic field out into interplanetary space (extending well beyond the orbit of Pluto). The magnetosphere size is measured from the center of the planet to the boundary of the planet's magnetic field in the direction towards the Sun. "Downstream" in the solar wind, in the direction away from the Sun, the solar wind pulls the planet's magnetosphere out into a long "magnetotail". The strong magnetic field of Jupiter produces a magnetosphere that even in its shortest dimension extends for 50 to 100 times the size of the planet, well beyond the orbits of the Galilean satellites. Jupiter's magnetosphere occupies a volume over 1000 times the volume of the Sun.

(5) The tail of the magnetosphere of Jupiter extends well beyond the orbit of Saturn. How many AU long is Jupiter's magnetosphere?

Summary

How do we know what is inside the giant planets?

• Average density gives a clue. The density of GPs is around 1 gm/cm³ which tells us that the GPs are not made of rock/iron
• Spectroscopic studies of the atmosphere tells us that the dominant gas is HYDROGEN with Helium, water, ammonia and methane.
• Water (H₂O), Ammonia (NH₃), and Methane (CH₄) comprise molecules made from the next most common elements after hydrogen and helium.

• Jupiter is 318 times the mass of the Earth (Saturn about 95 times mass of Earth, Uranus and Neptune ~20 mass of Earth) so that the gravity is much greater - holding in even light gas as hydrogen.
• The deeper inside, the greater the pressure.
• At greater pressures hydrogen acts more like a liquid, as the density increases

• At very high pressures inside Jupiter and Saturn hydrogen begins to act like a liquid metal (like mercury, for example). This provides an electrically conducting fluid in which a magnetic field is generated
• In Uranus and Neptune the magnetic fields are generated in an "ocean" of water, ammonia and methane.
• Jupiter and Saturn have very strong magnetic fields which are closely aligned with the planet's spin axis
• The magnetic fields of Uranus and Neptune are weaker, irregular and highly tilted with respect to the planet's rotation axis.

Atmospheres

Rotation

As we discussed above, gravity holds the giant planets together and pulls them into a spherical shape. But, these big blobs of fluid* are rotating fast enough that centrifugal forces pull out their equators--so that the radius at the equator is bigger than the polar radius. This is a noticeable effect - about 10% (for Earth it is about 0.3% effect - not measurable from a photo in a book).

*Rather than distinguish between the tenuous gases at the top of the planet's atmospheres or the liquid material at higher pressures deeper inside, we can consider the giant planets as fluid (to distinguish them from the solid, rocky terrestrial planets). Both gas and liquid flows so the giant planets are fluid except for small solid cores at their very centers.

(7) (a) What are the spin (rotation) periods of the giant planets? Jupiter? Uranus? Saturn? Neptune? (b) Use a ruler to measure the polar and equatorial diameters of Jupiter and Saturn in Figure 11.1. What are the ratios of equatorial to polar radii for (i) Jupiter and (ii) Saturn?

Colors

In Session 13 on Light we discussed why objects have different colors - what happens to the reflected sunlight that comes to an observer from a planet? (The "observer" could be you looking up at red Mars at night, an astronomer looking through a telescope or a camera on a NASA probe flying past a planet).

(8) Check back to Session 13 on Light. What part of sunlight is being absorbed and what part is reflected to make Mars look red? Similarly, what makes Venus look white?

Why do Jupiter and Saturn Look Red?

Since we are not looking at a surface on Jupiter and Saturn, then it must be the atmosphere that looks red. Really, it is not "red" but a range of colors from white, yellow, orange, red, to brown. There must be some chemicals in the atmospheres of Jupiter and Saturn that give them these colors. If you live in a large urban area (e.g., Los Angeles, Denver, etc.;) you may already be familiar with colored atmosphere - the infamous "brown clouds" of urban pollution. The chemistry in Jupiter's atmosphere is a little different from Earth's, but the process is the same - photochemical reactions. As the name suggests, this involves sunlight causing chemical reactions in the atmosphere.

The chemicals that are changed by sunlight are simple compounds containing carbon and hydrogen atoms - hydrocarbons. The sunlight (especially the ultraviolet part) turns simple hydrocarbons (such as methane) into increasingly complicated, larger molecules. Additional energy from lightening and the interior of the planet effectively "cooks" the hydrocarbons into a brown "gunk". These hydrocarbons are just very minor constituents of the atmosphere but they color the atmospheres of Jupiter and Saturn yellowy-brown. The white clouds are fresh clouds that have not been "dirtied" by the photochemical reactions. More about these later.

(9) Look at pictures of Jupiter and Saturn and compare the range of colors of the two planets. Watch out! Sometimes the pictures are in "false color" where the colors have been distorted to show some feature. You will find images in Chapter 11 of the textbook and at The Nine Planets and Views of the Solar System. Describe the range of colors that you see in the images of each planet.

Why do Uranus and Neptune Look Blue?

In the cases of Jupiter and Saturn, the color was produced by a colored substance (brown hydrocarbon "gunk") in the atmosphere. The reason for Uranus and, particularly, Neptune looking blue is a little more subtle. For this you need to remember some spectroscopy from Session 13. Here is a spectrum of Jupiter to remind you.

Now, let's look at spectra of Uranus and Neptune, compared with Saturn:

The wavelength range shown is in the RED part of the visible spectrum. The plot shows that there are strong absorption bands due to methane in ALL three of the atmospheres. But the absorption is much stronger in Uranus' and Neptune's atmosphere. So, with most of the red sunlight absorbed by methane, only the BLUE part of the spectrum gets back out of the atmosphere to an outside observer.

Clouds

On Earth you are probably familiar with the idea that as you climb up a mountain the air gets colder. This decrease in temperature with altitude is a general trend for all planets. Much higher up in the atmosphere (up above the "weather layer" - about the altitudes that commercial airplanes fly in the case of the Earth) the temperature increases with height. Figure 11.9 shows the temperature profiles for the atmospheres of the giant planets. Figure 10.9 shows the temperature profile for the terrestrial planets.

Considering just the lower layers of the atmosphere, where the temperature decreases with altitude, let us think about how clouds form. You know that the clouds on Earth are formed when moist air (that is, air that has lots of water vapor mixed in) rises up and cools down. When the parcel of air that is rising cools down to below the condensation temperature of water, the water vapor condenses into liquid droplets that together make up a cloud. If the temperature is colder (such as at very high altitudes or over the artic or antarctic) sometimes ice crystals form as clouds. The same process happens on the other planets. Because the giant planet atmospheres have a different composition to the Earth's atmosphere, the clouds have different chemical compositions. Note that the main constituents of the giant planet atmospheres - hydrogen and helium - do not condense except at VERY low temperatures. This property of gases to stay a gas and only condense at low temperatures has a name - a chemical is volatile if it has a low temperature for condensing (or freezing). The atmospheres of Jupiter and Saturn contain at least 3 different chemicals that form clouds. Because the 3 different chemicals condense at different temperatures, the cloud layers are at different altitudes. Specifically, ammonia condenses at very low temperatures so that ammonia ice clouds form at the top of Jupiter's and Saturn's atmospheres (see Figure 11.9). At intermediate temperatures ammonia hydrosulfide condenses to form a middle cloud deck. Deeper in the atmosphere, where the temperatures are around 0 degrees Celsius (273 Kelvin), water condenses to form water clouds (the high pressures, several Earth-atmospheres, at these depths in Jupiter's atmosphere changes the freezing point of water from our experience here on the surface of the Earth).

It is the high altitude ammonia clouds that look the whitest and the deeper ammonia hydrosufide clouds that have suffered the most photochemical darkening. The alternating white and reddish-orange stripes on Jupiter (and less-strikingly Saturn) are therefore clouds at different heights (Figure 11.7).

Uranus and Neptune also have clouds - since methane is the main constituent after hydrogen and helium, it is not surprising that the clouds - usually white - are made of condensed methane. The Voyager pictures (shown on this page) show Uranus with a uniform cloud coverage (and a thick haze layer above) while Neptune has a few white, high altitude clouds with some dark clouds deep in the red-light-absorbing atmosphere (which makes them look blue). The Hubble Space Telescope images of Uranus and Neptune, while not of such high resolution as the Voyager pictures, show that the clouds on Neptune seem to have disappeared since the Voyager 2 flyby in 1989. Uranus, on the other hand seems to have acquired high altitude white clouds since the Voyager 2 1986 flyby.

In December 1995, as part of NASA's Galileo Project, a probe carrying scientific instruments was dropped into the atmosphere of Jupiter. The instruments measured properties of the atmosphere as the probe descended.

(12) Go to the website on the Galileo Probe and find descriptions of the measurements that were made. Describe 3 properties of the atmosphere and how they were measured.

Weather

Seasons

Here is a press release that shows how Uranus' atmosphere is showing seasonal changes in Hubble Space Telescope pictures that have been taken over the past several years (Check out the animations - the image files could take some time to load - but they are worth the wait).

Summary

• Jupiter, Saturn, Uranus and Neptune are low-density "blobs" of gas/liquid (fluid) that are pulled into a spherical shape by gravity.
• The 10-15 hour rotation periods mean that rapid rotation pulls their equatorial regions out, making the planets oblate.
• Rotation is also a major factor in shaping the weather on the giant planets.

• The white clouds on Jupiter and Saturn are fresh Ammonia ice clouds
• The orange/red/brown clouds are produced by the same process as the photochemical smogs of big cities (e.g. Denver).
• Sunlight (particularly UV) hitting simple hydrocarbons (e.g. methane) produce more complex hydrocarbons. These react with molecules that contain sulfur and nitrogen to make dirty yellow/brown compounds.
• In the atmospheres of Uranus and Neptune there are higher concentrations of methane which absorbs the RED part of sunlight, so that only the BLUE part of the sunlight is reflected - making these planets look green/blue. Uranus and Neptune have high altitude, white clouds which are condensed methane.

• The striped pattern of alternating light and dark bands correspond to jets blowing in opposite directions, producing a strong WIND SHEAR
• The wind shear produces EDDIES - circulating storms, much like the cyclones and anti-cyclones on Earth.
• The Great Red Spot on Jupiter is such an eddy. It has been observed since the first telescopes were used to look at Jupiter in the 1600s.
• Saturn, Neptune and, to a lesser extend, Uranus, all show similar weather patterns:- strong east and west jets and eddies generated in the wind shears.

Seasons

• The tilt of the planet's spin axis determines the strength of the seasons experienced by the planet.
• Jupiter's equator is very close to the ecliptic (small spin axis tilt) so that it experiences very weak seasons
• Saturn and Neptune have spin axis tilts very similar to Earth - and hence have similar seasonal effects.
• Uranus is tipped on its side to that it has very strong seasons.