Jupiter

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Jupiter is the King of planets and commands your attention whenever it is visible. It is always one of the brightest objects in the sky and is captivating when viewed through a small telescope. Jupiter, as the most massive planet also is a dominant player in the solar system whose role is second only to the Sun itself.

Basic Facts

average distance from the sun 5.2028 AU orbital period is 11.867 a rotation: completes 1 rotation in only 9h 55.5m mass is 1.9 X 1027 kg or 318 X mass of Earth density is 1340 kg/m3 equatorial radius = 7.2 X 104 km albedo = 0.51 cloud-top temperature = 140 K magnetic field: Very strong, planet is source of strong radio emission Table 18.1 Basic properties of Jupiter
	Figure 18.1 jupiterGlobe

The Interior

Although we have not probed the structure of Jupiter in great detail there is considerable reason to expect that the interior regions consist of a molecular hydrogen shell encasing a liquid metallic hydrogen core. There may be a small, solid iron core at the center.

The best evidence for the liquid H core is the intense magnetic field that Jupiter displays. Metallic hydrogen would be an extremely good conductor and the very high rotation rate of the planet would conspire to generate a large magnetic field. Another reason to suspect that Jupiter's interior is quite different from the interiors of terrestrial planets can be observed even with a small telescope. Jupiter rotates in less than 10 hours. You can easily detect the rotation of the planet in a matter of hours just by watching the motion of surface features. The planet is also noticeably "flattened" at the poles which suggests that it is rotating more as a fluid than as a solid. Finally - a quick calculation of the average density of the planet tells us that is much different than a Terrestrial planet.

Figure 18.2 Compositional profile for Jupiter (Image courtesy NASA/STScI)

Example 18.1 Estimate the amount of flattening of the poles on Jupiter. This is formally called "measuring the oblateness" of the planet.

Solution: Oblateness can be defined as the difference between the long and short axes of an ellipse divided by the long axis of an ellipse.

	The roll-over image of Jupiter on the left shows how you could use any common image processing program to make a quick estimate of Jupiter's oblateness. Long axis = 350 pixels Short axis = 330 pixels   The oblateness of Jupiter is about 0.07 or it is "squished" by about 7%

Example 18.2 Estimate the average density of Jupiter and argue that this implies that the structure of Jupiter is much different than that of Earth.

Solution: Use the following fate from Table 18.1:

• M_Jupiter = 1.9 x 10²⁷ kg
• R_Jupiter = 7.15 x 10⁴ km = 7.15 x 10⁷ m

SInce density = Mass/Volume it follows that

Jupiter has a very low density! It is just marginally greater than that of water and this suggests that it cannot have an extensive, dense metallic core. This does not rule out the possibility of a smaller, rocky core however.

As Example 18.2 suggests, Jupiter is likely composed on an exotic form of liquid-metallic hydrogen rather than gas. The density and temperature of the planet suggest this.

The very strong magnetic field creates a protective magnetosphere around the planet that deflects the solar wind around the planet and traps some of the charged particles from the solar wind. This is shown in Figure 18.3. The particles trapped in the magnetosphere thread along the magnetic filed lines of Jupiter and form aurora in Jupiter's upper atmosphere in a fashion analogous to the aurora that form on Earth. The magnetosphere also emits intense radio waves which make Jupiter a very bright object in radio telescopes. An interesting oddity of Jupiter is that it actually emits about 2 X as much energy as it receives from the Sun. This energy excess is likely due to remnant energy released by the gravitational contraction of the planet as it formed. In that respect, Jupiter is still "cooling off".

Figure 18.3 The magnetosphere around Jupiter

Jupiter's Atmosphere

Jupiter's atmosphere consists of almost entirely Hydrogen (90% by weight) and Helium (10% by weight) with traces of heavier gases. This is very similar to what was believed to be the composition of the Solar Nebula. Figure 18.4 shows a temperature and pressure profile for the atmosphere of Jupiter. Although it is difficult to define where the "surface" of Jupiter starts the convention used by most planetary astronomers puts the bottom of the Jovian Troposphere at a position where the pressure is 10 times the pressure at the surface of the Earth and the temperature is approximately 340 K.

One of the most dramatic features of the Jovian atmosphere is the system of bright and dark parallel bands that circle the planet. The applet in Figure 18.5 shows these bands. The bright bands are called zones and the darker bands are belts. These are clouds in Jupiter's atmosphere and form in the Jovian troposphere at locations where the temperature and pressure allow ammonia (NH3), ammonium sulphate (NH4SH) and water (H2O) to condense and form droplets. Just as on Earth, convection causes larger, warm areas to rise in the atmosphere with neighbouring cooler cells to sink back into the atmosphere. The rapid rotation of the planet then stretches these convecting cloud movements into the zones and bands that we see.

Figure 18.4 The temperature profile for Jupiter's atmosphere.

The zones rise a bit higher in the atmosphere where it is a bit cooler while the belts are lower down where it is a bit warmer. These subtle temperature differences are enough to establish different chemical processes in the clouds which results in differences in colour. Another prominent feature of Jupiter's atmosphere is the Great Red Spot. This is believed to be a huge storm cell (analogous to a hurricane on Earth) that has existed for hundreds of years in the Jovian atmosphere. Its longevity is partly due to the enormous size of the planet and atmosphere but is likely also related to the rapid rotation of the planet and the heat excess being generated from below. Infrared observations tell us that the spot is slightly higher in the atmosphere than the normal cloud layer and this likely helps determine the chemistry of the cloud and its colour. Over the past 100 years the spot has diminished in size by almost a factor of 2 but it is premature to predict its demise. How long will the spot last? This is hard to say but smaller spots do come and go over periods of years to decades.

Figure 18.5 Globe showing the belts and zones that surround Jupiter.

Jupiter's Moons and Ring

Jupiter has an "entourage" of moons - at least 60 very small moons in addition to the famous four, Galilean Moons. Each of these last four tells its own remarkable story. Before looking at each one it is worth noting that the large size of these moons suggests that they likely formed together with Jupiter while the numerous minor moons may be material (asteroidal and cometary) captured later by Jupiter. Table 18.2 provides basic information on a small subset of these moons.

Example 18.3 Argue that the data in Table 18.2 suggests that there are distinct "families" within Jupiter's moon system.

Solution: The colour coding helps! But - careful inspection shows that there are 4 groupings of moons that have similar orbital distances and periods. The last 4 are quite interesting - they are orbiting in the opposite direction than the others. This argues that these families may have had different histories and origins.

Ganymede Ganymede is the largest moon in the solar system. It has a diameter of 5268 km which means that it is bigger than the planet Mercury! The density is quite low and this, coupled with gravimetric studies performed by flyby missions suggests that Ganymede is likely composed of an equal mixture of ice and rocky material. Figure 18.6 provides a globe of Ganymede. Ganymede is tidally locked so its orbital; period matches its rotation rate (7.15 d). The moon has a an albedo of 0.43. The surface is heavily cratered. Satellite observations suggest that there may be a salty liquid water layer approximately 100 km below the surface.
	Figure 18.6 ganymedeGlobe showing the moon Ganymede
Callisto Callisto is a large moon with a diameter of 4820 km and a density similar to that of Ganymede. The composition is similar to Ganymede's with numerous craters on the satellite's surface but the albedo is considerably lower at 0.22 Like Ganymede, Callisto is tidally locked and likely has a liquid water layer beneath its crust.
	Figure 18.7 callistoGlobe showing Callisto.
Europa Europa is an amazing, icy world! With diameter of 3100 km Europa is just smaller than our moon. The surface consists of a fine "cross-hatched" pattern of lines etched into an icy surface. These lines (called lineae) are very much like cracks that form on a sheet of ice on a lake. Periodically water from beneath seeps through the cracks and re-freezes. The lineae are the result of tidal stresses (fractures in the ice) created by the moons proximity to Jupiter. Europa has a very high albedo of 0.62. The density of Europa is 3.0 g/cm3 and it is generally believed that just below the surface there is a vast and surprisingly warm liquid-water ocean surround ing the moon. The heat to enable the existence of a liquid ocean is caused by the tidal interaction with Jupiter. The core of the moon is likely metallic. Europa also has a very thin, Oxygen atmosphere.
	Figure 18.8 europaGlobe showing Europa.
Io Io is one of the strangest places in the solar system! In many ways it acts like an extensive volcano with over 400 active volcanoes on its surface! The heat source for this is the continuous tidal stretching created by its tidal interaction with Jupiter. Io has a diameter of 3642 km which makes it the smallest of the Galilean moons. It has the highest density, however, at 3.6 g/cm3.
	Figure 18.9 ioGlobe showing Io.

Jupiter's Ring

Planetary rings will turn out to be a common motif for the Jovian planets. Jupiter's rings were first observed by the Voyager 1 spacecraft in 1979. The ring is invisible from Earth and is a faint band that circles the planet between 1.8 and 3.2 Jupiter radii. Observations of light scattering from the rings indicate that they are composed of extremely fine dust particles - quite similar to smoke particles. Figure 18.10 shows the rings along with the minor moons Metis, Adrastea, Amalthea and Thebe. These are "shepherd" moons whose own gravitational tugs help define the location of the rings.

Figure 18.10 Jupiter's Ring structure (image adapted from NASA)

Tidal Heating

Both Io and Europa illustrate the idea of tidal heating. The combined effect of Jupiter's gravitational pull and the gentle "nudges" from the other moons puts both Europa and Io in slightly eccentric orbits. When the moons are closest to Jupiter the tidal force that Jupiter exerts on the moons is greatest. When the moons are farthest away the tidal force is at its smallest. This cause a continuous stretching and relaxing of the moons. This creates stresses as well as frictional heating on the moons. The applet shown in Figure 18.11 illustrates this for Io.

Figure 18.11 TidalStretching on the moon Io as it orbits Jupiter.

In the applet the shape of Io is grossly exaggerated as it goes from a "football" shape to a sphere. Notice also that Io is tidally locked as it orbits Jupiter.

Practice

1. Explain why the four Galilean moons are in tidally locked orbits.
2. Explain how the composition of Jupiter supports one of the main ideas of the Solar Nebula Theory.
3. Speculate - why is Io both the smallest moon of the 4 Galilean moons and also the most dense? What would have prevented Io from accumulating ice and liquid water?
4. Speculate on why Europa has such a high albedo while Calisto has a low albedo.
5. Why do you know that the ring around Jupiter cannot be a solid object but must consist of small particle? (Give two reasons)
6. How do astronomers know that the ring around Jupiter is made up of minute, dust grain sized particles?