An Introduction to Astronomy Part VIII: Jovian Planets

1. An Introduction to AstronomyPart VIII: Jovian Planets Lambert E. Murray, Ph.D. Professor of Physics

4. Size and General Composition • Observations of the orbital period of Jupiter’s moons indicates that Jupiter has more than twice the mass of all the other planets, their moons, and the asteroids. • Jupiter’s diameter is about 10 times that of Earth, and its volume is 1,300 times Earth’s. • Thus, Jupiter’s density is only 1.3 g/cc, only 1/4 of Earth’s. This low density means Jupiter is composed of a high percentage of light elements. • The Galileo probe showed that Jupiter is about: • 86% hydrogen • 14% helium • small amounts of water (H2O), methane (CH3), and ammonia (NH3). • Thus, Jupiter is more like the Sun than the terrestrial planets.

5. Orbital and Rotational Motion • Jupiter is 5.2 AU from the Sun and takes 12 years to complete one orbit of the Sun. • Jupiter spins on its axis very quickly, once every 9h50m. Recall that all Jovian planets have much greater rotation rates than do terrestrial planets. • On Jupiter cloud bands near the equator rotate slightly faster (9h50m) than bands near the poles (9h56m). • Jupiter exhibits differential rotation—the rotation of an object in which different parts have different periods of rotation. This is possible only for gaseous objects. • Jupiter is very oblate, which is caused by its rapid rate of rotation. • Jupiter’s equatorial diameter is 6% greater than its polar diameter.

6. Jupiter’s Persistent Red Spot • Unlike terrestrial planets, surface features would have little effect on Jupiter’s upper atmosphere, allowing weather patterns to last for long periods. • Jupiter’s giant red spot, first seen by Galileo in the mid-1600s, has persisted to this day, over 300 years. • The red spot is approximately 40,000 km long and 15,000 km across, large enough to swallow the 13,000-km diameter Earth several times over!

7. Jupiter’s Giant Red Spot

8. Weather Patterns on Jupiter • The color patterns of Jupiter’s clouds are striking. • Colors seen in Jupiter’s upper atmosphere are likely due to chemical reactions induced by sunlight and/or lightning in its atmosphere. • The bright bands are called zones, while the dark bands are called belts. • As we will show in the next diagram, the zones are regions where the warmer gasses are rising, while the belts are regions where the cooler gasses are falling.

9. Zones and Belts on Jupiter

10. The Winds of Jupiter • Just as on Earth, there are different atmospheric cells generated by the convection currents in Jupiter’s atmosphere. • However, unlike the Earth, Jupiter appears to be emitting more energy than is incident upon it from the Sun – thus, it is being heated internally! • The boundaries between the convection cells exhibit high-speed winds, much like our jet streams. These high-speed winds cause a great deal of turbulence which can be seen in the upper atmosphere of Jupiter.

11. Close-up view of the Great Red Spot & associated turbulent flows rotates CCW ~ 6 day period High pressure region

14. Diagram of the possible internal structure of Jupiter

15. Jupiter’s Interior and Magnetic Field • The gaseous atmosphere on Jupiter is a few thousand miles thick. As one goes deeper in Jupiter’s atmosphere, gaseous hydrogen becomes liquid hydrogen. • At 15,000 km below the clouds, it is theorized that pressure and temperature create a state of liquid metal hydrogen. • Jupiter’s core, if it exists, is very small, contributing only 1% of the planet’s mass. • Jupiter’s magnetic field is quite strong— nearly 20,000 times stronger than Earth’s. • Jupiter’s magnetic field is believed to be generated by its large mass of liquid metal hydrogen and its rapid rotation rate.

16. Jupiter’s Internal Energy Source • Jupiter emits about twice as much energy as it receives from the Sun. • It is thought that Jupiter’s excess energy is left over from its formation; because of its great size, Jupiter is cooling very slowly. • The heat generated from the gravitational collapse of a large gas cloud is sufficient to start nuclear fusion in a star. • However, Jupiter would have to be 100 times more massive to release enough heat to support nuclear fusion, so it cannot act like a miniature star.

17. Jupiter’s Moons • Jupiter has at least 63 moons. (Dedicated searches for additional moons of both Saturn and Jupiter continue to turn up new, small moons, most of which are too small to be spherical. Saturn has at least for 60 moons.) • The smallest of the four larger moons – the Galilean moons – is 5000 times larger than the largest of the smaller moons. The Galilean moons are spherical, while the others are irregular in shape like asteroids. • The four large Galilean moons; and the four small inner moons (inside Io’s orbit), along with six of the outer moons all revolve around Jupiter in a counter-clockwise fashion (as observed from the North Pole). The orbits of many of the remaining outer moons are retrograde and often more eccentric.

18. The Galilean MoonsUp Close and Personal(in Order from Jupiter’s Surface) The Earth’s Moon is about the same size as Europa.

19. Io’s Active Volcanoes

20. The Surface of Io

21. Io’s Volcanic Activity • Io, the Galilean moon closest to Jupiter, has active volcanic geysers that spew hot sulfur onto the surface. • Io’s heat is produced by tidal forces caused by its eccentric orbit around Jupiter, and the influence of the gravitational pull of the other Galilean moons (Ganymede, the larges moon in the solar system, is outside Io’s orbit). • The surface of Io can rise and fall as much as 100 meters. • Io’s density is about 3.5, indicating that is it composed mostly of rock. It’s surface features indicate the erupted materials are sulfurous.

23. This is a picture of Io’s plasma torus, made up of charged sulfur ions trapped in Jupiter’s magnetic field. This demonstrates how volcanic activity could supply the gases needed for an atmosphere.

24. Jupiter’s Torus Quarter images of Io’s and Europa’s tori (also called plasma tori because the gas particles in them are charged—plasmas). Io is visible in its torus (green), while Europa is visible in its torus (blue). Some of Jupiter’s magnetic field lines are also drawn in. Plasma from tori flow inward along these field lines toward Jupiter.

25. Europa

26. Close-up of Europa’s Surface Close-up of icy region of Europa showing fractured ice similar to our polar ice caps. What looks like an impact crater in the ice pack.

27. Mosaic of Europa's Ridges, Craters Credit: Jet Propulsion Lab, NASA

29. Surface Features of Europa I

30. Surface Features of Europa II

31. Europa’s Icy World • Europa’s surface appears to be covered with ice, much like the polar ice pack at the North and South Poles. The moon’s moderate density indicates a rocky world covered by an ocean of frozen water. • Like Io, Europa also experiences some tidal heating, which may mean the interior of this moon is relatively warm; the seas below the ice pack may be warm enough to sustain life.

32. Ganymede and Callisto • Like Europa, Ganymede and Callisto both appear to be ice-covered moons. The ice may be thicker on these moons which are farther from Jupiter (perhaps thousands of miles thick). • Ganymede—larger than Mercury —is the largest moon in the solar system, and appears to be less active than Europa, with a darker surface. • However, Ganymede does appear to have some type of crustal activity which appears to be rejuvenating the surface. • Callisto, the outermost Galilean moon, shows more cratering than Ganymede, as would be expected from a less active surface, experiencing little tidal heating. • Callisto has the largest known impact crater—Valhalla— in the solar system.

34. Grooved Terrain of Ganymede

35. Higher Resolution of Galileo

36. Surface Features on Europa Lenticulae attributed to rising warmed ice and debris travel up from the moon’s interior by convection, arriving at and then leaking out at the surface. The white domes are likely to be rising material that has not yet reached the surface.

37. Callisto

38. Valhalla on Callisto

39. Irregularly Shaped Inner Moons The four known inner moons of Jupiter are significantly different from the Galilean satellites. They are roughly oval-shaped bodies. Although craters have not yet been resolved on Adrastea and Metis, their irregular shapes strongly suggest that they are cratered. All four moons are named for characters in mythology relating to Jupiter (Zeus, in Greek mythology).

40. Jupiter’s Ring • Only after the fly-by of Voyager I did we know that Jupiter had a thin ring. This ring had not been visible in telescopes from the Earth’s surface. • Most of the material in the ring is in a nearly flat plain, but there is a small amount of material scattered out around the ring as seen in the next picture. • The ring is relatively close to Jupiter, extending only to about 1.8 planetary radii. • We will discuss planetary rings more when we look at Saturn.

41. Jupiter’s Ring

42. Saturn

43. Saturn’s Vital Statistics • Saturn orbits the Sun at 9.5 AU; its distance from the Earth varies from 8.5 AU to 10.5 AU. • Saturn has an orbital period of 29.5 years. • Saturn is tilted 27° with respect to its orbital plane, so over time its rings appear in different orientations when viewed from Earth. • Like Jupiter, Saturn’s cloud belts rotate differentially. • Saturn’s rotation rate is 10h39m. • Saturn is even more oblate (0.102) than Jupiter, with its equatorial diameter 10% greater than its polar diameter.

44. Saturn’s Rings Cassini Division Enke Division

45. Description of Saturn’s Rings • As seen from Earth, Saturn’s rings look almost solid, except for two gaps. • The largest gap is called the Cassini division, believed to be caused largely by the gravity of Mimas acting synchronously on the orbital path of nearby ring particles. • The smaller gap is called the Enke division. • Saturn’s rings are very thin, as can be seen by the shadow they cast on the planet. In some cases they are less than 100 meters across. • The rings are made up of small ice covered particles ranging is size from centimeters to meters. • Each ring particle revolves around Saturn according to Kepler’s laws. • Because the plane of the rings is tilted, we see them with different orientations over time.

46. Numerous Thin Ringlets Constitute Saturn’s Inner Rings This Cassini image shows that Saturn’s rings contain numerous ringlets. Inset: As moons orbit near or between rings, they cause the ring ices to develop ripples, often like the grooves in an old-fashioned record.

47. The F Ring and One of its Shepherds Two tiny satellites, Prometheus and Pandora, each measuring about 50 km across, orbit Saturn on either side of the F ring. Sometimes the ringlets are braided, sometimes parallel to each other. In any case, the passage of the shepherd moons causes ripples in the rings. The gravitational effects of these two shepherd satellites confine the particles in the F ring to a band about 100 km wide.

48. Saturn’s Rings