The Jovian Planets

1. The Jovian Planets © Sierra College Astronomy Department

2. The Jovian PlanetsComposition, Structure, and Dynamics • Unlike terrestrial planets, Jovian planets are made of gas and liquid. • Jupiter and Saturn: mostly hydrogen and helium, with a few percent hydrogen compounds and a small fraction of rock and metal. • Uranus and Neptune: Less than half the mass is H and He, with most of the composition made of hydrogen compounds such as water, ammonia, methane. © Sierra College Astronomy Department

3. The Jovian PlanetsComposition, Structure, and Dynamics • The Jovian planets formed beyond the frost line and were thought to have grown from planetesimals of about the same mass – 10 Earth masses. • At greater distances, it took longer for small particles to accrete into large, icy planetesimals with gravity strong enough to pull in more material from the solar nebula. • Jupiter was first to form and was able to pull in the most material followed by Saturn and Uranus-Neptune. • Neptune is slightly more massive and denser than Uranus which suggests it formed from a slightly more massive ice-rich planetesimal. © Sierra College Astronomy Department

4. The Jovian PlanetsComposition, Structure, and Dynamics Density Differences • Saturn is considerably less dense than the other planets. • This makes sense if you compare what each planet is made of: Uranus and Neptune have less % of H and He than Saturn. • But then Jupiter should be the least dense of all! But it is not because its large gravity compresses the atmosphere. • This also, explains why Jupiter is only slightly larger in radius than Saturn © Sierra College Astronomy Department

5. Lecture 11: The Jovian PlanetsComposition, Structure, and Dynamics Shape of the Planets • The Jovian planets are non-spherical and have larger circumferences around their equators than around the great circles through the poles. • Saturn is the most oblate as it is about 10% wider than it is tall. • The strong gravity should make these large worlds spherical but rapid rotation makes the equatorial region bulge out. © Sierra College Astronomy Department

6. The Jovian PlanetsComposition, Structure, and Dynamics Inside Jupiter • The Galileo probe penetrated to a depth of 200 km (or 0.3% of Jupiter radius) before contact was lost. • At a depth of 80–100 km, predictions indicate the temperature is Earth-like and the pressure is 10 times greater than that at the Earth’s surface. • As one goes deeper in Jupiter’s atmosphere, gaseous hydrogen becomes liquid hydrogen (~7,000 km). The pressure here is 500,000 times that of the Earth surface. • At ~15,000 km below the clouds, it is theorized that pressure and temperature create a state of liquid metallic hydrogen (exists only in Jupiter and Saturn). • The core which contains roughly 10 Earth masses is only about the radius of the Earth (~6500 km). © Sierra College Astronomy Department

7. The Jovian PlanetsComposition, Structure, and Dynamics Comparing Jovian interiors • Saturn is the most similar to Jupiter and has liquid metallic hydrogen too, but much deeper beneath the visible clouds. • The pressures are not high enough to form liquid metallic hydrogen in Uranus and Neptune; however, hydrogen compounds reside in a layer above the central core of rock and metal. © Sierra College Astronomy Department

8. The Jovian PlanetsComposition, Structure, and Dynamics Magnetic Fields • Jupiter’s magnetic field is quite strong - nearly 20,000 times stronger than Earth’s. • Jupiter’s low density and distance from Sun implies that iron is not the source of the strong magnetic field. • Since hydrogen dominates and high pressures exist inside Jupiter, theorists predict that liquidmetallic hydrogenis best possible generator of the magnetic field. • Fast rotation period increases field strength. • Jupiter collects far more charged particles than the Earth does. • The other Jovian planets have smaller magnetic fields (though larger than Earth’s if compared side-by-side). • Uranus and Neptune’s magnetic field is generated by the core “oceans” of hydrogen compounds, rock, and metal. © Sierra College Astronomy Department

9. The Jovian PlanetsComposition, Structure, and Dynamics The Atmosphere (Weather) • Weather is driven on the Jovian planets by energy from the Sun and from within (plus the rotation of the planet). • Internal energy (present in all but Uranus) is likely coming from the conversion of potential energy to kinetic energy as gasses are slowly falling or condensing inside these planets. © Sierra College Astronomy Department

10. Jupiter from a ground-based telescope

11. The Jovian PlanetsComposition, Structure, and Dynamics Clouds and Colors • The Jovian planets have clouds which condense from a gas when the temperature becomes cold enough. • For the Earth only one gas – water vapor – can condense. • For Jupiter, from high to low altitude: ammonia, ammonium hydrosulfide, and water condense to form cloud layers, so most of the time we see ammonia clouds. This happens at about 30 to 100 km below the upper cloud tops. • For Saturn the same layers form but deeper in the atmosphere (200 km below) and farther apart . • Why? Saturn is colder and has weaker gravity. • For Uranus and Neptune, methane clouds dominate the atmosphere. These absorb red light very well and make the planets blue. © Sierra College Astronomy Department

12. The Jovian PlanetsComposition, Structure, and Dynamics Storms on Jupiter • Jupiter’s Giant Red Spot, first seen in the mid-1600s, has lasted for over 300 years (or at least 150 years). • The Giant Red Spot is a high-pressure storm system that rotates counterclockwise every 6 days. • The red spot is 40,000 km long and 15,000 km across, larger than the 13,000-km diameter Earth. • Cause of red color is still debated. • Several (12) zones and belts can be seen too. This banded structure is due to the Coriolis effect and rapid rotation. • A New Red Spot? • Oval BA formed in 2000 when three smaller spots collided and merged. • White in November 2005, brown in December 2005, and then red in February 2006. • As of March 2006, Red Jr. is about half the size of the Great Red Spot. © Sierra College Astronomy Department

13. The Jovian PlanetsComposition, Structure, and Dynamics Weather on other Jovian planets • Saturn has zone and belts which are harder to see since they are deeper in the atmosphere • Uranus had nearly no clouds when Voyager passed in 1986, but Earth observations has shown more weather when northern “spring” comes to Uranus. • Neptune had a Great Dark Spot seen by Voyager (1989) but it has since disappeared. © Sierra College Astronomy Department

14. Picture from Cassini: 9 February 2004 http://saturn.jpl.nasa.gov Cassini arrived 1 July 2004 Courtesy of NASA/JPL-Caltech

15. Voyager’s Uranus (1986) Enhanced Normal

16. Keck’s Uranus (2004) Rings and planet taken with separate exposures

17. Atmospheric Notes • Neptune’s winds - driven by an internal heat source - reach speeds of 2200 km/hr (1300 mph).

18. The Jovian PlanetsJupiter’s Moons Jupiter’s Moons • Jupiter’s family of 63 moons can be divided into 3 groups: • Outer moons, eccentric orbits, many retrograde, dark surfaces, captured asteroids. • 4 inner moons orbit very close to Jupiter and are probably fragmented moonlets (form and shape Jupiter’s ring). • 4 Galilean moons, nearly circular orbits, smallest is 5,000 times more massive than the largest of the other moons. © Sierra College Astronomy Department

19. The Jovian PlanetsJupiter’s Moons • Io, the Galilean moon closest to Jupiter, has active volcanic sulfuric geysers. • Creates many surface layers • But does not build high volcanoes (lava too fluid) • Io’s heat is produced by tidal forces caused by Europa and its eccentric orbit around Jupiter. • Io is surrounded by a halo of sodium atoms, which itself is embedded in a sodium torus that surrounds Jupiter. © Sierra College Astronomy Department

20. Io during eclipse The Jovian PlanetsJupiter’s Moons Volcanoes on Io • How? Should have cooler interior than Mercury and Mars (smaller object). • Io’s elliptical orbit forced by resonance with Europa and Ganymede causes differential tidal heating. • Io is tidally distorted more when closer to Jupiter than farther away. • This constant flexing heats the interior. • Spewed material from volcanoes forms torus of sodium(?) around Jupiter (Io Torus). © Sierra College Astronomy Department

21. The Jovian PlanetsJupiter’s Moons Resonances • Previous examples: spin-orbit • Moon (1:1) around Earth, Mercury (3:2) around Sun • New examples: spin-orbit • All other major satellites to parent planet (1:1) • New examples: orbit-orbit • Io-Europa (2:1) • Europa-Ganymede (2:1) • Later, in Saturn’s rings: Mimas-Cassini Division (2:1) © Sierra College Astronomy Department

22. The Jovian PlanetsJupiter’s Moons • Europa’s surface is ice; its moderate density indicates a rocky world covered by an ocean of frozen water. • Europa also experiences some tidal heating which has resurfaced it. • Ice rafts and lenticulae (100-m ice mounds) • This tidal heating of Europa also suggests that an interior liquid ocean of water may exist. • Europa is the smallest of the Galilean moons (and is smaller than Earth’s Moon). © Sierra College Astronomy Department

23. The Jovian PlanetsJupiter’s Moons • Ganymede has a surface that appears similar to our moon. • The surface is composed mostly of ice. • With fewer craters than Callisto, some resurfacing has occurred. • Ganymede is the largest moon in the solar system. • It also generates its own magnetic field • How? There may be a layer of salty-water buried 150 km beneath the surface © Sierra College Astronomy Department

24. The Jovian PlanetsJupiter’s Moons • Callisto also has a surface that appears similar to our moon. • The surface is composed mostly of ice. • There may be a water ocean below the surface. • Radioactive heating may contribute • There is a detectable magnetic field • Callisto is very heavily cratered implying that it is tectonically inactive. • May be the oldest surface in the solar system • Callisto’s interior is appears to be undifferentiated. © Sierra College Astronomy Department

25. Closer to Jupiter Further to Jupiter Dense Less Dense Younger Older Europa Ganymede Callisto Io Surface age determined by crater counts Size of Earth’s Moon Galilean Moon Stats

26. The Jovian PlanetsSaturn’s Moons Moons of Saturn • Saturn has 60 moons, second only to Jupiter in number. • Major moons include (from largest to smallest): Titan (second largest moon in the Solar System), Rhea, Iapetus, Dione, Tethys, Enceladus and Mimas. © Sierra College Astronomy Department

27. Some Moons of Saturn Rhea Mimas Enceladus

28. Close-up of surface Enceladus Enceladus has a very shiny surface (albedo = 0.9) and has just been discovered to have a “significant” atmosphere (which must be replenished) False-color image of anti-Saturn hemisphere

29. Dione Tethys

30. Iapetus (the two toned moon) Phoebe enlarged Phoebe Hyperion (next page too)

31. Hyperion

32. The Jovian PlanetsSaturn’s Moons Titan • Titan may be the most interesting moon in the solar system because it has an atmosphere (How?). • It is composed mostly of nitrogen with 1% methane and a trace of argon. • When sunlight strikes methane, it can cause the formation of organic molecules, which are a known precursor to life. © Sierra College Astronomy Department

33. Titan Rhea

34. Titan from Cassini Next Encounter: 19 November 2007 visible near-infrared

35. Titan as seen by Cassini Oct. 26, 2004 Polarized infrared light Picture from Huygens Probe on the surface of Titan (Jan 15, 2005

36. The Jovian PlanetsUranus’s Moons Uranus’s Moons • Five moons were known before Voyager (Miranda, Ariel, Umbriel, Titania, Oberon); now 22 more are known (total = 27). • Many moons named for Shakesperian characters. • All the moons appear to be low-density, icy worlds (but they appear to have had been more active than the Saturnian satellites of a similar size). • The innermost, Miranda, is perhaps the strangest looking object in the solar system. It appears as if it were torn apart by a great collision and then reassembled. © Sierra College Astronomy Department

37. The Jovian PlanetsNeptune’s Moons Neptune’s Moons • Before Voyager 2, Neptune was known to have 2 moons; 13 moons are now known. • Triton, Neptune’s largest moon, is the only major moon to revolve around a planet in a clockwise (retrograde) direction. • Causes significant enough tides on Triton. • Triton is also tilted 23 deg relative to Neptune’s equator • Triton has a very thin atmosphere of N2 and CH4. © Sierra College Astronomy Department

38. Triton © Sierra College Astronomy Department

39. The Jovian PlanetsNeptune’s Moons • Triton has a light-colored surface composed of water ice with some nitrogen and methane frost. • Its surface appears young, with few craters and active geyser-type volcanoes observed (nitrogen ice and carbon compounds). • Triton’s active volcanism is probably due to internal heating from tides, heating from the Sun or internal residual heat. © Sierra College Astronomy Department

40. The Jovian PlanetsPlanetary Rings - Saturn Planetary Rings • Saturn’s rings are very thin, in some cases less than 100 meters thick. • The rings are not solid sheets but are made up of small particles of water ice or water-ice mixed with dust. • Three distinct rings are visible from Earth, and were named (outer to inner) A, B, and C. © Sierra College Astronomy Department

41. Voyager approaching Saturn Saturn from Earth Voyager leaving Saturn

43. Mimas The Jovian PlanetsPlanetary Rings - Saturn • The largest division between rings is known as the Cassini division. • This space is caused largely by the gravity of Mimas acting synchronously (2:1 resonance) on the orbital path of nearby ring particles. • Some other ring features are explained by the presence of small shepherd moons. © Sierra College Astronomy Department

44. Close-up of Main Rings A Ring True C Ring Cassini Division B Ring

45. Cassini View of the main rings in true color C B A Cassini Division Shadows

46. The F ring: Confined by Shepherd Satellites Pandora and Prometheus The A ring Voyager Cassini

47. The Jovian PlanetsPlanetary Rings - Saturn The Origin of Rings • Saturn’s rings are probably about 100 million years old. • The origin of Saturn’s rings is not well understood, but is thought to be the result of: • A close-orbiting, icy moon that shattered in a collision with an asteroid . • A large comet which got too close to Saturn (much like Shoemaker-Levy 9 did at Jupiter in 1994). • Rings around the Jovian planets are not billions of years old and must be replaced or renewed on a much smaller time scale. • Tidal forces are greater on a moon in orbit close to a planet than they are on a moon in an orbit farther out. © Sierra College Astronomy Department

48. The Jovian PlanetsSaturn’s Rings • Roche limit is the minimum radius at which a satellite (held together by gravitational forces) may orbit without being broken apart by tidal forces. • Saturn’s rings are inside Saturn’s Roche limit, so no moons can form from the particles. © Sierra College Astronomy Department

49. Voyager from “behind” Jupiter The Jovian PlanetsPlanetary Rings - Jupiter Jupiter’s Ring • Voyager I discovered a thin ring (system) around Jupiter. • The ring is close to Jupiter, extending to only about 1.8 planetary radii. • The ring is thought to be replenished from the small moonlets within or near it. © Sierra College Astronomy Department

50. The Jovian PlanetsPlanetary Rings - Uranus and Neptune • The rings of Uranus and Neptune and are made of particles which are darker and smaller than that of Saturn. • The Uranian rings are narrow, a few of which are clearly confined by shepherding moons. • The Neptunian rings vary in width and are confined by resonances of some of the moons. © Sierra College Astronomy Department