Size Comparison of Outer Planets & Saturn's Moons: Cassini Spacecraft Discoveries

1. Size comparison of the Outer Planets Earth Neptune Uranus

2. Early telescopic observations of Saturn a. Galileo, 1610; b. Huygens 1655; c. Cassini 1676

4. Spacecraft that have visited Saturn • Pioneer 11 flew within 21,000 km of Saturn on 1 September 1979, discovered two new moons and an additional ring, charted Saturn's magnetosphere and magnetic field and found its planet-size moon Titan, to be too cold for life. • The Voyager 1 and 2 Saturn encounters occurred nine months apart, in November 1980 and August 1981. Discovers larger of internal to external heat: 2-to-1 compared to 1-to-1 for Jupiter. • Cassini orbital insertion on July 1, 2004. Took first image of a moon (Phoebe) on June 11, 2004. Still working in April 2006: http://photojournal.jpl.nasa.gov/mission/Cassini • Cassini dropped the Huygens probe into atmosphere of Titan on January 14, 2005.

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7. Cassini spacecraft; launched on Oct. 15, 1997 from KSC 7 Year cruise on Venus-Venus-Earth-Jupiter Gravity Assist Trajectory

8. The Cassini Spacecraft

10. Interior Structure of Saturn

11. Like on Jupiter, there are latitudinal bands of clouds and storms

12. Ribbon clouds in atmosphere – high-speed turbulence

13. Wind speeds at top of Saturn’s Atmosphere Opposite direction 1700 km/hr

15. Multiple Rings! Cassini Division Encke Gap D C B F A E Ring extends to 7.5 Saturn radii from cloud top

16. Structure: Non-symmetric on each side of Saturn

17. Braided F-Ring of Saturn due to co-orbiting “shepherd” moons

18. Two shepherd satellites confine Saturn’s narrow F ring. The outer shepherd gravitationally deflects ring particles inward, and the inner shepherd deflects ring particles outward.

19. Prometheus <R> = 47 km, Density = 0.42

20. Pandora <R> = 41 km, Density = 0.54

21. Ring Spokes – Dust above the ring plane

23. The Smaller Moons of Saturn Janus Telesto Prometheus Helene Atlas Calypso Pandora Epimetheus

24. Janus <R> = 90 km, Density = 0.61

25. Epimetheus <R> = 58 km, Density = 0.64

26. Mimas, 392 km in diameter Herschel Crater is 130 km wide and 10 km deep!

27. Enceladus – 500 km in diameter Diverse surface, with some tectonics

28. Enceladus

30. Tethys – 1,060 km in diameter. Heavily cratered, with some lineaments

31. Dione – 1,120 km in diameter. Bright surface swirls, many craters

32. Rhea – 1,530 km in diameter. Very heavily cratered

33. Various spacecraft views of Titan – 5,150 km in diameter Cassini visible light image Pioneer 11 Cassini near-IR image Voyager 1

34. What do we know about Titan? • It is the biggest of Saturn’s moons. • Titan’s diameter of 5,150 km makes it larger than the planet Mercury with a diameter of 4,880 km and Pluto (2,350 km). • Only Jupiter's moon Ganymede is larger. • Three-and-a-quarter Earth Moons could fit inside Titan. • Titan is the only moon known to have a large atmosphere – maybe thicker than Earth’s (>1,000 mb)! • Its atmosphere is mostly nitrogen, just like the atmosphere surrounding Earth, but thicker. • Its atmosphere contains methane and tiny amounts of oxygen.

35. Comparison of Titan’s atmosphere with Earth’s Important gases in Titan’s atmosphere

36. Ethane-methane lakes 2. Moist convection in methane clouds • 3. Sedimentation of aerosols 4. Elevated terrain washed by rain 3. 2. 1. 4.

37. January 14, 2005 – Huygens Probe sent to Titan’s surface

38. Huygens scientific instruments • Aerosol Collector and Pyrolyzer: collects Titan's aerosols for chemical composition analysis; • Descent Imager/Spectral Radiometer: makes spectral measurements and takes pictures of Titan's surface and atmospheric hazes; • Doppler Wind Experiment: uses radio signals to deduce wind speeds on Titan; • Gas Chromatograph/Mass Spectrometer: identifies and quantifies various atmospheric constituents on Titan; • Surface Science Package: determines the physical properties of Titan's surface.

39. Radar Image of Titan’s surface – valley systems

40. Few impact craters seen on Titan – implies young surface

41. Surface image from Huygens probe Horizon at 66 m 30 cm diameter Range 240 cm 15 cm diameter Range 85 cm

42. Iapetus – 1,460 km in diameter. Striking albedo differences

43. Cassini Views of Iapetus Iapetus is 1,460 km diameter Orbits 59.1 Saturn radii from planet 100 km

44. What’s Interesting about Phoebe? • Phoebe is roughly spherical and is~220 km in diameter. • Phoebe rotates on its axis every 9 hours and orbits Saturn in ~18 months. • Its irregular, elliptical orbit is inclined ~30 degrees toward Saturn’s equator. • Phoebe’s orbit is retrograde. • Phoebe’s average distance from Saturn is 13 million km, which is ~4 times farther away from Saturn than its nearest neighbor, the moon Iapetus. • Phoebe and Iapetus are the only major moons in the Saturn region that do not orbit close to the plane of Saturn’s equator. • Unlike most major moons orbiting Saturn, Phoebe is very dark and reflects only 6 percent of the sunlight it receives.

45. Saturn’s moon Phoebe

46. Identification of surface materials on Phoebe

47. Saturn Summary