Homework #4

1. Homework #4 Due on Thursday August 4 after class (2005) Chapter 11, p 300, Discussion 14 What is the cause of Io's volcanic activity? Chapter 11, p 300, Discussion 19 What might be the consequences of the discovery of life on Europa? Chapter 11, p 300, Problem 9 Illustrate the 1:2:4 "resonance" mentionned in the text by drawing a diagram showing the locations of Io, Europa, and Ganymede at various times over the course of one Ganymede orbit. Show the moons' locations at intervals of 1/4 Io's orbital period. Additional question: Calculate Jupiter' Mass using the revised Kepler's Third Law and the Period and orbit of Io. Why a moon such Io, which appears to be similar in size to the Moon and has a similar size orbit, should have a faster orbital period? Chapter 12, p 326, Discussion 11 What evidence supports the idea that a relatively recent catastrophic event was responsible for Saturn's ring? Chapter 12, p 326, Discussion 20 Imagine what the sky would look like from Saturn's moon Hyperion. Would the Sun rise and set in the same way it does on Earth? How do you imagine Saturn might look? + HW#4 Supplement

2. Uranus & NeptuneThe Outer World of the Solar System

3. Uranus Ouranos in Greek Personification of heaven and the sky Gaea (personification of Earth) is his mother and wife Was overthrew by his son Saturn (Cronos) A complex family….

4. Uranus • Semimajor axis = 19.2 AU, Period = 84.1 years • Mass = 8.68 ´ 1025kg = 14 Earths • Diameter = 51,200 km • Density = 1.2 g/cm3 • Escape velocity = 21 km/s • Rotation period = 17.2 hours • Axial tilt = 98o • Discovered by William Herschel in 1781 • Had been plotted as a star on several star charts • Visited by Voyager 2 (1986)

5. Discovery • Sir William Herschel, German astronomer, who lived in England in 1781 • His goal was to built a catalogue of faint stars using a 6-inch telescope • Discover a planet that he called Sidus Georgium and two satellites

6. A co-discovery? • William’s sister (Caroline Herschel) involved in the project • Participate to the observations, construction of telescopes • Catalogued the discoveries for the new generation of astronomers The planet will be finally named Uranus

7. Observing Uranus • Maximum angular diameter 4.1” • Magnitude in visible ~5.7 (limit of naked eye visibility) With a telescope • Appeared as a tiny pale greenish disk • Surrounding by several satellites (Ariel, Umbriel, Titania, Oberon, Miranda) (see Xephem) Voyager 1 mission (1986) • absence of atmospheric surface details

8. Uranus’ structure • Very few noticeable zones and belts • Clouds of methane and hydrogen sulfide are likely to form • Flow pattern in the same sense than rotation (wind speed 200-500 km/s) , faster in the North pole, T quite uniform • No metallic hydrogen near core, just ice and rock • Magnetic field still there • Off-axis and offset fromcenter • Probably due to polarized water • Axial tilt of 98o and time of Voyager 2 visit may be reason for lack of atmospheric bands • No measurable internal heat

11. 2007 Autumn equinox 2028 1986 2070 Northern Summer Solstice Spring Equinox 2049

12. Magnetic Field Generation • Earth/Mercury – spinning liquid iron near core • Mars – fossil magnetic field induced in rocks • J/S – rapidly spinning liquid metallic hydrogen • U/N – spinning, polarized water (not so) near core

15. Uranus False Color True Color

16. HST IR Image of Uranus (false color) Rings Very high clouds Sunlight reflecting by haze Deep atmosphere Inner satellites

17. Keck AO Infrared images of Uranus Images taken in Aug. 2004 Cloud velocity <-> wind speed = 200-500 km/h Clouds rotate in the same sense than planet Faster at the North pole -> Temperature nearly uniform on the whole planet Clouds

18. Neptune Poseidon in the Greek Mythology Son of Saturn God of the sea and of earthquakes Brother of Jupiter (Zeus) and Pluto (Hades)

19. Neptune • Semimajor axis = 30.1 AU, Period = 164.8 years • Mass = 1.02 ´ 1026 kg = 17 Earths • Diameter = 49,500 km • Density = 1.6 g/cm3 • Escape velocity = 24 km/s • Rotation period = 16.1 hours • Axial tilt = 29o • Discovered by using Newton’s Laws and examining perturbationsin Uranus’ orbit by Leverrier and Adams (independently) in 1846 • Visited by Voyager 2 (1989) • Radiates 2.7 times more heat than it receives from the Sun (cause is uncertain. CH4 thermal inertia?)

20. A controversial discovery • John Couch Adams, Cambridge mathematician, predicted the existence of an unseen planet. Uranus was being pulled slightly out of position in its orbit (1845). • His work has been ignored by the Royal observatory Greenwich • Until a French mathematician, Urbain Leverrier, published a similar prediction. • Search started in July 1846 lead by Prof. Challis at Cambridge university • Leverrier with the help of Johann Galle at Berlin Observatory found the planet on the 23rd September 1846 • Leverrier and Adams are now jointly credited with their “discovery”

21. Observing Neptune • P= 163 yr, the planet did not complete a revolution since its discovery • Cannot be seen with naked eye. magnitude =7.8 • With a small telescope: 2.4” diameter blue disk and satellites (largest moon Triton, and Nereid) • Cloud Structures are visible nereid triton

22. Neptune’s structure • Similar composition than Jupiter: 84% H and 14% He, CH4 (3%) (blue color). • No NH3 b/c Tcondensation = 70 K and Touttermostplanets ~58 K • Far more atmospheric activity than Uranus b/c of Interior heat (3 times more energy) + less haze • Modest axial tilt may allow for more atmospheric mixing • Storm activity seen and seems to change

23. Great Dark Spot

24. Cloud features: Methane ice crystal Lie 50km above the main Cloud top (cirrus) Wind speeds > 1500 km/h

25. Keck AO Infrared image No more Great Dark Spot Dynamic weather patterns Wind speed up to 1500 km/h (half the speed of sound) Where come from the energy driving such activity? (initial heat insulated by methane, other?)

26. Interior structure • Uranus and Neptune, just ice and rock in core (same size than Earth • Not enough pressure to force hydrogen into metallic hydrogen • Off-axis and offset magnetic field probably due to polarized water or ammonia dissolved in water in outer core

29. Magnetic fields

31. Moon Systems of Uranus • See Table book • Newly discoverer (ten smaller moons discovered by Voyager) • Small moons in retrograde orbits (debris captured) discovered recently • 5 largest moons (~ 6 midsized moons of Saturn) • Same density • Same size • No large “Galilean Moons” • Surface darker than Saturn moons due to “space weathering” or “radiation darkening” (construction of an organic layer due to impact of energetic particles or effect of radiation)

32. +2 new moons discovered since 1997

33. Surface cracks Activity due to tidal effect Darkest moon Bright spot on the pole Ridges, valley, oval faults. Catastrophically disrupted several times Neptune Moon

34. Moon Systems of Neptune • See Table book • 1 midsized moon (Proteus) D=440 km • 1 large moon (Triton) D=2710 km • Moons have no “regular” moon system • high inclination (Triton) • And/Or retrograde orbit (Triton) • And/Or High eccentricity (Nereid) • Triton: • 6th largest moon of the Solar System • Tsurface =37 K -> surface of Water ice and Nitrogen frost • Young surface (lack of craters) due to surface activity (+ fissures and geysers) -> thin atmosphere of N2

35. +5 new moons discovered since 2002

36. Triton Tsurface=37 K Tenuous N2 atm. Lakelike feature N2 Geysers seen! -> An icy volcanic activity Ridges N2 frost (pinkish)

37. Summary:Moons of Jovian Planets Fig 11.15

38. Scattering Basics • Relatively large wavelength photons hitting relatively small objects • Forward scattering • Relatively small wavelength photons hitting relatively large objects • Backscattering • Also have certain objects which scatter in all directions well (isotropically) • Large object with irregular surface

39. The Rings seen in occultation • As Voyager passed by Saturn it view a star which went behind the rings • How much light the got through the rings to Voyager depended on the thickness • The less of the light received the greater the optical depth of the rings • General properties and resonance feature are clearly seen

40. Rings of Uranus • Discovered in 1977 from the ground by star occultation • Confirmed by Voyager in 1979 • Orbit in the equatorial plane of the planet • Main ones are called Alpha, Beta, Gamma, Delta and Epsilon (44,000 - 51,000 km from the planet’s center in the Roche limit) • Dark (space weathering), narrow (less than 10km) and widely spaced (100-1000km) • Thin (less than 30m) • Epsilon ring is similar to F ring of Saturn. Shepherd by 1986U8 (Cordelia) and 1986U7 (Ophelia)

43. Epsilon ring

44. Rings of Uranus observed with Keck AO

45. Ring of Neptune • 5 main dark rings • Color due to radiation darkening • Seen now from Earth with Keck AO system • Structure in arcs not well understood

46. Summary: Rings of Jovian Planets

47. Summary: Rings of Jovian Planets

48. Why are the rings there? • Jupiter: dust supplied by inner 4 moons • Saturn: tides ripped up large comet which came to close to the planet • Rings remain shiny because constant collisions with other ring particles • Uranus & Neptune : Perhaps supplied by nearby satellites. • Rings are formed inside the Roche Limitwhere large moons would be torn apart by tidal forces • In 100 million years Triton will enter in Roche limit of Neptune -> new bright rings