1. Announcements Online calendar has been updated – check it out
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2. Two Kinds of “Classical” Planets
"Terrestrial"
Mercury, Venus,
Earth, Mars
"Jovian"
Jupiter, Saturn, Uranus, Neptune
Far from the Sun
Large
Close to the Sun
Small
Mostly Rocky
High Density ( g/cm3)
reminder: liquid water is 1 g/cm3
Mostly Gaseous
Low Density ( g/cm3)
Slow Rotation ( days)
Fast Rotation ( days)
Few Moons
No Rings
Main Elements Fe, Si, C, O, N:
we learn that from the spectra
Many Moons
Rings
Main Elements H, He

3. Mars' Moons Phobos and Deimos
Phobos: 28 x 20 km
Deimos: 16 x 10 km
Properties similar to asteroids. They are probably asteroids captured into orbit by Mars' gravity.

4. Clicker Question:
From Mars, Deimos has an angular diameter of 140 arcseconds. Would colonists on Mars ever see Deimos produce a total solar eclipse?
A: Yes, every day on Mars
B: Yes, every new moon
C: Yes, but rarely
D: Never

5. Clicker Question thoughts:
From Mars, Deimos has an angular diameter of 140 arcseconds.
The Sun has an angular diameter of 1300 arcseconds
Deimos is the smaller of the Mar’s two moons. Even the larger, Phobos, will never produce a solar eclipse. Phobos’ angular diameter is about 40% of the sun’s as seen from Mars

6. Saturn (from Cassini probe)
The Jovian Planets
Saturn (from Cassini probe)
Jupiter
Uranus
Neptune
(roughly to scale)

7. Clicker Question: Which gas giant has the lowest average density:
A: Jupiter
B: Saturn
C: Uranus
D: Neptune

8. Discoveries Jupiter and Saturn known to ancient astronomers.
Uranus discovered in 1781 by William Herschel.
Neptune discovered in 1845 by Johann Galle. Predicted to exist by John Adams and Urbain Leverrier because of irregularities in Uranus' orbit.

9. Basic Properties Major Missions Mass (MEarth) Radius (REarth)
Orbit semi-major axis (AU)
Orbital Period (years)
Jupiter
Saturn
Uranus
Neptune
318 95 15 17 11
9.5 4
3.9
5.2
9.5
19.2
30.1
11.9
29.4 84
164
(0.001 MSun)
Major Missions
Voyager 1
Voyager 2
Galileo
Cassini
Launch
Planets visited
1977
1979
1989
1997
Jupiter, Saturn
Jupiter, Saturn, Uranus, Neptune
Jupiter

10. Jupiter's Atmosphere and Bands
Whiteish "zones" and brownish "belts".
Optical – colors dictated by how molecules reflect sunlight
Infrared - traces heat in
atmosphere, therefore depth
So white colors from cooler, higher clouds, brown from warmer, lower
clouds. Great Red Spot – highest.

11. Composition: mostly H, some He, traces of other elements (true for all Jovians). Gravity strong enough to retain even light elements. Mostly molecular. Spectroscopy of reflected sunlight reveals which molecules present.
Altitude 0 km defined as top of troposphere (cloud layer)
NH4SH
(NH3)
These molecules should all give white clouds. Molecules responsible for colors actually not clear!

12. Other Jovian planets: banded structure and colors
More uniform haze layer makes bands less visible. Reason: weaker gravity allows clouds to
rise higher and spread out to create more uniform layer
Blue/green of Uranus and blue of Neptune due to methane. Colder than Jupiter and Saturn, their ammonia has frozen and sunk lower. Methane still in gas form. It absorbs red light and reflects blue.

13. - Zones and belts mark a convection cycle. Zones higher up than belts.
- Zones were thought to be where warm gas rises, belts where cooled gas sinks. Now less clear after Cassini, which found numerous upwelling white clouds in the dark belts.
- Jupiter's rapid rotation stretches them horizontally around the entire planet.
- Winds flow in opposite directions in zones vs. belts. Differences are hundreds of km/hr.

14. Clicker Question: Jupiter’s atmosphere is primarily made up of:
A: hydrogen
B: helium
C: carbon dioxide
D: ammonia

15. Clicker Question:
It takes 8 minutes for light to travel 1 AU, how long does it take for light to travel from Earth to Jupiter at its closest point to Earth in its orbit?
A: 1 minute
B: 5 minutes
C: 30 minutes
D: 2 days
E: 1 year

16. Storms on Jovian Planets
Jupiter's Great Red Spot: A hurricane twice the size of Earth. Has persisted for at least 340 years. Reaches highest altitudes.
New storm “Oval BA”

17. "white ovals" - may last decades
"brown ovals" - only seen near 20° N latitude. Not known why. May last years or decades
Neptune's Great Dark Spot: Discovered by Voyager 2 in But had disappeared by 1994 Hubble observations. About Earth-sized.
Why do these storms last so long?

18. Jupiter's Internal Structure
Can't observe directly. No seismic information. Must rely on physical reasoning and connection to observable phenomena.
(acts like a liquid metal, conducts electricity)
Core thought to be molten or partially molten rock,
maybe 25 g/cm3, and of mass about MEarth .
Other Jovians similar. Interior temperatures, pressures and densities less extreme.

19. Rapid rotation causes Jupiter and Saturn to bulge:
Gravity
Gravity
without rotation
with rotation
Jupiter and Saturn rotate every ~10 hours. Radius at equator several % larger due to bulge.

20. Differential Rotation
Rotation period is shorter closer to the equator:
Near poles
At equator
Jupiter
Saturn
Uranus
9h 56m
10h 40m
16h 30m
9h 50m
10h 14m
14h 12m
How do we know?

21. Differential Rotation
Rotation period is shorter closer to the equator:
Near poles
At equator
Jupiter
Saturn
Uranus
9h 56m
10h 40m
16h 30m
9h 50m
10h 14m
14h 12m
How do we know? Tracking storms at various latitudes, or using Spectroscopy and Doppler shift.

22. Uranus' rotation axis is tilted by 98o
Why? Unknown. Perhaps an early, grazing collision with another large body.

23. Clicker Question: The Great Red Spot is: A: A large basin on Mars
B: A long-lived high-pressure storm in Jupiter’s atmosphere.
C: The colored polar cap of Jupiter
D: Clouds of dust swirling around Jupiter’s largest volcano

24. Clicker Question:
Saturn is less massive than Jupiter but almost the same size. Why is this?
A: Saturn’s interior is hotter than that of Jupiter’s.
B: Saturn is composed of lighter material than Jupiter.
C: Saturn is rotating faster than Jupiter so the increased centrifugal force results in a larger size
D: Saturn’s smaller mass provides less gravitational force to compress it.

25. The Solar System Gas Giants
Massive: MJ = 318 Mearth ≈ MSun
Strongly influence dynamics/evolution of solar system
Terrestrial Planets – (land/water/air interface)
Moons and Rings
Comets & Kuiper Belt Objects – water and other materials
Asteroids – metals, water, other materials
Zodiacal Dust — eroding asteroids & KBOs (comets)
Small in size, but large in surface area
Intercepts sunlight – observable scattered and thermal signatures
Tdust ≈ 30K K (evaporation)
Tdust (Asteroid) ~ 160K - 200K
Tdust (KBO) ~ K

26. Moons of Jovian Planets
Io, Europa
Callisto, Iapetus

27. The Galilean Moons of Jupiter
(sizes to scale) Io
Europa
Ganymede
Callisto
Closest to Jupiter
Furthest from Jupiter
Radii range from 1570 km (Europa, slightly smaller than our Moon), to 2630 km (Ganymede - largest moon in Solar System).
Orbital periods range from 1.77 days (Io) to 16.7 days (Callisto).
The closer to Jupiter, the higher the moon density: from 3.5 g/cm3 (Io) to 1.8 g/cm3 (Callisto). Higher density indicates higher rock/ice fraction.

28. Io's Volcanism
More than 80 have been observed. Can last months or years.
Ejecta speeds up to 1000 m/s. Each volcano ejects about 10,000 tons/s
Rich in S, SO2. S can be orange, red, black depending on temperature.
Frozen SO2 snowflakes are white.

29. Activity causes surface to slowly change over the years:
Voyager 2 (1979) Galileo (1996)

30. First, Io and Europa are in a "resonance orbit":
Volcanic activity requires internal heat. Io is a small body. Should be cold and geologically dead by now. What is source of heat?
First, Io and Europa are in a "resonance orbit":
Day 0
Jupiter
Europa Io
Jupiter
Day 1.77
Europa Io
Day 3.55
Jupiter
The periodic pull on Io by Europa makes Io's orbit elliptical.
Europa Io

31. (exaggerated ellipse)
orbital speed faster
orbital speed slower Io
(exaggerated ellipse)
- Tidal bulge always points to Jupiter. So the angle of the bulge changes faster when Io is closer to Jupiter.
- But Io rotates on its axis at a constant rate.
- So bulge moves back and forth across surface => stresses => heat => volcanoes

32. Europa may have Warm Ocean beneath Icy Surface
Fissures suggest large moving ice sheets.
Hardly any impact craters.
860 km
Dark deposits along cracks suggest eruptions of water with dust/rock mixed in (Europa’s density => 90%
rock, 10% ice).
42 km
Icebergs or "ice rafts" suggest broken and reassembled chunks.

33. (exaggerated ellipses)
What is source of heat? Similar to Io: resonant orbits with Ganymede and Io make Europa's orbit elliptical => varying tidal stresses from Jupiter => heat.
Warm ocean => life?
Europa Io
Jupiter
Jupiter
Ganymede
Europa
(exaggerated ellipses)

34. Saturn's Titan: A Moon with a Thick Atmosphere
Taken during Huygens’ descent
Surface from Huygens probe
From Cassini-Huygens mission
Surface pressure is 1.6 atmospheres, T=94 K. Atmosphere 90% Nitrogen.
Evidence for methane rain, a few possible slushy lakes of methane/ethane, drainage channels, liquid-eroded rocks, icy volcanoes (replenishing the methane?), complex hydrocarbons in atmosphere (e.g. benzene C6H6). Mostly dry now - liquid flow may be episodic.
Origin of atmosphere: probably gases trapped in water ice at formation, released by heat from natural radioactivity and volcanos into atmosphere.
Trapped by Titan’s cold temperature and relatively high gravity.

35. Saturn's Rings (all Jovians have ring systems)
- Inner radius 60,000 km, outer radius 300,000 km. Thickness ~100 m!
- Composition: icy particles, <1 mm to >10m in diameter. Most a few cm.
- A few rings and divisions distinguishable from Earth.

37. Origin of Saturn's Rings:
If a large moon, held together by gravity, gets too close to Saturn, the tidal force breaks it apart into small pieces. The radius where this happens is called the Roche Limit.
Total mass of ring particles equivalent to 250 km moon.
Perhaps a collision between moons sent one inwards this way, or a captured stray body.
Rings expected to survive only million years.

38. Voyager probes found that rings divide into 10,000's of ringlets.
Structure at this level keeps changing. Waves of matter move like ripples on a pond.

39. Origin of Cassini Division: another resonance orbit
Approximate radius of Mimas' orbit
Mimas' orbital period is twice that of particles in Cassini division. Makes their orbits elliptical. They collide with other particles and end up in new circular orbits at other radii. Cassini division nearly swept clean.
Other gaps have similar origins.

40. Rings of other Jovian Planets
The rings of Uranus.
Discovered by "stellar occultation".
Jupiter, Uranus, Neptune rings much thinner, much less material. Formed by breakup of smaller bodies? Also maybe "sandblasting" of material off moon surfaces by impacts.
Given rings have short lifetime and all Jovian planets have them, their formation must be common.
Neptune's moon Triton is spiraling in to the planet and should produce spectacular ring system in 100 million years.

41. Clicker Question: The only Jovian planet without a large moon is:
A: Jupiter
B: Saturn
C: Uranus
D: Neptune

42. Clicker Question:
Jupiter’s moon Europa is thought to have a large ocean of liquid water under a frozen surface. What is the heat source that keeps it from freezing?
A: Heat trapped inside the moon since formation.
B: A strong greenhouse effect from a dense atmosphere.
C: Tidal forces exerted by Jupiter, Io and Ganymede.
D: Radioactive decay of heavy elements in the mantle.

43. Clicker Question:
Saturn’s rings are not perfectly uniform. What causes the observed gaps?
A: The gravitational influence of Saturn.
B: The gravitational influence of Saturn’s moons.
C: Radiation pressure from Saturn.
D: The gravitational influence of the Sun and Jupiter.

44. Zodiacal Dust (looking out)
View from the Earth
View in Galactic
Coordinates

45. Pluto Predicted to exist by remaining irregularities in Uranus' orbit.
Discovered in 1930 by Clyde Tombaugh ( ).
Irregularities later found to be incorrect!
Model created from HST images. This is the most detail we have.
Pluto may have two more moons, found in 2005
Discovery image of Pluto's moon Charon (1978)

46. Basic Properties of Pluto
Mass MEarth or 0.2 x mass of Moon
Radius 1150 km or 0.2 REarth
Density 2.0 g/cm3 (between Terrestrial and Jovian densities. More like a Jovian moon)
Icy/rocky composition
Eccentric, tilted orbit
Moons: Charon: radius about 590 km or 0.1 REarth . Pluto and Charon tidally locked. S/2005 P1 and S/2005 P2: about km.

47. The New “Dwarf Planet” (2003 UB313 = Eris)
It too has a moon (Keck telescope)
orbit
Very eccentric orbit. Aphelion 98 AU, perihelion 38 AU. Period 557 years. Orbit tilt 44°.
Radius 1200 ± 50 km so bigger than Pluto. Icy/rocky composition,
like Pluto. More massive than Pluto.

48. Origin of Pluto and Eris
Now known to be just the largest known of a class of objects in the outer reaches of the Solar System. These objects are:
The Kuiper Belt Objects
100's found since Probably 10,000's exist.
Icy/rocky.
Orbits tend to be more tilted, like Pluto's.
Leftover planetesimals from Solar System formation?

49. Bizarre Orbits of some of Saturn's Moons
Tethys
Janus and Epimethius
Telesto and Calypso share orbit with Tethys, and are always 60 deg. ahead and behind it! They stay there because of combined gravity of Saturn and Tethys.
Janus and Epimethius are in close orbits. When the approach each other, they switch orbits!

50. Shoemaker-Levy Impact

51. More Solar System Debris
Comets
Comet Halley (1986)
Comet Hale-Bopp (1997)
Short Period Comets
Long Period Comets
year orbits
Orbits prograde, close to plane of Solar System
Originate in Kuiper Belt
Few times 105 or 106 year orbits
Orbits have random orientations and ellipticities
Originate in Oort Cloud

52. Oort Cloud is a postulated huge, roughly spherical reservoir of comets surrounding the Solar System. ~108 objects? Ejected planetesimals.
A passing star may dislodge Oort cloud objects, plunging them into Solar System, where they become comets.
If a Kuiper Belt object's orbit takes it close to, e.g., Neptune, its orbit may be changed and it may plunge towards the inner Solar System and become a comet.

53. Comet Structure Nucleus: ~10 km ball of ice, dust
Coma: cloud of gas and dust around nucleus (~106 km across)
Tail: can have both gas (blue) and dust tails (~108 km long). Always points away from Sun.
Coma and tail due to gas and dust removed from nucleus by Solar radiation and wind.
Far from Sun, comet is a nucleus only.

54. Meteor Showers
Comets slowly break up when near Sun, due to Solar radiation, wind and tidal force.
e.g. Halley loses 10 tons/sec when near Sun. Will be destroyed in 40,000 years.
Debris spreads out along comet orbit.
IF Earth's orbit crosses comet orbit, get meteor shower, as fragments burn up in atmosphere.

55. Meteoroids
Even smaller rocky pieces left over from Solar System formation.
If one lands on Earth, called a Meteorite.
Note: Meteor is only the name of the visible streak as the rock burns in atmosphere.

56. Clicker Question: The Oort Cloud is:
A: a spherical solar system halo of icy objects far beyond the orbit of Pluto.
B: a flat region just outside the orbit of Neptune in which icy and rocky objects circle the Sun.
C: the collection of rocky objects orbiting the Sun between the orbits of Mars and Jupiter.
D: a swarm of small satellites around Jupiter.

57. Clicker Question: The Perseids meteor shower happens every year when:
A: the Earth passes through the constellation Perseus.
B: the Earth passes through the remnants of comet Swift-Tuttle.
C: the Oort cloud emits a burst of comets.
D: the Earth comes within closest approach to the asteroid belt.

58. Asteroids
Rocky fragments ranging from 940 km across (Ceres) to < 0.1 km. 100,000 known.
Most in Asteroid Belt, at about 2-3 AU, between Mars and Jupiter. The Trojan asteroids orbit 60 o ahead of and behind Jupiter. Some asteroids cross Earth's orbit. Their orbits were probably disrupted by Jupiter's gravity.

59. Asteroids and Kuiper Belt Objects
~ 5 AU
~ 45 AU

60. Gaspra
Ida and Dactyl
Total mass of Asteroid Belt only MEarth or 0.07 Mmoon. So it is not debris of a planet.
Probably a planet was trying to form there, but almost all of the planetesimals were ejected from Solar System due to encounters with Jupiter. Giant planets may be effective vacuum cleaners for Solar Systems.

61. Lagrange Points