1. The Solar System: Comparative Planetology 1
Chapter 6
The Solar System: Comparative Planetology 1

2. Learning Goals What is comparative planetology?
Describe the scale & structure of our solar sytem
Summarize the basic differences between the terrestrial & jovian planets
Identify & describe the major non-planetary components of our solar system
Describe some important solar system exploration spacecraft missions
Cosmogony – theories of solar system formation

3. Units of Chapter 6 An Inventory of the Solar System
Planetary Properties
Computing Planetary Properties
The Overall Layout of the Solar System
Terrestrial, Jovian & Dwarf Planets
Interplanetary Debris (asteroids, comets & meteors)

4. Units of Chapter 6, cont. Spacecraft Exploration of the Solar System
Gravitational “Slingshots”
How Did the Solar System Form?
The Concept of Angular Momentum

5. 6.1 An Inventory of the Solar System
Early astronomers knew the Moon, stars, Mercury, Venus, Mars, Jupiter, Saturn, comets, and meteors
Now known: Solar system has 166 moons, one star, eight planets (added Uranus & Neptune), many objects in the new class called dwarf planets (Pluto, Ceres, Eris, …), asteroids, comets, and meteoroids

6. 6.2 Planetary Properties 63 60 13

7. 6.2 Planetary Properties Distance from Sun known by Kepler’s laws
Orbital period can be observed
Radius known from angular size
Masses from Newton’s laws
Rotation period from observations
Density can be calculated knowing radius and mass

8. 6.3 The Overall Layout of the Solar System
All orbits paths are close to the ecliptic plane
Pluto’s orbit does not (17° tilt)

9. 6.4 Terrestrial and Jovian Planets
Relative sizes of the Sun & Planets
It would take 109
Earths to span the
Sun!

10. 6.4 Terrestrial and Jovian Planets
Terrestrial planets:
Mercury, Venus, Earth, Mars
Jovian planets:
Jupiter, Saturn, Uranus, Neptune
Pluto is neither but a new class called the
Dwarf planets

12. 6.4 Terrestrial and Jovian Planets
Differences (Comparative Planetology) between the terrestrial planets:
Atmospheres and surface conditions are very dissimilar
Only Earth has oxygen in atmosphere and liquid water on surface
Earth and Mars rotate at about the same rate; Venus and Mercury are much slower, and Venus rotates in the opposite direction
Earth and Mars have moons; Mercury and Venus don’t
Earth and Mercury have magnetic fields; Venus and Mars don’t
All four Jovian planets have surrounding magnetic fields 

21. 6.5 Interplanetary Debris
Asteroids and meteoroids have rocky composition; asteroids are bigger
Asteroid Eros is 34 km long:

22. 6.5 Interplanetary Debris
Comets are icy, with some rocky parts.
Comet Hale–Bopp
(1997)
ion tail
dust tail
Comet ISON

23. 6.6 Spacecraft Exploration of the Solar System
Mariner 10: flew by Mercury, 1974–75
MESSENGER: it’s there now!
Messenger
Mariner 10

24. 6.6 Spacecraft Exploration of the Solar System
Soviet Venera probes landed on Venus from 1970–1978:

25. 6.6 Spacecraft Exploration of the Solar System
Viking landers arrived at Mars in 1976:

26. 6.6 Spacecraft Exploration of the Solar System
Typical orbital path to Mars:

27. Spacecraft Exploration of the Solar System
The Sojourner Rover was deployed on Mars in 1997 as part of the Pathfinder Mission

28. 6.6 Spacecraft Exploration of the Solar System
Pioneer 10 & 11 and Voyager 1 & 2 flew through the outer solar system. This is Voyager:

29. 6.6 Spacecraft Exploration of the Solar System
The Cassini mission is now orbiting around Saturn, the ring system and its many moons; it used many gravity assists to get there:

30. 6.6 Spacecraft Exploration of the Solar System
Gravitational “slingshots” can change the trajectories of spacecraft, and also accelerate them:

31. Simulations Angular Momentum Example Kepler's 2nd Law Nebular Theory 1
Planetary Formation 2

32. 6.7 How Did the Solar System Form?
Nebular contraction:
Cloud of gas and dust contracts due to gravity; conservation of angular momentum means it spins faster and faster as it contracts

33. 6.7 How Did the Solar System Form?
Condensation theory:
Interstellar dust grains help cool cloud, and act as condensation nuclei
Why are rocky planets close to the Sun and gas giants far away?

34. 6.7 How Did the Solar System Form?
Conservation of angular momentum says that product of radius and rotation rate must be constant:
L = mvr
Lbefore = Lafter
m1 v1r1 = m2 v2r2
Think ice skaters, divers
& gymnasts

35. 6.7 How Did the Solar System Form?
Temperature in nebular cloud determines where various materials condense out:

36. Partial Summary of Chapter 6
Solar system consists of Sun and everything orbiting it
Asteroids are rocky, and most orbit between orbits of Mars and Jupiter
Comets are icy, and are believed to have formed early in the solar system’s life
Major planets orbit Sun in same sense, and all but Venus rotate in that sense as well
Planetary orbits lie almost in the same plane

37. Partial Summary of Chapter 6, cont.
Four inner planets – terrestrial planets – are rocky, small, and dense
Four outer planets – Jovian planets – (omitting Pluto) are gaseous and large
Nebular theory of solar system formation: cloud of gas and dust gradually collapsed under its own gravity, spinning faster as it shrank
Condensation theory says dust grains acted as condensation nuclei, beginning formation of larger objects