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

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

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)

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

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

6.2 Planetary Properties 63 60 13

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

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)

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

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

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 

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

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

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

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

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

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

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

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

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:

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

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

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

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?

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

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

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

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