1. Overview of the Solar System

2. Outline General properties of planets and Solar System
Spectroscopy, composition, density of planets
Why do some have atmospheres?
Terrestrial/Jovian planet distinction
Clues to interiors and history
NEAR Shoemaker (2001). Japanese too, on Itawaka, in 2005, returned in 2010, but not yet known whether sample collection worked properly.

3. You live in a special age
Landings on Moon, Venus, Mars, Titan, an asteroid and a comet
Returned rocks from the Moon
Atmospheres probed on Venus, Mars, Jupiter, Titan
Fly-bys past all planets and Pluto
Venus and Titan surfaces revealed by radar mapping
All of this in past ~45 years - and more to come!
NEAR Shoemaker (2001). Japanese too, on Itawaka, in 2005, returned in 2010, but not yet known whether sample collection worked properly.

4. Contents of the Solar System
Sun in center, contains most of the mass
Planets
Moons
Rings
Asteroids – mostly between Mars and Jupiter. Mostly rocky material
Comets – High eccentricity orbits. Icy material
“Trans-Neptunian Objects”
Meteoroids
Gas and dust

5. Key questions How did the Solar System and planets form?
How are planets similar to and different from Earth?
What are the planets made of and how do we know?
What range of properties do their moons show?
What is the origin and fate of rings?
What can we learn from asteroids and comets?

6. Solar System objects to scale

7. The Solar System is BIG!
It is difficult to make a correctly scaled model
Most of the Solar System is (nearly) empty space

8. If the Solar System were 10 km across…
Object
Distance
Diameter
Size example
The Sun
0 km
1.55 m
A large beach ball
Mercury
65 m
5.4 mm
A pebble
Venus
121 m
1.35 cm
A small marble
Earth
167 m
1.42 cm
Earth's Moon
43 cm
3.9 mm
Mars
254 m
7.6 mm
A pea
Jupiter
868 m
15.9 cm
A softball
Saturn
1.6 km
13.4 cm
Uranus
3.2 km
5.7 cm
A hen's egg
Neptune
5 km
5.5 cm

9. The Sun Average-sized star (mostly H, He)
99.8% of the mass of the Solar System
~4.6 Gyr old (middle-age)
Surface (photosphere) about 5800 K (emits mostly in visible, UV, IR)
Hot because of nuclear fusion in core
Builds He nuclei from H nuclei, a process that releases energy

10. Planetary orbits
All planets orbit the Sun in same direction and almost same plane
Orbits are close to circular
Main exception is Mercury:
orbital tilt 7o
eccentricity 0.21
Remember e related to b and a
All spin in same sense as orbit, except Venus and Uranus

11. Key concept: Density
= mass/volume. Depends on composition, compression by gravity.
Can compare to density of water:
water = 1000 kg/m3 = 1 g/cm3

12. (Earth=1) (Earth=1) (g/cm3)
Planet Diameter Mass 
(Earth=1) (Earth=1) (g/cm3)
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
(“Terrestrial”
Planets)
(“Jovian”
Planets)
Rocks at surface have density 3. So interiors must contain denser material.
Inner four are rocky, outer four are mostly gas and liquid. Must give
clue to formation.

13. How do we know what planets, moons, etc., are made of?
Chemical composition - determined by spectroscopic observations or sometimes direct chemical analysis.
The spectrum of a planet with a thick atmosphere reveals the atmosphere’s composition
If there is no atmosphere, the spectrum indicates the composition of the surface.
To a large extent, we must infer what the interiors are made of.
direct analysis – Venus, Moon, Mars (also asteroid, comet tail, interplanetary space).

14. Visible light from planets, moons, comets, etc. is dominated by
Can learn much from reflected spectrum, but must understand what features are due to Sun’s incident spectrum and Earth’s atmosphere. Compare with lab spectra. Wien’s law (for 10’s-100’s K, where is peak?)
Visible light from planets, moons, comets, etc. is dominated by
reflected sunlight. In IR, might see emitted blackbody radiation.

15. near IR spectrum

16. For context, consider overall abundances of elements in our part of the Milky Way

17. Where in the Solar System is this kind of abundance found?
Sun, Jupiter, Saturn, Uranus, Neptune (except planet cores)
Where is it NOT found?
Mercury, Venus, Earth, Mars, Moons, comets, asteroids

18. H and He abundant on the Jovian planets
H and He abundant on the Jovian planets. The terrestrial planets are composed mostly of heavier elements (e.g. Fe, Si, Mg, O, Ca, Al).
Can see solid surface of Mars. Color due to iron.

19. Planetary Atmospheres
Why do some planets have atmospheres, and others don't?
Why do they have different atmospheric compositions?

20. Now recall escape speed from a planet
V is the typical speed of particles of mass m in a gas of temperature T
Now recall escape speed from a planet
Particles colliding all the time. Some gain energy, some lose, so speeds change.
If a particle is moving too fast, the planet can't retain it
Rule of thumb: a gas will be retained in the atmosphere if Vesc > 6V

21. Example: For room temperature (293 K)
VH2 = 1.9 km/s, and 6 times 1.9 km/s = 11.4 km/s
VO2 = 0.5 km/s, and 6 times 0.5 km/s = 3 km/s
The escape speed from surface of Earth is 11.2 km/s.
 Molecular oxygen is easily retained, but hydrogen is not.

22. Guess which planets have atmospheres:
Planet Vesc (km/s)
__________________________________
Mercury 4.3
Venus 10.4
Earth 11.2
(Moon) 2.4
Mars 5.0
Jupiter 59.5
Saturn 35.5
Uranus 21.3
Neptune 23.5

23. Two Kinds of Planets "Terrestrial" Mercury, Venus, Earth, Mars
"Jovian"
Jupiter, Saturn, Uranus, Neptune
Close to the Sun
Small (D= km)
Far from the Sun
Large (D=50, ,000 km)
Mostly Rocky
High Density ( g/cm3)
Mostly Gas and Liquid
Low Density ( g/cm3)
Venus, Uranus retrograde
Slow Rotation ( days)
Fast Rotation ( days)
Few Moons
No Rings
Main Elements Fe, Si, Mg, O
Many Moons
Rings
Main Elements H, He

24. Also in the Solar System
Seven giant moons: size ~ size of Mercury
Many smaller moons
Comets
Asteroids (most orbit between Mars and Jupiter)
Meteoroids
“Trans-Neptunian” objects
Some moons were made in place, others captured asteroids. Asteroids in ecliptic, between Mars and Jupiter. Comets are generally not in the ecliptic

25. Chunks of rock and ice
Asteroids: small, rocky objects, most orbiting between Mars and Jupiter
Comets: small, dirty ice balls whose orbits bring them into inner Solar System
Trans-Neptunian Objects – icy bodies beyond Neptune’s orbit, including Pluto and Eris
Kuiper belt – zone AU from Sun containing most of the TNOs
All debris left over from planet making process

26. Trans-Neptunian Objects
Orbit of Eris

27. More definitions!
A planet is a spherical object orbiting a star that is not a
star itself, and has swept out its path
A dwarf planet is a spherical object orbiting a star that has not
swept out its path (Pluto, Eris, Ceres, a few other TNOs), and is
not a satellite. Note Pluto and Eris are also TNOs, and Ceres
is an asteroid.

28. Cratering on terrestrial planets
Result of impacts from interplanetary debris (but
some are volcanic)
Geologic activity =>
Many craters means old surface and low geological activity
Smaller objects lose heat faster: more cratered
Complications: external drivers of heat; water and wind erosion; atmospheric burn-up; gravitational attraction of impactors

29. Olympus Mons on Mars – largest volcano in Solar System. Volcanic
flows can fill impact craters. Its crater is a caldera.

30. Magnetic Fields – a direct indication of interior
The presence of a global, regular, magnetic field indicates a liquid, conducting interior
Need circulating currents to generate magnetic field, like in an electromagnet

31. The global magnetic field of the Earth is produced by metals, mainly iron, in the liquid state
The stronger fields of the Jovian planets are generated by liquid metallic hydrogen or by water with ionized molecules dissolved in it
Liquid metallic H: high pressures, electrons can move from one H atom to another. Venus, no field. Mars and Moon, weak fields in rocks at surface. Indicates they were earlier in molten state, and only field left is in rocks that were permanently magnetized when they solidified. B field period = rot rate of interior

33. Solar system formation
All objects formed from the same cloud of gas and dust
Composition determined by cosmic history
Different objects formed in different environments depending on their distance to the Sun

34. Problem 6.36
The four largest moons of Jupiter are roughly the same size as our Moon and are about 628 million km from Earth at opposition.
What is the size in km of the smallest surface feature that the HST can detect (resolution 0.1")?
How does this compare with the smallest feature that can be seen on the Moon with the unaided human eye (resolution 1')?
300 km ( (0.1”/206265)x6.28e8), 110 km ( (60”/206265)x384,400).

35. Copernicus crater on the Moon. Ejecta extends to 800 km away