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Lecture 1
Introduction
The Solar System - Basic Facts
- The Sun contains 99.9% of the mass.
- The Solar System is mostly empty space.
- The Solar System is a flattened disk.
- All planets revolve in the same direction
- Most planets also rotate in the same direction
- All objects have similar ages (about 4.6
billion years, when measurable).
Planetary Exploration
- Almost everything we know about planetary
physics has been learned in the last 30 years as a result
of space probes that have visited the planets.
- Only Pluto/Charon has not been visited
| Program |
Dates |
Destination |
| Mariner |
1960s |
Mercury, Venus, Mars |
| Venera |
1970s |
Venus (lander) |
| Viking |
1970s |
Mars (lander and orbiter) |
| Voyager |
1970s-1980s |
Jovian planets |
| Galileo |
now |
Jupiter (orbiter and probe) |
| Magellan |
1990s |
Venus (orbiter and probe) |
| Pathfinder |
1997 |
Mars (lander) |
| Cassini |
soon |
Saturn |
| |
|
|
| Many |
1960s - now |
The Moon |
| Many |
1960s - now |
The Earth |
Two Types of Planets
- Planets can be divided into two families
of similar bodies.
- Terrestrial planets are Earth-like.
- Jovian planets are Jupiter-like.
- Pluto/Charon does not fit in this scheme, and we
will have to treat this system separately.
- Terrestrial planets
- low mass (£
1 MÅ
)
- high density (rocky, metallic)
- slow rotators (P ³ 24 hours)
- few satellites
- close to Sun (a £
1.6 AU)
- Jovian planets
- high mass (³
15 MÅ
)
- low density (gaseous)
- rapid rotators (P £ 18 hours)
- many satellites
- far from Sun (a ³
5 AU)
Formation of the Solar System
- Any theory of formation of the Solar
System must explain all of the basic facts that we have
outlined so far.
- Nearly all orbits are nearly
co-planar - solar system is very flat
- Rotation and revolution of nearly
all major bodies are counter-clockwise
Star Formation
- Current thinking is that planets are
formed as part of the star formation process.
- star formation itself is poorly understood
- planet formation lets a forming star get rid of
enough angular momentum to collapse
- if this scenario is correct, planets should be
common throughout the Universe
- Star formation must occur in dark (dusty)
cold (T < 10 K) regions
- Gas cloud can collapse only if self-gravity
exceeds internal pressure
- Conservation of angular momentum
leads to disk formation
- Easier for collapse to take place parallel to
axis of rotation rather than perpendicular to it
- Central region collapses to form a star
Proplyds: Disk-Shaped Protostars
Four Important Processes
- Temperature Gradient:
Inner part of proto-planetary gas cloud is much
hotter than outer parts.
- Differentiation: In a
plastic (non-solid) body, denser material will settle
towards center: results in core, mantle, crust.
- Radioactivity: Source
of internal heat for terrestrial planets.
- Impacts:
Asteroids, comets, etc. hitting the surfaces of planets
Condensation Sequence
| Temperature (K) |
Condensate |
What |
| 1500 |
Metal Oxides |
Mercury |
| 1300 |
Fe (iron), Ni (nickel) |
| 1200 |
silicates |
|
| 1000 |
Aluminum oxides |
Venus |
| 680 |
FeS (iron sulfates |
| 175 |
H20 (water) |
Earth, Mars |
| 150 |
NH3 (ammonia) |
Jovian |
| 120 |
CH4 (methane) |
Pluto |
| 65 |
Ar (argon), Ne (Neon) |
Planet Formation
- As disk cools, solids condense
(snowflakes)
- inner disk is richer in less-volatile elements
- As central regions contract to form the
Sun, the "snowflakes" begin to stick together
and grow by accretion.
- Planetesimals form by
accretion (snowballs).
- "soft" collisions, since in similar
orbits (Kepler)
- planetesimals have sizes up to about 1 km
- Planetesimals coalesce to form protoplanets
Average Densities of Terrestrial
Planets
Planet
|
Density
(gm/cm3)
|
| Mercury |
5.4 |
| Venus |
5.2 |
| Earth |
5.5 |
| Mars |
3.9 |
| The Moon |
3.35 |
| Water |
1.00 |
Important Details
- In the outer disk, ices form
- larger masses can develop since more solids
- In disk, if M ³ 15 MÅ
, gravitational accretion works
- Jovian planets accrete H, He and get big
- Perturbations by biggest planets prevent
coalescence nearby
- asteroids
Important Details
- violent geological history of some planetary satellites
Miranda: Victim of a Violent
Past
- Miranda is fifth largest satellite of
Uranus, (diameter 485 km)
- Its surface shows evidence of violent
collisions late in its formation history.
Cleaning Up the Debris
- In later stages, many smaller particles
are removed from the Solar System
- Main mechanism: material is swept up
by protoplanets
- heavy bombardment phase, about 4 billion years
ago
- Cratering continues to present but at reduced
rate
- Also have gravitational "scattering"
- Icy bodies in the outer Solar System (10-30 AU)
have their orbits altered by Jovian planets
- Smaller orbits: objects evaporate
- Larger orbits: objects become comets
- Some planets develop rings (Saturn obvious one, but also
Jupiter, Uranus, Neptune)
Differentiation of Terrestrial
Planets
- Why are terrestrial planets chemically differentiated?
(heaviest elements are deep in the interior; lighter
elements are near the surface)
- Differentiation means that planets were at
one time molten.
- Even a 1 Msun lump of
rock will have a central temperature of only T =
1500 K due to pressure, not enough to melt iron.
- Radioactive decay (nuclear fission)
of uranium, thorium, potassium releases enough heat to
melt iron.
- Sinking iron releases additional heat,
precipitating an iron catastrophe.
Roche Limit
- There is a minimum separation R between
two bodies inside of which tidal forces exceed
self-gravity. This is called the Roche limit;
planetary rings are inside the Roche limit.
Planetary Magnetic Fields
- Hot, electrically charged matter wells up
in interior, and is deflected by rotation
- We expect large magnetic fields
- for larger bodies
- for faster rotators