Planets, Moons and Other Bodies
• Solar System
The Solar System
Chapter 15
Homework: All the multiple choice questions
in “Applying the Concepts” and Group A
questions in “Parallel Exercises”.
Mercury
• Innermost planet
• Highly elliptical orbit
• Average distance ~ 0.4
AU
• Orbital period ~ 3 months
• Rotational period ~ 59
days
• Visible shortly after
sunset or before sunrise
• Highly cratered; no
atmosphere
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• Terrestrial planets
Sun
9 planets
~100 moons
Thousands of asteroids,
millions of icy bodies,
comets, …
– Mercury, Venus, Earth and
Mars
– Mostly rocky materials,
metallic nickel and iron
• Giant planets
– Jupiter, Saturn, Uranus
and Neptune
– Mostly hydrogen, helium
and methane
• Astronomical unit (AU)
– Average Earth-Sun
distance
– 1.5x108 km
• Pluto
– In a class by itself
– True planet?
• Planet classification:
size, density and
atmosphere
Venus
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Orbital distance ~ 0.7 AU
Morning and evening “star”
Exhibits phases, like the Moon
Rotational motion opposite
orbital motion
Venusian “day” longer than
Venusian “year”
Visited by numerous probes
Mostly CO2 atmosphere, high
temperature and pressure
Surface mostly flat but varied
Mars
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Orbital distance ~ 1.5 AU
Geologically active regions
1.
2.
3.
4.
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Jupiter
• ~ 5 AU from Sun
• Most massive planet
Inactive volcanoes
Canyons
Terraced plateaus near poles
Flat regions pitted with craters
– 318 times Earth’s mass
• Mostly H and He with
iron-silicate core
• Dynamic atmosphere
Thin atmosphere, mostly CO2
Strong evidence for liquid water
in past
Numerous space probes
– H2, He, ammonia,
methane, water, …
– Great Red Spot
• 39 widely varying
satellites
Saturn
• 9.5 AU from Sun
• Rings of particles
• Density = 0.7 that of
water
• Surface similar to
Jupiter’s
• 30 satellites
– Titan: only moon with
substantial atmosphere
Uranus, Neptune and Pluto
• Uranus (~19 AU) and
Neptune: (~30 AU)
– Outermost giant planets
– Similar internal structures
• Pluto:
– Smaller than the Moon
– 70% rock; 30% water ice;
tenuous, thin atmosphere
– Unusual orbit
• Tilted 17o from ecliptic
• Crosses Neptune’s
Planetary System Summary
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Smaller bodies include comets, asteroids, meteorites
Leftover from solar and planetary formation that may bombard larger
objects
– Comet Shoemaker-Levy 9 fragments (bottom)…
– … and strikes Jupiter (July 1994)
Comet Origins
• Oort cloud
– Origin of long-period
comets (>200 years)
– 30 AU to light-year away
• Kuiper belt
– Origin of short-period
comets (<200 years)
– Disk-shaped region 30-100
AU from Sun
• Gravitational nudges
deflect objects toward
Sun
Smaller Bodies of the Solar System
• Comets, asteroids,
meteorites
• Leftover from solar and
planetary formation
• Mass of smaller bodies may
be 2/3 of total Solar System
mass
• Bombard larger objects
– Comet Shoemaker-Levy 9
fragments (bottom)…
– … and strikes Jupiter (July
1994)
Comet Structure
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•
Small, solid objects
“Dirty snowball” model
– Frozen water, CO2,
ammonia, and methane
– Dusty and rocky bits
•
Comet head
– Solid nucleus and coma
of gas
•
Two types of tails
1. Ionized gases
2. Dust
•
Tail points away from
Sun
Asteroids
• Located in belt between
Mars and Jupiter
• Sizes: up to 1,000 km
• Varied composition
– Inner belt: stony
– Outer belt: dark with
carbon
– Others: iron and nickel
• Formed from original
solar nebula
• Prevented from clumping
by Jupiter nearby
Origin of the Solar System
Protoplanet nebular
model
• Stage A
– Formation of heavy
elements in many
earlier stars and
supernovas
– Concentration in one
region of space as dust,
gas and chemical
compounds
Meteors and Meteorites
• Meteoroids
– Remnants of comets and
asteroids
• Meteor
– Meteoroid encountering
Earth’s atmosphere
– Meteor showers: Earth
passing through comet’s
tail
• Meteorite
– Meteoroid surviving to
strike Earth’s surface
– Iron, stony (chondrites and
achondrites) or stony-iron
Origin of the Solar System
• Stage B
– Formation of large, rotating
nebula
– Gravitational contraction,
spin rate increases
– Most mass concentrates in
central protostar
– Remaining material forms
accretion disk
– Material in accretion disk
begins clumping
Origin of the Solar System
• Stage C
– Protosun becomes a star
– Solar ignition flare-up may
have blown away hydrogen
and helium atmospheres of
inner planets
– Protoplanets heated,
separating heavy and light
minerals
– Larger bodies cooled
slower, with heavy
materials settling over
longer times into central
cores