Download Overview of the Solar System

Overview of the Solar System
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Outline
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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
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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!
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Contents of the Solar System
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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
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Key questions
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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?
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Solar System objects to scale
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The Solar System is BIG!
• It is difficult to make a correctly scaled model
• Most of the Solar System is (nearly) empty space
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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
A small marble
Earth's Moon
43 cm
3.9 mm
A pebble
Mars
254 m
7.6 mm
A pea
Jupiter
868 m
15.9 cm
A softball
Saturn
1.6 km
13.4 cm
A softball
Uranus
3.2 km
5.7 cm
A hen's egg
Neptune
5 km
5.5 cm
A hen's egg
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The Sun
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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
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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
• All spin in same sense as
orbit, except Venus and
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Key concept: Density
 = mass/volume. Depends on composition,
compression by gravity.
Can compare to density of water:
water = 1000 kg/m3 = 1 g/cm3
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Diameter Mass

(Earth=1) (Earth=1) (g/cm3)
-----------------------------------------------------------Mercury
0.383
0.055
5.4
Venus
0.949
0.815
5.2
(“Terrestrial”
Earth
1.000
1.000
5.5 Planets)
Mars
0.533
0.107
3.9
Jupiter
11.21
317.8
1.3
Saturn
9.45
95.2
0.7 (“Jovian”
Planets)
Uranus
4.01
14.5
1.3
Neptune
3.88
17.2
1.6
Planet
Inner four are rocky, outer four are mostly gas and liquid. Must give
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clue to formation.
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.
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Visible light from planets, moons, comets, etc. is dominated by
reflected sunlight. In IR, might see emitted blackbody radiation.
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For context, consider overall abundances of elements in our part of
the Milky Way
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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
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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).
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Planetary Atmospheres
• Why do some planets have atmospheres,
and others don't?
• Why do they have different atmospheric
compositions?
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Recall
3kT
V
m
V is the typical speed of particles
of mass m in a gas of
temperature T
Now recall escape speed from a planet
2GM
Vesc 
r
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
> 6V
Vesc
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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.
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Guess which planets have atmospheres:
Planet
Vesc (km/s)
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Mercury
Venus
Earth
(Moon)
Mars
Jupiter
Saturn
Uranus
Neptune
4.3
10.4
11.2
2.4
5.0
59.5
35.5
21.3
23.5
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Two Kinds of Planets
"Terrestrial"
"Jovian"
Mercury, Venus,
Earth, Mars
Jupiter, Saturn,
Uranus,
Neptune
Close to the Sun
Small (D=5000-13000 km)
Mostly Rocky
High Density (3.9 -5.5 g/cm3)
Slow Rotation (1 - 243 days)
Far from the Sun
Large (D=50,000-143,000 km)
Mostly Gas and Liquid
Low Density (0.7 -1.6 g/cm3)
Fast Rotation (0.41 - 0.72 days)
Few Moons
No Rings
Main Elements Fe, Si, Mg, O
Many Moons
Rings
Main Elements H, He
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Also in the Solar System
• Seven giant moons: size ~ size of Mercury
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• Many smaller moons
• Comets
• Asteroids (most orbit between
Mars and Jupiter)
• Meteoroids
• “Trans-Neptunian” objects
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 30-50 AU from Sun containing
most of the TNOs
• All debris left over from planet making process
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Trans-Neptunian Objects
Orbit of Eris
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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.
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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
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Olympus Mons on Mars – largest volcano in Solar System. Volcanic
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flows can fill impact craters. Its crater is a caldera.
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
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• 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
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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
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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')?
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