Download lecture9 Solar System1

Introduction to the
Solar System
Chapter 6
The Solar System
Ingredients?
The Solar System
Ingredients?
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1 Star: the Sun
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8 Planets + a few “minor planets”
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126 moons around these planets
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Asteroids, meteoroids, comets
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A lot of nearly empty space
Questions
 What percentage of the total mass of the solar system
does the Sun contribute?
 How is the solar system laid out in space? Spacing
between planets? Orbital directions?
Mass in Solar System
Sun
Jupiter
99.8%
0.1%
Comets
0.05%
All Other Planets
0.04%
Earth
0.0003%
Sun, Planets and Moon to scale
Sun accounts for 99.9% of solar system mass!
Solar System Temperatures
Planet
Distance
Temperature
(top of atmosphere)
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
0.38 AU
0.72 AU
1.00 AU
1.52 AU
5.20 AU
9.54 AU
19.22 AU
30.06 AU
39.5 AU
450 K
330 K
280 K
230 K
120 K
90 K
60 K
50 K
40 K
350 F
45 F
-390 F
Comparative Planetology
 Categorize planets by properties
 Compare similarities and differences
 Ask: What physical processes can explain these properties?
6.2 Planetary Properties
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60
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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)
Planet Orbits
Planet Orbits
 Orbits aligned in same plane (the ecliptic)
 Explains why planets always found in Zodiac
 Pluto’s orbit tipped the most (17 degrees)
 All planets orbit Sun counter-clockwise
 Planets rotate counter-clockwise
 except Venus
 Rotation axis roughly perpendicular to orbit
 except Uranus and Pluto
The Terrestrial Planets
Terrestrial Planets
 Terrestrial = Earth-like
 Mercury
 Venus
 Earth (and Moon)
 Mars
 Small, low mass
 No large moons (except Earth)
 Mars has two small ones…
 Close to Sun
Terrestrial Planets
 Rocky Surface
 High density (3-5 gm/cm3)
(water = 1 gm/cm3)
 Geologic Activity (volcanoes, continental drift)
 Present on larger planets (Earth and Venus)
 Absent on smaller planets (Moon, Mercury, and Mars)
 Atmosphere
 Little hydrogen and helium
 Mostly carbon dioxide (Venus and Mars)
or nitrogen (Earth)
 Smaller planets have no atmosphere (Mercury, Moon)
Origin of Pluto
Large member of a class of objects in the outer reaches of the
Solar System:
The Kuiper Belt Objects
100's found since 1992.
Orbits tend to be more tilted, like Pluto's.
Leftover planetesimals from Solar
System formation?
Asteroids
Mars
The Asteroid Belt
Asteroid Belt
Perhaps a planet was going to form there. But Jupiter's
strong gravity disrupted the planetesimals' orbits, ejecting
them out of Solar System. The Belt is the few left behind.
And Finally . . .
Remaining gas swept out by intense period of solar
wind activity.
The Jovian Planets
Jovian Planets
 Jovian = Jupiter-like
 Jupiter
 Saturn
 Uranus
 Neptune
 Large, massive
 Many moons
 Far from Sun
Jovian Planets
 Low density (1 gm/cm3)
 No obvious surface
 Atmosphere
 Mostly hydrogen and helium
 Other gases (methane, ammonia)
 may form ices
The Outer Solar System
Comets
Kuiper Belt and Oort Cloud
Let’s consider a scale model
of the Solar System!
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
The image at right shows a
picture of the Sun. The dark
spots located on this image
are sunspots. How does the
size of Earth compare to the
size of the sunspot that is
identified on the right side of
the image of Sun?
A) Earth and the sunspot
are about the same size.
B) The sunspot is much
larger than Earth.
C) The sunspot is much
smaller than Earth.
Sunspot
If you were constructing a scale model of the solar
system that used a Sun that was the size of a basketball
(approximately 12 inches in diameter), which of the
following lengths would most closely approximate the
scaled distance between Earth and the Sun?
A) 3 feet (length of an outstretched arm)
B) 10 feet (height of a basketball goal)
C) 100 feet (height of an 10 story building)
D) 300 feet (length of a football field)
Questions
 What are some of the smaller objects (or debris) found
in the solar system?
 What information do they contain that the planets and
moons do not?
 (Hint: What effects do erosion, geological activity,
vulcanism, etc. have on a planet?)
Questions
 What are some of the smaller objects (or debris) found
in the solar system?
 Comets, asteroids, meteoroids
 What information do they contain that the planets and
moons do not?
 Solar system debris is unevolved => gives direct evidence
of conditions during solar system formation!
Solar System Debris
Comets
Comet Halley (1986)
Short Period Comets
Comet Hale-Bopp (1997)
Long Period Comets
50-200 year orbits
Few times 105 or 106 year orbits
Orbits prograde, close to plane of
Solar System
Orbits have random orientations
and large ellipticities
Originate in Kuiper Belt
Originate in Oort Cloud
Oort Cloud is a huge, roughly spherical reservoir of comets
surrounding the Solar System. ~108 objects?
A passing star may redirect Oort cloud objects, creating long
period comets.
Kuiper Belt object can be redirected by Neptune, creating a shortperiod comet.
Question
 What causes the tail of a comet?
 (Hint: The tail always points directly away from the sun.)
Comet Structure
Nucleus: ~10 km ball of ice, dust
Coma: cloud of gas and dust
around nucleus (~106 km across)
Tail: Always points away from
Sun.
Coma and tail due to gas and dust
removed from nucleus by the
Solar Wind.
Far from Sun, comet is a nucleus
only.
Comet Trajectory
Meteor Showers
Comets break up when near Sun
- solar wind, evaporation, tidal
force.
e.g. Halley loses 10 tons/sec
when near Sun. Will be
destroyed in 40,000 years.
Debris spreads out along comet
orbit.
Intersection of orbits => meteor
shower
How did the Solar System Form?
What must be explained?
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Solar system is very flat.
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Planetary orbits are nearly circular.
Almost all moons and planets (and Sun)
rotate and revolve in the same direction.
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Planets are isolated in space.
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Terrestrial - Jovian planet distinction.
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Leftover junk (comets and asteroids).
Solar Nebula
 Start with rotating cloud of gas and
dust
 Collapses because of gravity
 spins faster
 flattens into disk-shape
 gets hotter
 Sun forms in center
 Temperature decreases outward
 As nebula cools, gas condenses
 Forms solid particles (dust grains)
Nebular Theory
• Nebula: Cloud of interstellar dust and gas about a light-
year across
• Condensing cloud heats up - star forms at center
• But why is solar system flat?
 Conservation of Angular Momentum!
 Ang. Mom. = mass x rotation speed x “size”
Conservation of angular
momentum
(Demo)
So, as nebula contracted it rotated faster.
It became a flattened disk, like a pizza crust. (Centrifugal hoops
demo)
But, clumps in rotating gas tend to disperse. Need modified theory.
Solar Nebula:
98% of mass is gas
2% in dust grains
Condensation theory:
1) Dust grains act as "condensation nuclei. Also radiate heat =>
help to cool gas => faster gravitational collapse.
2) Accretion: Clumps collide and stick
3) Gravity-enhanced accretion: objects now have significant
gravity => faster growth
Forming Planets
 Dust grains stick together
 form rocks
 Grow into planetesimals
 some still survive today
 asteroids
 comets
 Larger planetesimals attract
smaller ones (gravity)
 Planetesimals accrete
 form protoplanets / planet cores
 initially cold
 Collisions become violent
 heating melts protoplanet
 differentiation occurs
Forming Jovian Planets
 Snow line
 Location beyond which ices form
 Building blocks (solids)
 both silicates and ices
 Protoplanets / planet core
 grew larger
 gravity captured hydrogen & helium
 composition similar to Sun
 gaseous accretion disk forms around planet
 Moons form in disk around planet
Evolution of the Solar System
 Collisions dominate early-on
 produces early heavy bombardment
 comets collide with terrestrial planets
 Deposit volatiles that form atmosphere
(water, carbon dioxide, etc.)
 Planets sweep up / throw out remaining planetesimals
 Ones thrown out:
 Oort cloud
 Ones that remain:
 Comets (Kuiper belt)
 Asteroids (asteroid belt)
Planetary Ejection
Planetary Evolution - Geological
 Internal heating leads to geologic activity
 volcanism, tectonics
 active worlds
 As core cools & solidifies, activity slows, eventually stops
 e.g. Moon
 Earth, Venus large enough to still be active
Planetary Evolution - Atmosphere
 Atmosphere formed by
 gases escaping from interior
 impacts of comets (volatile-rich debris)
 Fate of water depended on temperature (distance from
Sun)
 Atmospheres changed chemically over time
 Life on Earth substantially changed the atmosphere