Inner Planets (Part I)
Sept. 16, 2002
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Science
Intro to Inner Planets
Four Main Processes
Planetary Comparisons
Intro to Atmospheres
Announcements
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If you are not here today due to Yom
Kippur, there will be an opportunity to
make up today’s quiz after class on Weds.
An extra credit problem will be available
on the course web site tonight or
tomorrow morning.
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It is due Thurs. Sep 19 at 5pm
Review
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Pieces of the Solar System
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Angular momentum
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Sun
inner planets
outer planets
other stuff
angular momentum is conserved
Solar System formation
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(in a few weeks)
(this week)
(next week)
(following week)
accretion disk, rotation, protostar
planetesimals
solar wind
Fact vs. Theory
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Facts are data which has been measured
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Theory is a model which describes/explains
data or predicts future events
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e.g. the Sun will set tonight
a thrown baseball will follow an arc
because of gravity and Newton’s Laws the Earth
revolves around the Sun and will continue to do so
It is impossible to prove a theory
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e.g. the Sun rose this morning
even Newton’s Laws are a theory
It IS possible to disprove a theory
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when facts do not agree with the model
Examples
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Fact: All major (currently observed)
planets/moons/asteroids are revolving around
the Earth in the same direction and in a
similar plane
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Theory: The Solar System was created from a
revolving sphere of gas and dust
Facts: The inner planets are composed
primarily from refractory materials while the
outer planets are mostly volatile materials
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Theory: The inner region was hotter and the
volatile materials did not survive there, but did in
the outer region, this contributed to the planets’
formations
NASA Solar System Missions
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Flyby missions - satellite to pass by another object
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Orbiters - satellite in orbit around a planet or moon
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more detailed studies, but not “hands-on”
examples: Galileo, Clementine, Magellan, …
Landers - lander on the surface of a planet or moon
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quick look, but cheap
examples: Voyager, Mariner, Pioneer, …
get rock samples and direct data, limited area can be covered
examples: Viking, Mars Surveyor, Mars Odyssey, …
Manned missions - humans on the surface of a planet or
moon
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can do advanced, complicated studies/experiments, but very
expensive
examples: Apollo 11 through Apollo 17
Differentiation
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During planetary formation, the rocks and
planetesimals compress together due to
gravity
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Differentiation is the process of the heavier
materials sinking towards the center of the
planet while lighter materials rise to the
outer edges
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energy is converted into heat
material melts and becomes fluid
materials become separated by type
Outer surface of planet cools fastest and
hardens
Planet Interiors
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Layered
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Hotter inside, cooler outside
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solid inner core
liquid outer core
solid outer mantle/crust
planet radiates heat into space
outer crust cooled and hardened
center is hottest and has highest pressure
Melting point depends on temperature and
pressure
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in the center, pressure wins and material is solid
farther out, temperature wins and material is liquid
outer edge, both lose and material is solid
Planet Interiors (cont)
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The inner planets have similar structure
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although we don’t have a lot of data on other
planets
Data on Earth’s interior comes from
seismic readings of earthquakes
Four Main Processes
These processes shape the surfaces of
planets
1) Tectonism
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movement of pieces of the planet’s crust
(plates)
2) Volcanism
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flow of material (lava) from beneath the
planet’s crust
3) Impact Cratering
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meteors hitting a planet’s surface
4) Gradation
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erosion of the surface
The first 3 processes build up structure
on the surface (mountains, valleys, etc)
The last process wears the surface down
Tectonics
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Major movements of the planets crust
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create mountain ranges, deep valleys
on Earth: tectonic plates rub against each
other
other planets: not plates, but major
cracking/shifting (fractures)
Interior Heating
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Radiative cooling alone should have cooled the
Earth’s interior more than observed
Friction adds some of the heat
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tidal forces due to the gravitational pull of the Moon
and Sun cause pieces of the interior to rub together
this rubbing generates heat (just like rubbing your
hands together)
Radioactive decays add most of the heat
The interior temperature is a balance between
original heat, radiative cooling and additional
heat
As the radioactive material disappears, the
Earth’s interior will cool
Volcanism
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Fissures in the planet’s crust can allow hot mantle to
flow to the top (lava)
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Long fissures cause shield volcanoes (large, long mounds
of cooled lava) to form over long time periods
Local “holes” can form mounds
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on Earth, plate movement limits the size and can result in a
chain of islands
Large flows of more fluid lava can create great plains
of lava
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the mantle is solid, but after relieving the pressure from the
crust, it can turn liquid
e.g. Lunar mares (seas)
Amount of volcanic activity indicates how active a
planet is
Comparative Volcanism
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Moon
 mares are volcanic in nature and indicate the
Moon once had a lot of lava flow
Mercury
 some visual indications of lava flow, not
enough known
Mars
 largest mountains in the Solar System (up to
25 km high) caused by volcanism
Venus
 evidence of a lot of complex volcanic activity
Earth
 lots of current and previous volcanic activity
(Pompeii, Hawaiian Islands, Mt. St. Helen’s)
Impact Cratering
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The number of collisions between objects depends on
how many objects there are
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Craters can be erased by tectonism, volcanism and
gradation
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Early in the Solar System there were many more small
planetesimals: more collisions
Number of craters can be used to “date” a planet
occurs on “active” planets
(e.g. Earth)
on “dead” planets, craters
remain (e.g. Moon)
Formation:
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heats and compresses
material thrown outward
surface rebounds
Comparative Cratering
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Moon
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Mars
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craters with impact craters which indicate there
might have been water on Mars once
Venus
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lots of craters in all sizes
dense atmosphere protects Venus
Earth
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protected by atmosphere (many meteors burn up)
large oceans leave no impact crater
most craters erased by gradation
Moon from the Earth
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Theory: the Moon comes from the Earth
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Mars size protoplanet hit Earth early in its history
this impact showered large amounts of material
into Earth orbit
volatile materials were lost
remaining materials condensed to form the Moon
Facts which are explained:
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Moon composed of same materials as Earth (moon
rocks)
Moon has no significant volatile materials (water,
air)
Moon is large fraction of Earth’s size
Gradation
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Surface leveling
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Moon & Mercury
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large dust storms observed, evidence of water
flow
Venus
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no atmosphere, possible ice, little gradation
Mars
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caused by blowing wind, flowing water and
water/ice freezing/melting
evidence of blowing wind, no evidence of water
Earth
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all processes present
e.g. dust/wind storms, rain, tides, glacier flow
Magnetic field
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Inner planets all have some magnetic field
This magnetic field is not caused only by
magnetized materials
At least partially caused by rotation of Earth
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spinning electric charges in core create magnetic
field
Facts:
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Earth has a strong magnetic field
Earth’s magnetic field moves with time (magnetic
north pole not the same as celestial north pole)
the Moon has no or very small magnetic field
Mercury has strong magnetic field
Venus and Mars have small magnetic field
Gases – Some Basics
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Lighter gases rise
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Temperature of a gas is really the speed of
the molecules
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Faster gases are hotter
Sunlight and heat from a planet’s interior
provide energy to heat atmospheres
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This is really because heavier gases sink and push
the lighter gases upward
Sunlight can also break up molecules
Fast atoms/molecules in the outer atmosphere
can escape the planet’s gravitational pull
Planets have a hard time hanging onto hydrogen
and helium
Primary Atmosphere
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A planet’s original atmosphere comes from the
gas of the accretion disk
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It is composed mainly of hydrogen and helium
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If a planet’s gravity isn’t strong enough, it
can’t hold onto these light gases
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same stuff the Sun is made of
They escape and leave the planet without an
atmosphere
Heating and solar wind help these processes
This happened to the inner planets
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We will see later it did not happen to the gas giants