Inner Planets (Part I)

1
Inner Planets (Part I)
Sept. 16, 2002

1. Science
2. Intro to Inner Planets
3. Four Main Processes
4. Planetary Comparisons
5. Intro to Atmospheres

2
Announcements

• If you are not here today due to Yom Kippur,
  there will be an opportunity to make up todays
  quiz after class on Weds.
• An extra credit problem will be available on the
  course web site tonight or tomorrow morning.
• It is due Thurs. Sep 19 at 5pm

3
Review

• Pieces of the Solar System
• Sun (in a few weeks)
• inner planets (this week)
• outer planets (next week)
• other stuff (following week)
• Angular momentum
• angular momentum is conserved
• Solar System formation
• accretion disk, rotation, protostar
• planetesimals
• solar wind

4
Fact vs. Theory

• Facts are data which has been measured
• e.g. the Sun rose this morning
• Theory is a model which describes/explains data
  or predicts future events
• e.g. the Sun will set tonight
• a thrown baseball will follow an arc
• because of gravity and Newtons Laws the Earth
  revolves around the Sun and will continue to do
  so
• It is impossible to prove a theory
• even Newtons Laws are a theory
• It IS possible to disprove a theory
• when facts do not agree with the model

5
Examples

• Fact All major (currently observed)
  planets/moons/asteroids are revolving around the
  Earth in the same direction and in a similar
  plane
• 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
• 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

6
NASA Solar System Missions

• Flyby missions - satellite to pass by another
  object
• quick look, but cheap
• examples Voyager, Mariner, Pioneer,
• Orbiters - satellite in orbit around a planet or
  moon
• more detailed studies, but not hands-on
• examples Galileo, Clementine, Magellan,
• Landers - lander on the surface of a planet or
  moon
• 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
• can do advanced, complicated studies/experiments,
  but very expensive
• examples Apollo 11 through Apollo 17

7
Differentiation

• During planetary formation, the rocks and
  planetesimals compress together due to gravity
• energy is converted into heat
• material melts and becomes fluid
• Differentiation is the process of the heavier
  materials sinking towards the center of the
  planet while lighter materials rise to the outer
  edges
• materials become separated by type
• Outer surface of planet cools fastest and hardens

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Planet Interiors

• Layered
• solid inner core
• liquid outer core
• solid outer mantle/crust
• Hotter inside, cooler outside
• planet radiates heat into space
• outer crust cooled and hardened
• center is hottest and has highest pressure
• Melting point depends on temperature and pressure
• 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

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Planet Interiors (cont)

• The inner planets have similar structure
• although we dont have a lot of data on other
  planets
• Data on Earths interior comes from seismic
  readings of earthquakes

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Four Main Processes

• These processes shape the surfaces of planets
• Tectonism
• movement of pieces of the planets crust (plates)
• Volcanism
• flow of material (lava) from beneath the planets
  crust
• Impact Cratering
• meteors hitting a planets surface
• Gradation
• erosion of the surface
• The first 3 processes build up structure on the
  surface (mountains, valleys, etc)
• The last process wears the surface down

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Tectonics

• Major movements of the planets crust
• create mountain ranges, deep valleys
• on Earth tectonic plates rub against each other
• other planets not plates, but major
  cracking/shifting (fractures)

12
Interior Heating

• Radiative cooling alone should have cooled the
  Earths interior more than observed
• Friction adds some of the heat
• 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
  Earths interior will cool

13
Volcanism

• Fissures in the planets crust can allow hot
  mantle to flow to the top (lava)
• the mantle is solid, but after relieving the
  pressure from the crust, it can turn liquid
• Long fissures cause shield volcanoes (large, long
  mounds of cooled lava) to form over long time
  periods
• Local holes can form mounds
• 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
• e.g. Lunar mares (seas)
• Amount of volcanic activity indicates how active
  a planet is

14
Comparative Volcanism

• 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. Helens)

15
Impact Cratering

• The number of collisions between objects depends
  on how many objects there are
• Early in the Solar System there were many more
  small planetesimals more collisions
• Number of craters can be used to date a planet
• Craters can be erased by tectonism, volcanism and
  gradation
• occurs on active planets
• (e.g. Earth)
• on dead planets, craters
• remain (e.g. Moon)
• Formation
• heats and compresses
• material thrown outward
• surface rebounds

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Comparative Cratering

• Moon
• lots of craters in all sizes
• Mars
• craters with impact craters which indicate there
  might have been water on Mars once
• Venus
• dense atmosphere protects Venus
• Earth
• protected by atmosphere (many meteors burn up)
• large oceans leave no impact crater
• most craters erased by gradation

17
Moon from the Earth

• Theory the Moon comes from the Earth
• 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
• Moon composed of same materials as Earth (moon
  rocks)
• Moon has no significant volatile materials
  (water, air)
• Moon is large fraction of Earths size

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Gradation

• Surface leveling
• caused by blowing wind, flowing water and
  water/ice freezing/melting
• Moon Mercury
• no atmosphere, possible ice, little gradation
• Mars
• large dust storms observed, evidence of water
  flow
• Venus
• evidence of blowing wind, no evidence of water
• Earth
• all processes present
• e.g. dust/wind storms, rain, tides, glacier flow

19
Magnetic field

• 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
• spinning electric charges in core create magnetic
  field
• Facts
• Earth has a strong magnetic field
• Earths 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

20
Gases Some Basics

• Lighter gases rise
• This is really because heavier gases sink and
  push the lighter gases upward
• Temperature of a gas is really the speed of the
  molecules
• Faster gases are hotter
• Sunlight and heat from a planets interior
  provide energy to heat atmospheres
• Sunlight can also break up molecules
• Fast atoms/molecules in the outer atmosphere can
  escape the planets gravitational pull
• Planets have a hard time hanging onto hydrogen
  and helium

21
Primary Atmosphere

• A planets original atmosphere comes from the gas
  of the accretion disk
• It is composed mainly of hydrogen and helium
• same stuff the Sun is made of
• If a planets gravity isnt strong enough, it
  cant hold onto these light gases
• They escape and leave the planet without an
  atmosphere
• Heating and solar wind help these processes
• This happened to the inner planets
• We will see later it did not happen to the gas
  giants