Announcements

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Announcements
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Outline

• The structure of the solar System

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The Layout of the Solar System

• Large bodies in the Solar System have orderly
  motions
• planets orbit counterclockwise in same plane
• orbits are almost circular
• the Sun and most planets rotate counterclockwise
• most moons orbit counterclockwise

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Comparative Planetology

• Studying the similarities among and differences
  between the planets
• this includes moons, asteroids, comets
• This approach is useful for learning about
• the physical processes which shape the planets
• the origin and history of our Solar System
• the nature of planetary systems around other stars

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The Layout of the Solar System

• Planets fall into two main categories
• Terrestrial (i.e. Earth-like)
• Jovian (i.e. Jupiter-like or gaseous)

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Mars
Neptune
Terrestrial
Jovian
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The Layout of the Solar System

• Swarms of asteroids and comets populate the Solar
  System

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A Few Exceptions to the Rules

• Both Uranus Pluto are tilted on their sides.
• Venus rotates backwards (i.e. clockwise).
• Triton orbits Neptune backwards.
• Earth is the only terrestrial planet with a
  relatively large moon.

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The Sun King of the Solar System

• How does the Sun influence the planets?
• Its gravity regulates the orbits of the planets.
• Its heat is the primary factor which determines
  the temperature of the planets.
• It provides practically all of the visible light
  in the Solar System.
• High-energy particles streaming out from the Sun
  influence planetary atmospheres and magnetic
  fields.

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Terrestrial Planet Surfaces
How do they compare to one another?
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Inside the Terrestrial Worlds

• After they have formed, the molten planets
  differentiate into three zones
• core - made of metals
• mantle - made of dense rock
• crust - made of less dense rock
• Lithosphere - the rigid, outer layer
• of crust part of the mantle which
• does not deform easily

determined by composition
determined by internal heat
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Inside the Terrestrial Worlds
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Inside the Terrestrial Worlds
active geology
inactive geology
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Heating the Terrestrial Worlds

• Planetary interiors heat up through
• accretion
• differentiation
• radioactivity

Supplies all the heat at the beginning
Supplies heat throughout the planets life
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Cooling the Terrestrial Worlds

• Planets cool off through
• conduction - heat flowing on the microscopic
  level
• convection - heat flowing on the macroscopic
  level (bulk motions)
• eruptions - hot lava bursts through crust
• the larger the planet, the longer it takes to
  cool off!

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Magnetic Fields

• Electric charges moving via convection in a
  molten iron core and spinning acts like an
  electromagnet ? magnetic field
• Earth has a magnetic field
• Venus, Mars, the Moon do not
• Mercury surprisingly has a weak magnetic field ??
• To have a magnetic field a planet must have
  significant rotation (which helps circulate the
  molten core) as well as a significant metallic
  core

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Shaping Planetary Surfaces

• Major geological processes that shape planetary
  surfaces
• impact cratering excavation of surface by
  asteroids or comets striking the planet
• volcanism eruption of lava from interior
• tectonics disruption of lithosphere by internal
  stresses
• erosion wearing down by wind, water, ice

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Impact Cratering

• objects hit planet at 10 70 km/s
• solid rock is vaporized
• a crater is excavated
• matter is ejected in all directions
• craters are circular
• large craters have a central peak

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Counting Craters to find Surface Age

• The older the surface, the more craters are
  present.
• Cratering rate decreased as Solar Systems aged.

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Volcanism

• Underground, molten rock, called magma, breaks
  through cracks in the lithosphere.
• Trapped gases are released
• H2O, CO2, N2 This provides planets with an
  atmosphere!
• Viscosity of lava (typically basalt) determines
  type of volcano

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Tectonics

• convection cells in the mantle cause both
• compression in lithosphere
• mountains are produced
• extension in lithosphere
• valleys are produced
• mountains valleys appear on the surface

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Erosion

• movement of rock by ice, liquid, or gas
• valleys shaped by glaciers
• canyons carved by rivers
• sand blown by wind
• erosion not only wears down features, it also
  builds them
• sand dunes
• river deltas
• sedimentary rock

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The Moon (?)
highlands older surface more craters
mare (sea) younger surface 3 4 billion
yrs fewer craters dark basalt
heavily cratered, no atmosphere, geologically
inactive
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Mercury

• dead planet with no atmosphere
• has no maria, but small lava plains
• has fewer craters than the Moon
• evidence for ice at the N pole
• tectonic stresses
• 3 km-high cliffs, 100s km long
• formed when crust contracted
• no evidence for expansion features
• implies the entire planet shrunk!

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Mars

• mountains canyons
• Valles Marineris- evidence of past tectonics
• no tectonics today
• thin atmosphere (CO2)
• evidence for water erosion
• Olympus Mons
• the largest volcano in our Solar System
• it is located atop the Tharsis Bulge along with
  several other volcanoes

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Four images of Mars in one Martian DaySummer in
North, Winter in South

• Mars has a rotation period axis tilt almost
  identical to Earths
• this implies that Mars has seasons
• look at the ice caps (CO2 H2O)

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Where are the Martians?

• These seasonal similarities fuelled speculation
  that Mars could be habitable.
• In 1877, Schiaparelli sketched a series of lines
  on Mars which he called canali.
• In the 1890s, Percival Lowell sketched a network
  of canals.
• Lowell published speculations about a Martian
  civilization
• early 20th Century conventional wisdom held that
  Mars was inhabited
• space probes sent to Mars in 1960s, 70s, 90s have
  proved this false

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Ancient Water on Mars

• Liquid water can not exist on Mars today.
• temperatures below freezing
• air pressure too low
• Dry river channels in southern highlands
• heavily cratered terrain ( 3 billion years old)

• Some craters are eroded.
• implies rainfall
• crater lakes
• Mars was warm wet over 3 billion years ago.

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Recent Water on Mars?

• Liquid water could exist temporarily with todays
  temperatures and air pressuresin a flash flood!
• Underground water seeps out to form erosion
  gullies
• these gullies were observed on a crater wall
• at their size, sandstorms would cover them in few
  million yrs
• such floods have occurred within the last few
  million years
• Maps of the hydrogen content of the soil suggest
  frozen water within a meter of the surface

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Venus

• Has a thick, cloudy atmosphere -- you can not
  visually see the surface
• we must image the surface using radar
• smooth plains with few mountain ranges
• few craters
• many volcanoes and domes of lava (corona)
• Venus is very active with tectonics volcanism

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Volcanism on Venus

• Impact craters are evenly spread over Venusian
  surface.
• implies that the planets entire surface is the
  same age
• crater counting suggests an age of 1 billion
  years old
• Volcanism paved over the surface 1 billion
  years ago.

• Two types of volcanism are observed
• shield volcanoes
• stratovolcanoes

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Tectonics on Venus

• The corona is a tectonic feature.
• rising plume in mantle pushes crust up
• cause circular stretch marks
• Plume forces magma to the surface.
• volcanoes are found nearby

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Lack of Erosion on Venus

• No erosion features are seen on Venus. (so far)
• This means no wind, rain, or ice on the surface.
• Such a lack of weather can be explained
• the surface of Venus is very hot (430 C) too hot
  for liquid or ice to exist
• Venus rotates very slowly (P 243 days), so no
  wind is generated

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Geological Destiny
A planets fundamental properties determine its
geological fate.

• Impact cratering
• important early on
• affects all planets equally
• Volcanism Tectonics
• become dominant later on
• require internal heat
• size determines how long a planet remains hot
• Erosion
• ultimately dominant
• requires volcanism for outgassing of atmosphere

planet size determines fate
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Earth

• most active geology
• volcanoes tectonics
• ongoing plate tectonics
• moderate atmosphere
• N2 O2 H2O
• H2O exists in liquid state
• rampant erosion
• few craters
• life

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Comparing Terrestrial Atmospheres
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What is an Atmosphere?

• A layer of gas which surrounds a world is called
  an atmosphere.
• they are usually very thin compared to planet
  radius
• Pressure is created by atomic molecular
  collisions in an atmosphere.
• heating a gas in a confined space increases
  pressure
• number of collisions increase
• unit of measure 1 bar 14.7 lbs/inch2 Earths
  atmospheric pressure at sea level
• Pressure balances gravity in an atmosphere.
• This is called hydrostatic or gravitational
  equilibrium

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Effects of an Atmosphere on a Planet

• greenhouse effect
• makes the planetary surface warmer than it would
  be otherwise
• scattering and absorption of light
• absorb high-energy radiation from the Sun
• scattering of optical light brightens the daytime
  sky
• creates pressure
• can allow water to exist as a liquid (at the
  right temperature)
• creates wind and weather
• promotes erosion of the planetary surface

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The Greenhouse Effect

• Visible Sunlight passes through a planets
  atmosphere.
• Some of this light is absorbed by the planets
  surface.
• Planet re-emits this energy (heat) as infrared
  (IR) light.
• planets temperature lower than Sun
• IR light is trapped by the atmosphere.
• its return to space is slowed
• This causes the overall surface temperature to be
  higher than if there were no atmosphere at all.

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Greenhouse Gases

• Key to Greenhouse Effectgases which absorb IR
  light effectively
• water H2O
• carbon dioxide CO2
• methane CH4
• These are molecules which rotate and vibrate
  easily.
• they re-emit IR light in a random direction
• The more greenhouse gases which are present, the
  greater the amount of surface warming.

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Planetary Energy Balance

• Solar energy received by a planet must balance
  the energy it returns to space
• planet can either reflect or emit the energy as
  radiation
• this is necessary for the planet to have a stable
  temperature

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What Determines a Planets Surface Temperature?

• Greenhouse Effect cannot change incoming
  Sunlight, so it cannot change the total energy
  returned to space.
• it increases the energy (heat) in lower
  atmosphere
• it works like a blanket
• In the absence of the Greenhouse Effect, what
  would determine a planets surface temperature?
• the planet's distance from the Sun
• the planets overall reflectivity
• the higher the albedo, the less light absorbed,
  planet cooler
• Earths average temperature would be 17º C (1º
  F) without the Greenhouse Effect

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What Determines a Planets Surface Temperature?
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Greenhouse Effect on the Planets

• Greenhouse Effect warms Venus, Earth, Mars
• on Venus it is very strong
• on Earth it is moderate
• on Mars it is weak
• avg. temp. on Venus Earth would be freezing
  without it

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Structure of Earths Atmosphere

• pressure density of atmosphere decrease with
  altitude
• temperature varies back and forth with altitude
  
• these temperature variations define the major
  atmospheric layers

• exosphere
• low density fades into space
• thermosphere
• temp begins to rise at the top
• stratosphere
• rise and fall of temp
• troposphere
• layer closest to surface
• temp drops with altitude

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Atmospheres Interact with Light

• X rays
• ionize atoms molecules
• dissociate molecules
• absorbed by almost all gases
• Ultraviolet (UV)
• dissociate some molecules
• absorbed well by O3 H2O
• Visible (V)
• passes right through gases
• some photons are scattered
• Infrared (IR)
• absorbed by greenhouse gases

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Reasons for Atmospheric Structure

• Light interactions are responsible for the
  structure we see.
• Troposphere
• absorbs IR photons from the surface
• temperature drops with altitude
• hot air rises and high gas density causes storms
  (convection)
• Stratosphere
• lies above the greenhouse gases (no IR
  absorption)
• absorbs heat via Solar UV photons which
  dissociate ozone (O3)
• UV penetrates only top layer hotter air is above
  colder air
• no convection or weather the atmosphere is
  stratified
• Thermosphere
• absorbs heat via Solar X-rays which ionizes all
  gases
• contains ionosphere, which reflects back human
  radio signals
• Exosphere
• hottest layer gas extremely rarified provides
  noticeable drag on satellites

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Structure of Terrestrial Planet Atmospheres

• Mars, Venus, Earth all
• have warm tropospheres (and greenhouse gases)
• have warm thermospheres which absorb Solar X rays
• Only Earth has
• a warm stratosphere
• an UV-absorbing gas (O3)
• All three planets have warmer surface temps due
  to greenhouse effect

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Global Wind Patterns

• air heated more at equator
• warm air rises at equator heads for poles
• cold air moves towards equator along the surface
• two circulation cells are created in each
  hemisphere

• cells do not go directly from pole to equator
  air circulation is diverted by
• Coriolis effect
• moving objects veer right on a surface rotating
  counterclockwise
• moving objects veer left on a surface rotating
  clockwise

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Global Wind Patterns

• On Earth, the Coriolis effect breaks each
  circulation cell into three separate cells
• winds move either W to E or E to W

• Coriolis effect not strong on Mars Venus
• Mars is too small
• Venus rotates too slowly
• In thick Venusian atmosphere, the pole-to-equator
  circulation cells distribute heat efficiently
• surface temperature is uniform all over the
  planet

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Four Major Factors which affect Long-term Climate
Change
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Gain Processes of Atmospheric Gas

• Unlike the Jovian planets, the terrestrials were
  too small to capture significant gas from the
  Solar nebula.
• What gas they did capture was H He, and it
  escaped
• Present-day atmospheres must have formed at a
  later time

• Sources of atmospheric gas
• outgassing release of gas trapped in interior
  rock by volcanism
• evaporation/sublimation surface liquids or ices
  turn to gas when heated
• bombardment micrometeorites, Solar wind
  particles, or high-energy photons blast
  atoms/molecules out of surface rock
• occurs only if the planet has no substantial
  atmosphere already

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Loss Processes of Atmospheric Gas

• Ways to lose atmospheric gas
• condensation gas turns into liquids or ices on
  the surface when cooled
• chemical reactions gas is bound into surface
  rocks or liquids
• stripping gas is knocked out of the upper
  atmosphere by Solar wind particles
• impacts a comet/asteroid collision with a
  planet can blast atmospheric gas into space
• thermal escape lightweight gas molecules are
  lost to space when they achieve escape velocity

gas is lost forever!
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Origin of the Terrestrial Atmospheres

• Venus, Earth, Mars received their atmospheres
  through outgassing.
• most common gases H2O, CO2, N2, H2S, SO2
• Chemical reactions caused CO2 on Earth to
  dissolve in oceans and go into carbonate rocks
  (like limestone.)
• this occurred because H2O could exist in liquid
  state
• N2 was left as the dominant gas O2 was exhaled
  by plant life
• as the dominant gas on Venus, CO2 caused strong
  greenhouse effect
• Mars lost much of its atmosphere through impacts
• less massive planet, lower escape velocity

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Origin of the Terrestrial Atmospheres

• Lack of magnetospheres on Venus Mars made
  stripping by the Solar wind significant.
• further loss of atmosphere on Mars
• dissociation of H2O, H2 thermally escapes on
  Venus
• Gas and liquid/ice exchange occurs through
  condensation and evaporation/sublimation
• on Earth with H2O
• on Mars with CO2
• Since Mercury the Moon have no substantial
  atmosphere, fast particles and high-energy
  photons reach their surfaces
• bombardment creates a rarified exosphere

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Martian Weather Today

• Seasons on Mars are more extreme than on Earth
• Mars orbit is more elliptical
• CO2 condenses sublimes at opposite poles
• changes in atmospheric pressure drive
  pole-to-pole winds
• sometimes cause huge dust storms

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Martian Weather N Polar Ice Cap Dust Storm
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Climate History of Mars

• More than 3 billion years ago, Mars must have had
  a thick CO2 atmosphere and a strong greenhouse
  effect.
• the so-called warm and wet period
• Eventually CO2 was lost to space.
• some gas was lost to impacts
• cooling interior meant loss of magnetic field
• Solar wind stripping removed gas
• Greenhouse effect weakened until Mars froze.

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Venusian Weather Today

• Venus has no seasons to speak of.
• rotation axis is nearly 90º to the ecliptic plane
• Venus has little wind at its surface
• rotates very slowly, so there is no Coriolis
  effect
• The surface temperature stays constant all over
  Venus.
• thick atmosphere distributes heat via two large
  circulation cells
• There is no rain on the surface.
• it is too hot and Venus has almost no H2O
• Venusian clouds contain sulfuric acid!
• implies recent volcanic outgassing?

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Climate History of Venus

• Venus should have outgassed as much H2O as Earth.
• Early on, when the Sun was dimmer, Venus may have
  had oceans of water
• Venus proximity to the Sun caused all H2O to
  evaporate.
• H2O caused runaway greenhouse effect
• surface heated to extreme temperature
• UV photons from Sun dissociate H2O H2 escapes, O
  is stripped

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Earths Carbon Dioxide Concentration

• The rise of carbon dioxide concentrations as
  measured on Mauna Loa, Hawaii.