Lecture 9: Planetary Atmospheres

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Lecture 9 Planetary Atmospheres
Earths atmosphere seen from space

• Claire Max
• May 10, 2007
• Astro 18 Planets and Planetary Systems
• UC Santa Cruz

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Planet hunter Michael Brown to speak at UCSC
Thurs, May 24

• "Pluto, Eris, and the Dwarf Planets of the Solar
  System
• 730 pm in the Humanities Lecture Hall
• Planet hunter Michael Brown will discuss his
  discoveries at the outer edge of the solar system
• Brown, a professor of planetary astronomy at
  Caltech, has been scanning the skies for planets
  beyond Pluto for the past 7 years.
• In 2005 his team discovered Eris, the largest
  object found in the solar system in the past 150
  years and the first new candidate for planethood
  to be discovered since Pluto.

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Practicalities Projects

• Presentations what do we expect?
• Time about 20 minutes per group
• Each person in group should speak about their own
  "questions"
• Format Your choice.
• PowerPoint
• Speak from written notes and hold up figures
• Make a poster and describe it to class
• In past years one group has done a dramatic
  presentation. Harder to do and still convey
  enough information.

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Projects, continued

• Written report on your projects what do we
  expect?
• Each group hand in your written contributions
  together as one paper
• Cover page listing overall title, all group
  members (with email addresses)
• Table of contents listing overall title and then
  topics that each person will write about
• 5 pages (or more if you want) from each person on
  "questions" each person is addressing

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Written project reports, continued

• Each person's five pages
• Introduction describing your "questions" and how
  they relate to the overall topic of your group
• Then describe your "questions" in more detail
• Then give a logical discussion of what you found
  out, including what your sources were for each
  potential "answer" you present
• use numbered references in text, referring to
  numbered bibliography at end of your section
• Summarize your overall conclusions describe what
  new questions your investigation has brought out

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Planetary atmospheres Outline

• What is an atmosphere? What is its structure?
• Temperature of a planet, neglecting effects of
  atmosphere (no-greenhouse temperatures)
• Generic atmospheric structure
• Global climate change
• Earth
• Venus
• Mars

Please remind me to take a break at 245 pm!
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The Main Points

• Planetary atmospheres as a balancing act
• Gravity vs. thermal motions of air molecules
• Heating by Sun vs. heat radiated back into space
• Weather as a way to equalize pressures at
  different places on Earths surface
• Atmospheres of terrestrial planets are very
  different now from the way they were born
• Formation volcanoes, comets
• Destruction escape, incorporation into rocks,
  oceans
• Huge changes over a billion years or less
• Prospect of human-induced global warming on Earth
  needs to be taken seriously

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The thin blue line

• Earth diameter
• 12,000 km
• Top of troposphere
• 12 km
• Thickness of atmosphere divided by Earth diameter
  1 / 1000

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Role of atmospheric pressure in "holding up" the
atmosphere
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In an atmosphere in equilibrium, pressure
balances gravity
volume
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Implications profile of pressure and density
with altitude

• Pressure, density fall off exponentially with
  altitude
• Higher temperature T ? larger scale height
  h0
• Stronger gravity g ? shorter scale height h0

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How big is pressure scale height?

• h0 kT / mg
• height at which pressure has fallen by 1/e
  0.368
• Earth h0 8 km
• the thin blue line
• Venus h0 15 km
• (g a bit lower, T higher)
• Mars h0 16 km
• (both g and T lower)

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Equilibrium atmospheric temperature
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Temperature of a planet balance solar heating
against cooling
No-greenhouse temperature

• albedo fraction of sunlight that is reflected
  by a surface

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No-greenhouse temperatures

• Conclusion for Venus and Earth, at least,
  something else is going on! (not just radiation
  into space)

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Lights Effects on the Atmosphere

• Ionization Removal of an electron
• Dissociation Destruction of a molecule
• Scattering Change in photons direction
• Absorption Photons energy is absorbed

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How do different energy photons interact with
atmosphere?
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The greenhouse effect
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Greenhouse gases

• carbon dioxide CO2
• water vapor H20
• methane CH4
• others too (NO2, ....)

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Generic atmospheric structure
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Temperature structure of Earths atmosphere
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Space shuttle view of top of troposphere
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Compare Earth, Venus, Mars
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Ozone and the Stratosphere

• Ultraviolet light can break up O2 molecules,
  allowing ozone (O3) to form
• Without plants to release O2, there would be no
  ozone in stratosphere to absorb UV light

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Role of ozone for Earths atmosphere

• Ultra-violet light from Sun dissociates oxygen
  molecules O2 to produce O in stratosphere
• O combines with O2 to form O3 (ozone)
• Ozone in stratosphere absorbs harmful ultraviolet
  light from Sun, providing land-based life with a
  protective shield
• Manmade aerosols (chlorofluorocarbons, CFCs)
  inhibit ozone formation
• Result Ozone hole

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Ozone hole over antarctica
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Antarctic ozone hole is worst in late southern
winter
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Ozone hole questions

• Why is ozone hole much deeper and larger in the
  antarctic than in the arctic?
• Why is it so tightly correlated with the south
  polar vortex?

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History of atmospheres on Venus, Earth, Mars

• Huge changes took place over the 4.6 billion
  years since planets formed!
• Early atmospheres didnt resemble current ones at
  all
• Question why are atmospheres of Venus, Earth,
  Mars so different?

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Sources of atmospheric gases
comets bring water, carbon componds
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Kilauea volcano outgassing
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Sinks of atmospheric gases
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Thermal Escape of atmospheric gases
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Components of atmospheres on Venus, Earth, Mars

• Why are they so different?
• Were they always this different from each other?

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The three atmospheres of EarthFirst Atmosphere

• First Atmosphere Primordial elements
• Composition - Probably H2, He
• Today these gases are relatively rare on Earth
  compared to other places in the universe.
• Were probably lost to space early in Earth's
  history because
• Earth's gravity is not strong enough to hold
  lightest gases
• Earth still did not have a differentiated core
  (solid inner/liquid outer core) which creates
  Earth's magnetic field (magnetosphere Van Allen
  Belt) which deflects solar wind. Protects any
  atmosphere.
• Once the core differentiated, gases could be
  retained

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Second atmosphere produced by volcanic
outgassing

• Gases similar to those from modern volcanoes
  (H2O, CO2, SO2, CO, S2, Cl2, N2, H2) and NH3
  (ammonia) and CH4 (methane)
• No free oxygen O2 (O2 not found in volcanic
  gases)
• Ocean Formation - As the Earth cooled, H2O
  produced by outgassing could exist as liquid
• Evidence - pillow basalts, deep marine sediments

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Third atmosphere Free oxygen came late

• Today, atmosphere is 21 free oxygen. How did
  oxygen reach this level?
• Oxygen Production
• Photochemical dissociation - breakup of water
  molecules by ultraviolet light
• Produced O2 levels 1-2 current levels
• At these levels O3 (Ozone) can form to shield
  Earth surface from UV
• Photosynthesis CO2 H2O sunlight organic
  compounds O2 - produced by cyanobacteria, and
  eventually higher plants - supplied the rest of
  O2 to atmosphere.
• Oxygen Consumers
• Chemical Weathering - through oxidation of
  surface materials (early consumer)
• Respiration (much later)
• Burning of Fossil Fuels (much, much later)
• Once rocks at the surface were sufficiently
  oxidized, more oxygen could remain free in the
  atmosphere

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• Source Kasting, Scientific American

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Cyanobacteria and stromatolites
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Evidence from rocks Earths oxygen increased
with time

• Iron (Fe) i s extremely reactive with oxygen. If
  we look at the oxidation state of Fe in the rock
  record, we can infer a great deal about
  atmospheric evolution.
• Archean - In sediments, find occurrence of
  minerals that only form in non-oxidizing
  environments Pyrite (Fools gold FeS2),
  Uraninite (UO2). These minerals are easily
  dissolved out of rocks under todays atmospheric
  conditions.
• So things must have been different back then.
• Red beds (continental siliciclastic deposits) are
  never found in rocks older than 2.3 billion years
  but are common during Phanerozoic time. Red
  because of the highly oxidized mineral hematite
  (Fe2O3).
• Conclusion - amount of O2 in the atmosphere has
  increased with time.

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Biological evidence

• Chemical building blocks of life could not have
  formed in the presence of atmospheric oxygen.
  Chemical reactions that yield amino acids are
  inhibited by presence of very small amounts of
  oxygen.
• Oxygen prevents growth of the most primitive
  living bacteria such as photosynthetic bacteria,
  methane-producing bacteria and bacteria that
  derive energy from fermentation.
• Conclusion - Since today's most primitive life
  forms are anaerobic, the first forms of cellular
  life probably had similar metabolisms.
• Today these anaerobic life forms are restricted
  to anoxic (low oxygen) habitats such as swamps,
  ponds, and lagoons.

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Earth hydrological cycle
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Did Earth get its water from comets?

• Potential source of the Earth's ocean water is
  comet-like balls of ice.
• Measure about 9 m (30 ft) in diameter.
• Enter atmosphere at rate of about 20/second.
• At the observed rate of occurrence, Earth would
  receive 0.0025 mm of water per year.
• Four billion years of such bombardment would give
  enough water to fill the oceans to their present
  volume.
• Possible problems with isotope ratios. Under
  active research.

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What factors can cause long-term climate change?
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Solar Brightening

• Sun very gradually grows brighter with time,
  increasing the amount of sunlight warming planets

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Changes in Axis Tilt

• Greater tilt makes more extreme seasons, while
  smaller tilt keeps polar regions colder

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Changes in Reflectivity

• Higher reflectivity tends to cool a planet, while
  lower reflectivity leads to warming

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Changes in Greenhouse Gases

• Increase in greenhouse gases leads to warming,
  while a decrease leads to cooling

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Global Warming on Earth

• Global temperatures have tracked CO2
  concentration for last 500,000 years
• Antarctic air bubbles indicate current CO2
  concentration is highest in at least 500,000
  years

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Global Warming on Earth

• Most of CO2 increase has happened in last 50
  years!

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Intergovernmental Panel on Climate Change

• IPCC - series of important reports
• International scientific consensus
• Website http//www.ipcc.ch/

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Direct Observations of Recent Climate Change
IPCC Report 2007
Global mean temperature Global average sea
level Northern hemisphere Snow cover
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Global mean surface temperatures have increased
IPCC Report 2007
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Sea Levels have risen
IPCC Report 2007
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Glaciers and frozen ground are receding
IPCC Report 2007
Area of seasonally frozen ground in NH has
decreased by 7 from 1901 to 2002
Increased Glacier retreat since the early 1990s
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IPCC Report 2007
Proportion of heavy rainfalls increasing in most
land areas
Regions of disproportionate changes in heavy
(95th) and very heavy (99th) precipitation
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Human and Natural Drivers of Climate Change

• CO2, CH4, N2O Concentrations
• - far exceed pre-industrial values
• - increased markedly since 1750
• due to human activities

Relatively little variation before the industrial
era
IPCC Report 2007
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IPCC Report 2007
Model Predictions Surface warming following
doubling of CO2 concentrations
Best estimate 3C likely 2-4.5C very
unlikely less than 1.5C higher values not
ruled out
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CO2 concentrations, temp., sea level continue to
rise long after CO2 emissions are reduced
IPCC Report 2007
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Constant emissions of CO2 do not lead to
stabilization of atmospheric concentrations
IPCC Report 2007
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Developing countries are the most vulnerable to
climate change
IPCC Report 2007

• Impacts are worse
• Already more flood and drought prone
• Larger share of the economy is in climate
  sensitive sectors
• Lower capacity to adapt
• Lack of financial, institutional and
  technological capacity
• Climate change is likely to impact
  disproportionately upon the poorest countries
• .. and the poorest people within countries
• Net economic effects expected to be negative in
  most developing countries

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Venus Climate
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Present-day tectonics very different on Earth and
Venus
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Venus tectonics, contd

• No evidence for plate tectonics on Venus
• No mid-ocean rifts
• No subduction trenches
• Volcanos spread evenly across surface instead of
  at plate boundaries, as on Earth.
• Lithosphere not broken into plates probably
  because heat at surface slightly softens the
  lithosphere.

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No carbon-silicate cycle on Venus
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Another graphic of Earths carbonate cycle
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• Source Kasting, Scientific American

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Resurfacing on Venus

• Venus has far fewer impact craters than Moon
  Mercury, but more than Earth.
• The atmosphere protects it from smaller impacts
• Geologic activity (volcanic resurfacing) has
  erased much of the evidence
• Surface age is only about a billion years.
• Rather uniform age implies that Venus was
  "resurfaced" by lava flows during a recent,
  relatively short period
• This differs profoundly from Earth's crustal
  history. What is it telling us?
• Could Venus' present crust only have formed that
  recently?
• Could there have been a growing crust before 1
  billion years ago that "turned over" as heat
  built up underneath, to lead to a new era of
  major lava flows?
• Why?

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There was once liquid water on Mars

• Geomorphological evidence (lots of it)
• Shape of ocean basins

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Why did Mars climate change?
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Climate Change on Mars

• Mars has not had widespread surface water for 3
  billion years
• Greenhouse effect probably kept surface warmer
  before that
• Somehow Mars lost most of its atmosphere

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Climate Change on Mars
One possible scenario

• Magnetic field may have preserved early Martian
  atmosphere
• Solar wind may have stripped atmosphere after
  field decreased because of interior cooling

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History of Mars atmospherea more complex
scenario

• Shortly after Mars formed, its surface
  temperature was equal to its blackbody
  temperature (around -55 C).
• As volcanoes dumped CO2 and H2O vapor into
  atmosphere, greenhouse effect increased
  temperature above 0 C (freezing) so liquid water
  could exist.
• Liquid water was present, so rocks could
  efficiently remove CO2 from atmosphere.
• Two competing effects determined amount of CO2 in
  atmosphere volcanoes adding CO2, and rocks
  absorbing CO2. Result moderate level of CO2 .
• Greenhouse effect could keep surface T gt 0 C, as
  long as volcanoes kept erupting.
• Eventually Mars' core cooled and solidified (Mars
  is small). Volcanic activity subsided. Magnetic
  field went away, solar wind particles eroded
  atmosphere.
• Once rate of eruptions tapered off, CO2 in the
  atmosphere started to fall.
• As the atmosphere thinned out, the greenhouse
  effect weakened. Eventually the average surface
  temperature dropped, and surface water froze.

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Mars vs. Venus key issues

• Balancing act between injection and removal of
  CO2 from atmosphere
• Role of liquid water in sequestering CO2
• Venus too hot
• Mars too cold
• Earth just right
• What is evidence for these scenarios? How could
  you test them?

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New space missions to Venus

• Venus Express (European)
• Orbiting Venus now
• Atmosphere, greenhouse gases, plasma environment
• Planet-C (Japan)
• Launch in 2010
• Atmosphere, volcanic activity, lightning

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New space missions to Mars

• Mars Reconnaissance Orbiter (NASA)
• At Mars now
• Atmosphere and climate are two of many goals
• Mars Climate Sounder producing global weather maps

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The Main Points

• Planetary atmospheres as a balancing act
• Gravity vs. thermal motions of air molecules
• Heating by Sun vs. heat radiated back into space
• Weather as a way to equalize pressures at
  different places on Earths surface
• Atmospheres of terrestrial planets are very
  different now from the way they were born
• Formation volcanoes, comets
• Destruction escape, incorporation into rocks,
  oceans
• Huge changes over a billion years or less
• Prospect of human-induced global warming on Earth
  needs to be taken seriously