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Lecture 9: Planetary Atmospheres


Planet hunter Michael Brown to speak at UCSC Thurs, May 24 ' ... Volcanic activity subsided. Magnetic field went away, solar wind particles eroded atmosphere. ... – PowerPoint PPT presentation

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Title: Lecture 9: Planetary Atmospheres

Lecture 9 Planetary Atmospheres
Earths atmosphere seen from space
  • Claire Max
  • May 10, 2007
  • Astro 18 Planets and Planetary Systems
  • UC Santa Cruz

Planet hunter Michael Brown to speak at UCSC
Thurs, May 24
  • "Pluto, Eris, and the Dwarf Planets of the Solar
  • 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.

Practicalities Projects
  • Presentations what do we expect?
  • Time about 20 minutes per group
  • Each person in group should speak about their own
  • 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.

Projects, continued
  • Written report on your projects what do we
  • 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

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

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!
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,
  • Huge changes over a billion years or less
  • Prospect of human-induced global warming on Earth
    needs to be taken seriously

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

Role of atmospheric pressure in "holding up" the
In an atmosphere in equilibrium, pressure
balances gravity
Implications profile of pressure and density
with altitude
  • Pressure, density fall off exponentially with
  • Higher temperature T ? larger scale height
  • Stronger gravity g ? shorter scale height h0

How big is pressure scale height?
  • h0 kT / mg
  • height at which pressure has fallen by 1/e
  • 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)

Equilibrium atmospheric temperature
Temperature of a planet balance solar heating
against cooling
No-greenhouse temperature
  • albedo fraction of sunlight that is reflected
    by a surface

No-greenhouse temperatures
  • Conclusion for Venus and Earth, at least,
    something else is going on! (not just radiation
    into space)

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

How do different energy photons interact with
The greenhouse effect
Greenhouse gases
  • carbon dioxide CO2
  • water vapor H20
  • methane CH4
  • others too (NO2, ....)

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

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

Ozone hole over antarctica
Antarctic ozone hole is worst in late southern
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?

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
  • Question why are atmospheres of Venus, Earth,
    Mars so different?

Sources of atmospheric gases
comets bring water, carbon componds
Kilauea volcano outgassing
Sinks of atmospheric gases
Thermal Escape of atmospheric gases
Components of atmospheres on Venus, Earth, Mars
  • Why are they so different?
  • Were they always this different from each other?

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
  • Once the core differentiated, gases could be

Second atmosphere produced by volcanic
  • 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
  • Ocean Formation - As the Earth cooled, H2O
    produced by outgassing could exist as liquid
  • Evidence - pillow basalts, deep marine sediments

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

  • Source Kasting, Scientific American

Cyanobacteria and stromatolites
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
  • 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
  • Conclusion - amount of O2 in the atmosphere has
    increased with time.

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 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.

Earth hydrological cycle
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
  • Possible problems with isotope ratios. Under
    active research.

What factors can cause long-term climate change?
Solar Brightening
  • Sun very gradually grows brighter with time,
    increasing the amount of sunlight warming planets

Changes in Axis Tilt
  • Greater tilt makes more extreme seasons, while
    smaller tilt keeps polar regions colder

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

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

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

Global Warming on Earth
  • Most of CO2 increase has happened in last 50

Intergovernmental Panel on Climate Change
  • IPCC - series of important reports
  • International scientific consensus
  • Website http//www.ipcc.ch/

Direct Observations of Recent Climate Change
IPCC Report 2007
Global mean temperature Global average sea
level Northern hemisphere Snow cover
Global mean surface temperatures have increased
IPCC Report 2007
Sea Levels have risen
IPCC Report 2007
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
IPCC Report 2007
Proportion of heavy rainfalls increasing in most
land areas
Regions of disproportionate changes in heavy
(95th) and very heavy (99th) precipitation
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
IPCC Report 2007
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
CO2 concentrations, temp., sea level continue to
rise long after CO2 emissions are reduced
IPCC Report 2007
Constant emissions of CO2 do not lead to
stabilization of atmospheric concentrations
IPCC Report 2007
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

Venus Climate
Present-day tectonics very different on Earth and
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

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

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
  • 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?

There was once liquid water on Mars
  • Geomorphological evidence (lots of it)
  • Shape of ocean basins

Why did Mars climate change?
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

Climate Change on Mars
One possible scenario
  • Magnetic field may have preserved early Martian
  • Solar wind may have stripped atmosphere after
    field decreased because of interior cooling

History of Mars atmospherea more complex
  • 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
  • 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.

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?

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

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

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,
  • 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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