Dave Paige Francis Nimmo

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ESS 250 MARS

• Dave Paige / Francis Nimmo

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Student Topic Signup Today After Class, 4710
Geology
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Lecture Outline

• Introduction to Mars Atmosphere and Climate
• Atmospheres and Atmospheric Processes
• Key Properties of the Martian Atmosphere
• Key properties explained
• Obliquity and Obliquity History
• Surface and Atmospheric Temperatures
• Atmospheric Pressure
• Atmospheric Composition
• Climate History
• Interannual Variability
• Secular Variations
• Astronomically Driven Climate Change
• Long-Term Atmospheric Evolution

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Atmospheres Are Integral Parts of Planets

• Four basic types of planets
• Jovian Planets(no distinct surface, massive
  hydrogen-rich atmospheres) Jupiter, Saturn,
  Uranus, Neptune
• Terrestrial Planets (distinct solid rocky
  surface, low mass oceans and atmospheres) Venus,
  Earth, Mars
• Icy Airfull Bodies (massive ice crusts with
  significant atmospheres) Titan, Triton, Pluto,
  Comets
• Airless Bodies (small solid rocky or icy
  surfaces, but negligible atmospheres)Mercury,
  Moon, Asteroids, Small Moons
• Atmospheric processes play important roles in the
  evolution of all types of planets except airless
  bodies

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Atmospheric Processes

• Atmospheres are the product of a number of
  complex and interacting processes
• Radiation (solar, infrared, orbit, spin axis)
• Chemistry (primordial composition, chemical
  interactions and mass exchange with solid planet,
  photochemistry)
• Space Interactions (loss or gain of matter
  through impact, escape)
• Thermodynamics (redistribution of materials due
  state changes, oceans, polar caps, condensate
  clouds)
• Dynamics (redistribution of materials due to
  creation of kinetic energy by heat engine)
• Biology (mass and energy cycling between
  non-living and living)
• Like their solid surfaces, the atmospheres of
  Earth and Mars share many key characteristics
  (except biology maybe)

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Martian Atmosphere Key Properties

• Mean Orbital Radius 1.5237  AU
• Orbital Period 687 Days
• Rotational Period 24.6 Days
• Surface Gravity 3.72 m/sec2
• Obliquity 25.19 deg
• Surface Temperature 148K-320K
• Surface Pressure 6 mbar
• Atmospheric Composition
• Carbon Dioxide (C02) 95.32 (variable)
• Nitrogen (N2) 2.7
• Argon (Ar) 1.6 
• Oxygen (O2) 0.13
• Carbon Monoxide (CO) 0.07
• Water (H2O) 0.03 (variable)
• Neon (Ne) 0.00025
• Krypton (Kr) 0.00003 
• Xenon (Xe) 0.000008 
• Ozone (O3) 0.000003 (variable)

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Why These Key Properties?

• Mean Orbital Radius? History of solar system
  formation (Bodes law)
• Orbital Period 687 Days? A consequence of 1. With
  Keplers 3d law P2 k r3
• Rotational Period 24.6 Days? History of late
  giant impacts, no large moons to cause tidal
  evolution
• 4. Surface Gravity 3.72 m/sec2 ? Mass and Radius
  of planet g G M / r2
• 5. Obliquity 25.19 deg? History of late giant
  impacts (mean), Spin Orbit Resonance Coupling and
  Chaotic Evolution (variability)
• 6. Surface Temperature 148K-320K? Consequence of
  1,2,3,5, surface thermal properties and
  atmospheric radiative properties
• 7. Surface Pressure 6 mbar? Consequence of 4,
  atmospheric escape history, climate history and
  carbonate formation, vapor pressure of permanent
  CO2 polar cap?
• 8. Atmospheric Composition? Consequence of 1-7,
  plus much more

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Obliquity Evolution

• Mars undergoes large-scale obliquity variations
  whereas Earth does not
• A planets obliquity is forced by resonances
  between the planets precessional period, and the
  periods inclination variations of the other
  planets.
• Mars periods are in resonance whereas the
  Earths are not.
• An Earth without the Moon would also have periods
  that would be in resonance
• Mars obliquity has varies chaotically, making it
  impossible to predict further and further back in
  time

Question If Earths orbital variations caused
the Ice Ages, what have Mars orbital variations
caused?
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What Determines Surface Temperatures?
Global Radiation Balance
Infrared Radiation
Solar Radiation
Sun
Mars
Day
Night
R
Instantaneously, assuming no atmosphere or heat
conduction, unit emissivity
Local Solar-Zenith Angle
Insolation
Surface Solar Reflectivity (Albedo)
Solar Const. at 1 AU
Surface Temperature
Stefan-Boltzmann Constant
Sun-Mars Distance (AU)
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Current Distribution of Insolation

• Martian seasons are hemispherically asymmetric
  due to eccentricity of orbit
• Currently, perihelion passage occurs close to
  southern summer solstice
• Southern spring and summer are shorter, but more
  intense than northern spring and summer
• Situation will reverse in 26,000 years due to
  precession of spin axis
• Both poles receive exactly the same insolation,
  regardless of orbital configuration because
  orbital angular velocity increases with 1/ r2 as
  insolation increases as 1/ r2

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Past Distribution of Insolation

• Low obliquity reduces insolation at poles (1/2
  times current insolation)
• High obliquity increases insolation at poles (2
  times current insolation)
• Annual average insolation at poles exceeds
  insolation at the equator for obliquities of
  greater than 50 degrees

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Current Martian Temperatures

• Latest MGS Thermal Emission Spectrometer (TES)
  data
• Current Mars Season is Ls336, Martian Southern
  Summer
• Ls is an angular measure of Martian Season, Ls0
  at Northern Spring Equinox

• Mars thin atmosphere and no oceans results in
  large daily temperature variations
• Atmospheric temperatures are intermediate
  between surface day and night temperatures
• The radiative time constant for the Martian
  atmosphere is 1 day, compared to weeks for the
  Earths atmosphere and months for the Earths
  ocean surface layer

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Mars Clouds and Thermal Structure
MGS MOC Global Cloud Map
Atmospheric Thermal Structure

• The Martian atmosphere is generally transparent
  to solar radiation, but local and global dust
  storms, and water ice clouds and hazes can
  obscure the surface at visible wavelengths
• Atmospheric dust absorbs solar radiation and
  heats the atmosphere
• Mars has no ozone layer (due mostly to lack of
  atmospheric oxygen), and no warm stratosphere
  like the Earth

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Infrared Radiation and Greenhouse Effect
Mariner 9 IRIS Spectra

• The surface and atmosphere of Mars emit
  radiation to space at IR wavelengths (10-30
  microns)
• CO2 gas is the dominant absorber of IR radiation
  when the atmosphere is clear
• Dust and water ice clouds also absorb IR
  radiation
• The absorption of IR radiation by the atmosphere
  results in a greenhouse effect, which elevates
  surface temperatures
• The Martian greenhouse effect is 5K, which is
  small compared Earth (25K) and Venus (450K)

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Martian Surface Pressure
What is pressure? A force per unit area. How
does pressure relate to atmospheric
mass? Newtons Second Law F m a Divide this
by area P (mass per unit area) g This
makes sense atmospheric surface pressure is the
weight of the overlying atmospheric
column How does pressure relate to temperature
and density? Equation of State P r R T
(Ideal Gas Law) How does pressure vary with
altitude? dP - r g dz (Hydrostatic
Law) Combine this with Ideal Gas Law dP -
(P/RT) g dz After integrating P Po exp
-(z/(RT/g)) RT/g is the atmospheric scale height
(10 km)
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CO2 Phase Relationships

• It is sometimes useful to think of planetary
  atmospheres as little sealed laboratory bottles
  containing soil and volatiles (substances that
  are liquids or gasses at room temperature and
  pressure) that can be stirred, heated or cooled
  etc.
• Real planetary atmospheres are sealed by
  gravity
• The pressures and temperatures of multi-phase
  systems in equilibrium follow phase relationships
• At the Martian CO2 surface pressure of 6 mbar,
  CO2 solid (ice) will form at T148K
• What causes the surface pressure to be 6 mbar?

• In 1966, Leighton and Murray proposed that the 6
  mbar Martian CO2 surface pressure was the
  consequence of the presence of a permanent CO2
  surface ice deposit at one of the Martian poles

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Seasonal CO2 Polar Caps

• At high latitudes during the cold fall and
  winter seasons, CO2 condenses out of the
  atmosphere to form surface deposits at T148K ,
  which then sublimate back into the atmosphere
  during spring and summer

Viking Lander 1 and 2 Pressure Data over 3 Mars
Years
Retreat of North Seasonal Polar Cap

• The condensation and sublimation of CO2 in both
  hemispheres results in a 20 seasonal variation
  in Martian surface pressure

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Permanent CO2 Polar Caps

• In Leighton and Murrays model, the total CO2
  pressure in the atmosphere was the consequence of
  the vapor pressures of permanent CO2 deposits at
  the poles
• Implication 1 Anything that changes the annual
  average temperatures of permanent CO2 deposits
  changes the equilibrium CO2 pressure locally in
  the overlying atmosphere
• Implication 2. Since atmospheric pressures
  equalize over the entire planet, the mass of the
  Martian atmosphere may undergo significant mass
  variations with obliquity, as long as there is
  sufficient CO2 in the cap-atmosphere system to
  support a permanent CO2 deposit

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Residual Polar Caps
Small residual caps are exposed at both poles at
the end of the summer season after seasonal CO2
frost has completely evaporated
North Residual Cap (larger, centered)
South Residual Cap (smaller, off-center)
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Residual Cap Observations
Orbiter observations show that the north and
south residual polar caps have contrasting
properties

• Implications
• Theories predict only one permanent CO2 deposit
  at any given time, since colder pole will rob
  CO2 from the warmer pole over time
• There may only be a very small amount of CO2
  remaining on the south residual cap today its
  importance as a significant source of atmospheric
  CO2 at high obliquity is questionable..

• North Residual Cap
• Composed of Water Ice
• High Summer Temperature (200K)
• High water vapor abundance
• Sponge Texture

Close up MOC Images

• South Residual Cap
• Covered by incomplete layer of CO2 frost
• Low Temperature (148K)
• Low water vapor abundance
• Swiss Cheese Texture

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Martian Atmosphere Key Properties
V R I P

• Mean Orbital Radius 1.5237  AU V
• Orbital Period 687 Days V
• Rotational Period 24.6 Days V
• Surface Gravity 3.72 m/sec2 V
• Obliquity 25.19 deg V
• Surface Temperature 148K-320K V
• Surface Pressure 6 mbar V
• Atmospheric Composition
• Carbon Dioxide (C02) 95.32 (variable)
• Nitrogen (N2) 2.7
• Argon (Ar) 1.6 
• Oxygen (O2) 0.13
• Carbon Monoxide (CO) 0.07
• Water (H2O) 0.03 (variable)
• Neon (Ne) 0.00025
• Krypton (Kr) 0.00003 
• Xenon (Xe) 0.000008 
• Ozone (O3) 0.000003 (variable)

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Nitrogen and Noble Gasses

• Nitrogen and noble gasses have high volatility
  and low chemical interaction with solid planet
• Tend to accumulate in atmosphere, and undergo
  isotopic fractionation due to atmospheric escape
  to space
• Atmospheric Thermal Escape

½ m V2 G M m / r k T v sqrt(2 G
M / r) sqrt (2 k T / m )
Kinetic Energy
Gravitational Potential Energy
Low Mass Molecules Escape at Lower Temperatures
Thermal Energy
Escape Velocity
Independent Of Mass, Higher For More Massive
Planets

• Atmosphere becomes enriched in heavy isotopes
  over time as lighter isotopes escape to space
• Non-thermal escape processes also important for
  Mars..

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Atmospheric Isotopic Ratios

• Measured by Viking Landers and in gas bubbles in
  Mars meteorites
• Atmospheric O formed phothemically by photolysis
  of water vapor by solar UV photons

Assuming Earth and Mars started out with the same
isotopic composition, then

• Mars atmosphere enriched in heavy isotopes of N
  and Xe relative to Earth, suggesting extensive
  atmospheric escape
• Mars atmosphere not enriched in heavy isotopes
  of O, suggesting current atmosphere is in
  isotopic equilibrium with a substantially larger
  O reservoir (CO2 or H2O ices, or O in rocks)

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H20 Phase Relationships

• Water is less volatile than CO2
• Found in lower concentrations in the atmosphere
• Water vapor concentration is an exponential
  function of temperature
• Liquid water requires pressures of 6.1 mbar
• 6.1 mbar is close to the current mean Martian
  surface pressure
• Liquid water could be stable on Mars close to
  the surface in the warmest regions during the
  warmest times of the day

Current Range Of Martian Temperatures
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Atmospheric Water Observations

• Both Viking and MGS measured column abundance of
  water vapor
• Scale is in precipitable microns of water
• Typical values are 15 microns at low latitudes,
  and up to 75 microns at the poles during summer

• Surface water vapor concentrations depend on how
  the water is mixed vertically in the atmosphere,
  but can never instantaneously exceed the frost
  point temperature from the water phase diagram

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Frost Point Temperatures

• If atmospheric water is well mixed with the
  atmosphere, we expect frost point temperatures of
  195K to 210K on Mars

• Since observed surface and atmospheric
  temperatures range from 148-300K, atmospheric and
  surface water is expected to change phases often,
  condensing during cold times of the day, and
  colder seasons, and evaporating during warmer
  times of the day or warmer seasons much like
  on Earth

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Water Exchange

• The water we observe in the Martian atmosphere
  represents a very small fraction of Mars
  exchangeable water
• We expect surface and subsurface reservoirs of
  water any places that are in good contact with
  the atmosphere where temperatures do not exceeded
  the frost point for significant periods of time

Surface Frost At Viking Lander 2 Site (45 N)
Ground Ice (terrestrial example)
Residual Polar Caps
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Near-Surface Water Distribution
Mars Odyssey Gamma Ray Spectrometer (GRS) Neutron
Spectrometer map of hydrogen abundance in
uppermost meter.

• Models predict that ground ice will be stable
  close to the surface at high latitudes where
  annual maximum temperatures never exceed the
  198K frost point
• Source of GRS equatorial water not uniquely
  determined (ice, hydrated minerals, etc)

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Carbonates (H20, CO2 and Rocks)

• Carbonates are chemical weathering products of
  volcanic rocks
• Carbonate form at low temperatures in aqueous
  environments
• Carbonates decompose at high temperatures
• Urey Reaction

MgCaSi2O6 2CO2 2H2O MgCO3 CaCO3 2SiO2
2H2O
Pyroxene (basalt)
Carbonic Acid
Carbonates
Quartz
Hot Cold
(reconstitution) (weathering)

• Ideas
• Early climate of Mars was warm and wet, but net
  carbonate formation decreased atmospheric CO2
  over time, resulting in todays cold climate
• Present 6.1 mbar atmospheric pressure is no
  coincidence, regulated by formation of carbonates
  in ephemeral liquid water environments
• Question Where are all the carbonates?
• Answer Limited spectroscopic evidence for
  carbonates on surface, and some Mars meteorites
  are 1 carbonate

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Climate and Climate Change

• Changes in observable properties and behavior of
  atmosphere occur on many time scales
• Weather (days to weeks, variations about a mean
  state)
• Seasons (months, forced by seasonal insloation
  variations)
• Interannual Variability (2-100 year variations
  about a mean state)
• Secular Variability (2-10000 year variations,
  not about a mean state global change)
• Orbital and Axial (10,000 10 million year,
  variations about a mean state forced by
  insolation variations)
• Long-Term (10 million 10 billion year
  variations, atmospheric and planetary evolution)
• Climate is usually defined to include variations
  at 2 year timescales
• We have fragmentary evidence for Martian
  variability on all these timescales

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Weather and Interannual Variations
Viking Lander Pressure
Telescopic Dust Storm Observations

• Weather variations at high latitudes can be
  large during fall and winter due to the passage
  of frontal systems
• Interannual variations in most aspects
  atmospheric parameters are small
• The occurrence and intensity of global dust
  storms varies from year to year

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Secular Climate Variations and Global Change

• High-resolution MOC images of the morphology of
  CO2 deposits on the south residual polar cap
  taken exactly one Mars year apart show
  significant interannual variations
• If the changes are interpreted as mass loss to
  the atmosphere, the atmospheric mass could double
  over the course of 100 years!
• There is no guarantee that the current
  configuration of Mars polar caps and subsurface
  ice deposits are in perfect equilibrium with the
  current climate

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Astronomically-Driven Climate Change

• Extensive layered deposits have been observed
  within both residual polar caps, and in
  mid-latitude craters
• Sedimentary layering is associated with changing
  depositional environments
• The ages and timescales associated with these
  layers are not known
• If the layers are due to astronomical climate
  forcing, then the exposed sections we can observe
  may represent incomplete records of climate
  variability..

North Polar Layered Deposits
Layered Deposits in Mid-latitude Crater
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Long Term Climate Change and Atmospheric Evolution

• The notion that early Mars was warm and wet and
  is now cold and dry was first popularized by
  Lowell at the turn of the 20th century
• This is an attractive hypothesis that has
  consciously or unconsciously influenced much of
  our thinking regarding Mars climate and biology
• Models show us that changing the global climate
  of Mars probably requires more than changes in
  the distribution of solar energy due to
  astronomical forcing, and that changes in
  atmospheric composition to give atmospheric CO2
  pressures of 1 atm are required to enable the
  stability of liquid water
• Unfortunately, most of the evidence cited for
  long-term climate change on Mars (minerals,
  runoff channels, outflow channels, layers,
  gullies etc.) can also be attributed to more
  local, short-lived processes that do not
  necessarily require a warmer global climate