ESS 250: MARS
Dave Paige / Francis Nimmo
ESS 250 Winter 2003
Student Topic Signup Today After Class, 4710 Geology
E S S 2 5 0 W in te r 2 0 0 4
M ARS
S tu d e n t T o p ic S ig n u p S h e e t
S u g g e s te d T o p ic s :
D id M a rs e v e r h a v e p la te te c to n ic s ?
D id M a rs e v e r p o s s e s s a N o rth e rn o c e a n ?
W a s e a rly M a rs w a rm o r c o ld ?
A re th e g u llie s e v id e n c e o f p re s e n t-d a y w a te r o n M a rs ?
A re th e re a n d e s ite s o n M a rs ?
Is th e h e m is p h e ric d ic h o to m y th e re s u lt o f g ia n t im p a c t(s )?
D id M a rs e v e r p o s s e s s life ?
A re th e re g la c ie rs o n p re s e n t-d a y M a rs ?
Y o u m a y c h o o s e o th e r to p ic s , b u t it is s u g g e s te d th a t y o u O K th e m w ith
o n e o f th e in s tru c to rs . S tu d e n ts a re e n c o u ra g e d to p a ir u p a n d a rg u e
th e p ro a n d c o n s id e s o f is s u e s o n th e s a m e d a y . If y o u d o n o t h a v e
a p a ir, y o u w ill b e e x p e c te d to a rg u e b o th s id e s o f th e is s u e . T w o s tu d e n ts
w ill p re s e n t p e r c la s s .
D a te
N am e
T o p ic
P ro , C o n o r B o th
_____________________________________________________________________
Feb 12
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ESS 250 Winter 2003
Lecture Outline
• Introduction to Mars Atmosphere and Climate
– Atmospheres and Atmospheric Processes
– Key Properties of the Martian Atmosphere
– Key properties explained
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•
•
•
Obliquity and Obliquity History
Surface and Atmospheric Temperatures
Atmospheric Pressure
Atmospheric Composition
– Climate History
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•
•
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Interannual Variability
Secular Variations
Astronomically Driven Climate Change
Long-Term Atmospheric Evolution
ESS 250 Winter 2003
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
1.
2.
3.
4.
5.
6.
7.
8.
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)
Nitrogen (N2)
Argon (Ar)
Oxygen (O2)
Carbon Monoxide (CO)
Water (H2O)
Neon (Ne)
Krypton (Kr)
Xenon (Xe)
Ozone (O3)
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95.32% (variable)
2.7%
1.6%
0.13%
0.07%
0.03% (variable)
0.00025%
0.00003%
0.000008%
0.000003% (variable)
Why These Key Properties?
1. Mean Orbital Radius? History of solar system formation (Bode’s “law”)
2. Orbital Period 687 Days? A consequence of 1. With Kepler’s 3d law:
P2 = k r3
3. 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 planet’s obliquity is forced by
resonances between the planet’s
precessional period, and the
periods inclination variations of
the other planets.
Mars’ periods are in resonance
whereas the Earth’s 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
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Question: If Earth’s orbital variations
“caused” the Ice Ages, what have Mars’
orbital variations caused?
What Determines Surface Temperatures?
Global Radiation Balance:
Infrared Radiation
Solar Radiation
Sun
Day
Mars
Night
R
Instantaneously, assuming no atmosphere or heat conduction, unit emissivity:
Insolation
Solar Const. at 1 AU
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Local Solar-Zenith Angle
Surface Solar Reflectivity (Albedo)
S 0 cos( (1  a)
4


T
R2
Sun-Mars Distance (AU)
Surface Temperature
Stefan-Boltzmann Constant
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 Ls=336, Martian
Southern Summer
• Ls is an angular
measure of Martian
Season, Ls=0 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 Earth’s atmosphere and months for the Earth’s 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
Mariner 9 IRIS Spectra
Effect
• 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?
Newton’s 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 multiphase systems in equilibrium follow
phase relationships
• At the Martian CO2 surface pressure of
6 mbar, CO2 solid (ice) will form at
T=148K
• What causes the surface pressure to be 6
mbar?
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•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
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
T~148K , 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
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• The condensation and sublimation of CO2 in both
hemispheres results in a ~20% seasonal variation in Martian
surface pressure
Permanent CO2 Polar Caps
• In Leighton and Murray’s 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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S 0 cos( (1  a)
4


T
R2
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)
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South Residual Cap
(smaller, off-center)
Residual Cap Observations
Orbiter observations show that the north and south residual polar caps have
contrasting properties:
Implications:
North Residual Cap
•Theories predict only one
• Composed of Water Ice
permanent CO2 deposit at
• High Summer Temperature (>200K)
any given time, since
• High water vapor abundance
colder pole will “rob”
• Sponge Texture
CO2 from the warmer
Close up
MOC
pole over time
Images
• There may only be a
South Residual Cap
very small amount of CO2
• Covered by incomplete layer of CO2 frost
remaining on the south
• Low Temperature (~148K)
residual cap today – its
• Low water vapor abundance
importance as a significant
• Swiss Cheese Texture
source of atmospheric
CO2 at high obliquity is
questionable…..
ESS 250 Winter 2003
Martian Atmosphere – Key Properties
1.
2.
3.
4.
5.
6.
7.
8.
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)
Nitrogen (N2)
Argon (Ar)
Oxygen (O2)
Carbon Monoxide (CO)
Water (H2O)
Neon (Ne)
Krypton (Kr)
Xenon (Xe)
Ozone (O3)
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95.32% (variable)
2.7%
1.6%
0.13%
0.07%
0.03% (variable)
0.00025%
0.00003%
0.000008%
0.000003% (variable)
V=RIP
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
Thermal
Energy
Escape
Velocity
Independent
Of Mass, Higher For
More Massive Planets
Low Mass Molecules
Escape at Lower
Temperatures
• 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
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Current Range
Of Martian Temperatures
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)
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Residual
Polar Caps
Ground Ice (terrestrial
example)
Near-Surface Water Distribution
Mars Odyssey Gamma Ray Spectrometer
(GRS) Neutron Spectrometer map of
hydrogen abundance in uppermost
meter.
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• 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…)
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
Carbonic
Carbonates
Quartz
(basalt)
Acid
Hot
(reconstitution)
Cold
(weathering)
Ideas:
1. Early climate of Mars was warm and wet, but net carbonate formation decreased
atmospheric CO2 over time, resulting in today’s cold climate
2. 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
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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
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
North Polar
Layered Deposits
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Layered Deposits in
Mid-latitude Crater
• Extensive layered deposits
have been observed within both
residual polar caps, and in midlatitude 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..
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
that2003
do not necessarily require a warmer global climate
ESS 250 Winter