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 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
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
ESS 250 Winter 2003 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…)
ESS 250 Winter 2003 Martian Atmosphere – Key Properties 1.Mean Orbital Radius AU 2.Orbital Period 687 Days 3.Rotational Period 24.6 Days 4.Surface Gravity 3.72 m/sec 2 5.Obliquity deg 6.Surface Temperature 148K-320K 7.Surface Pressure 6 mbar 8.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) % Krypton (Kr) % Xenon (Xe) % Ozone (O3) % (variable)
ESS 250 Winter 2003 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: P 2 = k r 3 3.Rotational Period 24.6 Days? History of late giant impacts, no large moons to cause tidal evolution 4. Surface Gravity 3.72 m/sec 2 ? Mass and Radius of planet: g = G M / r 2 5. Obliquity 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…
ESS 250 Winter 2003 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 Question: If Earth’s orbital variations “caused” the Ice Ages, what have Mars’ orbital variations caused?
ESS 250 Winter 2003 What Determines Surface Temperatures? Global Radiation Balance: Mars Sun Solar Radiation Infrared Radiation R Instantaneously, assuming no atmosphere or heat conduction, unit emissivity: Solar Const. at 1 AU Surface Solar Reflectivity (Albedo) Local Solar-Zenith Angle Sun-Mars Distance (AU) Stefan-Boltzmann Constant Surface Temperature Insolation DayNight
ESS 250 Winter 2003 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/ r 2 as insolation increases as 1/ r 2
ESS 250 Winter 2003 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
ESS 250 Winter 2003 Current Martian Temperatures 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 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
ESS 250 Winter 2003 Mars Clouds and 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 MGS MOC Global Cloud Map Atmospheric Thermal Structure
ESS 250 Winter 2003 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) Infrared Radiation and Greenhouse Effect Mariner 9 IRIS Spectra
ESS 250 Winter 2003 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 T (Ideal Gas Law) How does pressure vary with altitude? dP = - 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)
ESS 250 Winter 2003 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 T=148K 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 CO2 Phase Relationships
ESS 250 Winter 2003 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 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 Viking Lander 1 and 2 Pressure Data over 3 Mars Years
ESS 250 Winter 2003 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
ESS 250 Winter 2003 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)
ESS 250 Winter 2003 Residual Cap Observations Orbiter observations show that the north and south residual polar caps have contrasting properties: Close up MOC Images North Residual Cap Composed of Water Ice High Summer Temperature (>200K) High water vapor abundance Sponge Texture South Residual Cap Covered by incomplete layer of CO2 frost Low Temperature (~148K) Low water vapor abundance Swiss Cheese Texture 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…..
ESS 250 Winter 2003 Martian Atmosphere – Key Properties 1.Mean Orbital Radius AU 2.Orbital Period 687 Days 3.Rotational Period 24.6 Days 4.Surface Gravity 3.72 m/sec 2 5.Obliquity deg 6.Surface Temperature 148K-320K 7.Surface Pressure 6 mbar 8.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) % Krypton (Kr) % Xenon (Xe) % Ozone (O3) % (variable) = R I P
ESS 250 Winter 2003 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 V 2 = 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…..
ESS 250 Winter 2003 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 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) Assuming Earth and Mars started out with the same isotopic composition, then…
ESS 250 Winter 2003 H20 Phase Relationships Current Range Of Martian Temperatures 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
ESS 250 Winter 2003 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
ESS 250 Winter 2003 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 K, 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
ESS 250 Winter 2003 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) Residual Polar Caps Ground Ice (terrestrial example)
ESS 250 Winter 2003 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…)
ESS 250 Winter 2003 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: MgCaSi 2 O 6 + 2CO 2 + 2H 2 O = MgCO 3 + CaCO 3 + 2SiO 2 + 2H 2 O Pyroxene (basalt) Carbonic Acid Carbonates Quartz Hot Cold (reconstitution) (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
ESS 250 Winter 2003 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 ( 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
ESS 250 Winter 2003 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
ESS 250 Winter 2003 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
ESS 250 Winter 2003 Astronomically-Driven Climate Change North Polar Layered Deposits Layered Deposits in Mid-latitude Crater 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..
ESS 250 Winter 2003 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 20 th 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