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THE MARTIAN ATMOSPHERE Tracy Esman
BASICS
• a = 1.524 AU
• R = 3.3895x106 m = ~1/2 RE
• M= 6.42 x 1023 kg
• g=3.711m/s2
• A=0.250
• Obliquity = 25.19
• Teff=~212K
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ATMOSPHERIC LOSS
• Past wet and thick atmosphere
• Loss of dynamo and magnetic field led to higher loss
• Multiple processes expected to have caused loss of ~50-150 meter global water layer
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OBTAINING THE TEMPERATURE PROFILE
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• Only flybys and occultations until Mariner 9
• Orbiters and Landers followed: Viking 1 and 2, Mars Global Surveyor, Mars Odyssey, Mars Express, Mars Reconnaissance Orbiter, MAVEN
• And more…
SPECTROSCOPY
• SPICAM: Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars • Stellar Occultations • Limited to below 130 km altitude
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SPECTROSCOPY
• Mariner 6, 7, and 9 UV spectrometers and SPICAM day glow measurements
• Airglow features are produced by photon and photoelectron excitation of CO2 between 100-200 km in altitude
• Increases with atmospheric density
• Leblanc et al., [2006] found temperatures to be 201 ± 10 K (150-190km)
• Also observed by Earth orbiters and ground-based telescopes
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SPECTROSCOPY
• Thermal Emission Spectrometer (TES) on MGS
• (5.8-50 μm) interferometric spectrometer, along with a broadband thermal (5.1-150 μm) and a visible/near-IR (0.3-2.9μm) radiometer
• Temperature profile obtained during aerobraking and combined with Opportunity’s mini-TES
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ACCELEROMETERS
• Accelerometers are on all spacecraft that pass through atmosphere in order to measure drag experienced
• Density, temperature, and pressure profiles
• Landers are limited to a single profile
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MASS SPECTROMETERS
• Determine densities of species
• Temperatures can then be calculated from neutral densities (ie. Ar, CO2, O, and N2) by integrating down through the atmosphere while assuming hydrostatic equilibrium from an upper isothermal boundary temperature
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[S. Stone]
PRE-MAVEN VALUES
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MAVEN – “CURRENT” VALUES
• Variations in scale height
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MAVEN – “CURRENT” VALUES 13
[S. Stone]
MAVEN – “CURRENT” VALUES 14
[S. Stone]
DRY LAPSE RATE AND STRUCTURE
• Assume 100% CO2 atmosphere
• Dry lapse rate = -4.4 K/km
• Average observed lapse rate: -2.5 K/km
• Discrepancy is due to: • Heating of suspended dust particles in the
atmosphere • Large scale circulation on Mars helps
stabilize the temperatures
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[Haberle, 2002]
DRY LAPSE RATE AND STRUCTURE
• At altitudes lower than ∼6 km, the daytime lapse rate approaches the adiabatic lapse rate: this region is dominated by convection
• At altitudes greater than 15 km, the atmosphere is controlled radiatively
• O vibrationally excites CO2, which then radiatively cools at 15 μm
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ATMOSPHERIC COMPOSITION
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• 95.32% CO2
• 2.7% N2
• 1.6% Ar
• 0.13% O2
• 0.08% CO
• H2O variable, important for photochemistry
ATMOSPHERIC COMPOSITION
• Ionospheric chemistry is important
• So is ion escape
• 3 major ions: O+, O2+, CO2
+
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ATMOSPHERIC PARTICULATES - DUST
• ~1% of the dust composition is ferric oxides
• At least 60% is silicon dioxide
• Overall mixture of basalt and clay materials
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ATMOSPHERIC PARTICULATES - DUST
• Suspended dust matches surface dust, so has likely been lifted into the air
• Smaller particles are likely knocked into the air by saltation
• Only the particles of <20 μm diameters enter long term suspension
• Dust settling time of a major storm of 1971 that was observed by Mariner 9 suggests the particles in the storm were 1-10 μm or less in diameter, with an average diameter of around 2 μm
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ATMOSPHERIC PARTICULATES - DUST
• Typical velocities (Viking lander) were around 7 m/s
• Heights of storms calculated from shadows
• Storms can be as high as 70 km, but generally range from 10-30 km [McKim, 1996].
• The optical depth has a large range: ~0.2 to >15
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ATMOSPHERIC PARTICULATES - CLOUDS
• Both water and CO2 clouds
• Form via condensation onto dust particles
• Water clouds around 25 km
• CO2 clouds at 60-80 km above the surface.
• CO2 ice clouds are observed only at places where it’s cold enough for CO2 to condense (polar winter and total darkness)
• Rovers and spacecraft (ie IUVS occultations/limb scans [D. Lo]
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ATMOSPHERIC CIRCULATION
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• Mass flow associated with CO2 cycle
• Hadley cell varies in structure and intensity
• Equinox: two approximately symmetric Hadley cells
• Solstices: single cross-equatorial circulation pattern
Why?
ATMOSPHERIC CIRCULATION
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• Mass flow associated with CO2 cycle
• Hadley cell varies in structure and intensity
• Equinox: two approximately symmetric Hadley cells
• Solstices: single cross-equatorial circulation pattern
• Mass flux can change by an order of magnitude
• Regions of high and low pressure travel around the planet causing winds, eddies, and pressure change
CONCLUSIONS
• It’s a thin atmosphere, but a lot is going on.
• MAVEN still giving new results!
• Still lots of questions! • Where did the atmosphere go? • How does the magnetic environment alter escape rates? • Are there little Calvins all over the place?
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BACKUP SLIDES
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MARTIAN IONOSPHERE
• The ionosphere is an electrically conducting region in the upper atmosphere which is a part of the lower region of the magnetosphere [Russell, 1995].
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A representation of the dayside ionosphere of Mars (P. Withers, Boston University)
SCHUMANN RESONANCES
• Lightning creates broadband EM energy that resonates in wave guide: Schumann resonances • Extremely Low Frequencies (ELF) • Earth: fundamental frequency ~8 Hz,
harmonics up to 44 Hz (Nickolaenko and Hayakawa, 2002)
• Frequency dependent on distance and conductivity
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MARS
• Charge separation in dust devils and storms plausibly lead to electric discharges
• Ruf et al. (2009): Deep Space Network radio telescope detected non-thermal radiation at the same time as dust storm
• Anderson et al. (2012) used Allen Telescope Array, concluded that the non-thermal radiation was narrowband radio frequency interference (no storms)
• Esman et al. (prep) shows no evidence in Mars Global Surveyor MAG data of Ruf et al. (2009) scale
• Unlikely to be on terrestrial scales
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MARS GLOBAL SURVEYOR MAG DATA
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PLANETARY EMISSION
• Teff=~212 K
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