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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