The enigmatic polar caps of Mars

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The enigmatic polar caps of Mars

• Brief summary of history of observations
• Theory of seasonal cap behavior
• Residual (permanent) polar caps and evidence for
  climate change 105 to 107 years 10 200
  years
• Future space observations

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Christian Huygens
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Sir William Herschel

• Discovered Uranus
• Discovered IR radiation
• Deduced disc galaxy
• Recognized that polar caps were seasonal used
  observations to measure Mars obliquity (1784)

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G. Johnstone Stoney

• Suggested term electron for unit of charge in
  1891
• Applied kinetic theory of gasses to planetary
  atmospheres. Studied helium in earths
  atmosphere.
• Used this to suggest that Martian polar caps are
  CO2.

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Composition of Seasonal Cap

• Scientists (except Stoney) assumed that the caps
  were water ice snow
• This assumption was crucial to Percival Lowells
  canal theory, since he assumed the melting polar
  caps were the source of water.
• Kuiper used reflection spectra to identify water
  ice in cap. He also measured CO2 in atmosphere.

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Mariner 4 (1965)

• Radio occultations ? Martian atmospheric pressure
  is 600 pascals, over an order of magnitude less
  than most conservative previous estimates.
  Therefore, pATM pCO2.

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Robert Leighton and Bruce Murray
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Polar energy balance

• Absorbed insolation net energy advected into
  region conduction from subsurface IR radiance
  from atmosphere latent heat released by
  subliming CO2 energy radiated by surface
• L M (Science, 66) showed that CO2 will
  condense and that seasonal polar caps are carbon
  dioxide

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Viking Landers 1976-1982 Pressure
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1980s-1990s

• Hiatus in space exploration of Mars
• Modeling of polar caps using the Viking pressure
  curves as the primary constraint
• One D models gave way to GCM models based on
  primitive atmospheric equations
• Curves can be fit to pressure and predicts mass
  of CO2 condensed

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CO2 Condensed Mass

• During polar night the latent heat should be
  roughly equal to the radiation
• Unphysically low emissivities required to avoid
  having too much CO2 condense leading to large
  amplitude pressure curve
• Is there an additional source of energy (in
  addition to CO2 latent heat) in polar night?

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Conduction from sub-surface

• The ability of the surface to store energy is
  determined by thermal inertia vKT?cP
• Thermal inertia of surface traditionally
  determined from diurnal temperature observations
  that sample 1-10 cm. For that inertia,
  conduction is unimportant
• However, the seasonal penetration is much greater
  and samples 10 cm 1 m

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Gamma Ray Spectrometer on Mars Odyssey
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GRS determination of water
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Thermal storage in surface

• GRS discovered that in the polar regions there is
  nearly pure water ice just beneath the surface
• This enhances the conduction storage term and
  reduces CO2 condensation to match pressure

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Residual (permanent) Caps at both poles
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Buffering

• Suppose that CO2 remains at one of the poles all
  year.
• In equilibrium, the energy absorbed by the cap
  Energy radiated by cap sT(p)4
• Energy absorbed sina (obliquity)
• In pure CO2, sublimation temperature is a
  function of pressure
• So if there is big enough block of CO2 at poles,
  p will change with obliquity

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Obliquity

• The obliquity of Mars changes greatly over
  relatively short time scales due to the effects
  of other planets.
• Pressure change would bring about different
  climate (Sagan Malin, 73)

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• One of main goals of polar orbiting Viking
  Orbiter 2 was to determine composition of the
  much larger residual north polar cap.
• VO2 measured water vapor concentration (MAWD) and
  surface temperature (IRTM) of 220 K during
  summer.
• Result residual north polar cap water ice
• IRTM later showed that smaller residual south cap
  is CO2 ice because its temper-ature remains at
  150 K all summer.

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Polar Layered Terrain

• Viking discovered that ground underlying the caps
  is composed of many layers
• Possibly responds to variation of orbital
  parameters with T 105 107 years
• Layers composed of various mixtures of dust and
  water ice

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Mars Orbiter Camera on MGS

• Two wide angle (140 FOV) cameras make daily
  global map in red and blue wavelengths
• High resolution camera can resolve features as
  small as ½ meter at nadir minus blue filter

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Earth and Moon from MarsMars Odyssey from MGS
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Residual South Cap MOC

• Color images of the residual south polar cap at
  LS306º on (A) February 22, 2000, (B) January 9,
  2002, and (C) November 28, 2003.

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MGS Viking M9 RSPC
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Swiss cheese terrain
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One Mars Year Change
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Changes since Mariner 9 (16 MY)
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2001 Dust Storm
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Mountains of Mitchel 1999/01
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Effects of Atmospheric Dust on Sublimation
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Flux redistribution by dust

• Visible flux at surface CO2 frost decreases with
  increasing dust optical depth
• However, infrared flux increases with increasing
  optical depth because of emission by hot dust

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Albedo / Emissivity of CO2
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Effect of dust on sublimation

• Region with 0 dust sublimes more rapidly with
  increasing optical depth
• Region with large dust content and low visible
  albedo sublimes more slowly
• Effect on sublimation small for typical areas in
  the seasonal cap

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RSPC Albedo

• Measurements from HST HRC at 2003 opposition
• Dashed lines are albedos assuming t 0 solid
  lines t 0.2
• Albedos sufficient to stabilize residual cap
• Dust will increase sublimation rate

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Conclusions

• The RSPC is a unique feature totally unlike other
  portions of the polar caps
• The RSPC is dynamic on time scales of years.
  Stratigraphy suggests short deposition periods
  separated by longer periods of erosion
• The timeline, together with Mariner 9 B images,
  suggest that the last period of deposition was
  somewhere around 1970
• Late season dust storms could effect removal of
  RSPC units
• May also be connected with H2O ice distribution

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Mars Reconnaissance Orbiter

• HIRISE hi res with some color
• MARCI 180 FOV 5 vis 2 uv bands
• CTX 5 meters / pixel
• CRISM imaging spectrometer .4 - 4µm
• MCS atm profiles
• SHARAD 15 meter depth resolution

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Polar Observations

• 3pm orbit (compare 2 pm for MGS 5 pm Odyssey)
• Periapsis over south pole at 255 KM
• Apoapsis over north pole at 320 Km
• After one (earth year) lt 5 km between ground
  tracks at equator
• 12 orbits cross the poles every day

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MARCI Polar Science

• Acquire albedo maps of the poles in five bands
  and two UV channels
• Study behavior of dust storms and condensate
  clouds associated with the frost boundaries of
  both poles
• Search for interannual variability of and within
  seasonal caps
• Diurnal behaviors of storms and clouds
• Study frost phase functions at various
  wavelengths

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Polar Observations

• CTX is ideal for monitoring temporal changes in
  Swiss cheese features in RSPC, spiders, dark
  spots, etc. in the South Polar Region
• Similarly, CTX should be useful for monitoring
  albedo features and specific areas in the north
  polar region.
• CTX should reveal details of polar dust storm and
  cloud structures