Planetary Ices

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Planetary Ices
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In this lecture

• Ice around the solar system
• Abundance of ices
• Phases of ices
• Flow of ice
• Shapes of ice sheets
• How glaciers work
• Glacial features
• Periglacial features
• Expansion/contraction of ice
• Frost heave etc
• Pingos, polygons etc
• Sublimation of ice
• Mars CO2
• Triton N2
• Grain growth of ices

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Volatile depends on your point of view

• Behavior of ices is strongly temperature
  dependant
• Maxwell time
• Stress causes elastic deformation and creep
• Time after which creep strain equals elastic
  strain
• tM eel / (?ecreep/t) ?/µ
• µ is the shear modulus (rigidity), ? is the
  viscosity
• On Earth
• tM for rock gt109 years
• tM for ice 100s sec
• Water ice acts like rock outside Jupiters orbit
• Why so much water ice?

Arakawa and Kouchi, 2007
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Ice Abundances

• Ice is an increasing volumetric fraction (mass
  fraction is smaller) of bodies at increasing
  distances

• Venus fice0
• NONE
• Moon Mercury fice0
• Small deposits in permanently shadowed polar
  craters
• Earth/Mars ficesmall
• Large polar ice sheets
• Mountain glaciers
• Right regime for flow
• Asteroids fice0-50
• Completely rocky to ice/rock
• Some differentiated
• Galilean satellites fice70
• Io mostly devolatilized
• Subsurface oceans when differentiated

ROCK
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Water Ice Types

• Many crystal structures of water ice exist
• On Earth its all ice I
• Almost all ice Ih
• A little ice Ic in very high altitude clouds
  transition at 200K
• Under 1km of ice pressure in only 0.01 GPa
• In the outer solar system ice II III are
  possible
• Surface (almost) always ice I

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Water ice on planetary surfaces

• State of ice depends on the temperature and
  pressure when deposited
• Mostly low pressure for planetary surfaces
• Temperature is controlling factor
• Solar nebula ice-line
• Somewhere in asteroid belt T150K
• All ice interior to this was delivered later by
  impacts
• Low temperature phases are all meta-stable
• Presence of amorphous ice on comets indicate that
  they formed beyond Uranus orbit
• Much Cometary material never heated above 70K

Jenniskens et al., 1998
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• Underdense structure of ice Ih allows for
  clathrate hydrates
• Methane clathrates on Earths ocean floor
• CO2 clathrate hydrate on Mars
• Methane clathrates on Mars more speculative

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Big ice sheets of the inner solar system Earth
6 million km3
30 million km3
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Big ice sheets of the inner solar system Mars

• About 20 of the size of the Greenland ice-sheet

1.1 million km3
1.2 million km3
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• Modeling of ice cap flow
• Needs lots of assumptions
• Rates can vary by orders of magnitude
• Glens (Nye, 1953) Flow law

Greve and Mahajan, 2004

• Q is the activation energy
• n is the stress exponent
• A pre-exponential factor

Durham et al., 1997
Durham et al., 1997
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• Shape of ice sheet determined by
• Mass balance pattern
• Flow
• Modeling of Mars polar deposits
• Flow rates 0.1-1 mm yr-1

Greve and Mahajan, 2004
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• Mountain glaciers on Earth and Mars (and maybe
  Titan)

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• Geomorphologic products of glaciation
• Drumlins U-shaped valleys hanging valleys
• Eskers moraines
• Most of which suggested at some point for Mars

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Glacial control of Topography

• The glacial buzzsaw
• Earths high-altitude topography controlled by
  glaciers
• High-latitude mountains are smaller
• Mountain peaks correlate with snowline elevation

Egholm et al. 2009
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• Lobate debris aprons
• Now known to be almost pure ice

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Present Day Glaciers
Where are all icy features?
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Ground ice processes

• Permafrost covers a large portion of the Earth
• Ground ice on Mars theorized to exist for some
  time
• Regolith provides the thermal insulation
• Sharp latitude cutoff
• Stable ice distribution changes with climate
• Diffusive contact with the atmosphere

Mellon et al., 2004
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• Ground ice on Mars theorized to exist for some
  time
• Regolith provides the thermal insulation
• Stable ice distribution changes with climate
• Diffusive contact with the atmosphere

Water vapor
Surface
Up to a few feet down
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• Many models same behavior
• Shallow high-latitude ice
• Ice-free equatorial region
• Very sharp boundary
• Boundary position very sensitive
• Thermophysical properties
• Independently determined
• Global average water vapor
• Todays value is 10-14 pr µm

Chamberlain and Boynton, 2007
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• Gamma Ray spectrometer on Mars Odyssey spacecraft
  has detected hydrogen (from H2O) in near surface

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Ground ice processes

• Ice expands (a lot) when heated

• Model as a viscoelastic solid
• Short-term elastic behavior
• Long-term viscous behavior
• Heating ? expansion ? compression
• Some stress relieved by viscous creep
• Cooling ? shrinkage ? extension
• Less stress relieved viscously at lower T
• Tensile stress causes cracking
• Repeated thermal cycling
• Cracks propagate
• Propagation stops when it hits another crack
• Cracks fill with debris
• Compression cause ridges along cracks

Melon, 1997
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Frost Heave and Premelting

• Gibbs Thompson effect
• Freezing point depressed by surface curvature
• Liquid stays mobile around grain boundaries

Rempel, 2007
Taber, 1930
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• Modeled for pure water on the Earth
• A slow process on Mars?
• Film thickness on the order of monolayers (0.35
  nm)
• Mid-latitude ice on Mars (200k) would have about
  2 molecular layers of water
• Effects of perchlorates and other salts not
  included

Kereszturi et al., 2009
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• Pure ground ice on Mars

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• The Phoenix lander also discovered very pure
  buried water ice

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• Pingos on Mars??
• Water in the near subsurface freezes
• Frost heave produces growing ice lens causes
  uplift
• Water confined by either artesian pressure or
  trapped in lake sediments

Dundas et al., 2007
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• Differential frost heave
• Freezing front parallel to surface
• Freezing front deepens faster around buried rock
• Rock gets pushed up and downhill
• Form circles/stripes
• Phase change means this happens commonly on Earth
• HiRISE showing some possible cases on Mars

Kessler and Werner, 2003
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• Stone circles
• Labyrinthine terrain
• Stone stripes

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• An analogue for mars fretted terrain?
• Concentric crater fill
• Lineated valley fill

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• Regelation
• Thin films exist around impurities
• Films freeze on cold side
• Particle pushed along thermal gradient towards
  warmer temperatures
• Gravity irrelevant
• Impurities eventually expelled from ice
• Ground ice can self-purify itself

Miller and Black, 2003
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Ice Ablation

• Planetary ices sublimate at a (very) temperature
  dependant rate
• Depends on the vapor pressure
• Psat comes from the Clausius-Clapeyron relation
• i.e.

Andreas, 2007

• Sublimation rates are extraordinarily temperature
  sensitive
• 1m of water ice can survive 1 Gyr at 110K
• but temps of 130K can sublimate 1km of water ice
• lt 1m/Gyr is considered stable

Vasavada et al., 1999
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Sublimation Products

• Sublimation removes ice leaving debris behind
• Sublimation lag can cut off furthur sublimation
• Situation at Martian pole today
• Dry valley research on old terrestrial ice
• Ice forms like penitentes
• Blades of ice 1-2m high
• Found in driest, coldest snowfields
• Requires near-overhead solar radiation
• Mostly in Andies
• Martian north polar residual ice cap
• Undulating highs and lows
• Relief 1m, wavelength 10m
• Extremely homogeneous
• Slopes are low

Ng et al., 2005
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Ice Grain Growth

• On the Earth the snow to ice conversion timescale
  is of order years
• On Mars water ice is vapor deposited
• Very little porosity
• Modeling suggests grain-growth over 1000s of
  years
• Very dependant on thermal history

Micron sized grains
mm sized grains
Langevin et al., 2005
34

• Continual sublimation/condensation causes
  increase in grain size
• Molecules tend to evaporate from edges and
  corners and redeposit on flat-faces
• Surface energy larger on faces than edges

Clark et al., 1983

• E.g. for the Martian polar cap
• 210 K
• mm-sized grains can form in a few 1000 years

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• Cryptic ice on Mars
• CO2 ice grain growth to meter size

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• Grain growth for CO2 crystals
• Porous system pressureless sintering
• Non-porous grain boundary migration

• Martian CO2
• Deposited as micron-sized grains
• Grow to 10s of cm in 100s days

Eluszkiewicz et al., 2004
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• Same processes on Triton
• Nitrogen stable as frost and gas
• N2 atmosphere of 0.016 milli-bars
• Seasonal frost cap at 38K
• Also has an icy solid state greenhouse

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Summary

• Ice around the solar system
• Abundance of ices
• Phases of ices
• Flow of ice
• Shapes of ice sheets
• How glaciers work
• Glacial features
• Periglacial features
• Expansion/contraction of ice
• Frost heave etc
• Pingos, polygons etc
• Sublimation of ice
• Mars CO2
• Triton N2
• Grain growth of ices