EART160 Planetary Sciences

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EART160 Planetary Sciences
Francis Nimmo
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Last week crusts and impacts

• Planetary crustal compositions may be determined
  by in situ measurements or remote sensing
  (spectroscopy)
• Most planetary crusts are basaltic
• Impact velocity will be (at least) escape
  velocity
• Impacts are energetic and make craters
• Crater size depends on impactor size, impact
  velocity, surface gravity
• Crater morphology changes with increasing size
• Crater size-frequency distribution can be used to
  date planetary surfaces
• Atmospheres and geological processes can affect
  size-frequency distributions

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This Week

• Volcanism, tectonics and sedimentation
• What controls where and when volcanism happens?
• What kinds of tectonic features are observed on
  other planetary bodies, and what do they imply?
• How are loads on planetary bodies supported?
• What sedimentary features are observed?

Dont forget to look into summer undergraduate
research opportunities see the class website
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Volcanism

• Volcanism is an important process on most solar
  system bodies (either now or in the past)
• It gives information on the thermal evolution and
  interior state of the body
• It transports heat, volatiles and radioactive
  materials from the interior to the surface
• Volcanic samples can be accurately dated
• Volcanism can influence climate

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Volcanoes
Hawaiian shield
Sif Mons (Venus) 2km x 300km Note vertical
exaggeration!
Olympus Mons, Mars
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Dikes
Exhumed dikes (Mars Earth) Mars image width
3km MOC2-1249 Ship Rock, 0.5km high New Mexico
Radiating dike field, Venus
Dike Swarms, Mars and Earth
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Lava tubes and rilles
Venus, lava channel? 50km wide image
Hadley Rille (Moon) 1.5km wide
Io, lava channel? Schenk and Williams 2004
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Lava flows
Amirani lava flow, Io
500km

• Dark flows are the most recent (still too hot for
  sulphur to condense on them)
• Flows appear relatively thin, suggesting low
  viscosity

500km
Comparably-sized lava flow on Venus (Magellan
radar image)
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Example - Mars
Hartmann et al. Nature 1999
Olympus
Ascraeus
Pavonis
Arsia
Although the Tharsis rise itself may be ancient,
some of the lavas are very young (lt20 Myr). We
infer this from crater counts (see last lecture).
So it is probable that Mars is volcanically
active now. How might we test this? (Very) recent
methane results?
The Tharsis rise contains enormous shield
volcanoes. Most of them are about 25km high. What
determines this height? What about their slopes?
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Example - Io

• Whats the exit velocity?
• How do speeds like this get generated?
• Volcanism is basaltic how do we know?
• Resurfacing very rapid, 1cm per year

Loki
Pele
April 1997
July 1999
Sept 1997
Pillan
Galileo images of overlapping deposits at Pillan
and Pele
400km
Pele
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Why does it happen?
Temperature

• Material (generally silicates) raised above the
  melting temperature (solidus)
• Increase in temperature (plume e.g. Hawaii)
• Decrease in pressure (mid-ocean ridge)
• Decrease in solidus temperature (island arcs)

Reduction in pressure
Increase in temperature
Depth
Normal temperature profile
liquidus
solidus
Reduction in solidus

• Partial melting of (ultramafic) peridotite mantle
  produces (mafic) basaltic magma
• More felsic magma (e.g. andesite) requires
  additional processes e.g. fractional
  crystallization

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Eruptions

• Magma is often less dense than surrounding rock
  (why?)
• So it ascends (to the level of neutral buoyancy)
• For low-viscosity lavas, dissolved volatiles can
  escape as they exsolve this results in gentle
  (effusive) eruptions
• More viscous lavas tend to erupt explosively
• We can determine maximum volcano height

h
What is the depth to the melting zone on
Mars? Why might this zone be deeper than on Earth?
d
rc
rm
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Cooling timescale

• Conductive cooling timescale depends on thickness
  of object and its thermal diffusivity k

cold
hot

• Thermal diffusivity is a measure of how
  conductive a material is, and is measured in
  m2s-1
• Typical value for rock/ice is 10-6 m2s-1

Temp.
d

• Characteristic cooling timescale t d2/k
• How long does it take a metre thick lava flow to
  cool?
• How long to boil an egg?
• How long does it take the Earth to cool?

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Cryovolcanism

• Cryovolcanism was predicted on the basis of
  Voyager images to occur on icy satellites, but it
  appears to be rare
• Eruption of water (or water-ice slurry) is
  difficult due to low density of ice

This image shows one of the few examples of
potential cryovolcanism on Ganymede. The caldera
may have been formed by subsidence following
eruption of volcanic material, part of which
forms the lobate flow (?) within the caldera. The
relatively steep sides of the flow suggest a high
viscosity substance, possibly an ice-water slurry
(?).
Caldera rim
Lobate flow(?)
Schenk et al. Nature 2001
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Tectonics

• Global tectonic patterns give us information
  about a planets thermal evolution
• Abundance and style of tectonic features tell us
  how much, and in what manner, the planet is being
  deformed i.e. how active is it?
• Some tectonic patterns arise because of local
  loading (e.g. by volcanoes)

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Extensional Tectonics
Valles Marineris, Mars (8km deep)
Craters on Ganymede
37km diameter
Pappalardo Collins 2005 Diam. appx 40km
Crater on Venus
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Extension faulting

• Extension accommodated by normal faulting

q0
Stretching factor
L0
q
L

• Fault blocks rotate as extension proceeds
• Typical normal faults start with dips of 60o and
  lock up when dips30o, giving stretching factor
  1.7
• Stretching factor also controls amount of
  subsidence that happens during extension

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A Martian Rift Valley

• Looks similar to terrestrial continental rifts.
• Not been heavily studied, but may provide useful
  insights into crustal properties.

Hauber and Kronberg, JGR Planets, 2001
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Graben Systems
Steep scarp
Flat floor
Relay ramp?
Graben, Ganymede
Canyonlands graben, Utah, 2km across
across)
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Bands (Europa)
20km
from Sullivan et al., Nature (1998)
What mechanism drives the extension?
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Wrinkle Ridges and Lobate Scarps

• Compressional features, probably thrust faults at
  depth (see cartoon)
• Found on Mars, Moon, Mercury, Venus
• Possibly related to global contraction due to
  cooling?
• Spacing may be controlled by crustal structure

Mars, MOC wide-angle
Tate et al. LPSC 33, 2003
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Io compressional tectonics

• Burial leads to large compressive stresses due to
  change in radius
• Stresses easily large enough to initiate faulting
• Additional compressive stresses may arise from
  reheating the base of the crust

DR
After McKinnon et al., Geology 2001
stereo
Low-angle thrust faulting is probably responsible
for many of the mountain ranges seen on Io
550 km
10km
Schenk and Bulmer, Science 1998
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Strike-slip Motion
Explain pull-apart better!
Europa, oblique strike-slip (image width 170km)

• Relatively rare (only seen on Earth Europa)
• Associated with plate tectonic-like behaviour

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Mechanisms Compression

• Silicate planets frequently exhibit compression
  (wrinkle ridges etc.)
• This is probably because the planets have cooled
  and contracted over time
• (think railway tracks)
• Why do planets start out hot?
• Further contraction occurs when a liquid core
  freezes and solidifies
• Contractional strain given by

Hot mantle
Liquid core
Cool mantle
Where a is the thermal expansivity (3x10-5 K-1),
DT is the temperature change and the strain is
the fractional change in radius
Solid core
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Stress and strain

• For many materials, stress is proportional to
  strain (Hookes law) these materials are elastic
• Stress required to generate a certain amount of
  strain depends on Youngs modulus E (large E
  means rigid)
• You can think of Youngs modulus (units Pa) as
  the stress s required to cause a strain of 100

• Typical values for geological materials are 100
  GPa (rocks) and 10 GPa (ice)
• Elastic deformation is reversible but if strains
  get too large, material undergoes fracture
  (irreversible)

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Mechanisms Extension

• For icy satellites, one possible explanation for
  the ubiquitous extension is that they possess
  floating ice shells which thickened with time
  (see below)
• Why should the shell thicken?

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Tectonic Stresses Byerlees law

• Byerlees law says that faults dont move unless
  the shear stress exceeds the normal stress times
  the friction coefficient f
• For almost all geological materials, f0.6
  (unless the fault is lubricated somehow)

Shear stress
Normal stress
fault

• In general, the normal stress is simply the
  overburden pressure
• Prgh

• The shear stresses are provided by tectonic
  effects
• E.g. to cause a fault 10 km deep on Earth to move
  requires tectonic stresses of 3000 x 10 x 104 x
  0.6 180 MPa (a lot!)
• Typical tectonic stresses on Earth are usually
  10-100 MPa
• Why might Venus and Earth faults behave
  differently?

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Flexure and Elasticity

• The near-surface, cold parts of a planet (the
  lithosphere) behaves elastically
• This lithosphere can support loads (e.g.
  volcanoes)
• We can use observations of how the lithosphere
  deforms under these loads to assess how thick it
  is
• The thickness of the lithosphere tells us about
  how rapidly temperature increases with depth i.e.
  it helps us to deduce the thermal structure of
  the planet
• The deformation of the elastic lithosphere under
  loads is called flexure
• See EART162 for more details!

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Flexural Stresses
load
Crust
Elastic plate
Mantle

• In general, a load will be supported by a
  combination of elastic stresses and buoyancy
  forces (due to the different density of crust and
  mantle)
• The elastic stresses will be both compressional
  and extensional (see diagram)
• Note that in this example the elastic portion
  includes both crust and mantle

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Flexural Parameter (1)
load
rw

• Consider a load acting on an elastic plate

Te
a
rm

• The plate has a particular elastic thickness Te
• If the load is narrow, then the width of
  deformation is controlled by the properties of
  the plate
• The width of deformation a is called the flexural
  parameter and is given by

Here E is Youngs modulus, g is gravity and n is
Poissons ratio (0.3)
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Flexural Parameter (2)

• If the applied load is much wider than a, then
  the load cannot be supported elastically and must
  be supported by buoyancy (isostasy)
• If the applied load is much narrower than a, then
  the width of deformation is given by a
• If we can measure a flexural wavelength, that
  allows us to infer a and thus Te directly.
• Inferring Te (elastic thickness) is useful
  because Te is controlled by a planets
  temperature structure

a
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Example
10 km

• This is an example of a profile across a rift on
  Ganymede
• An eyeball estimate of a would be about 10 km
• For ice, we take E10 GPa, Dr900 kg m-3 (there
  is no overlying ocean), g1.3 ms-2

Distance, km

• If a10 km then Te1.5 km
• A numerical solution gives Te1.4 km pretty
  good!
• So we can determine Te remotely
• This is useful because Te is ultimately
  controlled by the temperature structure of the
  subsurface

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Te and temperature structure

• Cold materials behave elastically
• Warm materials flow in a viscous fashion
• This means there is a characteristic temperature
  (roughly 70 of the melting temperature) which
  defines the base of the elastic layer

• E.g. for ice the base of the elastic layer is at
  about 190 K
• The measured elastic layer thickness is 1.4 km
  (from previous slide)
• So the thermal gradient is 60 K/km
• This tells us that the (conductive) ice shell
  thickness is 2.7 km (!)

110 K
270 K
190 K
1.4 km
Depth
elastic
viscous
Temperature
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Te in the solar system

• Remote sensing observations give us Te
• Te depends on the composition of the material
  (e.g. ice, rock) and the temperature structure
• If we can measure Te, we can determine the
  temperature structure (or heat flux)
• Typical (approx.) values for solar system objects

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Erosion and Deposition

• Erosion and deposition require the presence of a
  fluid (gas or liquid) to pick up, transport and
  deposit surface material
• Liquid transport more efficient
• These processes tend to be rapid compared to
  other geological processes
• So surface appearance is often controlled by
  these processes
• Earth, Mars, Titan, Venus have erosional or
  sedimentary features

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Aeolian Features (Mars)

• Wind is an important process on Mars at the
  present day (e.g. Viking seismometers . . .)
• Dust re-deposited over a very wide area (so the
  surface of Mars appears to have a very homogenous
  composition)
• Occasionally get global dust-storms (hazardous
  for spacecraft)
• Rates of deposition/erosion almost unknown

Martian dune features
Image of a dust devil caught in the act
30km
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Aeolian features (elsewhere)
Namib desert, Earth few km spacing
Longitudinal dunes, Earth (top), Titan (bottom),
1 km spacing
Yardangs (elongated dunes) Mead crater, Venus
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Wind directions
Venus
Wind streaks, Venus
Mars (crater diameter 90m)
Global patterns of wind direction can be compared
with general circulation models (GCMs)
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Fluvial features

• Valley networks on Mars
• Only occur on ancient terrain (4 Gyr old)
• What does this imply about ancient Martian
  atmosphere?

100 km

• Valley network on Titan
• Presumably formed by methane runoff
• What does this imply about Titan climate and
  surface?

30 km
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Martian Outflow channels

• Large-scale fluvial features, indicating massive
  (liquid) flows, comparable to ocean currents on
  Earth
• Morphology similar to giant post-glacial floods
  on Earth
• Spread throughout Martian history, but
  concentrated in the first 1-2 Gyr of Martian
  history
• Source of water unknown possibly ice melted by
  volcanic eruptions (jokulhaups)?

Baker (2001)
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Martian Gullies

• A very unexpected discovery (Malin Edgett,
  Science 283, 2330-2335, 2000)
• Found predominantly at high latitudes (gt30o), on
  pole-facing slopes, and shallow (100m below
  surface)
• Inferred to be young cover young features like
  dunes and polygons
• How do we explain them? Liquid water is not
  stable at the surface!
• Maybe even active at present day?

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Lakes
Clearwater Lakes Canada 30km diameters
Gusev, Mars 150km
Titan, 30km across
Titan lakes are (presumably) methane/ethane Gusev
crater shows little evidence for water, based on
Mars Rover data
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Erosion

• Erosion will remove small, near-surface craters
• But it may also expose (exhume) craters that were
  previously buried
• Erosion has recently been recognized as a major
  process on Mars, but the details are still
  extremely poorly understood
• The images below show examples of fluvial
  features which have been exhumed the channels
  are highstanding. Why?

channel
meander
Malin and Edgett, Science 2003
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Sediments in outcrop
Opportunity (Meridiani)
Cross-bedding indicative of prolonged fluid flows
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Summary

• Volcanism happens because of higher temperatures,
  reduced pressure or lowered solidus
• Conductive cooling time t d2/k
• Planetary cooling leads to compression
• Elastic materials s E e

• Flexural parameter controls the lengthscale of
  deformation of the elastic lithosphere

• Lithospheric thickness tells us about thermal
  gradient
• Bodies with atmospheres/hydrospheres have
  sedimentation and erosion Earth, Mars, Venus,
  Titan

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Key Concepts

• Solidus liquidus
• Conductive cooling timescale
• Cryovolcanism
• Stretching factor
• Hookes law and Youngs modulus
• Contraction and cooling
• Byerlees law
• Flexural parameter and elastic thickness
• Valley networks, gullies and outflow channels

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End of Lecture
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Te and age

• The elastic thickness recorded is the lowest
  since the episode of deformation
• In general, elastic thicknesses get larger with
  time (why?)

McGovern et al., JGR 2002

• So by looking at features of different ages, we
  can potentially measure how Te, and thus the
  temperature structure, have varied over time
• This is important for understanding planetary
  evolution

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Compression on icy satellites

• Rarely observed. Why not?
• Is it hidden somewhere?
• Icy satellites are dominated by extension

The only example of unambiguously documented
compressional features on Europa to date
Prockter and Pappalardo, Science 2000
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Tidally-driven strike-slip faults

• How do they form? A consequence of the way tidal
  stresses rotate over one diurnal cycle (Tufts et
  al. 1999).

Vertical (map) view
Friction prevents block motion
Tidal stresses

• This ratcheting effect can lead to large net
  displacements
• Strike-slip motion will lead to shear heating if
  sufficiently rapid (c.f. San Andreas on Earth)