EART160 Planetary Sciences

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

gRv RGM

esc 22

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?

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

Volcanoes

Sif Mons (Venus) 2km x 300kmNote vertical exaggeration!

Hawaiian shield

Olympus Mons, Mars

Dikes

Exhumed dikes (Mars & Earth)

Mars image width 3kmMOC2-1249

Ship Rock, 0.5km highNew Mexico

Dike Swarms, Mars and Earth

Radiating dike field, Venus

Lava tubes and rilles

Hadley Rille (Moon)1.5km wide

Io, lava channel? Schenk and Williams 2004

Venus, lava channel?50km wide image

Lava flows - Moon

Lava flows - Moon

Hiesinger and Head (2003)

Lava flows - Moon

Hiesinger and Head (2003)Spectral based identification of mare basalt flows.

Lava flows on the Moon - ages

Hiesinger and Head (2003)

Lava flow – lunar stratigraphy

~2 meters in height

Lava flows on Io and Venus

• Dark flows are the most recent (still too hot for sulphur to condense on them)

• Flows appear relatively thin, suggesting low viscosity

500km

Amirani lava flow, Io

500km

Comparably-sized lava flow on Venus(Magellan radar image)

Example - Mars

Although the Tharsis rise itself may be ancient, some of the lavas are very young (<20 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?

Hartmann et al. Nature 1999

The Tharsis rise contains enormous shield volcanoes. Most of them are about 25km high.What determines this height?What about their slopes?

Arsia

Pavonis

Ascraeus

Olympus

Example - Io• What’s the exit velocity?• How do speeds like this get

generated?• Volcanism is basaltic – how

do we know?• Resurfacing very rapid, ~ 1cm

per year

April 1997 Sept 1997 July 1999

400kmPele

PillanGalileo images of overlapping deposits at Pillan and Pele

Pele

Loki

250km

Why does it happen?

• Why are Earth materials anywhere near their melting points?

Why does it happen?

• Volcanism is in many ways a result of planetary materials:-Being terrible thermal conductors.

Why does it happen?• 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 (hydration at island arcs)

Temperature

Dep

th

solidus

liquidus

Reduction in pressure

Increase in temperature

No

rmal tem

peratu

re

pro

file

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

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:

d

h

c

melt

What is the depth to the melting zone on Mars?Why might this zone be deeper than on Earth?

Cooling timescale• Conductive cooling timescale

depends on thickness of object and its thermal diffusivity

d

• 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

hotcold

Temp.

• Characteristic cooling timescale t ~ d2/• How long does it take a meter thick lava flow to cool?• How long does it take the Earth to cool?

Cryovolcanism

Schenk et al. Nature 2001

Lobate flow(?)

Caldera rim

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 (?).

• 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

Tectonics

• Global tectonic patterns give us information about a planet’s 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)

Wrinkle Ridges and Lobate Scarps• Compressional features, probably

thrust faults at depth (see cartoon)• Found on Mars, Moon, Mercury,

Venus• Some related to global contraction/

• Spacing may be controlled by

crustal structure

Tate et al. LPSC 33, 2003

50km

Krieger crater, Moon

25 km

Mars, MOC wide-angle

Strike-slip Motion

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

Europa, oblique strike-slip (image width 170km)

Mechanisms: Compression• Silicate planets frequently exhibit

compression (wrinkle ridges etc.)• This is probably because the planets

have cooled and contracted over time• 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

Solid core

T Where is the thermal expansivity (3x10-5 K-1), T is the temperature change and the strain is the fractional change in radius

The Moon IS shrinking

Watters et al. 2010

Stress and strain• For many materials, stress is proportional to strain

(Hooke’s law); these materials are elastic• Stress required to generate a certain amount of strain

depends on Young’s modulus E (large E means rigid)• You can think of Young’s modulus (units: Pa) as the

stress required to cause a strain of 100%

E • 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)

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!

Flexural Stresses

• 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

Elastic plateCrust

Mantle

load

Flexural Parameter (1)• Consider a load acting

on an elastic plate:Te

m

load

3

2

1

4

3 ( )(1 )e

m w

ET

g

w

• 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 approximate width of deformation is called the flexural parameter and is given by

Here E is Young’s modulus, g is gravity and is Poisson’s ratio (~0.3)

m

Flexural Parameter (2)

• Technically, the first zero crossing xo = 3πα/4, and the forebulge maximum is at xb = πα ~3α.

• Therefore, α is less than the width of deformation.

Flexural Parameter (3)• If the applied load is much wider than , then the load

cannot be supported elastically and must be supported by buoyancy (isostasy)

• If the applied load is much narrower than , then the width of deformation is given by

• If we can measure a flexural wavelength, that allows us to infer and thus Te directly.

• Inferring Te (elastic thickness) is useful because Te is controlled by a planet’s temperature structure

Example• This is an example of a profile

across a rift on Ganymede• An eyeball estimate of 3

would be about 10 km• For ice, we take E=10 GPa,

=900 kg m-3, g=1.3 ms-2 Distance, km

10 km

• If 3=10 km then Te=6.5 km

• So we can determine Te remotely

• This is useful because Te is ultimately controlled by the temperature structure of the subsurface

Load

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

110 K273 K

elastic

viscous

190 K

•E.g. for ice the base of the elastic layer is at about 190 K• The measured elastic layer thickness is 6.5 km (from previous slide)• So the thermal gradient is 12 K/km• This tells us that the ice shell thickness is 13 km•What’s wrong with these assumptions (convection changes geotherm).

Depth

6.5 km

Temperature

Liquid!

Surf. Temp.

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:Body Te (km) dT/dz

(K/km)Body Te dT/dz

(K/km)

Earth (cont.)

30 15 Venus (450oC)

~30 15

Mars (recent)

100 5 Moon (ancient)

15 30

Europa 2 40 Moon >100

Mascons and Compensation• Surprising result of the first lunar orbiters:

Lunar gravity anomalies

• They were being perturbed by a strong density anomaly, as inferred by small velocity changes

How to measure velocity/distance?

• Doppler shift of transmitter frequency.

• Round trip “time of flight” of packet of information.

Deep Space Network

Goldstone 34 and 70 meter dishesMadrid 70 meter dish

Canberra 70 meter dish

Aside: Voyager 1 in interstellar space?• 133 AU (fall 2015)

• How much transmitter power is received on Earth?

Mascons and Compensation• Surprising result of the first lunar orbiters:

Lunar gravity anomalies

• They were being perturbed by a strong density anomaly, as inferred by small velocity changes

But the density anomaly was over larger craters!

Mascons and Compensation

Expect something like this from a mass deficit

Mantle

Crust

Or, at least, why might you expect NOTHING?

Mascons and CompensationOr, at least, why might you expect NOTHING?

Mantle

Crust

Isostasy. “Compensated”

Level A

Mascons and compensation

Mascons and Compensation

Maybe it filled with dense basalt that is supported by lithosphere (uncompensated)?

Mantle

CrustLevel A

Lithostatically supported basalt

Mascons and CompensationNice idea, but models show the basalt fill is TOO THIN to explain the gravity anomaly

Mantle

CrustLevel A

Lithostatically supported basalt

Mascons and Compensation

Mantle

Crust

The mantle appears to have “overshot” the isostatic level and is superisostatic at level B. This possibly happened during “rebound” of the deep layers of the Moon during the impact, by some kind of

temporary weakening mechanism. The mantle then “froze” in place only hours later when the weakening mechanism terminated – otherwise, it would have fallen back down to the isostatic level.

Level A

Lithostatically supported basaltLevel B

Mascons and Compensation

Mantle

Crust

Ultimately:We have a load on the lithosphere, due to a combination of basalt

fill, and a superisostatic mantle plug.

Level A

Lithostatically supported basaltLevel B

Lunar tectonics lab!

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

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

30km

Image of a dustdevil caught inthe act

Martian dune features

Aeolian features (elsewhere)Namib desert, Earthfew km spacing

Mead crater, Venus

Longitudinal dunes, Earth (top),Titan (bottom), ~ 1 km spacing

Wind directionsVenus

Wind streaks, Venus

Global patterns of wind direction can be compared with general circulation models (GCM’s)

Mars (crater diameter 90m)

Mars rover solar panels• Initially concerned

that dust would accumulate, limiting mission life.– Opportunity rover

still going since January 2004!

• Some wind removes dust from panels.

Fluvial features• Valley networks on Mars• Only occur on ancient

terrain (~4 Gyr old)• What does this imply about

ancient Martian atmosphere?

30 km

• Valley network on Titan• Presumably formed by

methane runoff• What does this imply about

Titan climate and surface?

100 km

• 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?

Martian Outflow channels

50km

flowdirection

150km

Baker (2001)

Martian Gullies• A very unexpected discovery

(Malin & Edgett, Science 283, 2330-2335, 2000)

• Found predominantly at high latitudes (>30o), 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?

Lakes

Titan, 30km across

Gusev, Mars150km

Clearwater Lakes Canada~30km diameters

Titan lakes are (presumably) methane/ethaneGusev crater shows minor evidence for water, based on Mars Rover data

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.

Malin and Edgett, Science 2003

channel

Sediments in outcrop

Opportunity (Meridiani)

Evidence of fluid flows

Summary• Volcanism happens because of higher temperatures,

reduced pressure or lowered solidus

• Conductive cooling time t = d2/• Planetary cooling leads to compression• Elastic materials = E

3

2

1

4

3 ( )(1 )e

m w

ET

g

• 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

Key Concepts• Solidus & liquidus• Conductive cooling timescale• Cryovolcanism• Hooke’s law and Young’s modulus• Contraction and cooling• Mascons, superisostatic state, and compensation• Flexural parameter and elastic thickness• Valley networks, gullies and outflow channels

End of Lecture

h

30o

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 Small TeLarge Te

Decreasing age

• 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

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

Prockter and Pappalardo, Science 2000

The only exampleof unambiguously documented compressional features on Europa to date

Last class

• Volcanism happens because of higher temperatures, reduced pressure or lowered solidus

• Planets are poor heat conductors!

• Conductive cooling time t = d2/• Solidus & liquidus

• Cryovolcanism

• Eruption height controlled by ~buoyancy

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).Friction preventsblock motionTidal

stresses

Vertical (map) view

• 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)

UpdatesRosetta mission high-res, Nov. 12 landing