1. EART160 Planetary Sciences

2. 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

3. 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?

4. 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

5. Volcanoes Hawaiian shield Sif Mons (Venus) 2km x 300km
Note vertical exaggeration!
Olympus Mons, Mars

6. 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

7. Lava tubes and rilles Venus, lava channel? 50km wide image
Hadley Rille (Moon)
1.5km wide
Io, lava channel?
Schenk and Williams 2004

8. Lava flows - Moon

9. Lava flows - Moon
Hiesinger and Head (2003)

10. Lava flows - Moon Spectral based identification of mare basalt flows.
Plot iron absorption on board. Camera has several filters.
Spectral based identification of mare basalt flows.
Hiesinger and Head (2003)

11. Lava flows on the Moon - ages
Hiesinger and Head (2003)

12. Lava flow – lunar stratigraphy
~2 meters in height

13. Lava flows on Io and Venus
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)

14. 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 (<20 Myr). We infer this from crater counts (see last lecture). So it is probable that Mars is volcanically active now.
The Tharsis rise contains enormous shield volcanoes. Most of them are about 25km high.
What determines this height?
What about their slopes?

15. 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
250km
Loki
Pele
April 1997
Sept 1997
July 1999
Pillan
Galileo images of overlapping deposits at Pillan and Pele
400km
Pele

16. Why does it happen?
Why are Earth materials anywhere near their melting points?

17. Why does it happen?
Volcanism is in many ways a result of planetary materials:
-Being terrible thermal conductors.

18. 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
Reduction in pressure
Depth
Increase in temperature
profile
Normal temperature
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

19. 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: a h
Liquid forms of most materials are less dense than their solid forms. Might be deeper because Mars is cooler than the Earth.
What is the depth to the melting zone on Mars?
Why might this zone be deeper than on Earth? rc d
rmelt

20. Cooling timescale
Conductive cooling timescale depends on thickness of object and its thermal diffusivity k
hot
cold
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 meter thick lava flow to cool?
How long does it take the Earth to cool?

21. 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

22. 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)
Tectonics is the deformation of the crust/lithosphere of planets.

23. 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
Krieger crater, Moon
25 km
50km
Mars, MOC wide-angle
Tate et al. LPSC 33, 2003

24. Strike-slip Motion Relatively rare (only seen on Earth & Europa)
Europa, oblique strike-slip (image width 170km)
Relatively rare (only seen on Earth & Europa)
Associated with plate tectonic-like behaviour

25. 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
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

26. The Moon IS shrinking Watters et al. 2010
Such compressional features were hypothesized to exist, but required high resolution, global low-sun angled images to verify. Not until recently did we have a polar orbiter that could take the required photos (LRO).
Watters et al. 2010

27. 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 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)

28. 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!
Crust and lithosphere are different things.

29. 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

30. Flexural Parameter (1) Consider a load acting on an elastic plate:rw
load
Consider a load acting on an elastic plate: rm Te
~πa (see next slide) 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 is related to the flexural parameter:
Here E is Young’s modulus, g is gravity and n is Poisson’s ratio (~0.3)

31. Flexural Parameter (2)
The first zero crossing: xo = 3πα/4, and the forebulge maximum: xb = πα

32. Flexural Parameter (3)
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 planet’s temperature structure

33. Example This is an example of a profile across a rift on Ganymede
10 km
This is an example of a profile across a rift on Ganymede
An eyeball estimate of ~3a would be about 10 km
For ice, we take E=10 GPa, Dr=900 kg m-3, g=1.3 ms-2
Load
Distance, km
If ~3a = 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

34. 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 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).
Surf. Temp.
273 K
190 K
110 K
6.5 km
Depth
Liquid water below.
elastic
viscous
Temperature
Liquid!

35. 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 (approximate!) values for solar system objects:
Body
Te (km)
dT/dz (K/km) Te
Earth (cont.) 30 15
Venus (450oC)
~10-30
Mars (recent)
100 5
Moon (ancient)
Europa 2 40
Moon
>100

36. 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

37. How to measure velocity/distance?
Doppler shift of transmitter frequency.
Round trip “time of flight” of packet of information.

38. Deep Space Network Goldstone 34 and 70 meter dishes
How do we know where Mars rover will enter the martian atmosphere? How do we communicate with Voyager? Doppler and Ranging to spacecraft using the DSN dishes. SC transmitter power output is about 25 watts. Power at Earth for Voyager? 20 x 1012 meters.
Goldstone 34 and
70 meter dishes
Madrid 70 meter dish
Canberra 70 meter dish

39. Aside: Voyager 1 in interstellar space?
136 AU (fall 2016)
20 b.y. km, 1.6 light days
How much transmitter power is received on Earth?

40. 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!

41. Mascons and Compensation
Expect something like this from a mass deficit
Crust
Mantle
Or, at least, why might you expect NOTHING?

42. Mascons and Compensation
Or, at least, why might you expect NOTHING?
Crust
Level A
Mantle
Isostasy. “Compensated”

43. Mascons and compensation

44. Mascons and Compensation
Maybe it filled with dense basalt that is supported by lithosphere (uncompensated)?
Lithostatically supported basalt
Crust
Level A
Mantle

45. Mascons and Compensation
Nice idea, but models show the basalt fill is TOO THIN to explain the gravity anomaly
Lithostatically supported basalt
Crust
Level A
Mantle

46. Mascons and Compensation
Lithostatically supported basalt
Crust
Level B
Level A
Mantle
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.

47. Mascons and Compensation
Lithostatically supported basalt
Crust
Level B
Level A
Mantle
Ultimately:
We have a load on the lithosphere, due to a combination of basalt fill, and a superisostatic mantle plug.

48. Lunar tectonics lab!

49. 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

50. 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

51. Aeolian features (elsewhere)
Namib desert, Earth
few km spacing
Longitudinal dunes, Earth (top),
Titan (bottom), ~ 1 km spacing
Mead crater, Venus

52. Wind directions
Venus
Wind streaks, Venus
Mars (crater diameter 90m)
Global patterns of wind direction can be compared with general circulation models (GCM’s)

53. 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.

54. 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

55. 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?
50km
flow
direction
150km
Baker (2001)

56. Martian Gullies
A very unexpected discovery (Malin & Edgett, Science 283, , 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?

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

58. 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.
channel
Malin and Edgett, Science 2003

59. Sediments in outcrop
Opportunity (Meridiani)
Evidence of fluid flows

60. 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

61. 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

62. End of Lecture

63. h
30o

64. Te and age
Small Te
Large Te
Decreasing 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

65. 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

66. Last class
Volcanism happens because of higher temperatures, reduced pressure or lowered solidus
Planets are poor heat conductors!
Conductive cooling time t = d2/k
Solidus & liquidus
Cryovolcanism
Eruption height controlled by ~buoyancy

67. 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)

68. Updates
Rosetta mission high-res, Nov. 12 landing