EART 160 Planetary Sciences Last week crusts and

EART 160 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 sizefrequency distributions

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

Dikes Exhumed dikes (Mars & Earth) Mars image width 3 km MOC 2 -1249 Ship Rock, 0. 5 km high New Mexico Radiating dike field, Venus Dike Swarms, Mars and Earth

Lava tubes and rilles Venus, lava channel? 50 km wide image Hadley Rille (Moon) 1. 5 km wide Io, lava channel? Schenk and Williams 2004

Lava flows - Moon

Lava flows - Moon Hiesinger and Head (2003)

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

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

Lava flow – lunar stratigraphy ~2 meters in height

Lava flows on Io and Venus Amirani lava flow, Io 500 km • Dark flows are the most recent (still too hot for sulphur to condense on them) • Flows appear relatively thin, suggesting low viscosity 500 km Comparably-sized lava flow on Venus (Magellan radar image)

Example - Mars Hartmann et al. Nature 1999 Olympus Ascraeus Pavonis Arsia Although the Tharsis rise itself may be ancient, some of the lavas The Tharsis rise contains enormous shield volcanoes. Most are very young (<20 Myr). We infer this from crater counts (see of them are about 25 km high. last lecture). So it is probable that What determines this height? Mars is volcanically active now. What about their slopes?

Example - Io • What’s the exit velocity? • How do speeds like this get generated? • Volcanism is basaltic – how do we know? • Resurfacing very rapid, ~ 1 cm per year April 1997 Loki Pele July 1999 Sept 1997 Pillan Pele 250 km 400 km Galileo images of overlapping deposits at Pillan and Pele

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

Why does it happen? • Or, why is there so much heat inside of planets? – Planetary materials (rocks, ice) are excellent thermal insulators – Heat of accretion + heat from radioactive elements is retained for a long time.

Energy of Accretion • Let’s assume that a planet is built up like an onion, one shell at a time. How much energy is involved in putting the planet together? early later In which situation is more energy delivered? Total accretional energy = ? If all this energy goes into heat*, what is the resulting temperature change? a What do we conclude from this exercise? * Is this a reasonable assumption?

Types of energy transfer • Conduction • Convection • Radiation

Why are planets hot? - Cooling timescale • Conductive cooling timescale depends on thickness of object and its thermal diffusivity k • Thermal diffusivity is a measure of how conductive a material is, and is measured in m 2 s-1 • Typical value for rock/ice is 10 -6 m 2 s-1 hot cold Temp. d • Characteristic cooling timescale t ~ d 2/k • How long does it take a meter thick lava flow to cool? • How long does it take the Earth to cool?

Why does melting happen? Temperature Reduction in pressure Increase in temperature Reduction in solidus liquidus solidus re mperatu te Normal profile – Increase in temperature (plume e. g. Hawaii) – Decrease in pressure (midocean ridge) – Decrease in solidus temperature (hydration at island arcs) Depth • Material (generally silicates) raised above the melting temperature (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 a • We can determine maximum volcano height: h rc d rmelt What is the depth to the melting zone on Mars? Why might this zone be deeper than on Earth?

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 Caldera rim Lobate flow(? ) Schenk et al. Nature 2001 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 (? ).

Part II

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/ 25 km Krieger crater, Moon • Spacing may be controlled by crustal structure 50 km Mars, MOC wide-angle Tate et al. LPSC 33, 2003

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

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 Where a is thermal expansivity (3 x 10 -5 K-1), DT is the temperature change and the strain is the fractional change in radius Hot mantle Liquid core Cool mantle Solid core

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 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)

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 thermal structure of the planet • The deformation of the elastic lithosphere under loads is called flexure • See EART 162 for more details!

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

Flexural Parameter (1) • Consider a load acting on an elastic plate: rw load rm rm ~π a (see next slide) Te • 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)

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

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

Example: Mariana Trench • In this model, there is a bending moment in addition to the line load, as the plate is subducted From T&S

10 km Example: Ganymede Note vertical exaggeration Load • This is an example of a profile across a rift on Ganymede • An eyeball estimate of ~3 a would be about 10 km • For ice, we take E=10 GPa, Dr=900 kg m-3, g=1. 3 ms-2 Distance, km • If ~3 a = 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

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 Depth Surf. Temp. • E. g. for ice the base of the elastic layer 110 K 273 K 190 K is at about 190 K • The measured elastic layer thickness is 6. 5 km (from previous slide) elastic • So thermal gradient is 12 K/km • This tells us that the ice shell thickness viscous is 13 km Temperature • What’s wrong with these assumptions Liquid! (convection changes geotherm).

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) Earth (cont. ) Mars (recent) Europa 30 d. T/dz (K/km) 15 100 5 2 40 Body Te Venus (450 o. C) Moon (ancient) Moon ~10 -30 d. T/dz (K/km) 15 15 30 >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 dishes Madrid 70 meter dish Canberra 70 meter dish

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 Crust Mantle Or, at least, why might you expect NOTHING?

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

Mascons and compensation

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

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

Mascons and Compensation Crust Lithostatically supported basalt Level B Level A Mantle The mantle appears to have “overshot” the isostatic level and is at the “superisostatic” at level B. This possibly happened during a complex cooling and adjustment phase (see Melosh et al. 2013). The denser material then “froze” in place and remains rigidly supported by the lithosphere today.

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

Lunar tectonics lab!

Part III

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 Martian dune features Image of a dust devil caught in the act 30 km

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

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

Mars rover solar panels • Initially concerned that dust would accumulate, limiting mission life. – Opportunity operated for 15 years. • 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? 100 km 30 km • Valley network on Titan • Presumably formed by methane runoff • What does this imply about Titan climate and surface?

Martian Outflow channels • Large-scale fluvial features, indicating massive (liquid) flows, comparable to ocean currents on Earth • Morphology similar to giant postglacial 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? flow direction 50 km 150 km Baker (2001)

Martian Gullies • A very unexpected discovery (Malin & Edgett, Science 283, 23302335, 2000) • Found predominantly at high latitudes (>30 o), on pole-facing slopes, and shallow (~100 m 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 Clearwater Lakes Canada ~30 km diameters Gusev, Mars 150 km Titan, 30 km across Titan lakes are (presumably) methane/ethane Gusev 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. channel Malin and Edgett, Science 2003

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 = d 2/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

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

Extra slides

30 o h

Te and age Mc. Govern et al. , JGR 2002 Small Te Decreasing age • The elastic thickness recorded is the lowest since the episode of deformation • In general, elastic thicknesses get larger with time (why? ) Large Te • 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 The only example of unambiguously documented compressional features on Europa to date Prockter and Pappalardo, Science 2000

Last class • Volcanism happens because of higher temperatures, reduced pressure or lowered solidus • Planets are poor heat conductors! • Conductive cooling time t = d 2/k • 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). Vertical (map) view Tidal stresses Friction prevents block motion • 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)

Rosetta mission high-res, Nov. 12 landing Updates

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?