forming planetary crusts ii

PTYS 554

Evolution of Planetary Surfaces

Forming Planetary Crusts IIForming Planetary Crusts II

PYTS 554 – Forming Planetary Crusts II 2

Forming Planetary Crusts I Tour of planetary surfaces Terrestrial planet formation Differentiation and timing constraints

Forming Planetary Crusts II Giant impacts and the end of accretion Magma oceans and primary crust formation KREEP Late veneers and terrestrial planet water

Forming Planetary Crusts III One-plate planets vs. plate tectonics Recycling crust Plate tectonic changes over the Hadean and Archean


The first few 107 years to 108 years T0 = 4568.2 ± 0.6 Myr formation of the CAIs Rapid formation of planetesimals < 1Myr

Intense Al26 heating Melting and differentiation into iron meteorite parent

bodies

Formation of Chondrules and Chrondrites a few Myr later

No differentiation due to lower 26Al levels

Vesta-like bodies formed with volcanic activity in progress

Gas disk dissipates ~10Myr Mars in ~10 Myr

Silicate differentiation ~40 Myr

Earth in ~30-100Myr Ends with the moon-forming impact, 50-150Myr At 163Myr Earth has a solid surface (zircons)

Next phase (~50 Myr) involves giant impacts – the leading theory for…

Stripping of Mercury’s silicate mantle Formation of Earth’s moon Formation of Mars topographic dichotomy

Chambers et al., 2009

Kleine et al., 2009


Overview of a rocky planet Starts as homogeneous mix of rock & iron Molten state allows differentiation Iron core cools and solidifies (not yet complete for the Earth)

~12

,800

km

Millionsof years

Billionsof years


Planets start hot Gravitational potential energy of accreting mass

Minimum energy delivered as velocity might be more than the escape velocity

Integrate over the planets radius to get total energy delivered

Convert this energy to a temperature rise: Ignore cooling for now

ΔT for the Earth is very large >>> melting temperature

ΔT ~ melting temperature means R~1000 km Objects bigger than large asteroids melt during accretion

Differentiation also releases gravitational potential energy Amount depends on core/mantle density contrast and size of core Typically enough to melt the body Hf/W isotopes show differentiation essentially contemporaneous with accretion

Spread over the planet’s surface increasing radius by ΔR


Final phase High relative velocities

Low gravitational focusing An inefficient process Takes ~ 100Myr

Gas has disappeared now Jupiter and Saturn are fully formed

Heavily affects outcome in the asteroid belt Determines what regions contribute the

terrestrial planet material

Final number, masses and positions of terrestrial planets are essentially random.


Three possible impacts giant impacts to consider…

Formation of an iron-rich Mercury

Formation of Earth’s Moon

Mars Crustal dichotomy


Mercury’s uncompressed density (5.3 g cm-3) is much higher than any other terrestrial planet.

For a fully differentiated core and mantle Core radius ~75% of the planet Core mass ~60% of the planet Larger values are possible if the core is not pure iron

3 possibilities Differences in aerodynamic drag between

metal and silicate particles in the solar nebula.

Differentiation and then boil-off of a silicate mantle from strong disk heating and vapor removal by the solar wind.

Differentiation followed by a giant impact which can strip away most of the mantle.

Mercury’s Abnormal Interior


Basic story Mercury forms and differentiates Proto mercury is 2.25 times the

mass of the current planet

Impactor is ~1/6 of the mass

Fast, head-on, collision needed to strip off mantle material In contrast to slow oblique collisions at

Earth and Mars Head on collisions are less likely


Impact timescale A few hours to reform the iron rich Mercury

Magma ocean certain

Mercury must avoid re-accreting debris Half-life of debris is ~2 Myr

Poynting-Robertson drag

Dynamical models suggest Mercury can reaccumulate some small fraction of its old silicate material

No samples means no constraints

Benz et al., 2007


Facts to consider Moon depleted in iron & volatile substances

Bulk Earth 30% iron (mostly core) Bulk Moon 8-10% iron (mostly in mantle FeO)

Oxygen isotope ratios similar to Earth

Moon doesn’t orbit in Earth’s equatorial plane Orbital solutions show that original inclination was close to 10 degrees

Angular momentum of Earth-Moon system is anomalously high Corresponds to spinning an isolated Earth in 4 hrs

Geochemical evidence for magma ocean Floating anorthosite Uniform age of highland material – more on this later

Formation of the MoonFormation of the Moon


Possible theories (that didn’t work) Earth and Moon co-accreted

Explains oxygen isotopes Doesn’t explain iron and volatile depletion

Earth split into two pieces Spinning so fast that it broke apart (fission) …but the Moon doesn’t orbit in Earth’s equatorial plane …and present day angular momentum isn’t high enough

Capture of passing body Earth captures an independently formed moon as it passes nearby Pretty much a dynamical miracle (Very hard to dissipate enough energy to capture) Doesn’t explain oxygen isotope similarity to Earth

Current paradigm is Giant impact Earth close to final size Mars-sized impactor Both bodies already differentiated Both bodies formed at ~1 AU


Free parameters Late vs Early (mass of proto-Earth)

Early accretion poses compositional problem

Mass ratio ~9:1 for late accretion ~Mars-sized impactor

Impact parameter Controls angular momentum of final system Values 0.7-0.8 Rearth work best Most probable impact angle is 45°

(b~0.707Rearth)

Approach velocity Minimum is escape velocity Best results for v/vesc ~ 1.1

Canup, 2004

b


Canup, 2004


Isotopic ratios may have equilibrated through vapor cloud

Canup, 2004


Most material in the lunar disk comes from the impacting body

Yellows/greens Isotopic ratios shouldn’t

match without re-equilibration

Temperature of material that goes into the moon is coolest

Still several 1000K Enough to remove

volatile elements and water

Cores of bodies merge In the Earth

Canup, 2004


Disks are 1.5-2 lunar masses

Formation of a lunar sized body is possible in months

Tidal forces > self-gravity when inside the Roche limit ~2.9 Rearth for lunar density material

Optimum place to form moon is just outside this limit where disk is thickest

Conservation of angular momentum Moon ~15x times closer Earth’s rotation ~3.9x faster (~6 hours) Tides have removed some of this

angular momentum

Moon drifts outwards From disk interaction From terrestrial tides

Kokubo et al., 2000aR ~ 2.9 Rearth

Tk ~ 7 hours


Timeline constraints? Hf/W put the impact at >50Myr after CAIs

Anorthosite Sm/Nd 112 ± 40 Myr formation of lunar crust Norman et al. 2003

KREEP (Zircon Pb/Pb) 150 Myr Nemchin et al. 2009

Whole moon Rb/Sr 90 ± 20 Myr Halliday 2008

Earths magma ocean gone by 163Myr Zircons again


Northern and southern hemispheres of Mars are very distinct:

North Low elevation Few Craters – Young Smooth terrain Thin Crust No Magnetized rock

South High elevation Heavily cratered – Old Rough terrain Thick crust Magnetized rock

Dichotomy boundary mostly follows a great circle, but is interrupted by Tharsis

No gravity signal associated with the dichotomy boundary - compensated

Theories on how to form a dichotomy: Giant impact Several large basins Degree 1 convection cell Early plate tectonics

Mars: Crustal Dichotomy

Zuber et al., 2000


Despite all this the difference is only skin deep Buried impact basins in the northern hemisphere have been

mapped Before this burial the northern and southern hemispheres

were indistinguishable in age Rules out Earth-style plate tectonics

Northern hemisphere is a thinly covered version of the southern hemisphere

Mantled by 1-2 km of material (sediments and volcanic flows)

Frey et al., 200?


Borealis basin 208E, 67N 4250-5300 km in radius

Shares slope break with at ~1.5 basin radii with other basins

Largest impact structure in the solar system

Andrews-Hanna et al., 2008


Hydrocode modeling of a vertical and oblique impacts 3x1029 J impact, 6-10 kms-1 at 30-60° No global melting – melt layer 10s of km thick within basin Northern crust extracted from already depleted mantle

May correspond to Shergottites formed from a depleted reservoir 100Myr after most SNCs

Nimmo et al., 2008

Marinova et al., 2008


Lunar magma ocean was probably at least a few hundred km thick

Apollo 11 returned highland fragments, first suggestion of Magma ocean

Idea since extended to other terrestrial planets

Giant Impacts make Magma Oceans

A melt has a bulk chemical composition, but no crystals

Minerals are mechanically separable crystals with a distinct composition Terrestrial planets are dominated by silicon-oxygen based minerals – silicates

Silicate rocks are built from SiO4 tetrahedra


Depending on how Oxygen is shared Olivine

Isolated tetrahedra joined by cations (Mg, Fe) (Mg,Fe)2SiO4 (forsterite, fayalite)

Pyroxene Chains of tetrahedra sharing 2 Oxygen atoms (Mg, Fe) SiO3 (orthopyroxenes)

(Ca, Mg, Fe) SiO3 (clinopyroxenes)

Feldspars Framework where all 4 oxygen atoms are shared


What happens when you cool a melt? Bowens reaction series

Minerals begin to condense out in a certain order Dense minerals sink e.g. Olivine (Mg,Fe)2SiO4 Buoyant minerals rise e.g. Anorthite Ca Al2Si2O8

‘Undesirable’ elements get more concentrated in remaining liquid Potassium (K), Rare Earth Elements (REE), Phosphorus (P)

The reverse happens when you melt a solid

More on that in the volcanism lectures


Lunar Case


Fe poorLight

Less-dense Fe richDark

Dense

UltrabasicPrimative

Basic AcidicEvolved


End result is a chemically distinct skin of rock called a crust

10s of km thick Density ~3000 kg m-3

Two main consequences of crustal formation

Mantles depleted Upper mantle is more Olivine rich

Crusts enriched in isotopes The ‘undesirables’ are concentrated in the crust Radiogenic isotopes (heat sources ) mostly in the

crust

Mantle rocksAverage


The Moon has the ‘predicted’ anorthosite crust

Some resurfaced by later basaltic flows

Unexplained: crustal thickness variation Non-uniform KREEP distribution

Mercury should have lost any original anorthosite crust in its giant impact

Messenger indicates lower Ca/Si and Al/Si than the lunar highlands

…but abundant volatile species are a problem to explain

Very low Fe and Ti abundances

3.8 Ga 3.1 Ga

Nittler et al., 2011


Venus rock composition Sampled in only 7 locations by Soviet landers Composition consistent with low-silica basalt Exposed crust is <1 Gyr old though

Venera 14


Earth’s crust is continuously recycled by plate tectonics and so we don’t see any original crust

But we can see production of basaltic crust ongoing today

Characteristic stratigraphic sequence: Gabbro

(large grained basalt)

Sheeted dikes Each sheet was the wall of the inner ridge

Pillow basalts Blobs of basalt that are quickly quenched

Ocean sediments Fine-grained muds

Called an ophiolite sequence Can be obducted onto a continental setting Isua supracrustal belt – southern Greenland

3.8 Ga


Martian in-situ and orbital measurements Crust dominated by basalt With a thin weathered coating

McSween et al., 2009


Hydrocode modeling of a vertical and oblique impacts 3x1029 J impact, 6-10 kms-1 at 30-60° No global melting – melt layer 10s of km thick within basin Northern crust extracted from already depleted mantle

May correspond to Shergottites formed from a depleted reservoir 100Myr after most SNCs

Nimmo et al., 2008

Marinova et al., 2008


In decreasing order of severity… Mercury – head-on, high velocity, collision

Total planetary disruption

Earth – grazing, low velocity, collision Forms very large Moon Global magma oceans on both bodies

Mars – grazing, low velocity, collision Forms hemispheric dichotomy A baby magma ocean, no large moon

Vesta Distorted shape of object Ejected crustal and mantle samples to Earth

Giant impacts may have had other roles Formation of Pluto’s moons Rotation of Venus


Explaining Earth’s water is a problem Best done with Jupiter and Saturn on

circular orbits

Explaining a small Mars is a problem Best done with Jupiter and Saturn on

eccentric orbits, e ~ 0.1 Inconsistent with Nice model for later

giant planet migration

Raymond et al., 2009


Earth’s water 1 Earth ocean ~ 1.4 x 1021 Kg Estimates of Earth’s water content of ~5

oceans, about 0.1% MEarth

Inner nebula was too hot to allow water or hydrous minerals

Possible Sources Adsorbed on dust grains at 1 AU Comets Asteroids (either ice or as hydrated

minerals)

Constraints D/H of Earth’s water Late veneer of highly siderophile elements Moon is (mostly) dry

Surface water after moon-formation


D/H rules out comets But only 3 Oort cloud comets have been measured

Condensed near Jupiter’s current position

Bulk comet might be different than its coma Jupiter family comets might have a different D/H

Condensed in Kuiper belt

Mars D/H matches comets Lack of crustal recycling?

Asteroids match Earth’s D/H Only Carbonaceous Chondrites have significant

water But addition of these asteroids would produce the

wrong Os isotopes

Earth has a late veneer of highly siderophile elements (added post differentiation)

At ~0.003 of CI abundances (but in CI ratios) Ordinary chondrites are an isotopic match

Requires a ~1% MEarth addition after the moon forms

But late veneer and water delivery could come from different sources

Drake, 2005


Adsorbed onto dust grains? Simulated adsorption onto forsterite

grains shows a few oceans can be stored in this way

…but, not all adsorption sites would contain water (e.g. competition from H2)

Ordinary chondrites are not hydrated… Muralidharan et al., 2008

forming planetary crusts ii

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