History of the Moon and Mercury

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History of the Moon and Mercury
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In This Lecture

• Moon/Mercury (mostly 4.5 3.0 Ga)
• Formation events
• Basin sequences
• Plains volcanism
• Tectonics
• Atmospheres and polar volatiles

0.27 RE 1.00 AU
Surface activity on the Moon and Mercury mostly
died off about 3 Ga
as opposed to
Surface history of Venus is only available from
1.0 Ga onward
Surface activity and history of Mars spans its
entire existence
0.38 RE 0.39 AU
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Introduction to the Moon

• Moon is in a 11 spin orbit resonance with the
  Earth
• Two major surface units
• Heavily cratered light toned Terrae
• Less cratered dark toned Maria
• Crater size distribution follows a power law with
  an exponent close to -2
• Expected from fragmentation mechanics
• Extensive work on relative ages and stratigraphy
  relationships
• Apollo and Luna samples allowed that work to be
  attached to absolute dates

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Formation of the Moon

• Previous theories
• Co-accretion
• Fission of spinning Earth
• Capture of rogue planetisimal
• Apollo results (and common sense)
• Moon depleted in volatile elements
• Moon depleted in siderophile elements
• Oxygen isotope ratios similar
• Capture of a rouge planet would be a dynamical
  miracle
• Fission questionable since Moon doesnt orbit in
  equatorial plane
• Current paradigm is Giant impact
• Earth close to final size
• Mars-sized impactor
• Both bodies already differentiated
• Both bodies formed at 1 AU

From Robin Canup, SWRI Boulder
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Bulk composition and orbital state

• Iron cores of both bodies stay in the Earth.
• About 1 lunar mass of material goes into orbit
  outside the Earths Roche limit.
• Most of the matter in the Moon is from the
  impacting body.
• Heat of debris-disk removes volatiles

From Robin Canup, SWRI Boulder

• Earths spin and Moons orbit become locked in
  11 Cassini state
• Moons orbit expands by a few cm/year
• Earths rotation slows

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Magma Ocean

• Accretion of lunar material into the Moon within
  a few years!
• High-accretion rates mean surface is molten
• Magma ocean probably a few hundred km thick
• Apollo 11 returned highland fragments, first
  suggestion of Magma ocean
• Idea since extended to other terrestrial planets

• Different minerals condense at different times
• Pyroxene and Olivine sink
• Plagioclase-feldspar floats
• Moon gains global anorthosetic upper crust
• The leftover stuff sandwiched between these
  layers finally condenses
• Rich in incompatible elements such as potassium
  (K), rare-Earth elements (REE) and phosphorus (P)
• Collectively called KREEP 4.3 Ga
• Crust solidifies, sealing in radiogenic heat,
  which will become important 0.5 Gyr later

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Final Internal Structure

• Crustal Thickness Asymmetry
• Average crust 54-62km thick (45km at Apollo
  sites)
• Far-side crust is much (about 15km) thicker
• Crustal asymmetry is one the central questions in
  lunar science
• Mantle
• Ambiguous seismic results at 500-800km
• Upper 800km very low attenuation

KREEP
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• Core size?
• Remnant magnetism shows there was once a liquid
  core
• Moment of inertia (0.39) consistent with very
  small core (200-450 Km)
• Interaction with the solar wind suggests a solid
  conducting core of 340 90 km (1-3 of the lunar
  mass)
• Poor seismic data are consistent with liquid core
  (s-wave attenuation)
• but Apollo heat flow measurements are very low
• Free oscillation periods consistent with a single
  rigid body.

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Large Basins Form

• Bombardment of the Earth-Moon system continues
• Heavily cratered lunar highlands form
• Saturation equilibrium reached i.e. new impacts
  remove previous craters
• Several large basins form, which divide lunar
  stratigraphy into different epochs
• Fracture lithosphere to great depths
• Excavate lower crustal material e.g. KREEP

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The Moon at 3.8 Ga

• Crater saturation
• Lithosphere homogenized to depths of 20km
• Regolith generated to depths of 10s of meters
• Cratering rate declining dramatically

• Several large basins formed Aitken basin 2200
  Km
• Whole Moon has highland appearance
• Small amounts of mare material have appeared but
  were eliminated by basin impacts
• Radiogenic heat starting to produce large
  quantities of magma

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Formation of the Maria

• Mare material originates deep in the crust
• Maria lava fill pre-existing depressions (impact
  basins)
• High levels of pyroxene and olivine relative to
  the upper crust
• Very similar to terrestrial basalt
• Except that it is completely devolatilized
• Also abnormally high in titanium
• Darker color due to higher Fe content
• Amounts are small
• Most Maria 1-2km thick
• 5km in Imbrium, 0.6km in Orientale
• Individual flows 10-40m thick
• VERY low viscosity
• Some maria material interacts chemically with the
  KREEP layer as it rises
• Known as KREEPy maria
• Maria erupt mostly during the Imbrian period
  (3.8-3.1 Ga)
• A little late Mare formation into the
  Eratosthenian period but not much

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Evolution of the Maria

• Maria lava fill pre-existing depressions (impact
  basins)
• Maria does not reach surface on the far-side due
  to the thicker crust.
• Cryptomaria some maria can be buried
• Very smooth on scales of 100s of meters
• Weight of maria material causes subsidence
• Compression (wrinkle ridges) in the center
• Extension (graben) at the edges

• Edges of the Maria remain sharp
• Little lateral mixing from impacts

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Lunar Chronology

• Maria start forming as the heavy bombardment era
  ends.
• Maria crater density is much lower than the
  highlands
• Regolith is shallower than highlands probably a
  few meters deep
• Craters continue to accrue at a relatively slow
  rate until present day

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Lunar Timeline
Pre-Nectarian 4.6 3.92 Ga

• Giant impact between Earth and Mars sized body
  forms Moon
• Magma ocean
• Olivine rich rocks crystallize (sinks)
• Anorthosetic highland formation (floats)
• KREEP formation
• Heavy bombardment
• Homogenizes regolith up to 20 km
• Large basins form
• First maria destroyed by bombardment
• Impact rate declines significantly
• Maria erupt onto surface
• Mare material fills in preexisting basins
• Lighter cratering continues
• Recent craters still have bright rays
• Polar volatiles accumulate

Nectarian 3.92 3.85 Ga
Imbrian 3.85 3.15 Ga
Eratosthenian 3.15 1.0 Ga
Copernican 1.0 0 Ga
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Mercury
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Introduction to Mercury

• Orbital period 88 days is 3/2 times the
  rotational period
• Orbit is eccentric (e0.21)
• Leads to hot and cold poles on the equator

• Surface is lunar-like but with important
  differences
• Surface units
• Intercrater plains
• Smooth plains
• Caloris basin
• Global tectonic features

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Data acquired

• Mariner 10 had three fly-bys in 1974/5
• Equatorial pass _at_ 700 km (on dark side)
• South polar pass _at_ 50,000 km
• North polar pass _at_ 400 km
• Ironically the mission was not really designed
  for photogeology

• 45 photographic coverage of variable resolution
  and illumination
• Discovery of a dipole magnetic field

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Mercurys Abnormal Interior

• Mercurys 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.
• Geochemistry of mantle materials can distinguish
  between these hypothesis
• Lunar formation example shows there might be a
  better and more-surprising answer out there.

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• Core still liquid?
• Cooling models say probably not
• Unless theres a lot of (unexpected) sulfur
• Dipole field observed by Mariner 10 spacecraft
  says yes
• but that could be a remnant crustal field.
• New Earth-based radar observations of
  longitudinal librations core is still partly
  molten

• Core freezes into a solid inner core over time
• Slowed by sulfur
• Causes planetary contraction

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Mercurys Surface Almost Lunar

• Radar returns indicate regolith-like surface i.e.
  rough terrain composed of unconsolidated
  fragments
• Spectrally very similar to the lunar highlands
• Similar albedo and morphologies i.e. craters and
  basins everywhere

• Old surfaces (craters very degraded) not heavily
  cratered
• Smooth plains that look volcanic but have no
  basalt signature no maria
• Global sets of tectonic features preserved
• Global grid of aligned very old faults
• Global grid of unaligned compressional faults

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Geologic epochs on Mercury

• Mercurys history is divided into periods similar
  to the lunar examples
• Pre-Tolstojan
• Tolstojan
• Calorian
• Mansurian
• Kuiperian
• No absolute dates attached as there are no
  samples but crater counts yield some clues

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Spindown into a Cassini State

• Mercury likely started with a faster spin.
• Solar tides de-spun the planet to its current (59
  days) spin rate
• Ancient global lineament system observed
• Planet bulges less at the equator when spinning
  slowly
• Stresses created when rigid lithosphere readjusts
  to new shape
• Orientations of lineaments are a good match to
  model predictions

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Pre-Tolstojan Period

• Covers events occurring before the Tolstoj impact
  basin (500 km) was formed
• Mercury looks very much like the lunar highlands
• Similar number of large basins (gt500 km)

• Inter-crater plains are deposited
• Removes any basins lt 500 Km
• Plains material likely volcanic although theres
  no proof of this.
• A handful of other large basins accumulate after
  plains deposition.

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Tolstojan period

• Begins with formation of Tolstoj basin (500 km)
• Several more basins form
• Impact rate starting to significantly decline
• Smooth plains start to be emplaced
• Period ends with Caloris impact

Tolstoj impact basin
Smooth plains
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Global Contraction

• Extensive set of lobate scarps exist.
• No preferred azimuth
• Global distribution
• Sinuous or arcuate in plan
• Interpreted as thrust faults
• Evidence for an episode of global compression
• Planetary shrinkage of 1-2 Km
• Scarps are dated as Tolstoj through Calorian
  periods
• Probably formed over a few 100 Myr

Discovery Rupes
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Calorian Period

• Caloris impact was a major event for Mercury
• 3.9 Ga
• Impact structure is 1300 Km across
• Six concentric rings 630-3700 Km across
• Smooth plains material erupts after some delay
• Followed by compression (subsidence)
• Followed by extension (rebound)

Extensional Fractures
Compressional Ridges
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The Caloris Antipode

• Seismic waves from the Caloris impact all meet at
  the antipode at the same time.
• Modeling suggests vertical motions of up to 1km

• Terrain broken up into 1km sized blocks
• Official name is Hilly and furrowed terrain.
• Mariner 10 team called it weird terrain.

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Mercury Winds Down

• All the geological action for Mercury is now over
• Other geologic periods are relatively quiescent
• The rest of the Calorian sees last lobate scarps
  form
• Mansurian cratering rate similar to today
• Kuiperian period covers the most recent (freshest
  looking) craters (1.0 Ga to present)
• Ice accumulates in polar craters

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Mercurys Timeline

• Mercury forms, perhaps with a large core or
  suffers a giant impact
• Lithosphere forms
• Despinning results in shape change and global
  tectonism
• Heavy bombardment
• Homogenizes regolith up to 20 km
• Large basins form
• Volcanic flooding inter-crater plains
• Basins lt500km removed
• Core shrinks 1-2 km
• Global system of thrust faults forms lobate
  scarps
• Caloris impact structure forms
• Antipodal weird terrain
• Smooth plains form
• Subsidence and rebound in Caloris basin
• Lighter cratering continues
• Bright rayed craters

Pre-Tolstojan
Tolstojan
Calorian
Mansurian
Kuiperian
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Summing up

• The Moon and Mercury superficially have a lot in
  common
• Dominated by impacts with regolith surfaces
• Similar surface materials
• Both in a Cassini state
• Both geologically dead for Gyr
• Both currently have surface-bounded exospheres
• Both have permanent shadowing in their polar
  regions probably containing water ice
• But their histories and internal structure are
  different

LRO Fall 2008
Messenger Spring 2008