History%20of%20Venus

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History of Venus
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In this lecture

• Venus today
• Comparison to Earth
• Venusian atmosphere
• Water and magnetic fields
• Geologic record
• Volcanic resurfacing
• Tectonic features
• The lack of craters
• Putting events in order
• Resurfacing models

Surface history of Venus is only available from
1.0 Ga onward (not dissimilar to Earth)
as opposed to
Surface activity on the Moon and Mercury mostly
died off about 3 Ga
Surface activity and history of Mars spans its
entire existence
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Comparisons to Earth

• 81.5 of the mass of the Earth
• Slightly higher mean density (5230 kg m-3)
• Formed in a similar location 0.72 AU
• Implies a similar bulk composition

Venus
Earth
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Atmosphere of Venus

• Massive CO2 atmosphere with intense greenhouse
  effect
• 93 bars,740 K at mean surface elevation
• Altitude variations 45-110 bars, 650-755 K
• No day/night or equator/pole temperature
  variations
• 3 distinct cloud-decks
• Composed of sulfuric acid droplets
• Produced by photo-oxidation of SO2
• Effective scavenger of water vapor
• Layers differ in particle size
• Very reflective (albedo 70) keeps surface much
  cooler than it would otherwise be
• 100 ms-1 east-west at altitude of 65 km
• Drives cloud layer around planet in 4 days
• Reasons for super-rotating atmosphere are unknown
• True surface (243 day - retrograde) rotation
  period found with terrestrial radar.

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Topography

• Earth has obvious topography dichotomy
• High continents
• Low ocean floors
• Venus has a unimodal hypsogram
• No spreading centers
• No Subduction zones
• No plate tectonics
• How is this topography supported??

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What went wrong?

• Earth and Venus should be the same
• Venus absorbs roughly the same amount of sunlight
  as the Earth.
• Venus has roughly the same amount of carbon as
  the Earth
• but
• Venus has no plate tectonics
• Earths carbon get recycled through the crust
• Venusian carbon accumulates in atmosphere
  regulated by Urey reaction?

CaCO3 SiO2 CaSiO3 CO2 (calcite) (silica)
(wollastonite) log10PCO2 7.797
4456/T Equilibrium gives 92 bars at 742 K
All these differences can be traced back to the
lack of water on Venus
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• Why didnt this happen on the Earth ?
• Earth has water that rains
• Rain dissolves CO2 from the atmosphere
• Forms carbonic acid
• This acidified rainwater weathers away rocks
• Washes into the ocean and forms carbonate rocks
• Carbonate rocks eventually recycled by plate
  tectonics
• The rock-cycle keeps all this in balance
• Sometimes this gets out of sync e.g. snowball
  Earth stops weathering

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• Venus started with plenty of water
• Temperatures were just a little too high to allow
  rainfall
• Atmospheric CO2 didnt dissolve and form
  carbonate rocks
• Venus and Earth have the same amount of CO2
• Earths CO2 is locked up in carbonate rocks
• Venuss CO2 is still all in the atmosphere
• Same for sulfur compounds produced by volcanoes
• SO2 (sulfur dioxide) on Earth dissolves in the
  oceans
• SO2 on Venus stays in the atmosphere and forms
  clouds of sulfuric acid

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What happened to the water?

• Water CO2 build up in the atmosphere
• A very massive atmosphere
• A very hot surface
• No Magnetic field
• Slow spin
• Large early impact?
• Solar Tides?
• Little core convection
• Hot surface thick lithosphere keep core hot
• Water disassociated by sunlight
• H can thermally escape
• Solar wind impinges directly on Venusian
  ionosphere
• Ions can be easily stripped away
• Deuterium to Hydrogen ratio 0.024
• 150 times that of Earth
• Indicates significant loss of hydrogen
• Sun was 30 fainter in early solar system
• Venus may once have been more Earth-like

Venus
Earth
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Landers

• Only glimpse of the surface
• Soviets had 4 successful Venera landings on Venus
• Onboard experiments found basaltic surface
• Dark surface, albedo of 3-10
• Surface winds of 0.3-1.0 m/s
• Surface temperatures of 740 K
• Landers lasted 45-60 minutes

Venera 14 13 S, 310 E March 1982
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• Spherical images can be unwraped into a low-res
  perspective view
• Smooth-ish basaltic rock low viscosity magmas

Venera 13
Baltis Vallis 6800 km
Venera 9 A Blockier Appearance
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Venera 14
Venera 10
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• Venus rock composition
• Sampled in only 7 locations by Soviet landers
• Composition consistent with low-silica basalt
• Exposed crust is lt1 Gyr old though

Venera 14
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Interpretation of Radar Data

• Surface of Venus has been imaged by radar
• Pioneer Venus (late 1970s)
• Venera 15 and 16 (1980s)
• Magellan (1992 1994)
• Backscatter and altimetry
• 98 coverage
• Side-looking system
• No shadows observation at 0o phase
• Light/Dark tones dont correspond to albedo
• Strong radar return from
• Terrain that has roughness on the scale of the
  radar wavelength
• Large-scale slopes facing the spacecraft
• High-altitude shiny material
• High return due to unusual dielectric constant

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Physiography

• Surface dominated by volcanic material
• Plenty of tectonics but no plate tectonics
• Over 80 of Venus made up by
• Volcanic plains - 70 of surface, low-lying
• 9 Volcanic rises Rift zones and major
  volcanoes, dynamically supported
• 5 Crustal plateaus Dominated by Tesserae,
  isostatically compensated
• Unusual lack of impact craters
• Very young surface 0.5 1.0 Gyr

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• Low-lying Plains
• Ridged plains
• Smooth Plains
• Highlands
• Crustal Plateaus
• Volcanic Rises

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Volcanism on Venus

• Range of volcanic styles
• Low viscosity plains volcanism ? Shield volcanism
  ? highly viscous features

Sinuous rills Baltis Vallis 6800 km
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• Some viscous flow features may exist

Pancake domes Eistla region
South Deadman Flow Long valley, CA
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• Shield plains
• Usually only a few 100 km across
• Fields of gentle sloping volcanic shields
• Crossed by wrinkle ridges
• Shields usually constructed from non-viscous lava
• Some shields are steep implying more evolved lava
• Venera 8 lander probably sampled one of these
  areas

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Volcanic Plains

• Ridged plains 70 Venusian surface
• Emplaced over a few 10s Myr
• Deformed with wrinkle ridges (compressional
  faults)
• 1-2 km wide, 100-200 km long
• High-yield, non-viscous eruptions of basalt
• Gentle slopes and smooth surfaces
• Long run-out flows 100-200 km
• Chemical analysis Venera 9, 10, 13 Vega 1, 2
• Total volume of lavas close to 1-2 x 108 km3
• Contain sinuous channels
• 2-5 km wide, 100s km long
• Baltis Vallis is 6800 km long, longest channel in
  the solar system
• Thermal erosion by lava
• Smooth plains cover 10-15 of Venusian surface
• Superposed on ridged plains
• Not deformed by wrinkle ridges
• Consist of overlapping flows with lobate
  morphology

Sinuous rills Baltis Vallis 6800 km
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• Emplacement of plains material followed by
  widespread compression
• Solomon et al. (and some other papers) describe a
  climate-volcanism-tectonism feedback mechanism
• Resurfacing releases a lot of CO2 causing planet
  to warm up
• Heating of surfaces causes thermal expansion
  resulting in compressive forces.
• Explains pervasive wrinkle ridge formation on
  volcanic plains

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Coronae

• Morphologic term
• Quasi-circular raised feature
• Annulus of concentric fractures and ridges
• Radially orientated fractures in their interiors
• 360 Coronae identified
• Size ranges from 75 to 2000 Km
• Interiors raised about 1km
• Associated with large amounts of volcanism
• Occurred in parallel with volcanic plains
  formation
• Typical formation sequence
• Volcanism
• Topographic uplift
• Forming radial fractures
• Withdrawal of magma
• Topographic subsidence
• Forming concentric fractures

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• Highlands
• Crustal Plateaus
• Volcanic Rises
• Low-lying Plains
• Ridged plains
• Smooth Plains

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Volcanic Rises

• Nine major volcanic rises
• 1000-2400km across
• Containing
• Rift zones
• Lava flows
• Large volcanic edifaces
• Associated gravity anomalies
• Dynamically supported by a mantle plume
• Young
• Craters?
• Partly uplifted old plains
• Superposed features are young though
• Usually dominated by
• Rifts
• Large shield volcanoes
• Coronae

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• Rifts
• Extensional stress from volcanic rise uplift

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Crustal Plateaus

• Steep-sided, flat-topped, quasi-circular
• Isostatically compensated
• 1000-3000km across, raised by 0.5-4km
• Dominated by Tesserae
• Regions of complexly deformed material
• Contain several episodes of both extension and
  compression.
• Extremely rough (bright) at radar wavelength
• Origin of Tesserae
• Current thinking leans toward mantle plume origin
• Upwelling mantle plume causes extension
• Crust thickens
• Partial collapse when plume disappears causes
  compression

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Cratering

• Almost 1000 impact craters on Venus
• Very young surface
• Mean age 750 Myr
• 85 of the planets history is missing
• All craters at gt3 Km
• Atmosphere stops smaller impacts
• Craters 3-30 km in size have an irregular
  appearance
• Craters gt30 km in size appear sharp
• Tesserae are the old features
• 900 /- 220 Ma
• Volcanic plains have 2 units
• Old plains 975 /- 50 Ma
• Young Plains 675 /- 50 Ma
• Volcanic rises have young features
• Rifts and large isolated shields
• Also contain older uplifted terrain

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Crater-less impacts

• Impacting bodies can explode or be slowed in the
  atmosphere
• Significant drag when the projectile encounters
  its own mass in atmospheric gas
• Where Ps is the surface gas pressure, g is
  gravity and ?i is projectile density
• If impact speed is reduced below elastic wave
  speed then theres no shockwave projectile
  survives
• Ram pressure from atmospheric shock

• If Pram exceeds the yield strength then
  projectile fragments
• If fragments drift apart enough then they develop
  their own shockfronts fragments separate
  explosively
• Weak bodies at high velocities (comets) are
  susceptible
• Tunguska event on Earth
• Crater-less powder burns on venus
• Crater clusters on Mars

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• Powder burns on Venus
• Crater clusters on Mars
• Atmospheric breakup allows clusters to form here
• Screened out on Earth and Venus
• No breakup on Moon or Mercury

Mars
Venus
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• Distribution of craters
• Appears completely random
• Some plains units may be older
• Simulations taking in account atmospheric
  screening give ages of 700-800 Myr
• 26,000 impactors gt 1011 kg to produce 940 craters
• Atmosphere is very effective at blocking impacts

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Catastrophic resurfacing?

• Low crater population
• Catastrophic resurfacing
• Continual resurfacing (like Earth)
• Craters are indistinguishable from a random
  distribution
• 80 of craters are pristine
• Others have superposed tectonics or volcanic
  material

Heloise crater 38 km
Balch crater 40 km
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Catastrophic resurfacing?

• One timeline
• Tesserae form first
• Most craters on them are removed by tectonics
• Extensive Plains volcanism
• Resurfaces most of the planet
• Global compression creates ridged plains
• Additional volcanism makes smooth plains
• Back to extension
• Volcanic rises
• Rifts

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Not so catastrophic resurfacing?

• One timeline
• Volcanic rises and plains form continuously
• Focused mantle plumes for rises
• Diffuse upwelling for plains volcanism
• Volcanic rises evolve in Tesserae
• Transition to thick lithosphere
• 700Ma
• New volcanic rises can no longer evolve into
  tesserae
• Lack of transitional features means this occurred
  quite fast
• Extension allows for coronae and rifts
• Plains volcanism shuts off

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The future for Venus

• Can a thick lid break?
• Lack of water is a problem
• Thermal energy builds in the mantle
• Transient subduction?
• Happened in the past?

Venusian Geological Periods
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• Comparison to Earth
• Almost the same mass and bulk composition
• Only 2 Mars-masses apart (/- 1 giant impact)
• Probably the same water budget
• Asthenosphere likely in early history
• Basalt to eclogite transition is deeper on Venus
  (65 km)
• This could inhibit the initiation of plate
  tectonics
• Provides more time to outgas CO2 and initiate
  runaway greenhouse
• Water outgassed and destroyed over geologic time

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Summary

• Venus is like the Earth in a lot of ways
• Size, density, composition, orbit
• but
• A runaway greenhouse atmosphere has vaporized all
  the water
• Lack of a magnetic field means that the water is
  easily removable
• No water in the mantle means no plate-tectonics
  or carbon cycle
• So the atmosphere had a profound effect on
  surface processes
• Volcanic (low-viscosity basalt) plains dominate
  the surface
• Lengthy sinuous rills
• Ridged plains smooth plains, and shield plains
• Pancake domes might indicate some silica-rich
  volcanism
• 5 main crustal plateaus
• Contain extensively fractured tesserae
• High standing remnants, perhaps once supported by
  mantle plumes
• 9 main volcanic rises
• Currently supported by a mantle plume