Download PYTS 411– History of Venus

History of Venus
PTYS 411 Geology and Geophysics of the Solar System
Shane Byrne – [email protected]
Background is from Pioneer Venus
PYTS 411– History of Venus
In this lecture
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Venus today
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Geologic record
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Comparison to Earth
Venusian atmosphere
Water and magnetic fields
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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PYTS 411– History of Venus
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Comparisons to Earth
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81.5% of the mass of the Earth
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Slightly higher mean density (5230 kg m-3)
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Formed in a similar location – 0.72 AU
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Implies a similar bulk composition
Earth
Venus
PYTS 411– History of Venus
Atmosphere of Venus
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Massive CO2 atmosphere with intense
greenhouse effect
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93 bars,740 K at mean surface elevation
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Altitude variations 45-110 bars, 650-755 K
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No day/night or equator/pole temperature
variations
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3 distinct cloud-decks
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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
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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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PYTS 411– History of Venus
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Topography
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Earth has obvious topography
dichotomy
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Venus has a unimodal
hypsogram
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High continents
Low ocean floors
No spreading centers
No Subduction zones
No plate tectonics
How is this topography
supported??
PYTS 411– History of Venus
What went wrong?
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Earth and Venus should be the same…
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Venus absorbs roughly the same amount of
sunlight as the Earth.
Venus has roughly the same amount of carbon
as the Earth
…but…
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Venus has no plate tectonics
Earth’s 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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PYTS 411– History of Venus
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Why didn’t this happen on the Earth ?
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Earth has water that rains
Rain dissolves CO2 from the atmosphere
 Forms carbonic acid
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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
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Sometimes this gets out of sync e.g. snowball Earth – stops weathering
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PYTS 411– History of Venus
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Venus started with plenty of water
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Venus and Earth have the same amount of CO2
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Temperatures were just a little too high to allow rainfall
Atmospheric CO2 didn’t dissolve and form carbonate rocks
Earth’s CO2 is locked up in carbonate rocks
Venus’s CO2 is still all in the atmosphere
Same for sulfur compounds produced by volcanoes
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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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PYTS 411– History of Venus
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What happened to the water?
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Water & CO2 build up in the atmosphere
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A very massive atmosphere
A very hot surface
No Magnetic field
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Slow spin
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Little core convection
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H can thermally escape
Solar wind impinges directly on Venusian ionosphere
Ions can be easily stripped away
Deuterium to Hydrogen ratio: 0.024
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Hot surface & thick lithosphere keep core hot
Water disassociated by sunlight
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Large early impact?
Solar Tides?
150 times that of Earth
Indicates significant loss of hydrogen
Sun was 30% fainter in early solar system
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Venus may once have been more Earth-like
Venus
Earth
PYTS 411– History of Venus
Landers
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Only glimpse of the surface
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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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PYTS 411– History of Venus
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Spherical images can be unwraped into a low-res perspective view
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Smooth-ish basaltic rock – low viscosity magmas
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Venera 13
Baltis Vallis – 6800 km
Venera 9 – A Blockier Appearance
PYTS 411– History of Venus
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Venera 14
Venera 10
PYTS 411– History of Venus
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Venus rock composition
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Sampled in only 7 locations by Soviet landers
Composition consistent with low-silica basalt
Exposed crust is <1 Gyr old though
Venera 14
PYTS 411– History of Venus
Interpretation of Radar Data
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Surface of Venus has been imaged by radar
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Pioneer Venus (late 1970’s)
Venera 15 and 16 (1980’s)
Magellan (1992 – 1994)
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Backscatter and altimetry
98% coverage
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Side-looking system
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No shadows – observation at 0o phase
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Light/Dark tones don’t correspond to
albedo
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Strong radar return from:
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Terrain that has roughness on the scale of the
radar wavelength
Large-scale slopes facing the spacecraft
High-altitude ‘shiny’ material
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High return due to unusual dielectric constant
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PYTS 411– History of Venus
Physiography
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Surface dominated by volcanic material
Plenty of tectonics but no plate tectonics
Over 80% of Venus made up by
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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
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Very young surface 0.5 – 1.0 Gyr
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PYTS 411– History of Venus
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Low-lying Plains
 Ridged plains
 Smooth Plains
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Highlands
 Crustal Plateaus
 Volcanic Rises
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PYTS 411– History of Venus
Volcanism on Venus
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Range of volcanic styles
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Low viscosity plains volcanism  Shield volcanism  highly viscous features
Sinuous rills: Baltis Vallis – 6800 km
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PYTS 411– History of Venus
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Some viscous flow features may exist…
Pancake domes – Eistla region
South Deadman Flow – Long valley, CA
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PYTS 411– History of Venus
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Shield plains
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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
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Venera 8 lander probably sampled one of these areas
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PYTS 411– History of Venus
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Volcanic Plains
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Ridged plains – 70 % Venusian surface
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Emplaced over a few 10’s Myr
Deformed with wrinkle ridges (compressional
faults)
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High-yield, non-viscous eruptions of basalt
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1-2 km wide, 100-200 km long
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, 100’s 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
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Superposed on ridged plains
Not deformed by wrinkle ridges
Consist of overlapping flows with lobate
morphology
Sinuous rills: Baltis Vallis – 6800 km
PYTS 411– History of Venus
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Emplacement of plains material followed by widespread compression
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Solomon et al. (and some other papers) describe a climate-volcanismtectonism feedback mechanism
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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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PYTS 411– History of Venus
Coronae
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Morphologic term
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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:
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Volcanism
Topographic uplift
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Forming radial fractures
Withdrawal of magma
Topographic subsidence
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Forming concentric fractures
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PYTS 411– History of Venus
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Highlands
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Crustal Plateaus
Volcanic Rises
Low-lying Plains
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Ridged plains
Smooth Plains
PYTS 411– History of Venus
Volcanic Rises
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Nine major volcanic rises
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1000-2400km across
Containing:
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Rift zones
Lava flows
Large volcanic edifaces
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Associated gravity anomalies
 Dynamically supported by a
mantle plume
 Young
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Craters?
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Partly uplifted old plains
Superposed features are young
though
Usually dominated by:
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Rifts
Large shield volcanoes
Coronae
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PYTS 411– History of Venus
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Rifts
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Extensional stress from volcanic rise uplift
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PYTS 411– History of Venus
Crustal Plateaus
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Steep-sided, flat-topped, quasi-circular
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Dominated by Tesserae
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Isostatically compensated
1000-3000km across, raised by 0.5-4km
Regions of complexly deformed material
Contain several episodes of both extension
and compression.
Extremely rough (bright) at radar
wavelength
Origin of Tesserae
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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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PYTS 411– History of Venus
Cratering
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Almost 1000 impact craters on Venus
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Very young surface
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All craters at >3 Km
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900 +/- 220 Ma
Volcanic plains have 2 units
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Atmosphere stops smaller impacts
Craters 3-30 km in size have an irregular appearance
Craters >30 km in size appear sharp
Tesserae are the old features
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Mean age 750 Myr
85% of the planets history is missing
Old plains 975 +/- 50 Ma
Young Plains 675 +/- 50 Ma
Volcanic rises have young features
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Rifts and large isolated shields
Also contain older uplifted terrain
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PYTS 411– History of Venus
Crater-less impacts
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Impacting bodies can explode or be slowed in the atmosphere
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Significant drag when the projectile encounters its own mass in
atmospheric gas: i.e. Di  3PS 2 g P i
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Where Ps is the surface gas pressure, g is gravity and ρi is projectile density
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If impact speed is reduced below elastic wave speed then there’s no
shockwave – projectile survives
Ram pressure from atmospheric shock
Pram  v 2  atmosphere
if
T  const. Pram  v
where H  kT
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2
g ATM
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
 ATM
v 2 PS  z H
P z  
e
kT
gH
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PYTS 411– History of Venus
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‘Powder burns’ on Venus
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Crater clusters on Mars
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Atmospheric breakup allows clusters to form here
 Screened out on Earth and Venus
 No breakup on Moon or Mercury
Mars
Venus
PYTS 411– History of Venus
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Distribution of craters
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Appears completely random
Some plains units may be older
Simulations taking in account atmospheric
screening give ages of 700-800 Myr
26,000 impactors > 1011 kg to produce 940
craters
 Atmosphere is very effective at blocking impacts
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PYTS 411– History of Venus
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Catastrophic resurfacing?
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Low crater population
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Catastrophic resurfacing
Continual resurfacing (like Earth)
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Craters are indistinguishable from a random distribution
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~80% of craters are pristine
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Others have superposed tectonics or volcanic material
Heloise crater – 38 km
Balch crater – 40 km
PYTS 411– History of Venus
Catastrophic resurfacing?
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One timeline…
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Tesserae form first
 Most craters on them are removed by tectonics
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Extensive Plains volcanism
 Resurfaces most of the planet
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Global compression creates ridged
plains
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Additional volcanism makes smooth
plains
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Back to extension
 Volcanic rises
 Rifts
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PYTS 411– History of Venus
Not so catastrophic resurfacing?
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One timeline…
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Volcanic rises and plains form
continuously
 Focused mantle plumes for rises
 Diffuse upwelling for plains volcanism
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Volcanic rises evolve in Tesserae
Transition to thick lithosphere
~700Ma
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New volcanic rises can no longer
evolve into tesserae
 Lack of transitional features means this
occurred quite fast
 Extension allows for coronae and rifts
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Plains volcanism shuts off
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PYTS 411– History of Venus
The future for Venus
Can a thick lid break?
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Lack of water is a problem
Thermal energy builds in the mantle
Transient subduction?
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Happened in the past?
Venusian Geological Periods
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PYTS 411– History of Venus
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Comparison to Earth
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Almost the same mass and bulk composition
 Only 2 Mars-masses apart (+/- 1 giant impact)
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Probably the same water budget
 Asthenosphere likely in early history
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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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PYTS 411– History of Venus
Summary
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Venus is like the Earth in a lot of ways
Size, density, composition, orbit
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…but…
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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
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Volcanic (low-viscosity basalt) plains dominate the surface
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Lengthy sinuous rills
Ridged plains smooth plains, and shield plains
Pancake domes might indicate some silica-rich volcanism
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5 main crustal plateaus
Contain extensively fractured tesserae
High standing remnants, perhaps once supported by mantle plumes
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9 main volcanic rises
Currently supported by a mantle plume
Extension creates rifts
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Coronae are interpreted as collapsed upwellings
Cratering record indicate a very young surface
Lack of degraded craters has been interpreted as a catastrophic resurfacing < 1Ga …OR…
…surface geology can also be interpreted in terms of more gradual processes
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With a transition to a thick lithosphere within the past Gyr
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