History of Venus
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Transcript of History of Venus
PTYS 411 Geology and Geophysics of the Solar System
Shane Byrne – [email protected] is from Pioneer Venus
History of Venus
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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
In this lecture
Surface activity on the Moon and Mercury mostly died off about 3 Ga
Surface history of Venus is only available from ~1.0 Ga onward (not dissimilar to Earth)
Surface activity and history of Mars spans its entire existence
…as opposed to…
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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
Comparisons to Earth
Earth Venus
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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.
Atmosphere of Venus
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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??
Topography
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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 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/TEquilibrium gives 92 bars at 742 K
What went wrong?
All these differences can be traced back to the lack of water on Venus
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Why didn’t 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 didn’t dissolve and form carbonate rocks
Venus and Earth have the same amount of CO2 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 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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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
What happened to the water?
Venus
Earth
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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
Landers
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Spherical images can be unwraped into a low-res perspective view
Smooth-ish basaltic rock – low viscosity magmas
Baltis Vallis – 6800 km
Venera 9 – A Blockier Appearance
Venera 13
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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
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Surface of Venus has been imaged by radar Pioneer Venus (late 1970’s) Venera 15 and 16 (1980’s) Magellan (1992 – 1994)
Backscatter and altimetry 98% coverage
Side-looking system No shadows – observation at 0o phase Light/Dark tones don’t 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
Interpretation of Radar Data
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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
Physiography
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Low-lying Plains Ridged plains Smooth Plains
Highlands Crustal Plateaus Volcanic Rises
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Range of volcanic styles Low viscosity plains volcanism Shield volcanism highly viscous features
Volcanism on Venus
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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Ridged plains – 70 % Venusian surface Emplaced over a few 10’s 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, 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
Superposed on ridged plains Not deformed by wrinkle ridges Consist of overlapping flows with lobate
morphology
Volcanic Plains
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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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
Coronae
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Highlands Crustal Plateaus Volcanic Rises
Low-lying Plains Ridged plains Smooth Plains
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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
Volcanic Rises
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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
Crustal Plateaus
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Almost 1000 impact craters on Venus
Very young surface Mean age 750 Myr 85% of the planets history is missing
All craters at >3 Km 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 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
Cratering
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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 ρi is projectile density
If impact speed is reduced below elastic wave speed then there’s no shockwave – projectile survives
Ram pressure from atmospheric shock
Crater-less impacts
iPSi gPDei 23..
ATM
Hz
SATMram
atmosphereram
gkTHwhere
eHg
PvzP
TkvPconstTif
vP
22
2
.
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
MarsVenus
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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 > 1011 kg to produce 940 craters
Atmosphere is very effective at blocking impacts
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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
Balch crater – 40 km
Heloise crater – 38 km
Catastrophic resurfacing?
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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
Catastrophic resurfacing?
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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
Not so catastrophic resurfacing?
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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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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 Extension creates rifts
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
With a transition to a thick lithosphere within the past Gyr
Summary