History of Venus PTYS 411 Geology and Geophysics of the Solar System Shane Byrne – shane@lpl.arizona.edu Background is from Pioneer Venus PYTS 411– History of Venus In this lecture Venus today Geologic record 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 2 PYTS 411– History of Venus 3 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 Earth Venus PYTS 411– History of Venus 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. 4 PYTS 411– History of Venus 5 Topography Earth has obvious topography dichotomy Venus has a unimodal hypsogram 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? 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/T Equilibrium gives 92 bars at 742 K All these differences can be traced back to the lack of water on Venus 6 PYTS 411– History of Venus 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 7 PYTS 411– History of Venus Venus started with plenty of water Venus and Earth have the same amount of CO2 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 SO2 (sulfur dioxide) on Earth dissolves in the oceans SO2 on Venus stays in the atmosphere and forms clouds of sulfuric acid 8 PYTS 411– History of Venus 9 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 Little core convection H can thermally escape Solar wind impinges directly on Venusian ionosphere Ions can be easily stripped away Deuterium to Hydrogen ratio: 0.024 Hot surface & thick lithosphere keep core hot Water disassociated by sunlight Large early impact? Solar Tides? 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 PYTS 411– History of Venus 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 10 PYTS 411– History of Venus Spherical images can be unwraped into a low-res perspective view Smooth-ish basaltic rock – low viscosity magmas 11 Venera 13 Baltis Vallis – 6800 km Venera 9 – A Blockier Appearance PYTS 411– History of Venus 12 Venera 14 Venera 10 PYTS 411– History of Venus 13 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 PYTS 411– History of Venus Interpretation of Radar Data 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 14 PYTS 411– History of Venus 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 15 PYTS 411– History of Venus Low-lying Plains Ridged plains Smooth Plains Highlands Crustal Plateaus Volcanic Rises 16 PYTS 411– History of Venus Volcanism on Venus Range of volcanic styles Low viscosity plains volcanism Shield volcanism highly viscous features Sinuous rills: Baltis Vallis – 6800 km 17 PYTS 411– History of Venus Some viscous flow features may exist… Pancake domes – Eistla region South Deadman Flow – Long valley, CA 18 PYTS 411– History of Venus 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 19 PYTS 411– History of Venus 20 Volcanic Plains Ridged plains – 70 % Venusian surface Emplaced over a few 10’s Myr Deformed with wrinkle ridges (compressional faults) High-yield, non-viscous eruptions of basalt 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 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 Emplacement of plains material followed by widespread compression Solomon et al. (and some other papers) describe a climate-volcanismtectonism 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 21 PYTS 411– History of Venus 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 22 PYTS 411– History of Venus Highlands 23 Crustal Plateaus Volcanic Rises Low-lying Plains Ridged plains Smooth Plains PYTS 411– History of Venus 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 24 PYTS 411– History of Venus Rifts Extensional stress from volcanic rise uplift 25 PYTS 411– History of Venus Crustal Plateaus Steep-sided, flat-topped, quasi-circular Dominated by Tesserae 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 Current thinking leans toward mantle plume origin Upwelling mantle plume causes extension Crust thickens Partial collapse when plume disappears causes compression 26 PYTS 411– History of Venus Cratering Almost 1000 impact craters on Venus Very young surface All craters at >3 Km 900 +/- 220 Ma Volcanic plains have 2 units 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 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 Rifts and large isolated shields Also contain older uplifted terrain 27 PYTS 411– History of Venus 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: i.e. Di 3PS 2 g P i 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 Pram v 2 atmosphere if T const. Pram v where H kT 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 Pz e kT gH 28 PYTS 411– History of Venus ‘Powder burns’ on Venus Crater clusters on Mars 29 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 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 30 PYTS 411– History of Venus 31 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 PYTS 411– History of Venus 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 32 PYTS 411– History of Venus 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 33 PYTS 411– History of Venus 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 34 PYTS 411– History of Venus 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 35 PYTS 411– History of Venus 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 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 36