History of Venus

1. History of Venus

2. 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

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 Venus Earth

4. 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.

5. 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??

6. 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

7. 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

8. 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

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 • 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

10. 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

11. 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

12. Venera 14 Venera 10

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

14. 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

15. 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

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

17. Volcanism on Venus • Range of volcanic styles • Low viscosity plains volcanism  Shield volcanism  highly viscous features Sinuous rills: Baltis Vallis – 6800 km

18. Some viscous flow features may exist… Pancake domes – Eistla region South Deadman Flow – Long valley, CA

19. 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

20. Volcanic Plains • 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 Sinuous rills: Baltis Vallis – 6800 km

21. 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

22. 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

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

24. 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

25. Rifts • Extensional stress from volcanic rise uplift

26. 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

27. Cratering • 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

28. 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 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 • 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

29. ‘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

30. 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

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

32. 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

33. 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

34. 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

35. 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

36. 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