1. PTYS 411 Geology and Geophysics of the Solar System Shane Byrne – shane@lpl.arizona.edu Background is from Pioneer Venus History of Venus

2. PYTS 411– History of Venus 2 l Venus today n Comparison to Earth n Venusian atmosphere n Water and magnetic fields l Geologic record n Volcanic resurfacing n Tectonic features n The lack of craters n Putting events in order l 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…

3. PYTS 411– History of Venus 3 l 81.5% of the mass of the Earth l Slightly higher mean density (5230 kg m -3 ) l Formed in a similar location – 0.72 AU n Implies a similar bulk composition Comparisons to Earth Earth Venus

4. PYTS 411– History of Venus 4 l Massive CO 2 atmosphere with intense greenhouse effect l 93 bars,740 K at mean surface elevation n Altitude variations 45-110 bars, 650-755 K l No day/night or equator/pole temperature variations l 3 distinct cloud-decks n Composed of sulfuric acid droplets n Produced by photo-oxidation of SO 2 n Effective scavenger of water vapor n Layers differ in particle size n Very reflective (albedo 70%) keeps surface much cooler than it would otherwise be l 100 ms -1 east-west at altitude of 65 km n Drives cloud layer around planet in ~4 days n Reasons for super-rotating atmosphere are unknown n True surface (243 day - retrograde) rotation period found with terrestrial radar. Atmosphere of Venus

5. PYTS 411– History of Venus 5 l Earth has obvious topography dichotomy n High continents n Low ocean floors l Venus has a unimodal hypsogram n No spreading centers n No Subduction zones n No plate tectonics l How is this topography supported?? Topography

6. PYTS 411– History of Venus 6 l Earth and Venus should be the same… n Venus absorbs roughly the same amount of sunlight as the Earth. n Venus has roughly the same amount of carbon as the Earth l …but… n Venus has no plate tectonics n Earth’s carbon get recycled through the crust n Venusian carbon accumulates in atmosphere – regulated by ‘Urey reaction’? CaCO 3 + SiO 2 = CaSiO 3 + CO 2 (calcite) + (silica) = (wollastonite) log 10 P CO2 = 7.797 – 4456/T Equilibrium gives 92 bars at 742 K What went wrong? All these differences can be traced back to the lack of water on Venus

7. PYTS 411– History of Venus 7 l Why didn’t this happen on the Earth ? n Earth has water that rains n Rain dissolves CO 2 from the atmosphere wForms carbonic acid n This acidified rainwater weathers away rocks n Washes into the ocean and forms carbonate rocks n Carbonate rocks eventually recycled by plate tectonics l The rock-cycle keeps all this in balance n Sometimes this gets out of sync e.g. snowball Earth – stops weathering

8. PYTS 411– History of Venus 8 l Venus started with plenty of water n Temperatures were just a little too high to allow rainfall n Atmospheric CO 2 didn’t dissolve and form carbonate rocks l Venus and Earth have the same amount of CO 2 n Earth’s CO 2 is locked up in carbonate rocks n Venus’s CO 2 is still all in the atmosphere l Same for sulfur compounds produced by volcanoes n SO 2 (sulfur dioxide) on Earth dissolves in the oceans n SO 2 on Venus stays in the atmosphere and forms clouds of sulfuric acid

9. PYTS 411– History of Venus 9 l Water & CO 2 build up in the atmosphere n A very massive atmosphere n A very hot surface l No Magnetic field n Slow spin wLarge early impact? wSolar Tides? n Little core convection wHot surface & thick lithosphere keep core hot l Water disassociated by sunlight n H can thermally escape n Solar wind impinges directly on Venusian ionosphere n Ions can be easily stripped away l Deuterium to Hydrogen ratio: 0.024 n 150 times that of Earth n Indicates significant loss of hydrogen l Sun was 30% fainter in early solar system n Venus may once have been more Earth-like What happened to the water? Venus Earth

10. PYTS 411– History of Venus 10 l Only glimpse of the surface n Soviets had 4 successful Venera landings on Venus n Onboard experiments found basaltic surface n Dark surface, albedo of 3-10% n Surface winds of ~ 0.3-1.0 m/s n Surface temperatures of 740 K n Landers lasted 45-60 minutes Venera 14 – 13 S, 310 E – March 1982 Landers

11. PYTS 411– History of Venus 11 l Spherical images can be unwraped into a low-res perspective view l Smooth-ish basaltic rock – low viscosity magmas Baltis Vallis – 6800 km Venera 9 – A Blockier Appearance Venera 13

12. PYTS 411– History of Venus 12 Venera 10 Venera 14

13. PYTS 411– History of Venus 13 l Venus rock composition n Sampled in only 7 locations by Soviet landers n Composition consistent with low-silica basalt n Exposed crust is <1 Gyr old though Venera 14

14. PYTS 411– History of Venus 14 l Surface of Venus has been imaged by radar n Pioneer Venus (late 1970’s) n Venera 15 and 16 (1980’s) n Magellan (1992 – 1994) wBackscatter and altimetry w98% coverage l Side-looking system l No shadows – observation at 0 o phase l Light/Dark tones don’t correspond to albedo l Strong radar return from: n Terrain that has roughness on the scale of the radar wavelength n Large-scale slopes facing the spacecraft n High-altitude ‘shiny’ material wHigh return due to unusual dielectric constant Interpretation of Radar Data

15. PYTS 411– History of Venus 15 l Surface dominated by volcanic material l Plenty of tectonics but no plate tectonics l Over 80% of Venus made up by n Volcanic plains - 70% of surface, low-lying n 9 Volcanic rises – Rift zones and major volcanoes, dynamically supported n 5 Crustal plateaus – Dominated by Tesserae, isostatically compensated l Unusual lack of impact craters n Very young surface 0.5 – 1.0 Gyr Physiography

16. PYTS 411– History of Venus 16 l Low-lying Plains n Ridged plains n Smooth Plains l Highlands n Crustal Plateaus n Volcanic Rises

17. PYTS 411– History of Venus 17 l Range of volcanic styles Low viscosity plains volcanism  Shield volcanism  highly viscous features Volcanism on Venus Sinuous rills: Baltis Vallis – 6800 km

18. PYTS 411– History of Venus 18 l Some viscous flow features may exist… Pancake domes – Eistla regionSouth Deadman Flow – Long valley, CA

19. PYTS 411– History of Venus 19 l Shield plains n Usually only a few 100 km across n Fields of gentle sloping volcanic shields n Crossed by wrinkle ridges n Shields usually constructed from non-viscous lava n Some shields are steep implying more evolved lava wVenera 8 lander probably sampled one of these areas

20. PYTS 411– History of Venus 20 l Ridged plains – 70 % Venusian surface n Emplaced over a few 10’s Myr n Deformed with wrinkle ridges (compressional faults) w1-2 km wide, 100-200 km long n High-yield, non-viscous eruptions of basalt wGentle slopes and smooth surfaces wLong run-out flows 100-200 km wChemical analysis – Venera 9, 10, 13 & Vega 1, 2 wTotal volume of lavas close to 1-2 x 10 8 km 3 wContain sinuous channels w2-5 km wide, 100’s km long wBaltis Vallis is 6800 km long, longest channel in the solar system wThermal erosion by lava l Smooth plains cover 10-15% of Venusian surface n Superposed on ridged plains n Not deformed by wrinkle ridges n Consist of overlapping flows with lobate morphology Volcanic Plains Sinuous rills: Baltis Vallis – 6800 km

21. PYTS 411– History of Venus 21 l Emplacement of plains material followed by widespread compression l Solomon et al. (and some other papers) describe a climate-volcanism- tectonism feedback mechanism n Resurfacing releases a lot of CO 2 causing planet to warm up n Heating of surfaces causes thermal expansion resulting in compressive forces. n Explains pervasive wrinkle ridge formation on volcanic plains

22. PYTS 411– History of Venus 22 l Morphologic term n Quasi-circular raised feature n Annulus of concentric fractures and ridges n Radially orientated fractures in their interiors l 360 Coronae identified l Size ranges from 75 to 2000 Km l Interiors raised about 1km l Associated with large amounts of volcanism l Occurred in parallel with volcanic plains formation l Typical formation sequence: n Volcanism n Topographic uplift wForming radial fractures n Withdrawal of magma n Topographic subsidence wForming concentric fractures Coronae

23. PYTS 411– History of Venus 23 l Highlands n Crustal Plateaus n Volcanic Rises l Low-lying Plains n Ridged plains n Smooth Plains

24. PYTS 411– History of Venus 24 l Nine major volcanic rises n 1000-2400km across l Containing: n Rift zones n Lava flows n Large volcanic edifaces l Associated gravity anomalies n Dynamically supported by a mantle plume n Young l Craters? n Partly uplifted old plains n Superposed features are young though l Usually dominated by: n Rifts n Large shield volcanoes n Coronae Volcanic Rises

25. PYTS 411– History of Venus 25 l Rifts n Extensional stress from volcanic rise uplift

26. PYTS 411– History of Venus 26 l Steep-sided, flat-topped, quasi-circular n Isostatically compensated n 1000-3000km across, raised by 0.5-4km l Dominated by Tesserae n Regions of complexly deformed material n Contain several episodes of both extension and compression. n Extremely rough (bright) at radar wavelength l Origin of Tesserae n Current thinking leans toward mantle plume origin n Upwelling mantle plume causes extension n Crust thickens n Partial collapse when plume disappears causes compression Crustal Plateaus

27. PYTS 411– History of Venus 27 l Almost 1000 impact craters on Venus l Very young surface n Mean age 750 Myr n 85% of the planets history is missing l All craters at >3 Km n Atmosphere stops smaller impacts n Craters 3-30 km in size have an irregular appearance n Craters >30 km in size appear sharp l Tesserae are the old features n 900 +/- 220 Ma l Volcanic plains have 2 units n Old plains 975 +/- 50 Ma n Young Plains 675 +/- 50 Ma l Volcanic rises have young features n Rifts and large isolated shields n Also contain older uplifted terrain Cratering

28. PYTS 411– History of Venus 28 l Impacting bodies can explode or be slowed in the atmosphere l Significant drag when the projectile encounters its own mass in atmospheric gas: n Where P s is the surface gas pressure, g is gravity and ρ i is projectile density n If impact speed is reduced below elastic wave speed then there’s no shockwave – projectile survives l Ram pressure from atmospheric shock Crater-less impacts n If P ram exceeds the yield strength then projectile fragments n If fragments drift apart enough then they develop their own shockfronts – fragments separate explosively n Weak bodies at high velocities (comets) are susceptible n Tunguska event on Earth n Crater-less ‘powder burns’ on venus n Crater clusters on Mars

29. PYTS 411– History of Venus 29 l ‘Powder burns’ on Venus l Crater clusters on Mars n Atmospheric breakup allows clusters to form here wScreened out on Earth and Venus wNo breakup on Moon or Mercury Mars Venus

30. PYTS 411– History of Venus 30 l Distribution of craters n Appears completely random n Some plains units may be older n Simulations taking in account atmospheric screening give ages of 700-800 Myr n 26,000 impactors > 10 11 kg to produce 940 craters wAtmosphere is very effective at blocking impacts

31. PYTS 411– History of Venus 31 l Low crater population n Catastrophic resurfacing n Continual resurfacing (like Earth) l Craters are indistinguishable from a random distribution l ~80% of craters are pristine n Others have superposed tectonics or volcanic material Balch crater – 40 km Heloise crater – 38 km Catastrophic resurfacing?

32. PYTS 411– History of Venus 32 l One timeline… n Tesserae form first wMost craters on them are removed by tectonics n Extensive Plains volcanism wResurfaces most of the planet n Global compression creates ridged plains n Additional volcanism makes smooth plains n Back to extension wVolcanic rises wRifts Catastrophic resurfacing?

33. PYTS 411– History of Venus 33 l One timeline… n Volcanic rises and plains form continuously wFocused mantle plumes for rises wDiffuse upwelling for plains volcanism n Volcanic rises evolve in Tesserae Transition to thick lithosphere ~700Ma n New volcanic rises can no longer evolve into tesserae wLack of transitional features means this occurred quite fast wExtension allows for coronae and rifts n Plains volcanism shuts off Not so catastrophic resurfacing?

34. PYTS 411– History of Venus 34 The future for Venus l Can a thick lid break? n Lack of water is a problem n Thermal energy builds in the mantle l Transient subduction? n Happened in the past? Venusian Geological Periods

35. PYTS 411– History of Venus 35 l Comparison to Earth n Almost the same mass and bulk composition wOnly 2 Mars-masses apart (+/- 1 giant impact) n Probably the same water budget wAsthenosphere likely in early history n Basalt to eclogite transition is deeper on Venus (65 km) wThis could inhibit the initiation of plate tectonics wProvides more time to outgas CO 2 and initiate runaway greenhouse wWater outgassed and destroyed over geologic time

36. PYTS 411– History of Venus 36 l Venus is like the Earth in a lot of ways n Size, density, composition, orbit …but… l A runaway greenhouse atmosphere has vaporized all the water l Lack of a magnetic field means that the water is easily removable l No water in the mantle means no plate-tectonics or carbon cycle l So the atmosphere had a profound effect on surface processes l Volcanic (low-viscosity basalt) plains dominate the surface n Lengthy sinuous rills n Ridged plains smooth plains, and shield plains n Pancake domes might indicate some silica-rich volcanism l 5 main crustal plateaus n Contain extensively fractured tesserae n High standing remnants, perhaps once supported by mantle plumes l 9 main volcanic rises n Currently supported by a mantle plume n Extension creates rifts l Coronae are interpreted as collapsed upwellings l Cratering record indicate a very young surface n Lack of degraded craters has been interpreted as a catastrophic resurfacing < 1Ga …OR… n …surface geology can also be interpreted in terms of more gradual processes wWith a transition to a thick lithosphere within the past Gyr Summary