Venus

1. Venus Jewel of the Sky Earth’s Sister Planet

2. General Information • Named after the Roman goddess of love and beauty • Once thought to be two different celestial bodies • Thus known as the morning or evening star • The sun rises in the west and sets in the east • Slow retrograde motion causes this phenomenon • Slow rotation makes a Venusian day longer than a Venusian year • Features on Venus are named after goddesses of mythology, famous women, and common female first names • Atmosphere • Thick, dense cloud cover • Atmospheric pressure 92 times that of Earth at sea-level (equal to being ~1km beneath the ocean) • Clouds composed of CO2, H2O, and sulfuric acid droplets • Runaway greenhouse effect • Visible to naked eye • Always appears close to the Sun

3. Planetary Statistics • Mean distance from Sun 108,200,000km • 0.723 A.U. • Orbital Period = 224.7 days • Rotational Period = 243 days • Axis Tilt = 177.4 degrees (23.5) • Diameter = 12,104km (12,756km) • Mass = 4.869 x 1024 (5.97 x 1024) • (.815 of Earth) • Density = 5.25gm/cm3 (5.515gm/cm3)

4. Stats cont’d • Equatorial surface gravity = 8.87m/sec2 (9.78m/sec2) • Visual albedo = 0.65 (.37) • Moons = none • Surface = Rolling plains, lava flows and very little topographic relief • Atmospheric composition • CO2 96% • Nitrogen 3% • Sulfur dioxide, water vapor,carbon monoxide, argon, helium, neon,hydrogen chloride, and hydrogen fluoride • Mean surface temperature = 170C (330F) • Max temp = 482C (896F)

5. Atmosphere • It is thickness not density that hides surface • Expected composition formed by secondary processes of differentiation and outgassing • Some fluids may have been supplied to inner planets via cometary collisions • Venus too hot = water vapor • Earth cooler = water fluids • Cloud cover extends >50km • Main cloud deck betwn/25 & 75km • Level @ which H2SO4 (sulfuric acid) can condense • Potent acid rain doesn’t reach the surface (virga) because of evaporation ~30km altitude • Greenhouse Effect • Surface receives less solar radiation than Earth’s • Gas and fluid molecules in atmosphere absorb energy • Cloud cover traps heat from escaping • Heat distributed planet-wide (no cold polar regions) • Mass of water 100,000 times less than Earth’s

6. Atmosphere (cont’d) • Carbon cycle • Fossilized atmosphere trapped in Earth’s carbonate sedimentary rocks • If released the two planets atmos. would be much more similar • If carbonate rx existed on Venus temp’s now too hot – gas released back into atmosphere • Weathering • Varies with elevation (pressure and temperature zones) • Consistent boundary elevation • Varies by <100m over 100’s of km • Cuts across various types of terrain • Low temp & hi elev. basalt reacts w/sulfur to form pyrite • Bright signatures • Hi elev & lo albedo = young lavas not weathered into pyrite

7. Venus Atmospheric Profile • The first Soviet probe, Venera 4, descended by parachute through the atmosphere • Apparently was crushed by the dense atmosphere (~90 atm) and high temperatures • Did return information confirming that CO2 makes up about 97% of all gases present (very little water) • Detected droplets of sulphuric acid in the outer cloud deck • Venera data (refined by later Pioneer data) also lead to a general profile of temperature and pressure distributions in the atmosphere (at right)

8. Venus – Geologic Activity • Venus has a silicate (rocky) crust and mantle with a metallic core • No detectable magnetic field • Slow rotation does not preclude existence of a partial liquid core • Volcanism, weathering, aeolian transport tensional and compressional tectonic forces are dominant geologic processes • No water so weathering and erosion are very different from Earth • Greenhouse may have been process responsible for loss of primal water • Accretion, internal differentiation and out-gassing were important early events

9. Geologic Activity (cont’d) • Juvenile water • Water as a component of mantle • Small fraction lowers melting temps by ~200K (-100F) • Increases fluidization and formation of an asthenosphere • Mantle processes • No water, no asthenosphere, no plate tectonics • No water, no granite, no continents

10. Major Geologic Provinces • Lowlands • Cover ~60% of the planet • Rolling plains w/little local relief (<500m) • No evidence for formation by large impacts • Extensive basaltic flows w/few volcanoes • Exhibit compressional tectonic features (ridges, folds and faults) • Few impact craters so probably <1by old • Atalanta Planitia – largest area • Uplands • Isolated domes and broad swells (0-2km elev.) • Extensional tectonics resulting in broad domes capped by fault-bounded rifts systems and some shield volcanoes • Coronae • Volcanic and tectonic • Concentric horst and graben features • Some flow features • Beta Regio – largest upland region

11. Major Geologic Provinces • Highlands • Continental like • 3-5km elev. • <15% areal extent of planet • Compressional folds, ridges and troughs • Lateral movement of crust • Perhaps different composition • Never been sampled • Aphrodite Terra – largest area >9500km east to west

12. Magellan • In clean room at JPL • Primary instrument - multimode Radar Mapper (2.385 Ghz, or 12.6 cm wavelength) • SAR imaging mode (18° and 50° off-nadir), res = 360 m and 120 m (1181-394 ft) • It first established orbit on August 10, 1990 after 1 1/2 loops around the Sun • Altimeter mode achieved vertical accuracy < 50 m (164 ft) within a ground cell of 10 km (6.2 mi) diam. • Radiometer mode could sense surface radio-emission, whose signals can be converted to brightness temperatures with an absolute accuracy of ±20° K

13. Venus Topography

14. Venus – Geologic Activity • Relatively young surface - ~complete resurfacing about 300-500mya • Small population of craters • Extensive lava flows • Evidence of crustal movement (vertical & lateral) • Topography • Vast plains/lava flows • Highlands deformed by geologic activity (isostatic or compressional forces) • Craters • Distributed randomly across surface • Few by Lunar and Mercury standards • <2km diameters virtually non-existent • Exceptions • When large meteorites break up just before impact • Secondary impacts propelled from primary impact event • Tend to produce linear crater chains

15. Geologic Activity (cont’d) • Volcanism - >85% of surface is covered by volcanic rock • Very numerous – • >100,000 small shield volcanoes • 100’s of larger features • Lava flows • Long sinuous channels • Some > 7,000km long • Flooded lowlands creating vast plains • Shield volcanoes • Found on all terrain types except tessera • Mostly associated w/rifting (African Rift Valley) • Largest volcanoes >1000km across isolated • Smaller on flanks and in clusters • Calderas • Summit vents • Single and multiple • Larger than any similar features on Earth

16. Venus Visible vs Radar • Two different perspectives of Venus • Left is a mosaic of images acquired by the Mariner 10 spacecraft • Thick cloud coverage that prevents optical observation of the planet's surface • Surface remained hidden until 1978 • Pioneer Venus 1 spacecraft arrived • The spacecraft used radar to map the surface • The right image show a rendering of Venus from the Pioneer Venus and Magellan radar images

17. Venus Interior • What is known about the Venusian interior comes primarily from the Venera, Pioneer Venus and Magellan spacecraft • Before thought Venus would have tectonic processes similar to that of Earth's mantle convection • Venus showed no sign of plate tectonism & appears to have a single plate • Differentiated with a basaltic crust extracted from the mantle • Crust appears to be ~25 to 40 kms and in some areas more than 50 to 60 kms • Recent volcanism suggests still retains a partially molten core

18. Venus Interior • Venus’ slow rate of rotation precludes generation of a magnetic field like the Earth’s • Atmospheric composition that expected by outgassing • Crustal features expression of deep internal mantle convection • Mostly vertical components • No evidence of lateral or plate tectonics

19. On the Surface • Temperatures at surface mechanical strength of rocks significantly lowered • Over time (~1BY) many topographic features would “disappear” • Muting of many topographic effects • High topography (Ishtar Terra) must be young features • Composition of rocks at surface as measured by Venera landers • Basalts • Different from primordial meteorites • Alkali basalts or granites • Evidence of extensive differentiation, partial melting

20. On the Surface • Seven Soviet landers successfully reached surface and returned information • Rocks show evidence of geologic processes • Cratering • Volcanism • Layering • Fracturing (tectonics) • Chemical and mechanical weathering • Aeolian transportation • No hydrologic activity • Thin regolith compared to Lunar, Earth or Martian surfaces

21. Golubkina Crater • Magellan radar image of complex crater • Named after the Russian sculptor Anna Golubkina • 30 km diam. crater characterized by terraced inner walls and a central peak • Typical of large impact craters on the Earth, Moon and Mars • Terraced inner walls – • Take shape late in the formation of an impact crater • Due to the collapse of the initial cavity created by the meteorite impact • The central peak – • Forms due to the rebound of the inner crater floor

22. Golubkina Crater • This is a computer generated, 3D perspective view of Golubkina crater • Complex crater form w/central peak • Flat-floored crater interior (possible melt) • Vertical exaggeration in this image is about 20 x

23. Crater Mead • Largest known crater on Venus • Named after Margaret Mead, American anthropologist • Measures 280 km in diam • Classified as a multi-ring crater • Innermost ring is thought to be the rim of the original crater cavity • Irregular, radar-bright crater ejecta crossing the radar-dark floor terrace and adjacent outer radar-bright ring suggests that the terrace floor region is likely down-dropped and tilted outward, forming a concentric ring-fault

24. Balch Crater • This remarkable half crater is located in the rift between Rhea and Theia Montes in Beta Regio • (Radar illumination is from the left) • ~37 km in diam. • Cut by many fractures or faults since it was formed • Eastern portion was partially destroyed during the formation of a fault-valley • Measures up to 20 km wide • North-south profile through the center of this crater resulted from the down dropping and removal of most of the eastern half of the crater • Rifting shows evidence of vertical movement but no horizontal component indicative of tectonic forces

25. Wanda CraterAkna Mountains • Mtns. form the western edge of Lakshmi Planum • Wanda Crater, upper right • Diameter of 18 km (11 mi) • Doesn't appear deformed by tectonics but material from the Akna Mountains appears to have collapsed into it • Area of image is about 200 km long x 125 km wide

26. Alcott Crater • Magellan detected few impact craters in the process of being resurfaced by volcanism • Alcott is the largest of these craters with a diam. of 63 km • The trough-like depression (lower left) is a rille through which lava once flowed • Open lava tube • Remnants of rough radial ejecta is preserved outside the crater's southeast rim • Important to our understanding of resurfacing rates on Venus by volcanism • Note older, rougher terrain in upper right of image

27. Barton Crater • 54-km diam. crater • Size at which craters on Venus begin to possess peak-rings instead of a single central peak • The floor is flat and radar-dark • Indicates possible infilling by lava flows sometime following the impact • Central peak-ring is discontinuous and appears to have been disrupted or separated during or following the cratering process • Name has been proposed by the Magellan Science Team, after Clara Barton, founder of the U. S. Red Cross; the name is tentative pending approval by the IAU

28. Carson Crater • Venusian impact craters are frequently surrounded by radar-dark halos • Several of these special craters have halos that are parabolic in shape, and are very long, extending hundreds of kms • The darkness of the emissivity data indicates a smooth surface • Halos may be thick, smooth sediment deposits formed when incoming projectiles crashed into the surface • The black strips represent missing data

29. Atmospheric Effects on Ejecta Distribution • Large impact crater about 30 km in diam. surrounded by a fresh ejecta blanket • Extreme brightness of the blanket is due to its roughness and its ability to scatter the radar signals that are used to collect these images • Missing section of the ejecta blanket (NW) is due to atmospheric blast that followed the impactor as it crashed through the Venusian atmosphere

30. Crater Cluster • A small projectile broke up in the atmosphere to form four smaller impactors that struck nearly simultaneously to form this crater cluster • Note overlying ejecta blankets and indistinct, broken, irregular rims

31. Sapas Mons • Sapas Mons is a large volcano ~400 kms in diameter and 1.5 kms high • The summit consists of two mesas with flat to slightly convex tops and smooth surfaces that appear radar-dark • The sides of the volcano show bright overlapping flows that provide the edifice with a roughly radial outline • Many of the flows appear to be flank eruptions • Radial fractures transect the flows to the east and south • Darker flows in the southeast quadrant are smoother flows

32. Pyroclastic Cones • The cone volcanoes in this cluster are about 2 kms in diameter, 200m high, with 12° steep slopes overlying a fracture network in Niobe Planitia • Some cones are cut by younger, more widely spaced, north-striking fractures with curvilinear outlines

33. A) Lakshmi Planum light and dark surfaces equivalent to basalt flow types known as pahoehoe (smooth lavas; somewhat specular surfaces) and aa (chunky lavas, better backscatterers) • B) Series of Lava flows emanate from the Sils Mons volcanic source • C) A long channel filled with volcanic flow material, over which a younger flow has straddled is the Ammavaru flow sequence in the Lada region A C B

34. Myletta Fluctus Flow • One of the longest flows in SS occurs in Lavinia Planitia • ~1000km long • Must be low silica, low viscosity • Mafic basaltic composition • Infilling of crater between arrows

35. Anemones • “Anemone" volcano • Petal-like lava flows and radiating radar-bright patterns • Normally occur in association with fissure type eruptions • ~40 by 60 kms in size • Has a dark central edifice with bright central flows • Note elongated summit pits and an arcuate graben along southern summit

36. Cluster of four overlapping domes Domes average about 25 kms in diameter with maximum heights of 750m Pancakes • Interpreted as viscous or thick eruptions of lava coming from a vent on the relatively level ground allowing the lava to flow in an even lateral pattern

37. CalderasSacajawea Patera • Elliptical caldera 260 by 175 kms forming depression about 2 kms deep • Enclosed by zone of concentric troughs with radar-bright outlines extending outward from the caldera floor • Floor is covered with smooth mottled plains • A small shield measuring 12 kms in diam is transected by one of these features

38. Domes • This 17.4 km dome shows collapsed margins and landslide deposits • Landslide deposits show hummocky surfaces that extend up to 10 kms out on the plains • Dome is about 1.86 kms high and has a slope of about 23° • In general, the scale of lava domes and collapse features on Venus is orders of magnitude larger than that on Earth • Mt. Elden is ~2km across &

39. Mantle melting and convection • Different scales produce different features • Large shield volcanoes = small bodies of magma • Volcanic rises = large-scale mantle upwelling • Coronae = significant melt with smaller or more short-lived upwellings • Larger plumes also yield larger collapse features

40. Arachnids • As the name suggests, arachnoids are circular to ovoid features with concentric rings and a complex network of fractures extending outward • The arachnoids range in size from approximately 50 kms to 230 kms in diam

41. Arachnids • Believed to be igneous related • Buoyant plume rises causing crust to stretch and thin • Crust may not be pierced and plume widens, flattens and spreads • As plume cools it sinks causing secondary tensional deformation • Result is uplifted circular mountain ring surrounding central depression

42. Coronae • Unlike Earth, Venus shows no evidence of plate tectonics • Process that helps release interior heat • One way Venus releases heat is by the formation of a large number of features called coronae • Circular patterns of fractures thought to form when hot material beneath the crust pushes up, warping the surface • Often accompanied by vast lava flows. • Width of image area: 528 km

43. Fotla Corona • Named after the Celtic fertility goddess • It is located in a vast plain to the south of Aphrodite Terra • Just north (top) of this corona is a flat-topped pancake dome, about 35km in diam • Another pancake dome is located inside the western (left) part of the corona • There is also a smooth, flat region in the center of the corona, probably a relatively young lava flow • Complex fracture patterns like the one in the north-east (top-right) of the image are often observed in association with coronae

44. Novae • Nova - radial network of grabens • Radiate from central point • Extensional faulting • Upward pressure on crust by ascending magmatic plumes stretches overlying crust causing normal faulting • There have been about 50 novae identified on Venus • This Magellan radar image is from the Themis Regio • 250 km in diameter

45. Dunes • Pattern of bright and dark ripples in the center of this image is an area where loose material was deposited into dunes • The bright streaks of material curve away from small hills, revealing which way the winds were blowing • Stronger winds caused by meteorite impacts may also help create such features • Width of image area: 172 km

46. Wind Streaks • Comet-like tail trends NE from a volcanic edifice • Volcano, whose basal diameter is 5 km, is a local topographic high that has slowed down northeast trending winds enough to cause deposition of this material • The streak is 35 km long and 10 km wide • Not dust so why so bright? • Large vs small grain sizes

47. Fracture Network • Complex network of narrow (<1 km) fractures in the center of the image extends ~50 km • Network exhibits tributary-like branches similar to those observed in river systems on Earth (trellis drainage patterns) • Angular intersections of tributaries suggest tectonic control • Appear to be due to drainage of lava along preexisting fractures and subsequent collapse of the surface

48. Channels • 200km segment of a sinuous channel on Venus • 2 km wide • Common on plains • In some places appear to have formed by lava • Similar to rivers in some respects, with meanders, cutoff oxbows, and abandoned channel segments • Appear to be older as they are crossed by fractures and wrinkle ridges, and are often buried by other volcanic materials • In addition, they appear to run both upslope and downslope, suggesting that the plains were warped by regional tectonism after channel formation

49. Weathering on Venus • Type of weathering differs with altitude • Hi alt, lo temp • Sulfur in atmosphere reacts w/mafic lavas to form pyrite (FeS2) • Pyrite is highly reflective at radar wavelengths • May be why highlands and mtn. peaks are bright • Lo alt, hi temp • Magnetite and anhydrite are more stable and less reflective

50. Summary • Earth and Venus formed at approx. same distance from Sun during SS formation • Should be of similar composition and planetary evolution • Cratering, volcanism and vertical tectonics in common • Bulk density and composition in common • Contrast of two planets • Water • Silicic volcanism • Plate tectonics • Young surface • ~0.5 BY • Few impacts scattered evenly across surface • Resurfacing by volcanic events • Catastrophic vs gradual resurfacing • Lack of water • Outgassed early on • No carbonate rx • Thick, dense atmosphere rids Venus of water to space • No lowering of melting temps in mantle • Lack of large volume of granitic rx • No formation of asthenosphere no plate tectonics • Buoyant crust resists subduction • Up- and downwelling dominate • No magnetic field • Rate of rotation • Possibly still some liquid core material