Venus

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

2. General Information • Brightest object in sky after the Sun & Moon • 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, volcanoes & lava flows, extensive folding & faulting 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) • Hot enough to melt Pb

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 info confirming that CO2 makes up about 97% of all gases present (very little water) • Detected droplets of sulfuric acid in the outer cloud deck • Venera data (refined by later Pioneer data) also lead to a general profile of temp & press 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 (as we know it on Earth) • No water, no granite, no bimodal crustal composition (i.e. continental & oceanic crust)

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. Venus Visible vs Radar • This picture shows two different perspectives of Venus. On the left is a mosaic of images acquired by the Mariner 10 spacecraft. The image shows the thick cloud coverage that prevents optical observation of the planet's surface. The surface of Venus remained hidden until 1978 when the Pioneer Venus 1 spacecraft arrived and went into orbit about the planet. The spacecraft used radar to map the surface. The right image show a rendering of Venus from the Pioneer Venus and Magellan radar images.

13. 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

14. Venus Topography

15. Venus – Geologic Activity • Relatively young surface - ~complete resurfacing ~300-500mya • Small population of craters • Extensive volcanoes & 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 • Numerous but still few by Lunar and Mercury or even Mars 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

16. 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

17. Venus Interior • What is known about interior comes primarily from the Venera, Pioneer Venus and Magellan spacecraft • Before scientists thought Venus would have tectonic processes similar to that of Earth's mantle convection • However, no sign of plate tectonism & appears to have a single plate • Venus is differentiated • Basaltic crust extracted from mantle • Crust appears to be ~25 to 40 kms & more than 50 to 60 kms in some areas • Recent volcanism suggests Venus still retains a partially molten core • Slow rate of rotation precludes generation of a magnetic field like the Earth’s

18. Venus Interior • Differentiation of planet • Crust of lighter silicate mineralogy • Mantle of denser iron and magnesium silicates • Core, metallic, mostly iron, possibly liquid component • Composition of rocks at surface as measured by Venera landers • Basalts • Different from primordial meteorites • Alkali basalts • Evidence of extensive differentiation, partial melting • Atmospheric composition that expected by outgassing • Crustal features expression of deep internal mantle convection • Mostly vertical components • No evidence of 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 reduces topo extremes • High topography (Ishtar Terra) must be young features

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 (18 mi) 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, the American anthropologist • measures 280 km (168 mi) in diameter and is located north of Aphrodite Terra and east of Eistla Regio • 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 (23 mi) 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 (12 mi) wide • north-south profile through the center of this crater resulted from the downdropping 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 (124 mi) long x 125 km (78 mi) 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 (39 mi) • 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 (32-mi) 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 (19 mi) 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. Illumination is from the left at an incidence angle of 38 degrees • Note overlying ejecta blankets and broken, irregular rims.

31. Location of Major Volcanic Features on Venus

32. Smaller Volcanic Features

33. Sapas Mons • Large volcano ~400 kms diam & 1.5 kms • Located on a topo rise in Atla Regio • Summit has 2 mesas w/flat to slightly convex tops and smooth surfaces (radar-dark) • Sides of volcano show bright overlapping flows • 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.

34. Cinder (Pyroclastic) Cones • The cone volcanoes in this cluster are about 2 kms (1.2 mi) in diam, 200 m (660 ft) high, with 12° steep slopes overlying a fracture network in NiobePlanitia • Some cones are cut by younger, more widely spaced, north-striking fractures with curvilinear outlines • Figure at right shows cinder cone field in a corona structure with associated linear faulting

35. Lava Flows • 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 • The scene's dimensions are 450 by 630 km A C B

36. Basaltic Flows • A flow field south of Ozza Mons in Atla Regio consists of numerous adjacent and overlapping flows with varying degrees of brightness • Brightness in radar images is related to several factors such as surface roughness and emissivity • Approximately 300-500mya tremendous outpourings of lava like this resurfaced Venus creating topographically flat plains and burying evidence of previous impact record

37. 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

38. Crater w/melt or Volcano w/lava flow?

39. Anemones • Scientists have named this type of volcano "anemone" because of its petallike lava flows and radiating radar-bright patterns • Normally occur in association with fissure type eruptions • 40 by 60 kms (25 by 37 mi) in size • Dark central edifice with bright central flows • Elongated summit pits and an arcuate graben along the southern summit

40. This cluster of four overlapping domes is located on the eastern edge of Alpha Regio. The domes average about 25kms (16 mi) in diam with max heights of 750m (2,460ft) Pancakes • These features can be 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.

41. Calderas • Sacajawea Patera • Elliptical caldera 260 by 175 kms that forms a depression about 2 kms deep • Depression is enclosed by a zone of concentric troughs that show radar-bright outlines • Floor covered with smooth mottled plains • Brightest deposits occur around the periphery and near the center of the caldera floor

42. Domes • This 17.4km dome in Navka Planitia shows collapsed margins and landslide deposits in both the NW and & NE quadrants • Landslide deposits show hummocky surfaces extending up to 10km out on the plains • Dome is ~1.86 km high & 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

43. Volcanic Dome in Aino Planitia • Central dome ~100 km across • 1 km high • Note thick, fan-shaped lava flows and larger flow w/banded surface • Fan-shaped flow lobes are thick & vary between 120 and 540 m • Shape suggests the lavas had a hard time flowing away from the volcano • The banded flow looks like those produced by viscous lavas on Earth • Rarely basalts • Such lavas need water to form, however, water is scarce on Venus

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

45. Arachnids • Arachnoids are one of the more remarkable features found on Venus • They are seen on radar-dark plains • Arachnoids are circular to ovoid features with concentric rings and a complex network of fractures extending outward • Arachnoids range in size from ~50 kms (29.9 mi) to 230 kms (137.7 mi) in diam

46. 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

47. 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 (328 mi)

48. 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 diameter • 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

49. Extensional Fracturing/faultingEarth Analog : Cedar Mesa, Utah

50. Novae • Nova - radial network of grabens • Radiate from central point • Extentional 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