1. Venus

2. Venus basic data Semi-major axis 0.72 AU Eccentricity 0.0068
Orbital tilt °
Orbital Period Earth days
Rotation Period Earth days (retrograde)
Diameter 95% Earth’s
Mass 82% Earth’s
Density g/cm3
Atmospheric Pressure 90 atm at surface
Temperature K (hottest planet)
Temp is almost uniform due to thick atmosphere.

3. Strong upper level winds of up to 350 km/h
Atmosphere so thick, can’t see surface, at visible, UV or most IR wavelengths
Strong upper level winds of up to 350 km/h
Convection: hot air from equatorial regions to poles, cools, returns to equator. Keeps T very constant. Convection and winds give V-shape appearance.
Yet at surface winds are only < 5 km/h due to intense pressure.
Visible UV IR

4. Missions to Venus Soviet Venera 4 -18 (1967 - 1983)
Mariner 2, 5 and 10 (1962, 1967 and 1974)
Pioneer Venus (1978)
Magellan (1989)
Venus Express (ESA, launched 9 Nov 2005), in orbit since May Ended in 2014.
Venus Express studying atmosphere.

5. Venus' atmosphere
Hot, dry, dense: so hot at surface (~733K) it almost melts rocks
Thick atmosphere, 96.5% CO2 (strong greenhouse effect => high temps)
0.15% sulfuric acid (H2SO4), responsible for clouds and haze

6. So how can we study surface through the thick clouds?
Venus’ clouds consist of sulfuric acid (H2SO4)
Max height of Earth’s clouds
At lower levels, SO2 droplets evaporate, so no rain.
So how can we study surface through the thick clouds?
Radar  long wavelength radio waves will pass through clouds.

7. Radar echo measures altitude
space probe
time for signal to return tells you the altitude of surface feature.
Different substances also reflect radar to different degrees. Strength of return signal gives clue to surface composition.
Planet Surface

8. Topographic map of Venus
Flatter than Earth, no evidence of plate boundaries.
=> No large scale plate tectonics
Earth: continents and oceans differ by 4-6 km. Venus: 60% of surface within 500m of avg.
85% plains
15% highland plateaus (Ishtar Terra and Aphrodite Terra)

9. The surface of Venus: Magellan radar images
Mountains, volcanoes, lava flows and impact craters indicate active interior and a young surface

10. Impact Craters
Only ~1000 seen, spread randomly (unlike the Moon): All parts have about the same age.
None < 3 km across (no meteor impacts < 30 m), smaller ones burn up in atmosphere
Paucity of impact craters compared to Moon, Mercury implies that the surface is young, roughly 500 million years.

11. Field of craters and the largest crater found on Venus (280 km diameter)
Mead Crater
Rougher surface provides brighter radar echo => recent, unweathered event

12. More recent lava flows also rougher

13. Volcanism Volcanoes also randomly distributed – no plate tectonics.
SO2 varying in lower atmos at 40% level (Venus Express), but debated whether this means ongoing volcanism. SO2 should disappear in Myr, so volcanism as recent as that. Lightning not accepted as indication of volcanism.
Volcanoes also randomly distributed – no plate tectonics.

14. Shield volcano elevation map from Magellan radar data
Shield volcano elevation map from Magellan radar data. About 100 km across.
Vertical scale exaggerated by factor of 22.5
Sif Mons, and a ~ 5 km long lava flow. Estimate lava flowed within 10 million years.

15. "Pancake volcanoes" - eruptions of lava through vents close to ground.
"Coronae" - Concentric pattern, with a radial fracture => lava from inside caused surface to stretch.

16. Yet another type, a tick volcano, with ridges and valleys
Yet another type, a tick volcano, with ridges and valleys. A flat summit ~22 mi diameter.

17. More evidence for past (. ) volcanic activity:
More evidence for past (?) volcanic activity: Venera 13 (Soviet lander 1981) found rocks similar to Earth’s basalt.
Veneras 7-14 attempted to land. Didn’t last for long before heat and pressure got them. Venera 13 survived 2 hrs and sent photos

18. A 2 km-wide channel, caused by lava. Length is 6800 km (= 4200 miles)
A 2 km-wide channel, caused by lava. Length is 6800 km (= 4200 miles). Compare to Nile at 6600 km.
Carved by hot lava, which should remain liquid for a long time due to extreme surface temperatures

19. More evidence of small-scale surface deformation
1 km
Cross-hatched structure.
Faults and fractures?
Details not known.

20. Is volcanism still ongoing?
Venus Express infrared imaging in an atmospheric window at 1.02 microns reveals nine sites of hot-spot volcanism within past 2.5 million years and possibly 250,000 years
S02 levels in atmosphere indicate recent volcanism, but may not constrain time to < 10 million years ago.
High emissivity of basalt at IR apparently indicates young, unweathered flows. Gets dimmer as it ages.

21. Stronger, more recent (summer 2015) evidence from Venus Express:
Some near-IR wavelengths probe surface as well as atmosphere.
Dominated by blackbody radiation. IR spectrum yields temperature.
Some changes in brightness found on a few surface features:
Temperature is changing, suggesting volcanic activity.
“Object A” reached 1100 K and is about 1 km across.

22. Volcanism summary No volcanoes in chains => no plate tectonics
Much evidence for localized upwelling and fracturing
Volcanoes may still be active
Surface is young: roughly 500 million years. All parts have about same age. If volcanoes active, only small fraction of surface affected.

23. Why different from Earth?
One idea: more active volcanism keeps crust thin. Rock may even be soft due to surface heat. Easy to break through (“flake tectonics”).
Con: very few deformed craters

24. Second idea: the surface undergoes uniform upheaval about every ½ billion years. Heat builds up under thick, dead lithosphere until catastrophic surface meltdown.
Seismometers would be very nice to have on Venus…

25. Interior presumably like Earth’s in terms of densities
Interior presumably like Earth’s in terms of densities. Iron core, mantle, crust
 No magnetic field

26. Climate History of Venus
Like Earth, started with outgassing from volcanoes (mostly H2O, CO2, SO2) and perhaps additional water from cometary impacts.
Young Sun only 70% as luminous as now, so Venus’ early atmosphere would have been cooler. Probably liquid water? At least some water would have been gaseous. CO2 dissolved in water and rocks.
Water vapor is greenhouse gas, so T would rise. Plus aging Sun got brighter.

27. Runaway Greenhouse Effect!
Hotter Sun + water vapor increases atmospheric temperature.
Water could no longer be liquid as Sun got hotter (~ few 100 million years). Oceans start to evaporate, adding more H2O and CO2 (and SO2) to atmosphere.
Stronger Greenhouse effect => further evaporation => stronger Greenhouse effect, etc. Eventually, CO2 even baked out of rocks into atmosphere. Temperature stops rising when all CO2 removed.
Runaway Greenhouse Effect!

28. What happens to H2O? Solar UV radiation breaks H2O apart, H lost, O reacts to form molecules.
Chain of reaction(s) to make sulfuric acid, skip

29. Comparison of Venus and Earth
Both display volcanism and geological activity, BUT Venus has:
Slow rotational period
No plate tectonics
No magnetic field
No satellite
No water
High surface temperature and dense atmosphere
Rocks above Curie T, no frozen in B fields to be detected
Was Runaway Greenhouse Effect inevitable at this distance from Sun?

31. Venus fly-over

32. Venus Express Aim: to study atmosphere
Interactions surface
Interactions with solar wind
Circulation and composition as a function of depth
Radiative balance
IR image of doubled-eyed vortex at south pole
Will operate until May 2009
About 60 km

33. IR images, April 2006.
2.3 and 1.7m
Atmospheric structure at 35 and 20 km altitude respectively
Stripes: wave-like atmospheric motion (tidal forces?) but nature still unknown.

34. ASPERA (Analyzer of Space Plasma and Energetic Atoms), data taken when flying through interaction regions between Venus and solar wind.
Left: heated solar wind (protons and alpha-particles)
Right: regions of massive 'escape' of oxygen ions

35. Slow, retrograde motion

36. Retrograde rotation hard to understand just from planetary formation from pre-solar system nebula – almost everything spins and orbits in same sense
Possible explanations:
1) Rather complex mechanism involving tidal interaction Venus, Sun & Earth, and atmospheric braking ?
2) Massive impact de-spinning Venus (no moons?)

37. Rift valleys
Similar in size to East African Rift (largest on Earth, tectonic motion between African and Eurasian plates).
On Venus, rift valleys consequence of local activity – no large scale plate tectonics.