• Like Mercury – close to the Sun. • Maximum angle from Sun ~47° • Hence seen at most for 3 hours before sunrise or after sunset. Venus reflects ~70% of its sunlight. Property Earth Venus Radius 6378km 6052km Mass 5.97x1024 kg 4.87x1024 kg = = 1 ME 0.82 ME Average Density 5520kg/m3 5240kg/m3 Gravity 10.91 Earth’s Escape Speed 11.2km/s 10.4km/s Average Temperature 290K / 17C 720K / 447C Eccentricity 0.017 0.007 Venus’ Retrograde Rotation
• Hard to establish rotation rate due to dense clouds (use Radar Doppler techniques) • Sidereal rotation rate: 243 days – Retrograde ‘backwards’ with respect to Earth • Tilt angle is 177.4° -Earth is 23.5° Large angle due to definition that “Planets always rotate from East to West” ‘North’ is therefore below the plane of ecliptic. Venus’ Strange Rotation Consequences: Length of “Day”
• Slow backwards rotations means Solar Day (noon- to-noon) is 117 Earth days. • Sidereal rotation period is 243 Earth days. • Venus year is 225 Earth days. Why Retrograde Rotation? • Don’t really know! Possible that it was hit by a large object in its early evolution (like Earth?) which slowed it down to near zero. • Venus has near perfect 5:1 synchronism between rotational and synodic orbital motion (117 days to 584 days). • But no Earth-Venus gravitational interaction can account for this weird coincidence. This inhibits Earth based observations. Cloudy Venus
Best Earth-based photographs show an opaque planet in the visible wavelengths.
But origin is not due to water vapour! Venus in Ultra Violet Light
• Large fast moving clouds like our ‘jet streams’. • Move around the planet in ~4 days. • Hot ~ 730K – 457C • Sulphuric Acid cloud droplets! The Atmosphere of Venus
• 96.5% Carbon Dioxide • 3.5% Nitrogen
• Traces of Argon, water, CO, SO2 • No oxygen – very different from Earth • Clouds of sulphuric and hydrochloric acids, as well as Carbon Dioxide • Atmosphere 90x more massive than Earth. Earth – Venus Comparison The “Greenhouse” Effect
• Given its distance from Sun, why is Venus so hot? Carbon dioxide traps heat from the Sun.
• CO2 clouds absorb ~99% of all radiation released from the surface. • ~ 450C Temperature all over the planet (poles and equator) and little variation between day and night! Greenhouse Effect on Earth and Venus Why is Venus’ Atmosphere so Different from Earth’s?
• If all the CO2 that is either dissolved or chemically locked to minerals were to be released, Earth’s atmosphere would be
98% CO2 ! Ocean formation on Earth crucial. • Postulate that Venus and earth had similar atmospheres in the past…. Atmospheric Evolution on Venus • Venus was hotter than Earth due to it being ~30% closer to the Sun. More atmospheric water vapour on Venus. • Water is a greenhouse gas too – trapping more heat.
• Oceans never form. (CO2 stays in atmosphere) • Gets Hotter and Hotter :Runaway greenhouse effect. • Water “boils off” planet (Hydrogen into space). Venus has No Magnetic Field.
• Due to lack of rapid rotation which gives a ‘dynamo’ effect on the molten core.
No protection from solar wind of charged particles in upper atmosphere – hence a heating/ionisation process. The Earth’s Magnetic Field
Due to: Rapid Rotation of Liquid Conducting Core The Surface of Venus Radar Images reveal two “continents”: Ishtar Terra and Aphrodite Terra. Babylonian and Greek versions of Latin Venus – the Goddess of Love. Spatial resolution ~25km High Resolution Radar image of Aphrodite Terra
Magellan (False Colour) Continents are Stable
• Terra are ~8% of Surface Area (~25% Earth) • Sizes: Ishtar ~ Greenland, Aphrodite ~ Africa • No Signs of plate tectonics on Venus. Ishtar Terra: Maxwell Montes Mountain Range
Crater Cleopatra – meteoric – yet with signs of lava flow Mountains ~14km high above deepest depressions. (~20km Earth) Aphrodite Terra
• Ridges indicate surface buckling – Stresses • Lava channels common and 100’s km long Evidence of Volcanism on Venus
• Lava domes ~25km across. Lava oozes out of surface forms a dome…. Lava underneath withdraws leaving the crust to crack and subside. Large Volcanoes
• Sif Mons (left) and Gula Mons Similar type to Hawaii – local hot spot in crust. • Summit collapses –called a ‘Caldera’ in these ‘Shield volcanoes’. • Summits ~100km across! • Evidence of continued volcanic on Venus activity (Radio emissions on eruption) Sapas Mons, Pancake Domes, UV Venus Craters on Venus
• Few small < 3km craters due to atmosphere • Large Crater numbers < 10% of that on Moon • ‘Young’ planet or more erosion? Largest Crater: “Mead” Craters on Venus Soviet Landers: Venera 9, 10 (1975)
Spacecraft melted in ~1 hour!
• Small young - little erosion - rocks and ‘soil’. • Some volcanic basalt, others like granite. ESA’s “Venus Express” 2006
UV image
South pole vortex: Day and Night Temperatures (IR) The Turbulent and Chemically complex atmosphere of Venus (Thermal images)