venus: our sister planet
TRANSCRIPT
Venus: Our Sister Planet
• 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 VenusRadius 6378km 6052km
Mass 4.87x1024 kg = 0.82 ME
Average Density 5520kg/m3 5240kg/m3
Gravity 1 0.91 Earth’s
Escape Speed 11.2km/s 10.4km/s
AverageTemperature 720K / 447C
5.97x1024 kg= 1 ME
290K / 17C
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 117Earth days.
• Sidereal rotation period is 243Earth days.
• Venus year is 225Earth 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 synchronismbetween 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 VenusRadar 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 flowMountains ~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 MonsSimilar 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)