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)