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Lecture 4: Venus

(Goddess of love & beauty)

• Vital Statistics

• Orbit & rotation

• Interior

• Surface

• Atmosphere

• Greenhouse effect

Historical record

• Third brightest object

• First record of phases by

Galileo in 1610

• Transits observed: 1761,

1769, 1874, 1882, 2004

• Last one June 5-6 2012 –

next 11 December 2117)

• Transits used to measure

AU and detect atmosphere

Copernican

Ptolemaic

Venus - vital statistics

Orbital semi-major axis 0.723 AU

Orbital period 224.70 days

Rotational period 243.019 days (retrograde)

Eccentricity of orbit <1%

Inclination of rotation axis 177.4º

Diameter (at equator) 0.949 dEarth (6052 km)

Mass 0.815 MEarth

Mean density 5240 kg m-3 (0.95 Earth)

Most planets in the Solar System have prograde spin

Exceptions: Venus, Uranus (& Pluto). As they formed from

a single disk the planets should spin in the same direction.

Venus, Uranus (& Pluto) may have been hit early on – this

would explain their unusual tilt and slow rotation-rate.

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A Venusian Solar day (noon to noon) is 116.8 Earth days.

This is almost exactly 1/5 its synodic period (583.92d, the

orbital period viewed from Earth). This means Venus

presents nearly the same face to us on closest approach.

Is this near 5:1 resonance

a coincidence?

Yes.

No known way for

Earth to influence

Venus significantly.

Exploration of Venus

• Spectroscopy/imaging from Earth – CO2 found in 1932.

Imaging shows cloud “features”. Surface temp. ~750 K !!

• Radar data dominate, particularly Magellan (1990-94).

• Landers – all Russian. Venera 3 (1965). Crushed!

Venera 9 (1975) sent first picture. Others followed.

Venera 9 –

20-50 cm rocks.Appear young.

Venera 13 –

True-colour.

Similarity in size and density

to Earth implies a silicate-rich

mantle and an iron rich core.

Venera landers confirmed

Venus has a silicate surface.

No seismic data but we do

known it has no detectable

magnetic field (<0.1% Earth).

Interior of Venus Pioneer (1978) view of Venus

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Purple = low altitude

Green/Red = high altitude

25% “lowland” (≤0 km), 65% “rolling plains” (0-2 km),

only 10% “highland” (>2 km). Altitude range ~15 km.

(Earth: altitude range 20 km. 25% of surface continental.)

The Surface of Venus

Magellan: radar and gravity

mapped 99% of Venus at 100 m

resolution during 1990-94.

Venus gravity map (Magellan)

Very good correlation between the two.

Impact craters on Venus

No small craters (<3 km)

due to thick atmosphere.

Surface lightly covered –

relatively few craters

suggests recent resurfacing.

Surface age ~ 300 Myr.

Dickinson crater

diameter 69 km

Volcanic features - abundant

2 km shield volcano (middle)

+ coronae (front)700 km shield volcano

– but only 2 km high

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Coronae – roughly circular, can be very large (~600 km).

Arachnoids – smaller than coronae – precursor?

These features are unique to Venus and tend to be along

tectonic belts. Thought due to magma uplift.

Tectonic features

See mountain chains and fracture patterns in highland

areas. Here (dark) lava fills a 20 km wide valley.

Small fluctuations in atmospheric levels of SO2 indicate

that there may also be active volcanoes on Venus.

Active today?

Some evidence for

surface changes.

Landslide � Venus is much hotter than Earth

or Mercury

Cannot be explained by their

relative distances from the Sun.

Highly reflective atmosphere.

Sulphuric acid clouds!

The Atmosphere

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CO2 96.5 %

N2 3.5 %

SO2 150 ppm

Ar 70 ppm

CO 30 ppm

H2O 20 ppm

Present-day Atmosphere of Venus

Very different to Earth (N2 : O2 dominated – 78% : 20%).

Venus has a CO2 column mass x100 Earth’s atmosphere..

Structure of atmosphere on Venus

Profile simpler than Earth –

no stratosphere (no O3).

Mesosphere dominated by

sulphuric acid (H2SO4).

High cloud deck (30-70km).

Volcanic gases in the

atmosphere. Very high

surface temperature (mean

≈735 K).

Surface pressure ≈90x

Earth.

Thermosphere

Mesosphere

Runaway Greenhouse Effect

More CO2 – less heat escapes – warmer planet

∆Tgreenhouse ≈ +500°C on Venus, cf. +35°C on Earth.

Mean surface temperature on Venus ~ 460°C !!

Our only overall view of Venus

Radar maps (Magellan+Pioneer+Venera+Arecibo)

East at centre North at centre

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The End