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Lecture #16:

• Venus:

• Earth’s sister planet.

• General properties.

• Telescopic observations.

• Space missions.

• Interior, surface,

atmosphere.

• Readings:– Chapters 7.1(Venus), 9.5, 10.5

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The Main Point

Venus is very much like the Earth in

terms of its size, density and

presumably interior structure.

However its surface and atmosphere

are very different from Earth’s.

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Basic Properties of Venus• Average Distance from Sun: 108,200,000 km (0.72 AU).

• Orbital period: 225 days.

• Period of Spin around axis: -243 days!• Venus is spinning extremely slowly.

• And...it's spinning backwards relative to most everything else...

• Mass: 4.9x1024 kg = 0.81 ME ; Radius: 6052 km = 0.95 RE.

• Density = 5.3 g/cm3 (recall that Earth ~5.5 g/cm3).

• Surface Gravity = 8.9 m/sec2 (91% of Earth's).

• Thick CO2-rich atmosphere; Surface pressure ~90x Earth's.

• Average Surface Temperature: +482°C (900°F!)

• Complex surface geologic processes at work...

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Observations

• Venus is a prominent morning or evening

"star", always within ~50° of the Sun.

• Venus goes through

phases like the Moon,

visible in even small

telescopes.

• Venus is so bright

partly because it is so

close, but also

because we are seeing

sunlight reflected off

bright cloudtops!

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Venus Transit on June 8, 2004

Next transit on June 6, 2012.

The Royal Society hired

James Cook to travel to

Tahiti to observe and

record the 1769 Jun 03

transit of Venus across

the Sun. The goal was

to measure the value of

the astronomical unit.

This voyage later saw

the exploration of New

Zealand and Australia

aboard the Endeavour.

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Telescopic Observations• Venus hard to observe with optical telescopes from Earth:

– Usually close to the Sun (mucky air, poor "seeing").

– Can often be in thin crescent phase--rest of planet is dark.

– All we see is bland clouds, not the surface!

Venus near full phase, from a

ground based telescope.

Astronomers knew that some

cloud markings could be seen

in UV light, however.

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Telescopic Observations

• Venus much more observable with radio

telescopes: radio wavelengths see through clouds!

• Many surface features visible--

provided reliable method of determining

the spin rate: 243 days and backwards.

• Some very (radar) bright and very

(radar) dark areas clearly visible.

• Evidence for continent-scale geology.

• Much of this work done at Cornell!

• More details in Lecture 17…

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Space Missions

• 21 robotic space missions have either flown by, orbited, or

landed successfully on Venus (out of 36 attempts).

Mission Name Dates Goals and Results

Mariner 2,5 1962,1967 Venus flybys; gravity, radiometry

Venera 4-8 1967-1972 Venus hard and soft landings; photography

Venera 9,10 1974,1975 Venus orbiters and entry probes; atmospheric composition

Mariner 10 1974-1975 Venus and Mercury flybys; imaging

Pioneer Venus 1,2 1978 Venus orbiter and entry probes; gravity, atmospheric composition

Venera 11-14 1978-1982 Venus flybys and entry probes; soft landings; photography

Venera 15,16 1983-1984 Venus orbiters; radar imaging

Vega 1,2 1985 Venus balloons & atmospheric composition; Halley flybys

Galileo 1990 Venus flyby (then on to Jupiter); CCD images, spectra

Magellan 1990-1993 Venus orbiter; global radar mapping and gravity mapping

Venus Express 2005-2007 European atmospheric mapping mission

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Venus and Earth are Very Similar…

Property Earth Venus

Diameter (km) 12,756 12,104

Mass (kg) 6.0x1024 4.9x1024

Density (g/cm3) 5.52 5.25

Venus and Earth presumably have similar compositions

and interior structure. Both planets have a young surface

and active geology.

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Interior Structures of the Terrestrial Planets

Importance of planetary size: Larger planets generate more

interior heat and can retain interior heat longer, with important

consequences for convection in the mantle, the thickness of the

lithosphere, and surface geology (more in lecture 17).

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Venus and Earth are Very Different…

Property Earth Venus

Magnetic field Yes No

Surface Temperature ~ 15°C ~ 470°C

Surface Pressure (bars) 1 90

Atmospheric Comp. N2, O2, Ar,... CO2, N2 ,…

Importance of distance from the Sun: The atmospheres on

Venus and Earth have had wildly divergent evolutionary

histories (more in lecture 18).

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Planetary Magnetic Fields

• Importance of planetary rotation: Global magnetic field requires:

– Layer of electrically conducting fluid,

– Convection of that fluid,

– Rapid rotation.

• Slow spin of Venus does not allow for strong magnetic field.

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Planetary Evolution

Important factors are:

• Size,

• Distance from the sun,

• Rotation.

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Geological Histories of the Terrestrial Planets

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Summary

• Venus is a jewel in our evening or morning sky.

• Venus’ size, mass, & density are similar to Earth’s.

• Its interior structure is likely similar to Earth’s, but the

slow rotation does not allow for a global magnetic field.

• Its surface is geologically young.

• Its atmosphere is radically different from Earth’s!

• Venus has been studied from the ground and by

spacecraft, but there is still much we don’t know.

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Next Lecture...

• Surface of Venus:

• Radar Mapping Concepts.

• The Magellan mission.

• Surface properties.

• Reading:

– Chapter 9.5