Lecture #16:
The Main Point
• Venus:
• Earth’s sister planet.
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.
• General properties.
• Telescopic observations.
• Space missions.
• Interior, surface,
atmosphere.
• Readings:
– Chapters 7.1(Venus), 9.5, 10.5
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Basic Properties of Venus
Observations
• Average Distance from Sun: 108,200,000 km (0.72 AU).
• Orbital period: 225 days.
• Period of Spin around axis: -243 days!
• Venus is a prominent morning or evening
"star", always within ~50° of the Sun.
• Venus is spinning extremely slowly.
• And...it's spinning backwards relative to most everything else...
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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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• 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
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!
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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Astronomers knew that some
cloud markings could be seen
in UV light, however.
Venus near full phase, from a
ground based telescope.
Next transit on June 6, 2012.
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Space Missions
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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• 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
Venera 4-8
Venera 9,10
Mariner 10
Pioneer Venus 1,2
Venera 11-14
Venera 15,16
Vega 1,2
Galileo
Magellan
Venus Express
1962,1967
1967-1972
1974,1975
1974-1975
1978
1978-1982
1983-1984
1985
1990
1990-1993
2005-2007
Venus flybys; gravity, radiometry
Venus hard and soft landings; photography
Venus orbiters and entry probes; atmospheric composition
Venus and Mercury flybys; imaging
Venus orbiter and entry probes; gravity, atmospheric composition
Venus flybys and entry probes; soft landings; photography
Venus orbiters; radar imaging
Venus balloons & atmospheric composition; Halley flybys
Venus flyby (then on to Jupiter); CCD images, spectra
Venus orbiter; global radar mapping and gravity mapping
European atmospheric mapping mission
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Venus and Earth are Very Similar…
Property
Diameter (km)
Mass (kg)
Density (g/cm3)
Earth
12,756
6.0x1024
5.52
Interior Structures of the Terrestrial Planets
Venus
12,104
4.9x1024
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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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
Magnetic field
Surface Temperature
Surface Pressure (bars)
Atmospheric Comp.
Planetary Magnetic Fields
• Importance of planetary
rotation: Global magnetic
field requires:
Earth
Venus
Yes
No
~ 15°C
~ 470°C
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90
N2, O2, Ar,... CO2, N2 ,…
– Layer of electrically
conducting fluid,
– Convection of that fluid,
– Rapid rotation.
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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• Slow spin of Venus does
not allow for strong
magnetic field.
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Planetary Evolution
Geological Histories of the Terrestrial Planets
Important factors are:
• Size,
• Distance from the sun,
• Rotation.
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Summary
Next Lecture...
• 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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• Surface of Venus:
• Radar Mapping Concepts.
• The Magellan mission.
• Surface properties.
• Reading:
– Chapter 9.5
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