lecture #16: the main point - cornell...
TRANSCRIPT
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Astro 102/104 1
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