Download Venus - Overview Exploration of Venus Orbit and Rotation

Key Concepts: Lecture 14
Venus: structure, surface, magnetic field
Atmosphere (runaway greenhouse)
Exploration of Venus
• Telescopic observation of Venus
– Only see cloud layers
– reflect 76% of incoming sunlight
• Visited by ~ 20 spacecraft
Use of radar: transparency of atmospheres in radio; Doppler effect
Comparison of Terrestrial Planets
Venus - Overview
• Second planet from Sun
• Earth’s “sister planet”
– similar sizes, masses,
densities, cratering &
chemical compositions
Property
Radius
Mass
Density
Escape Speed
Atmosphere
Venus
6052 km
4.7 x 1024 Kg
5240 kg/m3
10.4 km/sec
Carbon dioxide,
Nitrogen
Earth
6378 km
6.0 x 1024 kg
5520kg/m3
11.2 km/sec
Nitrogen,
Oxygen
– Mariner 2 - first to visit in 1962
– Venera 7 - Soviet Space Craft - first to land on another
planet
– Venera 9 - first photographs of surface
– Magellan - detailed maps of surface from radar
Orbit and Rotation
• Orbit of Venus around Sun
– most circular orbit of all
planets
– 225 Earth days for 1
complete orbit
• Rotation of Venus
– Retrograde - in opposite
direction of most other
planets and most satellites in
solar system
– Very slow: 243 Earth days for
1 full rotation (siderial day);
117 Earth days for solar day.
Earth
Venus
Structure of Venus
• Interior structure similar to
Earth
– similar mass, size & density
• Metallic core
– somewhat lower density
than Earth
– somewhat smaller core than
Earth
• Large rocky mantle
• Rocky crust
Magnetic Field
• Very, very weak magnetic field - Why?
– Venus rotates much more slowly (243 times)
than Earth
• internal dynamo weaker
• weaker magnetic field - 10,000 times weaker than
Earth!
• Interaction with solar wind differs from Earth
– solar wind runs right into upper atmosphere of
Venus
– carries off some of the atmosphere
Surface of Venus
• Varied Terrain
–
–
–
–
–
mountains
high plateaus
canyons
ridges
craters
• Overall relatively flat compared to
Earth
– Only 10% of Venus’ surface above 10 km
Radar to map
surface…
• Atmospheres
are quite
transparent in
the radio
wave band.
…and measure rotation rates
• Uses Doppler effect:
waves are compressed
if emitting object is
moving towards us;
expanded if moving
away from us.
Surface of Venus - Volcanoes and Lava Domes
• Volcanoes occur in complex groups
• Lava Domes
• Shield volcanoes
• relatively flat
• often having a collapsed
central volcanic crater at
summit
Surface of Venus
Surface of Venus - Impact Craters
• Few craters
• Several upland
plateaus,
resembling
continents
• Low-land lava
plains
• Some volcanoes,
maybe active, as
revealed by
variable gas
emissions in
atmosphere
• Relatively few impact craters young surface overall
• No small craters
Radar map:
– small meteoroids burn up in
Venus’ dense atmosphere
• Craters come in bunches
– large meteoroids that reach
surface break up in atmosphere
Atmosphere of Venus
• Constituents:
– 96% carbon dioxide
– 3.5% nitrogen
– water, sulfuric acid clouds,
hydrochloric acid
– Variability may indicate
volcanic eruptions
– Fast winds in upper
atmosphere, almost no wind
at surface
Temperature & Pressure
• Temperature increases
as you get closer to the
surface
– surface temperature
~800 K
• Pressure increases as
you get closer to the
surface
– 90 times greater than
Earth’s surface
pressure!
Greenhouse Effect on Venus
• ~76% of sunlight reflected by clouds & never reaches surface
• Yet surface temperature extremely high!
• Surface temperature high due to strong greenhouse effect
• No oceans or life to remove CO2
Questions about Venus
• Why is its rotation retrograde? Perhaps due
to giant impact.
• Why did Venus’ atmosphere evolve so
differently than the Earth’s? Probably
because conditions never arose for large
amounts of liquid water oceans (too hot
initially, or not enough supplied by comets).
Summary: Terrestrial Planets
Evolution of Terrestrial Planets:
atmospheres
Mercury
Mercury
Venus
Mars
Earth
• All terrestrial planets are differentiated
• Larger planets take longer to cool and still have
active mantles (plate tectonics on Earth, active
volcanoes on Earth & Venus) and liquid cores
Earth
Venus
Mars
Mercury
Tsurface ~ 750K
Earth
Mars
Tsurface ~ 300K
Tsurface ~ 218K
• Volcanic activity creates secondary atmosphere
• Gravity holds atmosphere. Light atoms escape most easily. Higher temperatures
allow heavier atoms to escape
• Water and life remove CO2 and life creates O2
Evolution of Terrestrial Planets:
interiors
Evolutionary Stage of Terrestrial Planets
Tsurface ~ 623K (day)
~ 100K (night)
Venus
Moon
Planet accretes from planetesimals
Solid crust forms. Heavy
infall of planetesimals -> cratering
Major cratering ends.
Mare-type basins flood with lava.
Surface tectonically active.
Volcanoes, plate motions or
other mantle motions.
Mantle solidifies.
Tectonic activity ends on surface.
Interior cold.
All tectonic activity stops.
• Small planets cool fastest and go through this planetary
evolution faster