Chapter 9: Venus Earths Sister Planet

1
Chapter 9 VenusEarths Sister Planet

• Orbital Properties
• Bulk Properties
• Internal Structure
• Atmosphere
• Surface Features

2
Earth-based Observations

• Venus observed by Galileo to have phases.
• Always appears close to the Sun.
• Maximun elongation of 470.
• Best viewed above the horizon for 3 hours before
  the Sun rises or after it sets.
• Third brightest object in the sky
• Sun and Moon are first and second.
• 10 x brighter than Sirius.
• Highly reflective reflects 70 of incident
  sunlight.
• Appears brightest at 390 separation from Sun.

3
Venus Day

• Established by radar techniques
• Rotation period -243 Earth days
• Retrograde rotation opposite sense to Earths
  rotation and to orbital direction
• Solar day (noon to noon) 117 Earth days
• Orbital period 225 Earth days

4
Venus Retrograde Rotation

• Most planets and moons move in pro-grade motion.
• (counter-clockwise as viewed
  from N-pole of Solar System)
• Venus' rotation is retrograde (clockwise).
• One way to explain this motion is to assume that
  Venus' rotation axis is tilted about 177o, so its
  north pole points "down".
• This great tilt may have been caused by a
  gigantic impact shortly after the planet formed.

animate
5
Exploration of Venus from Space

• First soft landing on surface in 1970 by Venera
  7.
• Four of the most successful missions in revealing
  the Venusian surface are NASA's Pioneer Venus
  mission (1978), the Soviet Union's Venera 15 and
  16 missions (1983-1984), and NASA's Magellan
  radar mapping mission (1990-1994). As these
  spacecraft began mapping the planet a new picture
  of Venus emerged.
• Missions
• showed surface dry and dusty.
• rocks at surface show sharp edges, slab-like
• implying young age and lack of surface erosion.
• photographs of smaller rocks and finer materials.
• chemical analysis of surface rock indicated
  basalt and some
  terrestrial-like granite.

6
U.S. Exploration of Venus

• In 1978, Pioneer Venus mission
• photographed Venuss upper atmosphere in UV
• dropped probes to study T, P and
  chemical
  composition of atmosphere
• mapped surface with radar
• found
• large, fast-moving cloud patterns resembling
  Earths jet streams
• upper cloud decks rotate around planet faster
  than planet
• Magellan probe (1990-1993) mapped the surface of
  Venus with radar
• Resolve 120 m horizontally and 50 m vertically.
• Provided more data than all previous planetary
  missions combined.

7
Venus in UV from Pioneer Venus
8
Venus in Radar from Magellan
Dark areas smooth Light areas rough
9
Venus from Magellan data
Center view of N-pole outside four views of
equator
10
Venus Express

• Nov 2005 Launch
• Apr 2006 Venus Arrival
• Status In Development
• Journey through space will last 153 days.
• Once it is captured by Venusian gravity, Venus
  Express will take 5 days to maneuver into its
  operation orbit, looping around the poles of the
  planet.
• At its closest, it will reach an altitude of 250
  kilometers and at its furthest, it will be 66,000
  kilometers away from the planet.
• The mapping mission is due to last for 2 Venusian
  days, roughly 500 Earth days.

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Atmosphere of Venus Composition and Surface
Pressure

• Atmosphere by volume
• 96.5 carbon dioxide
• 3.5 nitrogen
• Trace amounts of other gases
• water vapor, carbon monoxide, sulfur dioxide,
  argon
• No oxygen or ozone detected.
• Surface pressure 90x Earths
• Earth 14.5 lb/in2
• Venus 1300 lb/in2
• Equivalent to 0.6 mile below ocean surface on
  Earth.

12
Atmosphere of Venus Structure

• Troposphere
• extends 100 km altitude
• temperature decreases with altitude up to 50-km
  level of clouds
• reflective, sulfuric-acid clouds in three layers
• 65-, 53-, and 48-km above surface.
• below clouds, to 30 km altitude, sulfuric acid
  haze.
• below 30 km, air is clear, surface winds gt 2
  km/hr
• Jet-stream above clouds
• 300-400 km/hr, west to east

13
Circulation and Dynamics

• Clouds
• appear yellowish because of suspended sulfur
  particles.
• are composed of droplets of sulfuric acid (H2SO4)
  and
• produce acid rains.
• rotate around the planet once every 4 days
  (retrograde).
• occur in stable layers with little mixing.
  Appears to be very little change in
  weather.
• Cloud patterns
• are observed well in ultraviolet light.
• appear to be very stable because of their slow
  rotation, uniform temperature, and high density.

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Atmospheres Earth vs. Venus
15
Where did all the water go?

• Venus approximately same size and overall
  composition as Earth.
• Size indicates interior heat would produce
  volcanic activity which, in turn, outgasses
  atmosphere.
• Carbon dioxide
• Nitrogen
• Sulfur dioxide
• Water vapor
• most common product outgassing on Earth today

16
Atmospheric Evolution

• The present atmosphere probably formed from gases
  released in volcanic eruptions, similar to
  Earth's.
• Originally, Venus and Earth probably had similar
  atmospheres, but Venus' slightly initial warmer
  temperature (because it is closer to the Sun)
  caused water to evaporate, so no vast oceans
  accumulated on Venus.
• The water vapor would have been susceptible to
  solar UV radiation, which can break
  apart the water into hydrogen and oxygen.
• Thus, Venus could have lost most of its water.
• Carbon dioxide was dissolved in Earth's oceans
  and eventually incorporated into carbonate rocks
  by the carbon dioxide-water cycle.
• But carbon dioxide was allowed to accumulate in
  the atmosphere of Venus because of the lack of
  liquid water.

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Atmospheric Evolution

• Water vapor, carbon dioxide, and sulfuric acid
  all act as greenhouse gases in the atmosphere of
  Venus.
• Venus has a "runaway" greenhouse effect that is
  self perpetuating, and that
  is why the surface temperature of
  Venus is so high.
• The sulfur cycle can explain how sulfuric acid
  came to replace water in the clouds of Venus.
• H2O vapor solar UV light --------gt
  H2 O
• 6S 9O2 6H2O --------gt
  6H2SO4
• Only about 7 of the incoming sunlight reaches
  the surface of Venus. Sulfuric acid forms a
  smoggy layer near the surface, which may
  experience an acid rain drizzle.

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Greenhouse Effect
Works on Venus as it does on
Earth, but Venus has 1 million x more CO2 ,
so effect is much stronger.
19
Venus Surface Features

• Mapped by radar from Earth and Venera, Pioneer
  Venus, and Magellan spacecraft.
• Relatively smooth surface
• Lowlands (92)
• rolling plains
• Highlands ( 8)
• continents
• No obvious tectonic plate boundaries.
• Extensive volcanic features.
• Impact basins.
• Earth-sized mountain ranges.

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Comparison of Topography
21
Venus Radar Map
Click to animate
22
Continents of Venus

• Ishtar Terra
• southern high latitudes
• size of Australia
• large plateau ringed by mountain ranges
• Maxwell Mons, highest point on Venus (14 km)
• Aphrodite Terra
• equatorial
• size of Africa
• extensive lava flows
• compressional ridges

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Venus Surface Features

• Highlands (8)
• Much smaller area than Earth's continents.
• Some very high mountains are observed.
• Maxwell Montes is taller than Mt. Everest.
• Volcanoes have been observed.
• Indirect evidence for current volcanic eruptions.
  
• SO2 decline lightning .
• Volcanic calderas have been observed.
• Rift valleys have been observed.
• Impact craters.
• Geologically active.
• volcanic domes and folded mountains

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Venus Surface Features

• Lowlands (92)
• Some large impact basins have been observed.
• Relatively flat compared to Earth's ocean basins.
  
• Total relief is about 13 km (Earth 20 km).
• No evidence of sea floor spreading.
• Basaltic rock crust.
• Crust may be less than 500 million years old.
• Crust is within 50 of melting temperature.
• No plate tectonics.

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Surface Rocks

• Color photos of surface rocks. These photographs
  were obtained by the Soviet Venera 13 and 14
  landers. The yellowish color seen here is an
  artifact of the camera response reprocessing of
  these images has shown that the surface rocks are
  gray, like similar rocks on Earth. (TASS from
  SOVFOTO)

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Shield Volcano Gula Mons
Most common volcanic feature on Venus similar to
Earths Hawaiian Islands characterized by
formation of a caldera
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Volcanic Caldera

• A large volcanic caldera. This is a depression
  called Sacajawea, in the Lakshmi Plenum region of
  Venus. The enormous caldera is 1 to 2 km deep,
  120 km wide, and 215 km long. It is thought to
  have formed as the result of drainage and
  collapse of a large underground magma chamber.
  (NASA/JPL)

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Volcanic Corona on Venus

• largest volcanic structures on Venus
• unique to Venus
• caused by upwelling of mantle material
• Characterized by
• large fracture zones
• lava flows.

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Pancake domes at Tinatin Planitia
62 km diameter domes from by extrusion of high
viscosity lava at surface
30
3-D Pancake Domes
23x vertical exaggeration 2.4 km
diameter 750 m high False color from
Venera mission information.
31
Tick Volcano at Alpha Regio
Rim 30 km diameter
32
Mountains

• Tesserae. This complex pattern of intersecting
  ridges and cracks is thought to be the result of
  repeated episodes of horizontal motion.
  (NASA/JPL)

33
Meteorite Craters on Venus

• Thick atmosphere protects surface from small
  meteorites.
• all crater diameter gt 3 km
• few crater diameter lt 25 km
• Rate of formation 1/10 rate in lunar maria.
• Estimated surface age, 1 billion years old
  (relatively young)

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Interior Structure

• Venus' interior is probably similar to Earth's,
  but there are no direct observations
  (seismographs) to confirm this.
• The surface rocks are basaltic,
  suggesting that the interior must be
  differentiated.
• However, there is no strong evidence from radar
  mapping to suggest active plate tectonics.
• Venus interior structure may resemble that of a
  young Earth, before convection was established in
  the mantle.

35
Venus Internal Structure

• No direct measurements to constrain models of
  interior.
• Earth-like average density implies similar
  overall composition
• relatively thin crust
• mantle
• partially molten iron-rich core
• Lack of detectable magnetic field slow
  rotation, no dynamo
• High surface temperature, lack of water
  may inhibit Earth-like plate tectonics.

36
Venus Magnetosphere

• Venus has no magnetic field.
• Although Venus may have a substantial liquid
  metallic core, its rotation rate is so slow that
  a magnetic field can not be generated.
• An ionosphere is formed by solar UV striking the
  upper atmosphere.
• The solar wind is at least partially deflected by
  these charged particles.
• The thickness of the atmosphere prevents many, if
  any, of the solar wind particles from reaching
  the surface.

37
Venus Biosphere

• Surface conditions on Venus do not permit life
  (as we know it) to exist.
• It is possible (although it does not seem likely)
  that life formed but then became extinct as the
  conditions on Venus changed.
• Whether the latter is possible depends upon how
  long liquid water existed on Venus in an earlier
  environment.

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Venus Geologic History

• Gravitational condensation from the solar nebula
  4.6 billion years ago.
• Accretion of planet from planetesimals.
• Tremendous glancing impact alters tilt of axis.
• Complete melting and differentiation
  of its interior.
• Cooling and solidifying of mantle and crust.
• Partial re-melting of the upper mantle by
  radioactive decay.
• Global runaway greenhouse effect heats the crust
  and makes it soft.
• Geologic activity begins, but no plate tectonics.

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Properties of Earth and Venus
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Venus Statistics

• Satellites 0
• Diameter 12,140 km (7,545 miles or 0.95xEarth)
• Mass 0.82 x Earth
• Density 5.2 gm/cm3 (water1,Earth5.5)
• Surface Gravity 0.9 x Earth
• Length of Solar Day 117 Earth days
• Length of year 225 Earth days
• Tilt of Axis 177o
• Orbit eccentricity 0.007
• Minimum Distance from Sun 108 million km (67
  million miles)
• Maximum Distance from Sun 109 million km (68
  million miles)
• Minimum Distance from Earth 40 million km (25
  million miles)
• Temperature 730 K (860oF) on surface
• Surface magnetic field lt0.001 (Earth1)

41
Distance from Mars to Earth

• Mars becomes easily visible about once every two
  years (780 days OR the period between
  oppositions), when it is
• closest to the Earth,
• visible all night long, and
• highest in the sky at midnight.
• At maximum brightness, it is the second brightest
  planet.

42
Earth-based Studies of Mars

• First telescopic observations by Galileo.
• Small angular size of Mars, Earths atmospheric
  turbulence limit observations
• show daily changes in surface due to rotation
• seasonal changes in colors of regions
• length of day
• tilt of rotation axis
• 19th century observations by Schiaparelli
  interpreted surface as criss-crossed by straight
  lines, called canali.
• U.S. astronomer Percival Lowell also observed the
  features in 1896 others did not.