Chapter 9: Venus
Earth’s Sister Planet
Orbital Properties
Bulk Properties
Internal Structure
Atmosphere
Surface Features
Objectives
• After completing this chapter, you should be able to:
• compare the general physical properties of Venus to Earth and
the Moon.
• compare the orbital and rotational properties of Venus to Earth
and the Moon.
• describe the atmosphere, hydrosphere, lithosphere,
magnetosphere, and biosphere of Venus and
• compare to Earth's, Mercury's and the Moon's.
• describe Venus' cycle of visibility as seen from the Earth.
• compare the atmospheric histories of Venus and the Earth.
• explain the "runaway" greenhouse effect on Venus.
• compare the "geologic" history of Venus and the Earth.
• explain the lack of a strong magnetic field around Venus.
Venus’ Place in the Solar System
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.
Venus’ Day
• Established by radar
techniques
• Rotation period :
-243 Earth days
• Retrograde rotationopposite sense to
Earth’s rotation
and to orbital
direction
• Solar day
(noon to noon):
117 Earth days
• Orbital period:
225 Earth days
Venus’ Retrograde Rotation
• Most planets and moons move in prograde 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.
Exploration of Venus from Space
•Early exploration dominated by Soviets.
•First soft landing on surface in 1970 by Venera 7.
•Later Venera missions
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–
–
–
–
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.
•Detailed picture of atmosphere, detected lightning
U.S. Exploration of Venus
•In 1978, Pioneer Venus mission
– photographed Venus’s 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
Earth’s jet streams
• upper cloud decks rotate around planet faster than planet
•Magellan probe (in orbit August, 1990)
mapped the surface of Venus with radar yielding
10 x spatial resolution of previous missions
– resolve 120 m horizontally and 50 m vertically
Venus in UV from Pioneer Venus
Venus in Radar from Magellan
Dark areas:
smooth
Light areas:
rough
Venus from Magellan data
Center view
of N-pole;
outside four
views of
equator
Venus
(false color map from Magellan data)
Venus Statistics
• Satellites: 0
• Diameter: 12,140 km (7,545 miles or 0.95xEarth)
• Mass: 0.82 x Earth
• Density: 5.2 gm/cm3 (water=1,Earth=5.5)
• 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: <0.001 (Earth=1)
Questions: Motions, Observation
• How does Venus appear to move as viewed from
Earth? Explain.
• What is peculiar about Venus’ rotation?
– Why does it rotate this way?
• Why does Venus appear so bright when viewed by
eye from Earth?
– How would Earth look as viewed from Venus?
• What is the best way to map the surface of Venus?
Explain.
– How did radio observations of Venus made in
the 1950’s alter our conception of the planet?
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 Earth’s
– Earth: 14.5 lb/in2
Venus: 1300 lb/in2
– Equivalent to 0.6 mile below ocean surface on Earth.
Venus’ Atmosphere
• COMPOSITION
–
–
–
–
–
–
–
Carbon Dioxide - 96%
Nitrogen - 3.5%
Sulfur Dioxide - 0.015%
Water Vapor - 0.010%
Oxygen - 0%
Hydrogen - 0%
Helium - 0%
• Carbon dioxide is a strong greenhouse gas. The
extreme greenhouse effect raises Venus' surface
temperature to 900oF.
• The density of Venus’ atmosphere at the
surface is about 90x the Earth's.
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 3 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 > 2 km/hr
•Jet-stream above clouds
– 300-400 km/hr, west to east
The Clouds of Venus
The sulfuric acid clouds are separated into three distinct layers.
These layers occur at altitudes where the combination of
temperature and pressure causes sulfuric acid to condense.
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.
There 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.
Atmospheres: Earth vs. Venus
VENUS
EARTH
Carbon dioxide
Nitrogen/Oxygen
90 atmospheres
1 atmosphere
Very strong greenhouse
Moderate greenhouse
Extremely hot (900o F)
Moderate (70o F)
Dry
Wet
Sulfuric acid clouds
Water clouds
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 in this manner.
• 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.
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 --------> H2 + O
•
6S + 9O2 + 6H2O --------> 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.
Greenhouse Effect
Works on Venus as it does on Earth,
but Venus
has ~1 million x more CO2 , so effect is much stronger.
Questions: Atmosphere
• Name three ways that the atmosphere of
Venus is different from that of Earth.
• What are the main constituents of
Venus’ atmosphere?
• What are the clouds in the upper atmosphere
made of?
• Why is the surface of Venus so hot?
• Why is the CO2 component of the atmosphere
of Venus is so great compared to the Earth’s?
• Why is there no water on Venus?
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.
Smooth areas - dark
Rough areas - light
Comparison of Topography
Venus: Radar Map
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
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 .
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–
–
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Volcanic calderas have been observed.
Rift valleys have been observed.
Impact craters.
Geologically active.
• volcanic domes and folded mountains
Venus: Surface Features
LOWLANDS (92%)
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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.
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)
Volcanoes on Venus
This is a Magellan reconstruction of a region in Eistla Regio.
At the left is the volcano Gula Mons; at the right is Sif Mons.
Lava flows extend from Gula Mons toward lower right. (NASA/JPL)
Shield Volcano: Gula Mons
Most common volcanic feature on Venus; similar to Earth’s Hawaiian Islands;
characterized by formation of a caldera
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)
Volcanic Domes: Coronas
The large, circular feature seen here is a corona. It is a raised structure, approximately
200 km in diameter, thought to be the result of uplift due to upwelling magma from
below. The smaller circular feature is a "pancake" dome, about 35 km in diameter,
formed by the eruption of very viscous lava. Another pancake is seen to the left; these
features are seen only on Venus. (NASA/JPL)
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.
Tick Volcano at Alpha Regio
Rim = 30 km diameter
Pancake domes at Tinatin Planitia
62 km diameter domes from by extrusion of
high viscosity lava at surface
3-D Pancake Domes
23x vertical exaggeration
2.4 km diameter
750 m high
false color from Venera mission information
Rift Valley
This is the highland region Ovda Regio, near the equator of Venus.
The complex structure seen here is thought to be the result of compression (in the
upper left-lower right direction) followed by stretching (upper right to lower left),
caused by flowing motions in the mantle of Venus that are similar to the motions
that cause continental drift on the Earth. (NASA/JPL)
Mountains
Tesserae. This complex pattern of intersecting ridges and cracks is thought
to be the result of repeated episodes of horizontal motion. (NASA/JPL)
Meteorite Craters on Venus
• Thick atmosphere protects surface from small meteorites.
– all crater diameter > 3 km
– few crater diameter < 25 km
• Rate of formation ~1/10 rate in lunar maria.
• Estimated surface age, ~1 billion years old (relatively young)
Questions: Lithosphere
• How do the “continents” of Venus differ
from Earth continents?
• How are the impact craters of Venus different from
those found on other bodies in our solar system?
• What evidence supports the supposition that
volcanism in various forms has changed
the surface of Venus?
• What is the evidence for active volcanoes on Venus?
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.
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.
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.
Question: Magnetosphere
• Given that Venus probably has a partially
molten core, like Earth, why doesn’t it
also have a magnetic field?
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.
Questions: Biosphere
• Do you think there might be life on
Venus? Explain.
• Is Earth in danger of experiencing a
runaway greenhouse effect like Venus?
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.
Properties of Earth and Venus
Earth Venus
Semimajor axis (AU)
1.00
0.72
Orbital period (yr)
1.00
0.61
12756
12140
Ave. density (g/cm3)
5.5
5.2
Surface gravity (Earth=1)
1.00
0.91
Escape velocity (km/s)
11.2
10.4
23.9 hrs
-243 days
Surface area (Earth=1)
1.00
0.94
Atmospheric pressure (bar)
1.00
90
300 K
730 K
Diameter (km)
Rotation period
Ave. surface temperature
Overview of Venus
• Closest approach to Earth of any planet.
• Mass and size nearly the same as Earth.
• Dense clouds covering planet,
coupled with 480 maximum elongation,
make visual observation difficult.
• Radar penetrates clouds to map surface and
determined Venus’s retrograde rotation about
an axis nearly  to its orbital plane.
– Almost no seasonal variations.
Overview of Venus
• Atmosphere mostly carbon dioxide.
– Greenhouse effect produces average surface
temperature of 730K.
– Surface pressure 90x Earth’s sea-level pressure.
– Clouds contain concentrated sulfuric acid droplets.
– Clouds move at high speeds with circulation
patterns determined by solar heating.
• No water on planet surface and only a trace of
water vapor in upper cloud layers.
• No planet-wide magnetic field observed.
• No natural satellites.
Overview of Venus
• Unique features
–slow, retrograde rotation
–carbon dioxide atmosphere
–runaway greenhouse effect
–high surface temperature
–high surface atmospheric pressure
–some evidence for current volcanism
• variable SO2 concentrations, lightning
• Important features
–sharp-edged rocks imply few surface erosion processes
–lightning occurs in atmosphere
Spheres: Earth, Venus, Mercury,
and the Moon
REALM
EARTH
VENUS
MERCURY
MOON
Atmosphere
Very Active
Active
Very thin
None
Hydrosphere
Very Active
None
Magnetosphere
Very Active
Very Weak
Lithosphere
Very Active
Active
Biosphere
Very Active
None
Very inactive Very inactive
Very weak
None
Very inactive Very inactive
None
None