Chapter 9: VenusEarth’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 rotation- opposite 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 – 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 .

– Volcanic calderas have been observed.

– Rift valleys have been observed.

– Impact craters.

– Geologically active.• volcanic domes and folded mountains

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.

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 VenusEarth Venus

Semimajor axis (AU) 1.00 0.72

Orbital period (yr) 1.00 0.61

Diameter (km) 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

Rotation period 23.9 hrs -243 days

Surface area (Earth=1) 1.00 0.94

Atmospheric pressure (bar) 1.00 90

Ave. surface temperature 300 K 730 K

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 Very inactive Very inactive

Magnetosphere

Very Active

Very Weak Very weak None

Lithosphere

Very Active

Active Very inactive Very inactive

Biosphere

Very Active

None None None