View from Earth
Chapter 9 Part a: Venus
Evening Star
Basic Properties
West
!View from Earth
!Orbit
!Physical properties
!Rotation
!Interior & Magnetic Field
MorningStar
StarWest
Evening
Morning
EveningStar
StarEast
Atmosphere
!Temperature
!Clouds and Atmosphere
!Acid Rain
!Greenhouse effect
!Atmospheric Origin
!Loss of Water
!Earth/Venus--- Comparison
Greeks thought Venus at evening
!"Phosphorus#$ and morning
!"Hesperus#$ were different stars
Since Venus' orbit lies within
Earth's, it can never be seen
much longer than 3 hr after
sunset or 3 hr before sunrise
Measurement of Rotation Rate
Radar exploration of Venus started in the
1960s astronomers succeeded in
bouncing waves off the surface found an
extremely slow backward motion.
Distance = 0.72 AU
Orbit Period= 225 days
Rotation period = - 243 days
Cloud Cover
Mass = 0.82 Earth
Radius = 0.95 Earth
Density = 5.24(Earth=5.52)
Venus is similar to Earth in size, density, and mass! Because of this
similarity Venus is often called Earth’s sister planet.
However, Venus is anything but Earth's twin. Its surface is a blistering
inferno and weather which rains sulfuric acid!
Cloud Cover prevents Earth based observations
of its surface
Doppler Shift
It takes 243 days to
rotate (‘backwards’) on
its axis, but only 225
days to orbit the Sun.
Possible Reasons for Planets “Backward”
Rotation
Almost all planets rotate counterclock wise, i.e.
in the same sense as orbital motion. Exceptions:
Venus, Uranus, and “Pluto”.
No measurements available for internal
structure---however it should be much
like the Interior of the Earth: a central
iron core and a rocky mantle and crust,
similar in composition to Earth’s.
Venus intrinsic magnetic field is less
than 10-5 times that of Earth.
Two Reason for Venus:
1. Tidal interaction between Venus, Sun, & Earth with
complex braking by the atmosphere.
Problem: The Earth does not exert a strong enough
gravitational pull on Venus to reverse its spin
2. Massive glancing impact virtually despinning Venus and making it go slowly
backwards.
Interior
Venus with no intrinsic magnetic field
has no shield to protect it from the
solar wind.
Venus (CW)
Interaction with Solar Wind
Its ionosphere separates the atmosphere from the solar wind,
giving Venus a distinct magnetic environment.
On Venus, the core is perhaps solid,
and furthermore the rotation of the
planet is very slow - 243 days. This may
explain the lack of a magnetic field
The Temperature
1930s---- (-43 C)
The solar wind penetrates relatively deep into the planetary
atmosphere and causes substantial atmosphere loss.
The temperature of the cloud tops was measured in the 1930s to be 230
K (-43 C), similar to the stratosphere of the Earth. These were assumed to
be water clouds, like the Earth.
The main ion types being lost are O+, H+ and He+.
1958-1962----- (480 C)
Radio astronomers in 1958 and Mariner 2 in 1962 found
that Venus was very hot----The carbon dioxide
atmosphere, a well known greenhouse gas, keeps the
planet extremely warm.
The surface temperature of Venus is 480
degrees C (900 degrees F). It is so hot that
the ground may be slightly plastic and emit
a red glow at night.
Venus' surface is actually hotter than Mercury's( 423 C), despite being
nearly twice as far from the Sun.
Origin of Venus’ Atmosphere
Clouds and Atmosphere
The upper clouds require only 4 Earth days to make
one complete circuit, traveling at 360 km/hour, much
faster than the planet rotates -- 243 days.
The early atmosphere contained the gasses
in the early Solar System: Hydrogen, helium,
methane, ammonia. These early gases
escaped from Venus
At the surface the winds are a mere breeze.
The secondary atmosphere created from volcanic activity
contained the gasses- H2O, CO2, and SO2
The atmosphere contained substantial amounts of water vapor and
CO2 which caused a strong greenhouse effect.
Its pressure is 90x the Earth's atmosphere--- about the same as the water
pressure at a depth of 3,000 feet in Earth's oceans. The closest the Earth
comes to matching this combination of extreme pressure and
temperature is in the realm of deep undersea volcanic vents.
While there is no erosion due to water, the atmosphere is so dense that a
surface "breeze" of only a few mph would pack the force of a river
current.
Build up of CO2
The elevated temperature evaporated Venus’ oceans and drove more
carbon dioxide out of the rocks. This caused a runaway greenhouse
effect
The Earth has roughly as much carbon dioxide as Venus, but it has
been dissolved in the Earth’s oceans and chemically bound into its
rocks
Although the cloud tops of Venus
are around (-43 C) 230 K, the
temperature increases rapidly
toward the surface (480 C).
Runaway Greenhouse Effect
the initial solar heating
kept oceans from
forming (Or caused
them to evaporate).
The loss of water and
of rainfall kept the
carbon dioxide in the
atmosphere rather
than binding it in the
rocks as is the case for
Earth
Radio signals indicate that lightning
occurs during the afternoons in a layer
of clouds 35 miles above the surface.
230 K,(-43 C)
H2SO4
CO2 atmosphere is a very
effective greenhouse gas
caused the temperature to
rapidly increase---driving
more CO2 from the rocks
(Venus=480 C)
480 C
Comparison of the Atmospheres of Venus and
Earth
Venus and Earth are worlds truly apart in many major aspects. Walking on
Venus’s surface would be difficult, like walking under water at 900 meter
depth (the atmospheric pressure at Venus is 90 times higher than on
Earth at sea level) - and as uncomfortable as staying in an oven at 465 °C!
Venus’s surface is indeed the hottest in the Solar System. Despite only 10%
of the solar flux reaching the surface, enough energy is trapped by gases
and particles present in the lower atmosphere, to raise the temperature at
the surface dramatically. A catastrophic ‘greenhouse effect’ is taking
place.
1. On Earth, CO2 is absorbed in the oceans and
rocks. If the absorbed CO2 on Earth were released
into the atmosphere, 98% would be CO2 and the
atmospheric pressure would be 70 X what it is
now---so, except for the O2 and water, Earth's
atmosphere would be similar to Venus' if the CO2
had not gotten absorbed.
2. The oxygen on Earth is a product of life. The
water on Venus has disappeared due to the
extreme temperatures brought on by the runaway
greenhouse effect. Water vapor rose to high
elevations and was broken down into oxygen and
hydrogen by ultraviolet radiation hydrogen, being
light, escaped from Venus and oxygen combined
with other atmospheric gases
3. Water on Venus was lost forever
Sec 2
End of Chapter 9a
Venus
Go to Chapter 9b