Venus - the Sulphurous Greenhouse

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Module 11 Venus - the Sulphurous Greenhouse

Activity 1 The HellishAtmosphere of Venus
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Summary
In this Activity, we will investigate (a) Venus
vital statistics (b) the atmosphere of Venus
- clouds, atmospheric circulation, surface
pressure, temperature variations (c)
water on Venus? and (d) Venus and the runaway
greenhouse effect.
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(a) Venus Vital Statistics

• First lets look at the bulk properties of our
  nearest planetary neighbour, Venus, and compare
  them to those of Earth - continuing with our
  theme of comparative planetology.

Structurally, they have much in common
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Venus
Earth
crust
core
mantle
M 0.81 M?
M M?
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• As Venus has no natural satellites, its mass
  could only be well determined once spacecraft
  were sent to visit it.

If a spacecraft is placed in orbit around a
planet, itsorbital period and orbital radius can
be used in KeplersThird Law to determine the
planets mass. Even if the spacecraft is not
placed into orbit, as it doesa flypast of the
planet its acceleration due to the
planetsgravitational attraction can be measured
- by measuringthe Doppler effect on radio
signals it sends to Earth - and Newtons Law of
Gravitation can be used to deduce the planets
mass.

Click here to be reminded about the
ElectromagneticSpectrum and Doppler shifts
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Earth
Venus
Av. Distancefrom Sun
0.72 AU
1 AU
Venus, our closest planetary neighbour, is 28
closerto the Sun than is the Earth.
As we will see, this has profound implications
for theclimates of Venus and Earth.
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Earth
Venus
Av. Distancefrom Sun
0.72 AU
1 AU
1 y ?
Length of Year
0.62 y ?
A year on Venus takes about seven and a
halfEarth months.
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Earth
Venus
Av. Distancefrom Sun
0.72 AU
1 AU
1 y ?
Length of Year
0.62 y ?
117 d? (retrograde)
Length of solar day
1 d?
Years may be short on Venus, but days are very
long! - and Venus rotates (very slowly) in the
opposite direction to Earth and the other planets
(excepting Uranus, which also exhibits retrograde
rotation.)
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Earth
Venus
Av. Distancefrom Sun
0.72 AU
1 AU
1 y ?
Length of Year
0.62 y ?
117 d? (retrograde)
Length of solar day
1 d?
Inclinationof axis
177
23.5
This large inclination of rotation axis to the
ecliptic resultsfrom Venus retrograde orbit.
This can be thought of as a-3 tilt, which means
Venus does not experience seasons.
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Earth
Venus
Av. Distancefrom Sun
0.72 AU
1 AU
1 y ?
Length of Year
0.62 y ?
117 d? (retrograde)
Length of solar day
1 d?
Inclinationof axis
177
23.5
Accelerationdue to gravity
1 g?
0.90g?
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• The acceleration due to gravity on Venus is very
  similar to that on Earth.

So why havent we sent more space missions to
visitthe surface of Venus, our nearest neighbour?
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Earth
Venus
av. albedo
0.39
0.76(cloud tops)
Remote observing of Venus is difficult, due to
its thickcover of highly reflective clouds.
Its proximity to Earth and the Sun, combined
withits high albedo, makes Venus the brightest
object in thesky (as seen from Earth) after the
Sun and the Moon.
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This sunset imaged from low Earth orbit by the
Atlantis space shuttle crew in May 1989 features
Venus above the Earths horizon.
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Earth
Venus
av. albedo
0.39
0.76(cloud tops)
atmosphere
96 CO2 3.5 N2 0.2 H2O acids
78 N2 20 O2 0.03 CO2 2 H2O
The atmosphere
The atmosphere of Venus consists almost entirely
of carbon dioxide - very different to Earths
present-dayatmosphere, but similar to what we
believe made up Earths primeval atmosphere.
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Earth
Venus
av. albedo
0.39
0.76(cloud tops)
96 CO2 3.5 N2 0.2 H2O acids
78 N2 20 O2 0.03 CO2 2 H2O
atmosphere
surface temperature
472C
- 50C ? 50C
The surface temperature of Venus is extreme -
several times the boiling point of water.
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(b) The Atmosphere of Venus

• In the Activities on Earth, the Moon and Mercury,
  our primary focus was on the nature of their
  planetary surfaces.

On Venus, however, the atmosphere
dominateseverything, including the evolution of
the planetary surface.
So for the rest of this Activity, we will study
the hellishatmosphere of Venus.
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• Clouds

Venus is permanently hidden by clouds - probably
made of sulphuric acid sulphur crystals, with
water dropletsmixed in.
(The Earth has a layer ofsulphuric acid droplets
in itsstratosphere too, but lowerdown its
atmosphere is washedclean of any sulphur
compounds by rain.)
False colour ultraviolet image highlighting cloud
patterns
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This false colour image,taken by the
spaceprobeGalileo in February,1990, highlights
structure in the swirling sulphuric acid
clouds. The bright area is sunlight glinting
off the upper cloud deck.
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The cloud banks on Venus are much higher more
stablethan those on Earth
100 km
Height of highest clouds on Earth
Sulphurouscloud layers
Haze
68 km
Mostly clear air (96 CO2)
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• Atmospheric circulation

Atmospheric circulation is much less turbulent on
Venus than on Earth. This gives the cloud banks
on Venus their stability.
Remember that we saw in the Activity Earths
Atmosphere Magnetic Field that the Earths
rotation twists convection currents in the
atmosphere to establish global atmospheric
circulation patterns.
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Venus, however, rotates very slowly
(approximately once every 243 days) and so its
rotation has a much smaller effect on atmospheric
circulation.
The atmosphere itself rotates much faster than
that, takingapproximately 4 days to circle the
planet - in the samedirection to the planets
retrograde rotation.

• Venus atmospheric circulation is dominated by
  two main weather patterns
• stable, 300 km/h jet streams exist in the upper
  atmosphere
• winds blow from the equator to the poles in huge
  100 to 500 km diameter cyclones.

The strong winds at the cloud tops decrease
rapidly withaltitude, decreasing to only a few
km/h at the surface.
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• Surface Pressure

The atmospheric pressure at the surface of Venus
is 90patm - i.e. 90 times that on Earth!
(If all the carbon dioxide trapped in the Earths
oceans androcks were released into the air, the
Earths atmospherewould be about as thick as on
Venus, with a similar surfacepressure.)
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• Temperature Variations

The temperature variation in Venuss atmosphere
is extreme
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(c) Water on Venus?
As we have seen, possible signs of water ice have
beenfound on the Moon and Mercury. As Venus is
not as close to the Sun as is Mercury, one might
expect it to be cooler and more likely to contain
water ice. Instead, its surface is so uniformly
hot (approx. 745K, or 472C) that even liquid
water cannot exist.
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The atmosphere of Venus is quite dry - it
contains only 1 of the amount of water vapour
that exists in the Earths atmosphere.
However testing of the isotopic composition of
Venus atmosphere indicates that it once
apparently contained significant amounts of
water. Whether this water was ever liquid, we do
not know. Presumably the water - liquid or
vapour - was broken up by UV radiation, as Venus
has no protective ozone layer.
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We asked earlier why havent we sent more space
missions to visit the surface of Venus, our
nearest neighbour?
Weve now seen several reasons for this (1)
thick cloud cover makes surface imaging from
orbit impossible (except for radar imaging,
as we will see in the next Activity) (2)
the furnace-like surface temperature (472C) (3)
the extreme surface pressure (90 times that of
Earth) (4) the chemical composition of the
atmosphere - almost all carbon dioxide, with
sulphuric acid clouds.
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Venus is an exceedingly inhospitable planet, and
as we willsee in the next Activity, the
conditions on the surface quicklydestroy space
probes sent to land on it.
Why is Venus so hot and inhospitable, when its
vitalstatistics are so similar to those of Earth?
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(d) Venus and the Runaway Greenhouse
Effect
Why is Venus so hot? Due mainly to its carbon
dioxideatmosphere, Venus has apparently
undergone a runaway greenhouse effect.
Lets first review what we know about the
greenhouseeffect on Earth
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All planets absorb energy from the Sun, but long
agothey reached equilibrium - that is, the
amount of energy they absorb per second is equal
to the amount per second they re-radiate out
into space.
Incident sunlight
Reflected sunlight
Re-radiated energy
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The water vapour and (to a lesser extent) carbon
dioxide in the Earths atmosphere are good
absorbers of infrared radiation, so they trap
much of the re-radiated energy inside the
atmosphere.
Most of the re-radiated infrared radiationis
trapped within the atmosphere
The result is that the Earth is
significantlywarmer than it would be without an
atmosphere.
Water vapour and carbon dioxideare called
greenhouse gases.
To find out why the term greenhouse is (mis)used,
click here.
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Water vapour is the main greenhouse gas on Earth.
Its levels in the Earths atmosphere vary from
time to time, but remain roughly constant on
average.
The current scientific debate about the
greenhouseeffect centers on the rising levels of
carbon dioxidein the Earths atmosphere, due to
sources such as the burning of fossil fuels and
effects such as deforestation.
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• Although the water vapour in the Earths
  atmosphere is a major greenhouse gas, liquid
  water in the Earths oceans has absorbed most of
  the carbon dioxide which was originally in the
  Earths atmosphere - and so, overall, the
  continuing presence of water on Earth has acted
  to limit the Earths greenhouse effect.

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• However Venus, in its warmer location closer to
  the Sun, has no liquid water to remove carbon
  dioxide from its atmosphere - and, as we will see
  in the next Activity, its many volcanoes have
  added more carbon dioxide to its atmosphere.

Venus atmosphere has been warmed by the
greenhouseeffect caused by the levels of
atmospheric carbon dioxide. As it has become
hotter and hotter at the planets surface,more
carbon dioxide (and sulphurous compounds)
haveliterally been baked out of the planets
surface.
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• These effects conspire to give us a vicious
  circle

rising temperatures
increased carbon dioxide in the atmosphere of
Venus
increased Greenhouse Effect
-an example of what scientists call positive
feedback, known as the runaway Greenhouse Effect
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• In the next Activity we will go on to look at
  what is known about the surface of Venus, and
  what techniques have been used to investigate it.

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Image Credits
NASA Venus globe http//nssdc.gsfc.nasa.gov/image
/planetary/venus/venusglobe.jpg NASA Earth
globe http//pds.jpl.nasa.gov/planets/welcome/eart
h.htm NASA Venus on the Horizon http//antwrp.gsf
c.nasa.gov/apod/ap971014.html NASA Just passing
by http//antwrp.gsfc.nasa.gov/apod/ap980501.html
Hubble Venus in ultraviolet light http//oposite.
stsci.edu/pubinfo/PR/95/16.html
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• Now return to the Module 11 home page, and read
  more about the atmosphere of the Venus in the
  Textbook Readings.

Hit the Esc key (escape) to return to the Module
11 Home Page
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The Electromagnetic Spectrum

• Visible light is made up of a whole spectrum of
  colours

Each of which corresponds to a characteristic
frequency f (or wavelength ?) range, because
light, as an electromagnetic wave which travels
at the speed of light c, obeys the equation
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Optical astronomers observing in visible
lightwork between wavelengths of about
? 400 nm (or frequency f 7.5 x 1014 Hz) -
the blue end of the visible spectrum
And? 700 nm (or frequency f 4.3 x 1014 Hz)
- the red end of the visible spectrum.
Forgotten what 1014 means? Click here to revise
scientific notation.
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300 kHz 3x105 Hz
1 km
The visible spectrum is a small part of the
whole electro -magnetic spectrum.
3 GHz 3x109 Hz
10 cm
wavelength
frequency
3x1013 Hz
10-5 m
Visible
1 nm 10-9 m
3x1017 Hz
3x1021 Hz
10-13 m
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Astronomers try to access as much of the
electromagnetic spectrum as possible with their
telescopes detectors, ranging from radio waves
to gamma rays.
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Doppler Shift

• If an emitter of electromagnetic radiation
  (light, radio waves, etc) is heading towards us,
  then the waves of the electromagnetic radiation
  it emits are compressed. If the electromagnetic
  radiation is visible light, it will look bluer.

Um... you were movingrelative to us when you
made it, so we got a blueshifted signal instead
Here, have some em radiation!
Ah, well, thats the Doppler shift for you
Well, thats relativity for you
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Doppler Shift

• If an emitter of electromagnetic radiation
  (light, radio waves, etc) is heading away from
  us, then the waves of the electromagnetic
  radiation it emits are expanded. If the
  electromagnetic radiation is visible light, it
  will look redder.

This time the signal is redshifted.
Well, thats relativity for you
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Click here to return to the Activity!
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Scientific notation 1

• In order to save writing heaps of zeroes,
  scientists use a system of notation where very
  large numbers are written with the number of
  factors of ten as an exponent.
• For instance 5 000 is written 5 x 103
• 6 000 000 000 is written 6 x
  109
• 42 700 is written 4.27 x 104

In scientific notation the aim is to present the
number as a number between 1 and 10 multiplied
by a power of tene.g. 4.27 x 104
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Scientific notation 2

• Also, in order to save writing heaps of decimal
  places, scientists use a system of notation where
  very small numbers are written with the number of
  factors of ten as an exponent.
• For instance .007 is written 7 x 10-3
• 0.00000010436 is written 1.0346 x
  10-7
• 0.000060001 is written 6.0001 x 10-5

In scientific notation the aim is to present the
number as a number between 1 and 10 multiplied
by a power of ten e.g. 6.0001 x 10-5
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Click here to return to the discussion onthe
Electromagnetic Spectrum and Doppler Shift!
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The Greenhouse Effect

• Greenhouses maintain a higher temperature than
  their surroundings - which is why delicate plants
  are kept in them in cold winters.

They achieve this due to their glass (or plastic)
walls, which let light in which is largely
absorbed by the plants and surfaces inside the
greenhouse. These re-radiate infrared radiation,
which warms up the air in the greenhouse.
This sounds pretty similar to the situation of
the Earth andits atmosphere, which is why the
term greenhouse effectis used.
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• There is an important difference though. Although
  the air and walls of a greenhouse do absorb
  infrared radiation, the main reason that a
  greenhouse stays warmer than its surroundings in
  winter is that its walls trap the warm air,
  preventing cooling drafts.

So the Earths atmosphere is not exactly like a
greenhouseit has no walls. Our atmosphere is
relatively warm becauseit traps re-radiated
infrared radiation by absorbing most of it
before it reaches space.
Back to the Activity!
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