Activity 1:

The HellishAtmosphere of Venus

Module 11: Venus - the Sulphurous Greenhouse

SummaryIn 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.

(a) Venus’ Vital Statistics

First let’s 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:

EarthVenus

D0.95 D

M = 0.81 M M = M

core

mantle

crust

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 Kepler’sThird Law to determine the planet’s mass.Even if the spacecraft is not placed into orbit, as it doesa flypast of the planet its acceleration due to the planet’sgravitational attraction can be measured - by measuringthe Doppler effect on radio signals it sends to Earth - and Newton’s Law of Gravitation can be used to deduce the planet’s mass.

*

* Click here to be reminded about the ElectromagneticSpectrum and Doppler shifts

EarthVenusAv. Distancefrom Sun 1 AU0.72 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.

EarthVenusAv. Distancefrom Sun 1 AU0.72 AU

Length of “Year”

1 y 0.62 y

A year on Venus takes about seven and a halfEarth months.

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.)

EarthVenusAv. Distancefrom Sun 1 AU0.72 AU

Length of “Year”

1 y 0.62 y

Length of solar day

117 d

(retrograde)1 d

* A solar day on Venus is “only” 117 d

*

* A sidereal day on Venus is 243 d - longer than its year!

*

EarthVenusAv. Distancefrom Sun 1 AU0.72 AU

Length of “Year”

1 y 0.62 y

Length of solar day

117 d

(retrograde)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.

EarthVenusAv. Distancefrom Sun 1 AU0.72 AU

Length of “Year”

1 y 0.62 y

Length of solar day

117 d

(retrograde)1 d

Inclinationof axis

177° 23.5°

Accelerationdue to gravity

1 g0.90g

The acceleration due to gravity on Venus is very similar to that on Earth.

So why haven’t we sent more space missions to visitthe surface of Venus, our nearest neighbour?

EarthVenus

av. albedo 0.390.76(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.

This sunset imaged from low Earth orbit by the Atlantis space shuttle crew in May 1989 features Venus above the Earth’s horizon.

EarthVenus

av. albedo

atmosphere 96% CO2

3.5% N2

0.2% H2O & acids

78% N2

20% O2 0.03% CO2

~2% H2O The atmosphereThe atmosphere of Venus consists almost entirely of carbon dioxide - very different to Earth’s present-dayatmosphere, but similar to what we believe made up Earth’s primeval atmosphere.

0.390.76(cloud tops)

EarthVenus

av. albedo 0.390.76(cloud tops)

atmosphere 96% CO2

3.5% N2

0.2% H2O & acids

78% N2

20% O2 0.03% CO2

~2% H2O

surface temperature

472°C - 50°C + 50°C

The surface temperature of Venus is extreme - several times the boiling point of water.

(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.

Venus is permanently hidden by clouds - probably made of sulphuric acid & sulphur crystals, with water dropletsmixed in.

• Clouds

(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

This false colour image,taken by the spaceprobe

Galileo in February,1990, highlights structure

in the swirling sulphuric acid clouds.

The bright area is sunlight glinting

off the upper cloud deck.

68 km Haze

Mostly clear air (96% CO2)

Sulphurouscloud layers

100 km Height of highest clouds on Earth

The cloud banks on Venus are much higher & more stablethan those on Earth:

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 Earth’s Atmosphere & Magnetic Field that the Earth’s

rotation twists convection currents in the atmosphere to

establish global atmospheric

circulation patterns.

• Atmospheric circulation

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 planet’s 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.

The atmospheric pressure at the surface of Venus is 90patm

- i.e. 90 times that on Earth!

• Surface Pressure

(If all the carbon dioxide trapped in the Earth’s oceans androcks were released into the air, the Earth’s atmospherewould be about as thick as on Venus, with a similar surfacepressure.)

Haze

Mostly clear air (96% CO2)

Sulphurouscloud layers

100 km

745°K

330°K

250°K

The temperature variation in Venus’s atmosphere is extreme:

• Temperature Variations

170°K

(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. 745°K, or 472°C) that even liquid water cannot exist.

The atmosphere of Venus is quite dry - it contains only ~1% of the amount of water vapour that exists in the Earth’s 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.

We asked earlier “why haven’t we sent more space missions to visit the surface of Venus, our nearest neighbour?”

We’ve 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 (472°C)(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.

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?

Why is Venus so hot? Due mainly to its carbon dioxideatmosphere, Venus has apparently undergone a “runaway greenhouse effect”.

(d) Venus and the Runaway Greenhouse Effect

Let’s first review what we know about the greenhouseeffect on Earth:

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 energythis re-radiated energy isinfrared radiation

Most of the re-radiated infrared radiationis trapped within the atmosphere

The water vapour and (to a lesser extent) carbon dioxide in the Earth’s atmosphere are good absorbers of infrared radiation, so they trap much of the re-radiated energy inside the atmosphere.

Water vapour and carbon dioxideare called “greenhouse gases”.

The result is that the Earth is significantlywarmer than it would be without an atmosphere.

To find out why the term greenhouse is (mis)used, click here.

Water vapour is the main greenhouse gas on Earth. Its levels in the Earth’s 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 Earth’s atmosphere, due to sources such as the burning of fossil fuels and effects such as deforestation.

Although the water vapour in the Earth’s atmosphere is a major greenhouse gas, liquid water in the Earth’s oceans has absorbed most of the carbon dioxide which was originally in the Earth’s atmosphere - and so, overall, the continuing presence of water on Earth has acted to limit the Earth’s greenhouse effect.

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 planet’s surface,more carbon dioxide (and sulphurous compounds) haveliterally been “baked out” of the planet’s surface.

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

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.

NASA: Venus globe

http://nssdc.gsfc.nasa.gov/image/planetary/venus/venusglobe.jpg

NASA: Earth globe

http://pds.jpl.nasa.gov/planets/welcome/earth.htm

NASA: Venus on the Horizon

http://antwrp.gsfc.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

Image Credits

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

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

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.

The visible spectrum is a small part of the whole electro -magnetic spectrum. w

av

ele

ng

th

1 km

10 cm

10-5 m

1 nm = 10-9 m

10-13 m

freq

uen

cy

Gamma rays

X rays

Ultraviolet

Radio waves

Microwave &Infrared

Visible

300 kHz= 3x105 Hz

3 GHz= 3x109 Hz

3x1013 Hz

3x1017 Hz

3x1021 Hz

Astronomers try to access as much of the electromagnetic spectrum as possible with their telescopes & detectors, ranging from radio waves to gamma rays.

Doppler ShiftIf 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.

Here, have some em radiation!

Um... you were movingrelative to us when you

made it, so we got a blueshifted signal instead:

Ah, well, that’s the Doppler shift for you

Doppler ShiftIf 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.

Click here to return to the Activity!

Scientific notation 1In 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 ten:e.g. 4.27 x 104

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

Click here to return to the discussion onthe Electromagnetic Spectrum and Doppler Shift!

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.

The Greenhouse Effect

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 Earth’s atmosphere is not exactly like a greenhouse:it 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!