Download Chapter 3: Venus

THE COMPLETE COSMOS
Chapter 3: Venus
Beneath the clouds - planetary hell. A poisonous, crushing atmosphere, searing heat, volcanoes
and a runaway greenhouse effect. Why?
Outline
Venus is a planet of lowlands, shaped by volcanoes and shrouded in dense, poisonous
clouds. The brightest planet in our night sky, Venus is beautifully observed in the evening
twilight sky - a factor determined by Venus's orbit of the Sun. Observations of a transit of
Venus in 1769 were used to refine our knowledge of distances within the Solar System.
Radar signals from Earth, which pierced the clouds, revealed surface features for the first time plus the planet's very slow backwards rotation. Russia's Venera probes showed the planet's hostile
nature - early spacecraft were crushed. Later, radar mapping uncovered a surface of plains,
pockmarked by volcanoes. Venus has only two continents, in contrast to Earth's seven.
In the 1990s, images from the Magellan spacecraft yielded recognizable landscapes – after
computer processing. Gravity mapping provided more information about the interior of Venus.
The chapter concludes by suggesting that Venus, possibly, could once have been Earth-like. But
then, alas, the Venusian oceans evaporated and the atmosphere thickened, leaving the baked,
hostile Venus of today.
Sub-chapters
An Unlikely Twin
• A tour of Venus, swooping towards the planet, down through the clouds, over plains and
volcanoes, finally traveling back through the toxic skies.
• Comparisons with Earth, highlighting their similar size, and their differences – the
searing temperatures of Venus, its crushing atmosphere and poisonous clouds.
• How Venus may once have looked - more like Earth.
Venus Unveiled
• Venus viewed from Earth - why Venus is often fairly low in the dusk or dawn twilight
sky.
• The orbits of Venus and Mercury, as viewed from Earth.
• Transits of Venus across the solar face. How observations of a transit by England's
Captain James Cook in 1769 led to a more accurate measure of the distance between
Earth and the Sun.
Mapping Venus by Radar
• How radar provides the first information about the surface of Venus, and reveals its slow
backwards rotation.
• Early spacecraft discover an atmosphere 90 times denser than Earth's, clouds of sulphuric
acid, and very high surface temperatures.
• Venus as a world of extensive lowland plains, with few uplands and only two continents.
Venusian Volcanism
• 1990: America's Magellan mapper arrives at Venus. How computer animation
transformed its radar imagery, enabling us to "view" the surface and "fly" the planet.
• How Magellan's radar scans the planet, strip by strip.
• A tour of the surface of Venus: impact craters, volcanoes with lava flows, evidence for
volcanic activity over billions of years.
Inside Venus
• How gravity mapping reveals the structure and composition beneath the surface.
• Inside, Venus is rather like Earth, but without a molten outer core, so today there is no
magnetic field.
Pressure Cooker
• Venus orbiting within the habitable zone of the Solar System, but closer to the Sun than
the Earth.
• How Venus may once have resembled Earth. Then, billions of years ago, the oceans
evaporate.
• Carbon dioxide is released from the oceans, the atmosphere thickens, and heat is trapped
near the surface.
• A runaway greenhouse effect creates the hostile world we see today.
Background
The Greenhouse Effect on Venus
The great amount of the gas carbon dioxide in the dense cloudy atmosphere of Venus has
led to what is called a "runaway greenhouse effect". Although the thick cloud layers
keep out up to 80% of the Sun's rays, some visible sunlight does reach the surface, where
it is absorbed by the rocks. This sunlight, however, is re-radiated as infrared
radiation, and the carbon dioxide clouds do not let this through, trapping the heat near
the planet's surface.
The effect is very similar to what happens in a greenhouse in the summer. The greenhouse glass
lets through visible sunlight, but does not allow the infrared radiation, re-radiated by the plants
and soil, to escape. Heat is trapped inside. Temperatures soar - hence the term "greenhouse
effect".
The runaway greenhouse effect on Venus has caused the planet's surface temperature to
rise to nearly 480 degrees Celsius. Venus is, therefore, the hottest planet, even though Mercury is
closer to the Sun.
Measuring Distances in the Solar System
One way to calculate the average distance between the Earth and the Sun is for a number of
people in different parts of the world to observe and measure a transit of the planet Venus. A
transit is when the planet passes exactly between Earth and the Sun, appearing in silhouette as a
black dot crossing the brilliant face of the Sun.
The British explorer Captain James Cook led one of many expeditions to observe the transit of
Venus in 1769. His group studied the event from the island of Tahiti in the Pacific Ocean.
Calculations made from Cook's observations of the transit - and many others elsewhere - enabled
astronomers to determine the average distance between Earth and the Sun. This measurement is
known as the "astronomical unit", and is approximately 150 million kilometers. Knowing the
astronomical unit enabled astronomers to calculate the relative dimensions of the entire Solar
System. Today, the length of the astronomical unit can be determined far more accurately by
other methods.
Radar and the Strange Rotation of Venus
Radar astronomy involves transmitting a beam of radio waves that are then bounced off a target for example, the surface of Venus - and picked up by a radio receiver, usually the transmitter
working in reverse. A great advantage of radar for investigating Venus is that radio waves can
penetrate the dense clouds to reach the planet's surface very easily.
When astronomers first made radar contact with Venus, they were surprised. They discovered
that Venus spins on its axis more slowly than any other planet. It turns once on its axis in 243
Earth days, and, even more strangely, Venus spins from east to west. Most of the other planets,
including Earth, rotate from west to east. In the course of a day on Venus, the planet moves a
considerable distance along its orbit around the Sun. For this reason its "day" - measured from
one sunrise to the next (unfortunately not visible through the dense clouds) - is far less than its
period of rotation. This "day" on Venus lasts 117 Earth days.
Links for Further Information
Venera Mission to Venus site, containing basic description of Venus, information on personnel,
launch/orbital information, and images from the Venera missions.
http://nssdc.gsfc.nasa.gov/planetary/venera.html
Comprehensive Magellan page. Includes mission data, the geology of Venus, information on the
Magellan craft, its radar system, science personnel and images.
http://pds.jpl.nasa.gov/mveg/guide.html
A good Venus site with history of the planet, size and structure, missions to the planet, its
atmosphere and retrograde rotation, plus other links to Venus.
http://seds.lpl.arizona.edu/nineplanets/nineplanets/venus.html
Impressive site for Venus images and movies.
http://seds.lpl.arizona.edu/nineplanets/nineplanets/pxvenus.html
Another good Venus site, featuring significant dates for Venus, images of Venus, surface
information, Mariner 2 background and general information.
http://www.acim.usl.edu/SOLAR/venus/venus.html
The Magellan spacecraft home page, including links to numerous images, technical data, and
animations.
http://www.jpl.nasa.gov/magellan/
Questions and Activities for the Curious
1. As viewed from Earth, the apparent brightness of Venus changes as it moves along its orbit.
Describe the main factors which determine the variations in the planet's brightness as seen from
Earth.
2. Captain Cook led an expedition to Tahiti to observe the transit of Venus in 1769. Imagine that
you were a member of this expedition, and describe your experiences on the voyage.
3. What two main reasons explain the lack of large numbers of impact craters on the surface of
Venus?
4. How might the activity of the volcanoes on Venus affect the atmosphere and conditions on the
planet's surface?
5. Research information on the greenhouse effect. Why it is important for the future of the Earth's
atmosphere.
6. Why is the greenhouse effect relevant to the reason why dogs should not be locked in a car,
with all the windows closed, in hot weather?
7. Scientists believe there were once oceans on the surface of Venus. How would you account for
the disappearance of all this water?
8. Venus is often referred to as Earth's twin sister. What properties do the two planets have in
common? In what ways are they dissimilar?