VENUS

VENUS
OBJECTIVES
In this exercise you will learn about the planet Venus and study how a radar
mapping satellite works.
EQUIPMENT
A box with a lid, shapes that can be placed inside the box, something to punch holes
in the lid with, straws, a ruler, graph paper, colored pencils or crayons.
INTRODUCTION
Venus has been known since prehistoric times and is the brightest object in the sky
except for the Sun and the Moon. At times, Venus is so bright that it can be seen in the
daytime, if you know where to look, and venuslight can cast a shadow at night. Like Mercury,
it was popularly thought to be two separate bodies: the evening star (Hesperus), and the
morning star (Eosphorus). But the Greek astronomers knew it was one body, and called it
Aphrodite. The Babylonians called it Ishtar. Venus, Aphrodite, and Ishtar were all female gods
of love and beauty.
But Venus has not been a woman in all cultures. For example, the Maya called it
Kulkulcan, and said it was the brother of the Sun. The Aztecs also said it was a man and
called it Quetzalcoatl. Both of these cultures worshiped Venus and built elaborate
observatories to allow them to keep track of its movements in the sky, which they could predict
with great accuracy. The Mayan calendar based on the observed 260-day cycle of morning
star to morning star is still used in some parts of Central America.
Venus was one of the first objects observed by Galileo when he turned his telescope
to the sky. It was his observation of phases, similar to the phases of the Moon, that helped him
prove the Copernican theory. These phases could be possible only if Venus was orbiting the
Sun, and not Earth.
Venus is the second planet from the Sun and the sixth largest. Venus has no moon. Its
orbit is the most nearly circular of any planet, with an eccentricity of less than 1%. It orbits the
Sun once every 224 earth days at a distance of .72 AU (108 million kilometers). Because it
is moving around the Sun in the same direction as Earth, it takes 584 earth days for it to go
around the Sun and catch up to us again. It’s diameter is about .95 earth diameters (12,100
km), and its mass is .82 earth masses (4.9 x 10 kg). This gives it an average density of 5.
24
2
gm/cm ,
3 as compared to Earth’s 5.5 gm/cm3. These figures make Venus at least appear to
be the planet most like Earth, and Venus is frequently called Earth’s Sister.
But despite the fact that it is the nearest planet to us, we knew almost nothing about
Venus for most of history. This is because it is surrounded with a thick, uniform layer of clouds
that prevents us from seeing any features. This lack of information inspired many people to
fantasize about what lay beneath the clouds. Such accomplished writers as Edgar Rice
Burroughs, Ray Bradbury, Robert Heinlein, and Isaac Asimov all wrote stories
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taking place on a fictional Venus. These fictional planets were typically wet and warm, the
idea being that if Venus was surrounded by clouds there must be rain. And since it was closer
to the Sun than us, it must be warm, even tropical. But with the advent of the space age, we
were to learn that Venus is very different from anything we ever imagined.
We first began to learn more about what lay beneath the clouds in 1956, when
astronomers detected microwaves being emitted by the planet. The source of these
emissions was a mystery and some astronomers proposed that they were being emitted by
a hot surface, as hot as 350
o C. Then, in 1961, radar was used to determine the rate of
rotation. It was found the planet rotates on its axis once every 243 days, but in a retrograde
motion. That is, if you were above the north pole of Venus looking down, it would appear to
be rotating clockwise, where most planets (including Earth) would appear to be rotating
counterclockwise. In addition, the periods of Venus' rotation and of its orbit are synchronized
such that it always presents the same face toward Earth when the two planets are at their
closest approach. Whether this is a resonance effect or merely a coincidence is not known.
This slow rotational rate (19 days longer than its orbital period) combined with the
orbital period results in a ‘day’ that is 117 earth days long. You wouldn’t be able to see the
Sun because of the clouds, but if you were on the surface of Venus you would be able to sense
the Sun rising in the west, slowly crossing the sky, and setting in the east about 59 earth days
later. As a result, there are about 2 venusian days per venusian year.
After this, the invasion of Venus began. The first spacecraft to visit Venus was Mariner
2 in 1962. It has since been visited by more than 20 spacecraft from the United States and the
Soviet Union. More spacecraft have been sent to Venus than any other planet (remember the
Moon is not a planet), although it is likely that Mars will assume this distinction within a few
years. Among these spacecraft were the
Soviet Venera 7, which was the first
spacecraft to land on another planet;
Venera 9, which sent back the first
images of the planet’s surface (Figure
1); Pioneer Venus, which sent several
probes into the atmosphere and orbited
the planet for nearly two decades; and
Magellan, which provided high
resolution radar images of nearly the
Figure 1
Surface of Venus from Venera 9 entire surface. Venus is now the best
mapped planet in the solar system, even
better than Earth. With the help of the data collected by instruments on these spacecraft, we
have been able to put together a much more comprehensive picture of the planet.
THE ATMOSPHERE
The atmosphere, it was found, has a high concentration of sulfuric acid at high altitudes.
Earth’s atmosphere does also, although at much lower concentrations. In both cases this
results in acid rain. Although the acid rain in the clouds of Venus is much stronger, acid rain
here on Earth causes a great deal of damage. Acid rain is responsible for the destruction of
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forests and freshwater ecosystems, as well as the rapid erosion of manmade structures. Even
though the acid rain on Venus evaporates long before it reaches the surface, it provides us
with a laboratory for studying the chemical processes that create and possibly destroy acid
rain.
There are strong (350 kph) winds at the cloud tops but winds at the surface are very
slow, no more than a few kilometers per hour. The clouds are
several kilometers thick, but not dense, more like fog than a
cloud and you would be able to see for several kilometers if
you were in them. By observing in different wavelengths we
are able to see features in these clouds and also to observe
at different altitudes. In this way, we have learned that the
clouds are not as uniform as they appear, but actually vary
greatly in thickness and structure. Observing in ultraviolet light
reveals a mystery by revealing large areas in the cloud tops
that are dark (Figure 2). These areas are ultraviolet dark
because they contain something that absorbs ultraviolet light, Figure 2
but what that substance could be is unknown.
Venus in Ultraviolet light
The composition of the atmosphere was found to be
about 96.5% CO , most of the rest is nit
2
rogen. The surface pressure is about 90 times the
surface pressure on Earth, about the same as at an ocean depth of one kilometer. Traces of
gases in the atmosphere have led many scientist to believe there was once an ocean on
Venus, but others dispute it. What is certain is that there is no water now. That’s because the
surface temperature is an amazing 480 C, hot enough to melt lead!
o
Venus' surface is actually
hotter than Mercury's despite being nearly twice as far from the Sun, making Venus the hottest
planet in the solar system. There is virtually the same temperature everywhere on the surface,
from pole to pole and even on the nightside. This is very puzzling because calculations based
on Venus’ distance from the Sun showed the temperature should be no more than 330 C an
o
d
possibly as low as 80 C. The explanation for this is the greenhouse effect.
o
It used to be thought the way a greenhouse works is by allowing light to enter, where
it heats up objects within. These objects then emit infrared radiation, which heats the air but
can’t pass through the window panes. We now know this is not the way the greenhouse
works, but the name stuck. However, it is very close to the way a carbon-dioxide atmosphere
works. The CO in the atmosphere is a very good absorber o
2
f infrared radiation. In Venus’
case it absorbs 99% of the infrared radiation emitted by the surface.
If you were to throw extra blankets on your bed at night you would find it to be hotter,
even though you are not adding anymore heat to it than you were before. The extra blanket
allows less heat to escape. Likewise, Venus’ thick carbon-dioxide atmosphere acts as a
blanket, trapping heat and making the atmosphere hotter, even though no more heat is being
added to the system than the calculations showed there was. What was not taken into
consideration is that something might be keeping the heat from escaping once it entered into
the atmosphere.
But how can this account for the temperature being so high? This is a case of what is
called the runaway greenhouse effect. Carbon dioxide can be trapped in rocks and water.
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But if the temperature is raised a little then this carbon dioxide is released into the
atmosphere. As the concentration of CO in the air goes up, more heat is
2
captured and the
temperature goes up, causing more CO to be released, causing
2
the temperature to go up,
and on and on. Studying the atmosphere of Venus makes us very concerned about what we
are doing here on Earth. Are our activities raising the level of carbon dioxide in the
atmosphere? And are we threatening ourselves with a runaway greenhouse effect? Earth's
carbon dioxide is trapped in rocks and the oceans. If it was all released our atmosphere would
have about 98% CO and 2% O
2
2. The surface pressure would be 70 bar. Both of these figures
are close to those found on Venus. By studying Venus, we will learn more about the
greenhouse effect and its threat to us.
SURFACE FEATURES
By international convention, surface features are named after women, either real or
mythical. The dominant features are two large areas with elevations higher than their
surrounding, although we don’t yet know if they are truly continents (a continent has different
rock composition than the surrounding basin area). Ishtar Terra, in the north, is about the size
of Australia, has a large high plateau, Lakshmi Planum, and the highest mountain on Venus,
Maxwell Montes. Aphrodite Terra lies along the equator and is about the size of Africa. There
are two smaller areas, Alpha Reggio and Beta Reggio, and several other smaller reggios.
Maxwell Montes (named for the 19 century physicist James Clerk
th
Maxwell), Alpha Reggio
and Beta Reggio were all identified with Earth based radar and named before the female
convention was established. There are also several broad depressions: Atalanta Planitia,
Guinevere Planitia, Lavinia Planitia.
Magellan's images show a wide variety of
interesting and unique features including pancake
volcanoes (Figure 3) which seem to be eruptions of
very thick lava and coronae which seem to be
collapsed domes over large magma chambers.
Although compressive forces are evident, we
do not see evidence of large scale plate tectonics,
such as we see on Earth. This raises a question, what
does Venus do to release its internal energy? It’s
possible that the internal energy has been focused
elsewhere, but it is also believed that water acts as a
lubricant between plates and mantle and is a critical
Figure 3
Pancake Volcanoes ingredient in plate tectonics. Studies of the distribution
of meteor impact craters have revealed the surface
was nearly completely reworked about 800 million years ago. It is now believed that some
kind of massive, cataclysmic event occurred 800 million years ago that released much of the
internal energy and reworked the entire surface of Venus at once. We don’t fully understand
what this event may have been, so we don’t know if it happens repeatedly. If so, then it could
occur again.
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VOLCANISM
Magellan’s radar images clearly showed the presence of volcanoes and lava flows,
proving the existence of volcanism on Venus. However, it is not known for sure if there is
current volcanism or if these images are of ancient, extinct volcanoes. But the evidence for
current volcanism includes the detection of radio emission associated with lightning (which
is associated with volcanism). Also, Soviet Venera missions did not detect any sulfur dioxide
in the air in the early 1970's. But, Pioneer Venus detected SO
2 when it arrived in 1978.
Measurements taken with instruments on the Hubble Space Telescope indicated in the early
1990's that the level of SO had decreased to
2
one-tenth as much. It is believed this indicates
a large volcanic eruption occurred in the early 1970's, after the Veneras but before Pioneer
Venus.
THE PLANET’S INTERIOR
By studying the density of the planet and the way satellites behave when they are in
orbit about it (this is called gravity data), we can deduce a lot of information concerning the
structure of the interior. The interior of Venus is probably very similar to that of Earth with an
iron core of about 3000 km in radius, and a molten rocky mantle comprising the majority of the
planet. It is believed that Venus' crust is much stronger and thicker than Earth’s. Although
Venus has a molten interior, there is no magnetic field. This is probably due to the planet’s
slow rate of rotation.
Exploration of Venus with spacecraft has greatly increased our understanding of the
planet. In the process, we have become aware of, or increased our understanding of, three
major environmental problems facing us today: global warming, acid rain and ozone depletion.
But many questions remain concerning our neighboring planet. These questions include:
C
How did the environment come about? Did it evolve through natural, gradual
processes? Or is it the result of a catastrophe?
C
What is the mysterious ultraviolet light absorber in the atmosphere?
C
Did Venus once have oceans?
C
Why did Venus evolve so differently from Earth?
C
Why does Venus rotate backwards?
It will probably require new missions in the future to answer these and many other questions
about Venus. And history has shown we may learn a lot about Earth by learning why the
basically similar Venus turned out so differently.
BIBLIOGRAPHY
Arnett, Bill; The Nine Planets, A Multimedia Tour of the Solar System;
http://seds.lpl.arizona.edu/nineplanets/nineplanets/venus.html
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Chaisson, Eric, and Steve McMillan; Astronomy Today, 3rd Ed.; Prentice Hall; Upper Saddle
River, NJ, 1998.
Kahn, Ralph; Comparative Planetology and the Atmosphere of Earth, NASA, Jet
Propulsion Laboratory, Pasadena, CA, 1989.
Grinspoon, David Harry; ‘Venus Revealed’, Helix Books, Reading, MA, 1997.
Pasachoff, Jay M., Astronomy: From the Earth to the Universe, 5th ed. Saunders College
Publishing, Orlando, 1997.
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QUESTIONS
1. Why is Venus so bright in the sky?
2. What causes the temperature to be so high on Venus?
3. Why would Venus’ slow rotation be blamed for the absence of a planetary magnetic field?
4. Some critics of global warming theories argue that the observed warming trend in Earth’s
temperatures is a natural result of the end of the last ice age. Others argue that the
environment is simply going through a warm period, similar to what it has done many times
in the past. Suppose these claims are correct, why is it still important for us to reduce
emissions of greenhouse gases (gases that trap heat, such as carbon dioxide)?
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5. Why did we have to use radar mappers to obtain a good picture of the surface of Venus?
6. The Soviet Union put several Venera spacecraft on the surface of Venus. These probes
lasted only a few minutes to a couple of hours before failing. Why do you think this is?
7. What is the difference between the greenhouse effect and the runaway greenhouse effect?
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EXERCISE
Radar mapping satellites, such as Magellan, send a radar signal, very similar to the kind used
in television weather broadcasts, towards the surface. This signal may be absorbed by some
material, it may be reflected away from the satellite, or it may be reflected back towards the
satellite where it is received. These signals can then be processed to give us an ‘image’ of
the surface. Areas that reflect the most signal back to the satellite are bright, areas that
absorb or reflect the signal away from the satellite are dark. Likewise, by timing how long it
takes the signal to return it is possible to determine the topography of the surface.
1. Make a ‘mystery’ planet surface by putting a variety of objects inside a box. These objects
can be made of styrofoam, Legos, clay, or some sturdy three-dimensional object. Arrange
the objects to make a varied typography. If you are doing this with multiple groups, one group
can make the landscape for another and then exchange boxes. Be sure the lid is on the box
so the landscape is a mystery.
2. Place a piece of graph paper on the lid and punch holes all the way through the lid about
every inch. The holes need to be large enough for a straw to fit through.
3. Make a height-color key for mapping the surface. For example, use warm colors for high,
cool colors for medium, and dark colors for low topography. Make the increments .5
centimeters in width.
4. Perform a low-resolution mapping pass by using every other hole. Place a straw through
the hole until it lightly touches the bottom. Grasp the straw at the lid with your fingers and
remove it. Measure how far down the straw went and lightly write that measurement on a piece
of graph paper. By using every other hole, each measurement will correspond to an area of
about two square inches.
5. Using the height-color key, select the color that corresponds to the measured height and
color in the entire two-inch square.
6. Continue doing this until you have mapped the entire box.
7. Next, repeat the measurements at high resolution by using every hole, instead of every other
hole. Otherwise, use the same technique as before.
8. Compare the two maps to each other. Open the box lid and compare the objects to what
you have mapped.
9. Describe your observations.
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