PHYS-1000
Mapping The Surface Of Venus
Fall 2012
Name:
Introduction
The planet Venus is nearly the same size, mass, and density as Earth, and for many years we considered it to
be Earth’s slightly warner “sister planet”—a slightly warmer version of our own world. Space missions and
Earth-based spectroscopic and radio astronomy studies have shown, however, that Venus is a very di↵erent
world from our own.
The atmosphere of Venus is almost entirely carbon dioxide, and is so thick that the atmospheric pressure
at the surface is ⇡ 90 times greater than what we experience at sea level on Earth. Carbon dioxide is a
greenhouse gas that traps heat energy near the surface, so with this thick insulating blanket, the surface
temperature of Venus has reached nearly 900 F! The planet is completely covered with clouds, which reflect
75% of the incoming sunlight, making the surface a dimly lit place. The cloud deck is quite high—45 70
km above the surface—and unlike on Earth, where clouds are formed from water droplets or ice crystals,
the clouds of Venus are made of sulfuric acid droplets.
Under such conditions, visually observing the surface of Venus with telescopes is impossible, and landing
spacecraft there presents quite a challenge, so astronomers have turned to alternative methods of exploring
the terrain. During the 1970s, the Soviet space program sent several landers to the surface of Venus, given
the harsh conditions, none of these returned data for more than a couple of hours. Images of Venus from the
Hubble Space Telescope and various Earth-bound observatories reveal only clouds, but images of the surface
can be constructed by sending radar waves through the atmosphere, then receiving the reflected “echoes” of
these signals. In 1990, the Magellan mission was sent to Venus to obtain a detailed radar map of the surface.
It spent three years collecting radar data and building a global map (Figure 1(a)).
(a) Venus’s surface (courtesy NASA).
(b) Radar-mapping method (courtesy U.S. Geological Survey).
Figure 1:
In the radar-mapping method, the spacecraft sends a radio wave toward the surface, then detects the reflected
signal, as depicted in Figure 1(b). In the simplest approximation, the distance between the radar-emitting
spacecraft and the surface below can be calculated by timing how long it takes for the reflected waves to
travel to the surface and back to the orbiter. If the time is measured. the distance the waves travel can be
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PHYS-1000
Mapping The Surface Of Venus
Fall 2012
calculated if the speed of the radar waves is known. But radar waves are simply radio waves—which are
electromagnetic waves—whose speed is well known.
Venus rotates slowest of all the planets, taking 243 days to spin once on its axis. Magellan took advantage
of the slow rotation in compiling its radar map of the surface. The spacecraft was oriented so that it moved
around the planet in an orbit that took it from pole to pole, rather than around the equator. With each
orbit of the spacecraft, the planet shifted slightly underneath, allowing a new strip of surface to be mapped.
After 243 days, the entire planet had rotated under the spacecraft.
If the time is measured, the distance the waves travel can be calculated if the speed of the radar waves is
known. The formula for such a calculation is
distance = speed ⇥ time
Procedure
You will be given a sealed container with holes in it. The container is a model of Venus in the sense that
you cannot see what lies below without sending some sort of probe through the opaque container lid. DO
NOT OPEN THE CONTAINER UNLESS THE INSTRUCTOR TELLS YOU TO DO SO.
The grid of holes provides access to the “surface” hidden below. You will attempt to construct a topographic map of the surface by sending your “radar beams” (wooden skewers) through the “cloud layer” (the
lid).
Select a wooden skewer and,if it is not already marked, carefully mark o↵ 1 cm gradations, starting with the
pointed end. If it is marked, verify that each of the marks are 1 cm apart. Push the skewer through each
hole in the lid until it touches the surface below, making sure that it is completely vertical at the same time.
For each hole, record on the graph sheet provided how many marks passed through the lid.
Work with other members in your group to divide the work and then combine your data.
Ue the grid in Figure 2 to record the values for all the holes in the lid. (Near the comers of the boxes there
are some positions with no holes:record “⇥” for these positions, meaning no data. Occasionally this happens
in space missions.
Each person will construct a complete grid for the whole box, using the assembled data from the members
of the group. In the space below, list your group members as well as the number that is on the side of your
box.
Group Member
Group Member
Group Member
Group Member
Box Number
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PHYS-1000
Mapping The Surface Of Venus
Fall 2012
Questions
1. What is the speed of the radar waves (in km/s)?
2. A signal is sent from a spacecraft to a mountain on the surface of Venus. The signal returns to the
spacecraft 0.002 seconds after it was emitted. How far below the spacecraft is the mountain? (Be sure
to show your work and specify the appropriate units.)
3. Why is it better to put a radar-mapping mission like Magellan in a polar orbit, rather than an orbit
around the equator of Venus?
4. Does a large number for a square in Figure 2 mean low elevation or high elevation?
5. After the complete grid has been constructed, use the following color code to construct a topographic
map of the hidden surface.
9 or greater—Black
8—Gray (pencil)
7—Violet
6—Blue
5—Green
4—Yellow
3—Orange
2—Red
1 or under—Brown
Use the space on the sheet containing your color-coded map (Figure 2) to carefully label and describe
the terrain for the section of the planetary surface contained in your box.
6. What could be done to increase the resolution of your map (i.e., to increase the level of detail you can
plot)?
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PHYS-1000
Mapping The Surface Of Venus
Fall 2012
7. Examine the Venus maps provided by the instructor. They show
1. a volcanic edifice of Venus called the Erstla Region, and
2. a mountainous region known as Akna Montes.
Both have been divided into eight rectangular sections, each of which is depicted in a box used in this
lab. Examine your map and determine which section of the map matches your box. The colors on the
map might be a bit di↵erent from those on your own map, so pay attention to changes in elevation and
specific features to make your judgement.
Which section of the map is represented by your box? Justify your answer.
Figure 2: Record skewer mark values in this grid.
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