www.astrosociety.org/uitc
No.
18 - Fall 1991
©
1991, Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco,
CA 94112.
The
Magellan Spacecraft at Venus
by
Andrew Fraknoi, Astronomical Society of the Pacific
"Having finally
penetrated below the clouds of Venus, we find its surface to be naked [not hidden],
revealing the history of hundreds of millions of years of geological activity.
Venus is a geologist's dream planet.''
—Astronomer David Morrison
This fall, the
brightest star-like object you can see in the eastern skies before dawn isn't
a star at all — it's Venus, the second closest planet to the Sun. Because
Venus is so similar in diameter and mass to our world, and also has a gaseous
atmosphere, it has been called the Earth's "sister planet''. Many years ago,
scientists expected its surface, which is perpetually hidden beneath a thick
cloud layer, to look like Earth's as well. Earlier this century, some people
even imagined that Venus was a hot, humid, swampy world populated by prehistoric
creatures! But we now know Venus is very, very different.
New radar images
of Venus, just returned from NASA's Magellan spacecraft orbiting the
planet, have provided astronomers the clearest view ever of its surface, revealing
unique geological features, meteor impact craters, and evidence of volcanic
eruptions different from any others found in the solar system. This issue of
The Universe in the Classroom is devoted to what Magellan is teaching
us today about our nearest neighbor, Venus.
Venus orbits the Sun
in a nearly circular path between Mercury and the Earth, about 3/4 as far from
our star as the Earth is. Venus takes about 7 and one-half months to go around
the Sun once, but it turns so slowly on its axis that one complete rotation lasts
243 Earth days! Astronomers have learned that Venus's surface is hotter than any
other planet in the solar system, including Mercury (which orbits much closer
to the Sun). The average ground temperature is around 900 degrees Fahrenheit —
much hotter than a typical oven broiler, and hot enough to melt metals like lead
and zinc. Venus' atmosphere is 97 percent carbon dioxide, and is over 90 times
heavier than the Earth's. Most of the intense heat on Venus is due to this thick
blanket of carbon dioxide, which traps heat very well. (See the box on back page).
And astronomers have determined that Venus' clouds are made of — at least
in part — corrosive sulfuric acid droplets. Venus is truly an inhospitable
place!
 |
| This
80-kilometer (50-mile) long trough (dubbed "gumby" by project
scientists because of its resemblance to the pliable toy) is at least one
kilometer deep. It probably formed when lava channels beneath the surface
collapsed after the lava had drained out through vents at the surface. |
Before Magellan,
more than 20 spacecraft from the U.S. and the U.S.S.R. had already probed Venus'
surface and atmosphere. No other planet has received so many terrestrial visitors
and yet managed to remain so elusive. The Soviets' early Venera craft were
crushed by the high pressure of Venus' atmosphere, but later models survived to
land on the surface, and sent back the first pictures ever taken on another planet.
In the late 1970s and early 1980s, American and Soviet orbiters made the first
global radar maps of Venus, but their instruments could only "see'' features larger
than a kilometer or more. About 75 percent of Venus seemed to be covered by lava
plains, resembling the dark lunar maria we see on our Moon. There were
two continent-sized land masses visible on Venus, as well as some large mountains,
possibly volcanic. And there were indications of meteor impact craters.
These tantalizing
early observations of Venus told us that the planet had some features similar
to our Earth, but the pictures and maps were too coarse to answer many of our
questions. Thus astronomers built a new spacecraft, Magellan, that would
use modern radar technology to make images of the surface much sharper than
ever before.
The Magellan
mission was originally approved in 1980, but had to be simplified due to budget
pressures. The modified spacecraft was based on existing designs, and its construction
included spare parts left over from other space missions. For example, Magellan's
medium-sized antenna comes from the Mariner 9 project, and its main 3.7-meter
(12-foot) mapping and communications antenna is a spare from Voyager.
Magellan
was launched from Cape Canaveral on May 4, 1989, aboard the space shuttle Atlantis.
The spacecraft arrived at Venus on August 10, 1990, after traveling more than
1.3 billion kilometers (806 million miles) in a path that took the probe around
the Sun one and one-half times.
Although visible light
will not penetrate Venus' thick atmosphere, longer wavelength radio waves easily
pass through the clouds and reflect off the ground, revealing surface elevations
and details such as craters, canyons, cliffs, lava flows, and volcanoes. Different
types of terrain reflect more or less radio energy, allowing scientists on Earth
to determine not only the shape of the Venusian surface, but its approximate composition,
roughness, and the size of its irregularities as well. Electronic systems that
use radio waves to measure distances and heights are called radar, short
for radio detection and ranging.
Most typical forms
of radar systems — called real aperture radar — send out
one signal at a time, and process each echo by itself, before sending out another
pulse of radio energy. The Magellan spacecraft uses a special technique
called Synthetic Aperture Radar to gather extremely detailed pictures
of the Venusian terrain. Magellan sends out several thousand radar pulses
each second, and its SAR system uses fast computers to accumulate multiple echoes,
received from many locations simultaneously, as the spacecraft flies over the
surface. Magellan stores the data, and transmits it back to Earth at
the end of each 3 hour, 9 minute orbit. Scientists then process the data, factor
in how fast Magellan was moving as it received the echoes, and create
wonderfully detailed images. In a sense, using SAR is like taking many pictures,
from many different angles, of every feature along a path, creating a very precise,
almost three-dimensional image. Magellan can see features on Venus more
than 10 times sharper than any previous radar images taken from Earth or orbiting
spacecraft.
Magellan's
Synthetic Aperture Radar allows the spacecraft to photograph details as small
as 120 meters across. Magellan's computers can take in 36 million bits
of data each second. In fact, the spacecraft has already sent back more data
on Venus alone than had been acquired in total from all previous space missions,
to all of the planets.
Magellan has
photographed a remarkable assortment of new features and terrain on Venus; some
have never before been seen in the solar system. Unlike Earth, Venus' heavy atmosphere
and lack of liquid water or ice keep the planet's surface from eroding substantially.
Many of these unique surface features are hundreds of millions of years old —
far older than much of our planet's surface, but still much, much younger than
the heavily cratered ancient surfaces of Mercury.
Unique
kinds of impact craters
 |
| The
impact crater Aurelia, about 20 miles across, is surrounded by a thick blanket
of ejected material. A small "tail" of melted rock flowed away
from the crater to the lower right. Aurelia was Julius Caesar's mother. |
Magellan has
sent back beautiful images of impact craters on Venus, formed when large meteors
plummeted through the planet's atmosphere and struck the ground. Smaller meteors
burn up completely in the thick Venusian air. Colliding with Venus at about 20
kilometers per second (12 miles per second, which is over 43,000 miles per hour!),
the surviving meteors release a tremendous amount of energy. The collision utterly
vaporizes the incoming object and the surrounding ground. Surface material is
blasted out, forming a crater, and lava trapped miles below the ground can be
released. On Venus, the atmosphere is so thick that the material thrown out by
the impact doesn't fly very far, but the surface temperature is so hot that it
stays molten, and oozes away from the crater. Thus the craters on Venus look different
from those on any other planet.
One way to tell
the age of Venus' surface is to count the number of impact craters. On the plains
we see only about 15 percent of the number of craters on the Moon's lava plains;
these results tell planetary geologists that lava must have covered many of
the craters in the "recent'' past, and that Venus was geologically active. Magellan
also revealed multiple craters in a tight grouping, formed when the dense atmosphere
broke up an incoming meteor into pieces that struck near each other. And still
other craters were surrounded by streaks of dark material blown by a slow wind.
In the extremely thick, 900 degree Venusian air, a "wind'' of 3-4 miles per
hour is really more like a slow current at the bottom of the Pacific Ocean.
Volcanoes,
Pancakes, and Coronae
 |
Pancake
domes. |
Venus is literally
covered with volcanoes, but Magellan has not yet observed any actively
erupting. Hundreds of thousands of small domes 2 — 3 kilometers across,
and perhaps one hundred meters high, dot the plains. These are shield volcanoes,
similar to those that formed our planet's Hawaiian Islands, and they form over
hot spots in Venus' crust. In other locations, volcanoes appear where thick, sticky
lava oozed into 25- kilometer wide pancake-like puddles, typically less than a
mile high. As the pancake dome of lava cools, the hot material underneath pushes
and cracks the surface. On Earth, geologists have identified similar kinds of
features near Mono Lake in California, but those on Venus are much larger. And
Magellan has revealed hundreds of broad circular features called coronae,
which appear to be blobs of hot lava that rose, created a large bulge or dome
in the surface, and then sank, collapsing the dome and leaving a ring that looks
like a "fallen souffle.'' These are also unique, never before seen on any other
planet or satellite in the solar system. Other new volcanic features seen by Magellan
included what some astronomers have dubbed "arachnoids'', because they look like
spider bodies connected by a web of fractures.
Venus also has
some very large volcanoes. One of the largest, called Sif Mons (named after
the Scandinavian Grain Goddess Sif, whose long golden hair is the autumn grass),
is about 500 km across, but only 3 km high; Sif Mons is broader but lower than
the Hawaiian volcano Mauna Loa. The lava flows from Sif are up to 500 km (more
than 300 miles) long — longer than any on Earth.
On our planet, the
crust is really a set of lighter "plates'' floating on heavier molten lava. Where
the plates buckle or collide, mountain ranges can form; where they rub together,
zones of earthquakes and volcanoes occur. We use the term "plate tectonics'' to
describe these motions of our surface. But Venus does not appear to have plates
like Earth. Instead, Magellan has shown us that the geology of Venus seems
dominated by hot currents and hot spots beneath the crust, which push and stretch
the surface. In the process, blobs of hot lava bubble up to form large land masses,
mountains, and the volcanic pancakes and coronae mentioned earlier. Scientists
have called the unique geology of Venus "blob tectonics.''
Magellan has
successfully concluded its primary mission, and has mapped 84 percent of the surface
of Venus over 243 days (one complete rotation of the planet). The probe is now
continuing to map Venus, filling in the gaps from the first pass. The spacecraft
will look for changes in terrain that might indicate large-scale winds or active
volcanoes. Scientists will also try to measure the slight changes in the planet's
gravity as the probe flies over areas of high density. Magellan speeds
up and rises a few meters as it passes over high density areas (which have more
gravity), and it slows down and falls closer to Venus as it flies over a lower-density
region. Data can be analyzed to help us determine the large-scale composition
of Venus' crust. Eventually, scientists will change Magellan's orbit to
make it follow a close circle above Venus' atmosphere, allowing even more sensitive
gravitational measurements to be made.
True, Venus is
not likely to be a place for people to colonize — or even visit on vacation
— since its surface conditions are so uninviting. But Magellan
is helping us to understand our own planet's history by providing a glimpse
of Venus' very different terrain. Without Earth's effective weathering processes
of wind and rain, Venus maintains its tectonic and volcanic features for long
periods. Scientists can study the planet's carefully preserved history, and
learn more about volcanoes and lava flows. Magellan's observations are
also teaching us about the movement of a planet's crust as heat escapes from
the interior. Furthermore, if — as some scientists suspect — human
activity and pollution on Earth is loading our atmosphere with material that
could slowly increase our own global temperature, studying the extreme conditions
on Venus may help us understand our own future. And, of course, Venus is the
closest example of another world — and another global arena in which to
exercise our curiosity about where we live, and how we got here.
Reading
List
- Beatty, K.,
"Venus in the Radar Spotlight'', Sky & Telescope, July 1991, p. 24
- Burnham, R.,
"Venus: Planet of Fire'', Astronomy, Sept. 1991, p. 32. (See also "Update
on Magellan'', Astronomy, Feb., 1991, p. 44.)
- Saunders, S.,
"The Exploration of Venus: A Magellan Progress Report'' in Mercury,
Sept/Oct. 1991.
- Saunders, S.,
et. al., "Magellan at Venus'', Science, Vol. 252, pp. 247 —
297, 12 April 1991. (Technical)
Soviet and American
unmanned probes that landed on the surface of Venus reported temperatures around
900 degrees Fahrenheit, day or night. Part of the reason for the high temperature
is that Venus is closer to the Sun than we are — but that's by no means
the whole story! Most of the heat on Venus is due to the peculiar heat-trap its
atmosphere provides, caused by the greenhouse effect.
Venus' massive
atmosphere is made mostly of carbon dioxide, a gas that is transparent
to visible light from the Sun but which almost completely blocks infrared
light (sometimes called heat radiation). On Venus, sunlight filters through
the clouds and heats up the rocky surface. The rocks then "glow'', and re-radiate
infrared heat, but the carbon dioxide atmosphere doesn't allow that infrared
light to get out into space. Instead, its energy is trapped in the atmosphere,
adding to the heat provided by sunlight, and turning Venus into a gigantic furnace.
Ordinary window
glass behaves in much the same way on Earth as carbon dioxide does on Venus.
Imagine a car parked in a sunny parking lot with the windows rolled up. Sunlight
gets through the windows, and warms up the car's interior. The seats then radiate
that heat as infrared light, which is trapped by the windows; glass doesn't
allow the heat radiation to escape easily. Heat builds up inside the car, and
when you open the door . . . Whew! It's hot. In a way, Venus can be thought
of as a car that's been left in a sunny parking lot for four billion years with
the windows rolled up!
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