ÐÓ°ÉÔ­´´

VENUS is one of the most visited planets in the Solar System. Since
the beginning of the space age in 1957, no fewer than 15 Soviet and five
American unmanned spacecraft have travelled there. Orbiting craft and probes
on parachutes have measured the structure and composition of its atmosphere.
Automatic landers have photographed parts of the surface of Venus and analysed
the chemical composition of its rocks. Yet we have only a sketchy knowledge
of the planet’s surface, because of its constant, dense cloud cover. This
prevents the surface from being photographed from space by ordinary cameras.

From August 1990, if all goes well, a new American spacecraft will study
the surface of Venus in unprecedented detail. Called Magellan, after the
famous 16th-century Portuguese explorer Ferdinand Magellan, it is due for
launch from the cargo bay of the space shuttle Atlantis this week. When
it arrives at Venus, Magellan will spend eight months using radar to map
most of the planet at a resolution (a measure of the smallest surface features
that can be seen) 10 times as fine as images of the surface obtained by
previous spacecraft.

In the early 1960s, scientists first began to use radar to study the
surface of Venus, with radio telescopes based on the Earth. Unlike visible
light, radio waves can penetrate the thick clouds of Venus. The planet’s
solid surface reflects the waves back to Earth. With the aid of computers,
astronomers can turn these radar reflections into pictures of the surface.
The early work with Earth-based radar provided reliable data about the diameter
of Venus, its period of rotation, and showed some of the large-scale surface
features. But such studies are limited. Venus rotates at a rate such that
when it is closest to Earth, the planet always shows the same hemisphere
to us, so only a fraction of the planet’s surface can be explored at high
resolution from Earth.

The early Mariner and Venera spacecraft, sent by the US and the Soviet
Union respectively to study Venus at close range, answered many questions
about the planet’s atmosphere and the conditions at its surface. Although
it is the planet that is most like our own Earth in size, mass and distance
from the Sun, Venus is very different in many other respects. The venusian
atmosphere is much denser than our own, and consists almost entirely of
carbon dioxide, with a high layer of cloud composed of droplets of sulphuric
acid. This thick blanketing atmosphere traps outgoing infrared radiation
between the solid surface and the atmosphere, so the surface temperature
is a searing 470 Degree C – hot enough to melt lead. The atmospheric pressure
at the surface is 90 times that of the Earth’s at sea level, equal to that
found at a depth of about 900 metres in the ocean. Any liquid water that
might once have existed on Venus has long since disappeared.

In 1978, NASA launched the Pioneer Venus Orbiter and Multi-probe missions
to conduct the most comprehensive studies of Venus undertaken to date. Most
of the experiments concerned the planet’s atmosphere, but the orbiting spacecraft
also carried a radar system that mapped more than 90 per cent of the surface
between latitudes 75 Degree north and 63 Degree south, with a horizontal
resolution of about 50 to 140 kilometres. The system could measure heights
to an accuracy of about 200 metres. For the first time, planetary scientists
had a global map of Venus, which showed the existence of three broad types
of terrain.

Most of the planet is covered by rolling upland plains, which rise no
more than 2000 metres above the mean level of Venus. (The ‘mean level’ takes
the place of ‘sea level’ on the Earth as a convenient zero for measuring
heights: it is at a radius of 6051.4 kilometres from the centre of Venus.)
The rolling plains cover 65 per cent of the planet’s surface. There are
also clearly distinguished continent-like highlands (about 8 per cent),
that rise above 2000 metres. The third type of terrain consists of smooth
lowlands (about 27 per cent), lying below the mean level.

However, although this radar map was important, it showed only large-scale
features. The hills, valleys, craters and lava flows – the telling details
of venusian geology – remained uncharted, and the questions about the extent
to which the venusian surface has been shaped by volcanoes, plate tectonics,
impact craters, and water and wind erosion remained unanswered.

The best radar images to date have come from the latest in the successful
series of Soviet Venera probes, Veneras 15 and 16, which the Soviet Union
placed in orbit around Venus in late 1983. From the time of their arrival
in October 1983 until July 1984, Veneras 15 and 16 mapped the planet with
a different type of radar, which provided a resolution of at best 2 kilometres.
Because of the nature of their elliptical orbits around the planet, these
spacecraft could map only about a quarter of Venus’s surface, near the north
pole. For comparison, Magellan’s high-resolution radar will map 70 to 90
per cent of the planet at a resolution varying from 250 to 750 metres.

Combining the results from the Earth-based radar studies, Pioneer Venus
Orbiter, and the Venera 15 and 16 missions, we can produce an outline picture
of the large-scale geography of Venus. There are two major ‘continents’,
or elevated highland plateau regions, Ishtar Terra and Aphrodite Terra,
that appear to be younger than the rest of the surface.

Ishtar Terra, with a diameter of 2900 kilometres, is about the size
of Australia. It is centred at a latitude of about 65Degree north. Jutting
up from the Ishtar highlands is one of the highest mountains in the Solar
System, the Maxwell Montes region. It is 11 000 metres high, more than 2000
metres higher than our own Mount Everest. There is some evidence that Maxwell
Montes is a huge volcano, probably now inactive. The radar images reveal
features that resemble old lava flows. The summit, Cleopatra Patera, is
marked by a large double depression, which is probably a volcanic caldera;
the outer ring is 100 kilometres across and the inner depression 50 kilometres
across.

The western part of Ishtar Terra is a vast smooth plateau called Lakshmi
Planum, rising 3000 to 5000 metres above the mean level of Venus. It is
bordered by the high mountain ridges of Akna Montes to the west, Freyja
Montes to the north and Maxwell Montes in the east. Lakshmi Planum is thought
to consist of thin lavas overlying an uplifted section of older crust. Two
prominent circular depressions on the plateau, Colette (9600 square kilometres)
and Sacajawea, which is larger (39 200 square kilometres), may be collapsed
volcanic craters. On the southern flank of Ishtar Terra are giant cliffs,
Ut Rupes and Vesta Rupes, which descend to vast lowland plains.

To the east of Maxwell Montes is a complex zone of intersecting ridges
and grooves, covering 2 million square kilometres, which extends towards
the lower lying, smoother plains nearer the planet’s equator. Because of
its distinctive crisscross pattern, people call this the ‘parquet’ terrain.
We can see the same pattern elsewhere on Venus. It seems to have been caused
by stresses, which fractured the crust of the planet. (Tectonism is the
name given to processes that cause fracturing in a planet’s crust.) The
plains east of Ishtar Terra contain several regions of higher upland terrain,
notably Tethus Regio. This region is similar to the southern part of Ishtar,
but has groups of dome-shaped hills and several unusual elliptical structures
300 to 500 kilometres in diameter known as ‘coronae’. The central parts
of these features are irregular in height; parts of the interior lie above
and parts below the level of the surrounding plains. They are surrounded
by a belt of concentric ridges and grooves, about 200 kilometres wide and
up to 2000 metres high. The origin of these peculiar structures, which seem
confined to a zone between latitudes 55Degree and 80Degree north, will remain
speculative until radar data of higher resolution become available, but
they could be the product of some unusual tectonic process.

Aphrodite Terra, stretching along and south of the equator, is the largest
elevated ‘continental’ land mass on Venus. It covers an area of 32 million
square kilometres, about the size of Africa. Aphrodite is even more rough
and complex than Ishtar Terra and contains an enormous, nearly circular
feature, about 2400 kilometres across, of unknown origin. There are two
major mountain regions on opposite sides of the ‘continent’, separated by
an area of lower terrain. Aphrodite also contains the deepest valleys on
Venus, the gigantic trenches of Diana Chasma, which are hundreds of kilometres
wide, over 1000 kilometres long, and lie 2000 metres below the mean radius
of Venus – 4000 metres below the adjoining ridges. This may be a rift valley
caused by the movement of two blocks of crust away from each other.

Two other highland areas, Alpha Regio and Beta Regio, stand out conspicuously
in the radar maps, and may be a result of volcanic activity. Beta Regio
lies southwest of Ishtar Terra, separated from it by rolling plains. It
contains two prominent volcanic peaks, Theia Mons and Rhea Mons, both about
5000 metres high, with streaks resembling lava flows on their flanks. The
Soviet lander probes Veneras 9 and 10 landed at the edge of Beta Regio.
They carried out chemical analysis showing that the rocks of the highland
area at the landing sites are basaltic, similar to those on the Earth’s
ocean floor or rocks formed from oozing volcanic lava flows.

There is a lot of evidence indicating that volcanic activity continues
in this region today. Spacecraft have recorded low-frequency radio emissions
clustering around both Beta Regio and Phoebe Regio, another highland region
to its south. These are probably lightning discharges, similar to those
observed in the dust plumes of erupting volcanoes on Earth. Levels of sulphur
dioxide and sulphuric acid droplets in the atmosphere also fell between
1978 and 1983. This may show that large-scale volcanic eruptions took place
in the mid 1970s, sending large quantities of sulphurous gases high into
the Venusian atmosphere.

The rolling plains that cover most of Venus appear undramatic in the
low-resolution images so far acquired, when compared to the rough highland
terrain. They contain, however, a wide range of features, including many
approximately circular structures, measuring up to 1000 kilometres in diameter.
These could be craters caused by impacts or may have been produced by tectonic
activity. The plains are apparently composed of older crustal rock. Clearly,
Venus has a complex geological history and, until high-resolution radar
images become available for most of its surface, many puzzles will remain.
This is why planetary geologists await with great interest the results of
the Magellan radar-mapper – the next big step in the exploration of the
planet’s surface.

Carried aloft aboard the space shuttle Atlantis, Magellan marks the
return of the US to planetary exploration after a gap of 11 years. The Challenger
accident delayed Magellan for a year. It was originally to be boosted by
a Centaur upper stage. But mission planners now deem the Centaur too dangerous
to be carried aboard the shuttle because it contains highly explosive liquid
fuels.

Following the launch, and after several revolutions around the Earth,
the Magellan spacecraft , with an Inertial Upper Stage (IUS) booster attached
to its base, is sent off from the shuttle cargo bay into its own orbit.
After two thirds of another revolution around the Earth, the IUS fires its
solid-fuelled engine and propels Magellan on its way towards Venus. The
IUS is then jettisoned. Due to the relative motions of the Earth and Venus,
the launch window, which began on 28 April, lasts only 26 days, so any protracted
delays to the launch would have resulted in the launch being postponed for
at least 18 months until the two planets were once again in the right positions.
During this launch period, Venus is approximately 255 million kilometres
from Earth.

After launch, it will take 15 months for Magellan to reach its destination.
During this interplanetary cruise, Magellan will follow an elliptical path
one-and-a-half times around the Sun before intercepting Venus. A traditional
trajectory to the planet would use approximately one-half of a revolution
around the Sun; the scheduling of the shuttle launch, however, has dictated
the longer route for Magellan. Three mid-course corrections along the way
will keep Magellan on time and on target for its rendezvous with Venus.
The date of arrival at Venus is fixed for 10 August 1990, and NASA planned
that the cruise period could be anything from 442 to 468 days depending
on the precise date of the launch.

On arrival at Venus, a Star-48 solid rocket motor will fire to place
Magellan into the correct orbit around the planet. After a few minor adjustments,
the spacecraft will follow an elliptical path, with its lowest point 250
kilometres above the planet’s surface and its highest point at 8030 kilometres.
The time taken for Magellan to complete one orbit around Venus will be three
hours and nine minutes. Because the orbit will be tilted at four degrees
to the rotation axis of Venus, the spacecraft will pass nearly, but not
quite, over both poles.

One unusual aspect of Venus is that its day (243.0 Earth days) is longer
than its year (224.7 Earth days). Even stranger is the fact that, unlike
most other planets, Venus rotates in a retrograde or ‘wrong-way’ direction
(clockwise as seen from above the north pole). To an hypothetical observer
on its surface, the Sun, if it could be seen, would appear to rise in the
west and set in the east 58 days later. Because Magellan will be in a fixed,
nearly polar orbit, around a slowly turning planet, it will take 243 days
to complete one cycle during which it will map the planet’s surface. Mapping
will begin on 28 August 1990, 18 days after the spacecraft arrives in orbit
around Venus.

Magellan will collect several additional types of information while
mapping. Between the radar pulses that it sends out, the antenna on Magellan
will measure the heat emissions from the surface of Venus. These are generated
(like infrared radiation) by the planet’s heat. From this study, called
radiometry, scientists will build up a map of the way the temperature varies
on the planet’s surface.

They can also gain extra information about the surface by studying in
detail the pulses of radar reflected back to Magellan. The surface rocks
do not just reflect the radar pulses like a mirror: they interact with the
transmitted radar pulses, and modify them before the pulses are reflected
back to the receiving antenna. Factors that modify the radar pulses include
the material from which the surface is made, the degree of roughness, its
electrical properties, and the slope of the illuminated terrain (radar beams
can produce shadows just like beams of visible light). An area that appears
bright in the radar image indicates a fairly rough surface, which can reflect
a fair amount of the radar signal directly back towards Magellan; a radar-dark
area would indicate a smoother surface, which would reflect most of the
radar signal away in another direction. So, analysis of the way in which
the radar signals are reflected will yield important data on the properties
of the surface materials.

During each orbit, Magellan will pass closest to the surface at a latitude
of 10Degree north, and will then move down towards the south pole and around
the planet in a wide loop. This elliptical path means that Magellan will
map the surface for only 37.2 minutes of each orbit. The rest of the orbital
period will be spent transmitting the recorded raw data from the completed
mapping pass, receiving telemetry instructions from Earth, and calibrating
the spacecraft’s system for controlling which direction it points in. Magellan
has only one radio antenna, which it will use both for the radar-mapping
and for communicating with the Earth. During the part of the orbit where
it is not mapping Venus, the spacecraft must, therefore, reorient itself
to point the antenna towards Earth. The transmissions will be picked up
by the large dish antennas of NASA’s Deep Space Network located around the
world and then relayed to the Jet Propulsion Laboratory at Pasadena in California.
While Magellan is in touch with the Earth, the mission scientists can make
precise measurements of the slight changes in the spacecraft’s orbital motion,
produced by variations in the gravitational field of Venus. These will provide
important clues about the nature of the planet’s interior.

After completing its ‘call home’, Magellan will reorient itself so that
the antenna once again points down towards the surface and mapping can start
again. As Venus rotates slowly beneath Magellan’s orbit, the spacecraft
will map the surface in a series of slightly overlapping strips, each about
25 kilometres wide and 16 000 kilometres long. Near the north pole the strips
will converge, so they will overlap. This means that the spacecraft can
cover the north pole completely by using every other swathe, so the mission
planners have arranged an alternating pattern of northern and southern mapping
passes. On one orbit, Magellan will map a swathe from the north pole to
a southern latitude for about 37 minutes. On the next orbit, mapping will
begin 4.7 minutes later, and stop 4.7 minutes further south. Magellan’s
orbit and the time taken to transmit data to Earth make it impossible, however,
to cover both poles completely during one 243-day mapping cycle. Because
the Ishtar Terra region, situated at high northerly latitudes, is very important
geologically, scientists decided to map the northern hemisphere completely.
Mapping of the southern hemisphere will extend to about latitude 70Degree
south.

Magellan arrives at Venus about three months before the planet passes
behind the Sun as viewed from Earth. At this time, radio interference from
the Sun will make it impossible to communicate with the spacecraft and conduct
radar mapping. After one mapping cycle, Magellan will have acquired data
from 70 to 90 per cent of the planet’s surface, and fulfilled its nominal
mission, which ends on 28 April 1991. The gap in coverage in early November
1990 can be filled during early July 1991, in the subsequent 243-day cycle.
The spacecraft has only a limited lifetime, because of the finite amount
of fuel for the thrusters that control its orientation. When this is exhausted,
ground controllers will no longer be able to point the craft precisely and
the mission will end. Magellan will use its thruster fuel at a fairly low
rate, however, if it accurately enters the correct orbit around Venus in
the first place, and if a second system for controlling its orientation
– a set of momentum wheels that spin around in the opposite direction to
the spacecraft’s motion – function as intended. In this case, the nominal
mission length of one 243-day mapping cycle may be extended by six to nine
cycles or even more. In this additional time, Magellan could ‘spotlight’
particularly interesting surface features and view them from several angles
to provide more detailed information.

ÐÓ°ÉÔ­´´s are looking to the data from Magellan to answer some important
long-standing questions about the geological history of our planetary neighbour.
The upper crust of the Earth is divided into irregular, flat tectonic plates
that move around the planet’s surface, driven by convection in the hot,
fluid rocks beneath the crust. Where plates collide, mountain ranges are
thrust upwards, and where they pull apart, rift valleys and ocean basins
form. We would like to know whether similar tectonic plate movements have
shaped the surface of Venus. No conspicuous plate boundaries were visible
in the map from the Pioneer Venus Orbiter, suggesting that if a system of
plate tectonics does exist, it must be a different kind from the one on
Earth. Evidence of plates would be only marginally visible, however, at
the resolution of the radar images obtained by Pioneer Venus. Also, the
limited coverage that Venera 15 and 16 achieved may not have been extensive
enough to reveal a global pattern of tectonic plates.

Magellan will also provide detailed information about the volcanic craters
and other volcanic features on Venus, so that scientists can reconstruct
the planet’s geological history. Venus may even still be volcanically active,
so by counting the volcanoes and identifying where and what kind they are,
Magellan will provide data on the planet’s internal processes. High-resolution
images of individual lava flows will also help todetermine the sequence
of volcanic events that shaped the surface. From the slopes and shapes of
the volcanic flows, geologists can assess the composition of the lava and
obtain further clues about the planet’s interior and the thickness of the
crust. The high-resolution survey of Venus’s gravity that Magellan will
make will show any correlation between the gravitational field and surface
topography. Earlier data indicated that the gravitational field is stronger
above the planet’s elevated plateaux – evidence that these features are
related to internal structure.

Magellan’s global inventory of impact craters will have much to tell
scientists about the history of the planet and the ages of different geological
regions. The rate at which craters formed may also reveal how the density
of the planet’s atmosphere has varied throughout the planet’s history. The
present radar maps of Venus do not reveal whether the circular features
are the scars of old impacts, collapsed volcanic craters or domes of rock
warped upwards by tectonic forces. Large stretches of old, cratered terrain
would argue against crustal motion in these areas, and would indicate that
erosion processes proceed more slowly on Venus than on Earth. Some kind
of compression or extension of the crust evidently occurs on Venus, forming
the folded ridges and rift-like valleys revealed in the Venera radar images.
Studying large rift valleys, such as Diana Chasma in Aphrodite Terra, will
reveal if they were formed by tectonic motions or by volcanic processes.

Another critical question is whether Venus once had water on its surface.
Measurements of the ratio of deuterium to hydrogen in the atmosphere of
Venus suggest that in the past there was more water in the planet’s atmosphere.
Some scientists have suggested that the atmosphere was once very much thinner
than today, before the runaway greenhouse effect took control, and the surface
temperatures correspondingly lower. At that time, there may have been large
oceans of liquid water on the surface of Venus. Magellan will look for evidence
of ancient marine terraces, river beds and deltas or other features that
would point towards the past existence of running water on the planet. Such
discoveries would have profound implications for the evolution of the planet’s
atmosphere as well as its surface. Magellan will also look for large wind-blown
dunes on the surface; the sizes and shapes of these would provide clues
about the direction and the strength of the winds on Venus.

In just 243 days, Magellan will map most of the surface of Venus with
detail that far exceeds that of the best radar images previously obtained.
The resulting maps will reveal the traces, if they exist, of a whole range
of planetary processes: volcanism, wind, water and meteorite impacts – in
short all the important forces that shape an evolving planet’s surface.
By giving us this new information, Magellan may help to tell us why Venus,
in some ways our planetary twin, is at the same time such a complete stranger.

John Mason is a freelance writer and a scientific and technical consultant
specialising in space issues. He wishes to acknowledge the help of the Public
Information Office at NASA’s Jet Propulsion Laboratory.

* * *

HOW MAGELLAN’S RADAR WORKS

RADAR, developed for military purposes during the Second World War,
is an acronym for RAdio Detection And Ranging. Radar imaging devices are
‘active’ remote-sensing systems in that they provide their own source of
energy. With conventional radar, the resolution of the image depends on
the size of the antenna: the bigger the antenna, the smaller the details
that it can resolve. A large antenna on board a spacecraft, however, would
be both expensive and difficult to manipulate. Instead, the signals from
Magellan’s synthetic aperture radar (SAR) system will be processed by computer
on Earth so that they will imitate, or synthesise, the behaviour of a large
antenna on the spacecraft. By this means, the onboard radar system will
operate as though it had a huge antenna, hundreds of metres long, yielding
very high-resolution images, even though the dish antenna is only 3.7 metres
in diameter. This computerised process, known as ‘aperture synthesis’, is
what gives SAR its high resolving power as well as its name.

As Magellan passes above the Venusian surface, its antenna will look
downwards and to the left side of the spacecraft’s orbital path. The radar
beam will thus ‘illuminate’ the surface of the planet along a strip that
will be perpendicular and to one side of the spacecraft. The radar images
will, therefore, be built up from a series of successive, slightly overlapping
strips, which have to be pieced together like a mosaic. For a period of
37.2 minutes in each orbit, during which Magellan gathers data, the SAR
will emit several thousand radar pulses each second. Travelling at the speed
of light, these pulses of radar energy will strike and illuminate a swathe
of the planet’s surface 25 kilometres wide. They will immediately bounce
back and be received at the instrument. The radar system will measure the
strength of the received signals (brightness), how long each signal took
to make the round trip (the range to the target point and back), and changes
in the signal frequency (pitch) of the signals caused by the doppler effect,
resulting from the spacecraft’s motion over the surface.

By recording the range and change of pitch for each returned radar pulse,
it is possible to locate each point on the planet’s surface precisely. The
time taken for the radar signal to return to Magellan gives the distance
or range of the spacecraft from that point. The doppler shift gives the
location of the point with reference to the spacecraft’s line of flight,
because Magellan will be either approaching or receding from the point at
any given time. Because each point in the radar image will have a unique
range and shift in frequency, these two coordinates, together with knowledge
of the antenna’s line-of-sight with respect to the surface, are all that
is needed to determine the location of any returned signal. The brightness
of the image at that point then becomes a single picture element of the
two- dimensional image of the map.

Using this technique, radar data acquired by Magellan will be transmitted
back to Earth, where complete high-resolution images of the venusian surface
will be constructed by computers at NASA’s Jet Propulsion Laboratory in
Pasadena, L California. In these images we will be able to distinguish features
as small as 250 metres across in the equatorial regions (where Magellan
will pass closest to the planet) and about 750 metres across near the poles.
The best existing ground-based and spacecraft radar maps of Venus show no
features smaller than about 2 kilometres across.

Part of the Magellan SAR is its altimeter, which sends radio signals
straight down and receives them back again to determine the altitude of
features directly below the spacecraft. It uses a separate, smaller fan-beam
horn antenna attached to the dish antenna. Magellan will eventually produce
a topographic map showing height variations as small as 30 metres for the
entire mapped portion of the planet.

* * *

THE MAGELLAN SPACECRAFT

A KEY feature of the Magellan spacecraft, built by Martin Marietta in
Denver, Colorado, is the economy and relative simplicity of its design and
construction. Engineers have assembled it largely of spare parts from the
Voyager, Galileo, Viking and Ulysses spacecraft to save costs. The spacecraft,
which weighs 3.45 tonnes, carries just one scientific instrument, a radar
system of innovative design, constructed by the Hughes Aircraft Company.
It will perform three functions: it will collect imaging data of the surface
of Venus, acquire altimetric data of the planet’s topography, and measure
the thermal emissions from the planet’s surface.

Magellan is stabilised on three axes and has two types of moving parts:
solar panels to provide electrical power and three gyroscopic momentum wheels
to control the attitude (orientation) of the spacecraft. The pair of solar
panels, 3.5 metres square, that collect the solar energy for charging the
spacecraft’s two nickel-cadmium batteries will always point towards the
Sun, and will provide over a kilowatt of electricity during the mapping
phase of the mission. The batteries will provide backup power when the Sun
is temporarily hidden behind Venus.

Precise control of the orientation of the spacecraft will be crucial
for the success of the mission. While mapping, Magellan’s dish antenna (used
both for radar imaging and radiometry and for communicating with the Earth)
must be pointed towards Venus. While transmitting data, the spacecraft must
point the same antenna towards Earth. Small deviations in the orientation
of the spacecraft can result in the loss of data. The spacecraft’s attitude
will be controlled by three momentum wheels, which will act as gyroscopes,
maintaining the correct L orientation of Magellan relative to the planet.
These will be assisted, when necessary, by 12 small gas thrusters, which
will be fired to turn the spacecraft and release the excess speed that builds
up in the momentum wheels.