THE LEGACY OF MARINER
If intelligent life had existed on Venus on the afternoon of the Earth’s
December 14, 1962, and if it could have seen through the clouds, it
might have observed Mariner II approach from the night side, drift down
closer, cross over to the daylight face, and move away toward the Sun to
the right. The time was the equivalent of 12:34 p.m. along the Pacific
Coast of the United States, where the spacecraft was being tracked.
Mariner II had reached the climax of its 180-million-mile, 109-day trip
through space. The 35-minute encounter with Venus would tell Earth
scientists more about our sister planet than they had been able to learn
during all the preceding centuries.
SPACE WITHOUT DUST?
Before Mariner, scientists theorized about the existence of clouds of
cosmic dust around the Sun. A knowledge of the composition, origin, and
the dynamics of these minute particles is necessary for study of the
origins and evolution of the solar system.
Tiny particles of cosmic dust (some with masses as low as 1.3 × 10⁻¹⁰
gram or about one-trillionth of a pound) were thought to be present in
the solar system and have been recorded by satellites in the near-Earth
regions.
These microcosmic particles could be either the residue left over after
our solar system was formed some 5 billion years ago, possibly by
condensation of huge masses of gas and dust clouds; or, the debris
deposited within our system by the far-flung and decaying tails of
passing comets; or, the dust trapped from galactic space by the magnetic
fields of the Sun and the planets.
Analysis of the more than 1,700 hours of cosmic dust detector data
recovered from the flight of Mariner II seems to indicate that in the
region between the Earth and Venus the concentration of tiny cosmic dust
particles is some ten-thousand times less than that observed near the
Earth.
During the 129 days (including the post-encounter period) of Mariner’s
mission, the data showed only one dust particle impact which occurred in
deep space and not near Venus. Equivalent experiments near Earth (on
board Earth satellites) have yielded over 3,700 such impacts within
periods of approximately 500 hours. The cause of this heavy near-Earth
concentration, the exact types of particles, and their source are still
unknown.
The cosmic dust experiment performed well during the Mariner mission.
Although some calibration difficulty was observed about two weeks before
the Venus encounter, possibly caused by overheating of the sensor
crystal, there was no apparent effect in the electronic circuits.
THE UBIQUITOUS SOLAR WIND
For some time prior to Mariner, scientists postulated the existence of a
so-called plasma flow or “solar wind” streaming out from the Sun, to
explain the motion of comet tails (which always point away from the Sun,
perhaps repelled by the plasma), geomagnetic storms, aurorae, and other
such disturbances. (Plasma is defined as a gas in which the atoms are
dissociated into atomic nuclei and electrons, but which, as a whole, is
electrically neutral.)
The solar wind was thought to drastically alter the configuration of the
Sun’s external magnetic field. Plasma moving at extreme velocities is
able to carry with it the lines of magnetic force originating in the
Sun’s corona and to distort any fields it encounters as it moves out
from the Sun.
It was believed that these moving plasma currents are also capable of
altering the size of a planet’s field of magnetic flux. When this
happens, the field on the sunlit face of the planet is compressed and
the dark side has an elongated expansion of the field. For example, the
outer boundary of the Earth’s magnetic field is pushed in by the solar
wind to about 40,000 miles from the Earth on the sunward side. On the
dark side, the field extends out much farther.
The solar wind was also known to have an apparent effect on the movement
of cosmic rays. As the Sun spots increase in the regular 11-year cycle,
the number of cosmic rays reaching the Earth from outside our solar
system will decrease.
Mariner II found that streams of plasma are constantly flowing out from
the Sun. This fluctuating, extremely tenuous solar wind seems to
dominate interplanetary space in our region of the solar system. The
wind moves at velocities varying from about 200 to 500 miles per second
(about 720,000 to 1,800,000 miles per hour), and measures up to perhaps
a million degrees Fahrenheit (within the subatomic structure).
With the solar plasma spectrometer working at ten different energy
levels, Mariner required 3.7 minutes to run through a complete energy
spectrum. During the 123 days, when readings were made, a total of
40,000 such spectra were recorded. Plasma was monitored on 104 of those
123 days, and on every one of the spectra, the plasma was always
present.
Mariner showed that the energies of the particles in the solar winds are
very low, on the order of a few hundred or few thousand electron volts,
as compared with the billions and trillions of electron volts measured
in cosmic radiation.
The extreme tenuousity or low density of the solar wind is difficult to
comprehend: about 10 to 20 protons (hydrogen nuclei) and electrons per
cubic inch. But despite the low energy and density, solar wind particles
in our region of the solar system are billions of times more numerous
than cosmic rays and, therefore, the total energy content of the winds
is much greater than that of the cosmic rays.
Mariner found that when the surface of the Sun was relatively inactive,
the velocity of the wind was a little less than 250 miles per second and
the temperature a few hundred thousand degrees. The plasma was always
present, but the density and the velocity varied. Flare activity on the
Sun seemed to eject clouds of plasma, greatly increasing the velocity
and density of the winds. Where the particles were protons, their
energies ranged from 750 to 2,500 electron volts.
The experiment also showed that the velocity of the plasma apparently
undergoes frequent fluctuations of this type. On approximately twenty
occasions, the velocity increased within a day or two by 20 to 100%.
These disturbances seemed to correlate well with magnetic storms
observed on the Earth. In several cases, the sudden increase in the
solar plasma flux preceded various geomagnetic effects observed on the
Earth by only a short time.
The Mariner solar plasma experiment was the first extensive measurement
of the intensity and velocity spectrum of solar plasma taken far enough
from the Earth’s field so that the Earth would have no effect on the
results.
HIGH-ENERGY PARTICLES: FATAL DOSAGE?
Speculation has long existed as to the amount of high-energy radiation
(from cosmic rays and particles from the Sun with energies in the
millions of electron volts) present within our solar system and as to
whether exposure would be fatal to a human space traveler.
This high-energy type of ionizing radiation is thought to consist of the
nuclei of such atoms as hydrogen and helium, and of electrons, all
moving very rapidly. The individual particles are energetic enough to
penetrate considerable amounts of matter. The concentration of these
particles is apparently much lower than that of low-energy plasma.
The experiments were designed to detect three types of high-energy
radiation particles: the cosmic rays coming from outside the solar
system, solar flare particles, and radiation trapped around Venus (as
that which is found in the Earth’s Van Allen Belt).
These high-energy radiation particles (also thought to affect aurorae
and radio blackouts on the Earth) measure from about one hundred
thousand electron volts up to billions of volts. The distribution of
this energy is thought to be uniform outside the solar system and is
assumed to move in all directions in a pattern remaining essentially
constant over thousands of years.
Inside the solar system, the amount of such radiation reaching the Earth
is apparently controlled by the magnetic fields found in interplanetary
space and near the Earth.
The number of cosmic rays changes by a large amount over the course of
an 11-year Sun-spot cycle, and below a certain energy level (5,000 Mev)
few cosmic rays are present in the solar system. They are probably
deflected by plasma currents or magnetic fields.
Mariner’s charged particles experiment indicated that cosmic radiation
(bombardment by cosmic rays), both from galactic space and those
particles originating in the Sun, would not have been fatal to an
astronaut, at least during the four-month period of Mariner’s mission.
The accumulated radiation inside the counters was only 3 roentgens, and
during the one solar storm recorded on October 23 and 24, the dosage
measured only about ¼ roentgen. In other words, the dosage amounts to
about one-thousandth of the usually accepted “half-lethal” dosage, or
that level at which half of the persons exposed would die. An astronaut
might accept many times the dosage detected by Mariner II without
serious effects.
The experiment also showed little variation in density of charged
particles during the trip, even with a 30% decrease in distance from the
Sun, and no apparent increase due to magnetically trapped particles or
radiation belts near Venus as compared with interplanetary space.
However, these measurements were made during a period when the Sun was
slowly decreasing in activity at the end of an 11-year cycle. The Sun
spots will be at a minimum in 1964-1965, when galactic cosmic rays will
sharply increase. Further experiments are needed to sample the charged
particles in space under all conditions.
The lack of change measured by the ionization chamber during the mission
was significant; the cosmic-ray flux of approximately 3 particles per
square centimeter per second throughout the flight was an unusually
constant value. A clear increase in high-energy particles (10 Mev to
about 800 Mev) emitted by the Sun was noted only once: a flare-up
between 7:42 and 8:45 a.m., PST, October 23. The ionization chamber
reading began to increase before the flare disappeared. From a
background reading of 670 ion pairs per cubic centimeter per second per
standard atmosphere, it went to a peak value of 18,000, varied a bit,
and remained above 10,000 for 6 hours before gradually decreasing over a
period of several days. Meanwhile, the flux of the particles detected by
the Geiger counter rose from the background count of 3 to a peak of 16
per square centimeter per second. Ionization thus increased much more
than the number of particles, indicating to the scientists that the
high-energy particles coming from the Sun might have had much lower
average energies than the galactic cosmic rays.
[Illustration: _Data obtained by microwave radiometer are
illustrated at left; results of infrared radiometer experiment are
shown at right. Note how moving spacecraft sees more of atmosphere
along limb or edge of planet, less in center._]
In contrast, the low-energy experiment detected the October 23 event,
and eight or ten others not seen by the high-energy detectors. These
must have been low-penetrating particles excluded by the thicker walls
of the high-energy instrument. These particles were perhaps protons
between 0.5 and 10 Mev or electrons between 0.04 and 0.5 Mev.
At 20,000 miles from the Earth, the rate at which high-energy particles
have been observed has been recorded at several thousand per second.
With Mariner at approximately the same distance from Venus, the average
was only one particle per second, as it had been during most of the
month of November in space. Such a rate would indicate a low planetary
magnetic field, or one that did not extend out as far as Mariner’s
21,598-mile closest approach to the surface.
Mariner II measured and transmitted data in unprecedented quantity and
quality during the long trip. In summary, Mariner showed that, during
the measuring period, particles were numerous in the energy ranges from
a few hundred to 1,000 electron volts. Protons in the range 0.5 to 10
Mev were not numerous, but at times the flux (density) was several times
that of cosmic rays.
Almost no protons were shown in the 10 to 800 Mev range, except during
solar flares when the particles in this range were numerous. Above 800
Mev (primarily those cosmic rays entering interplanetary space from
outside the solar system) the number decreased rapidly as the energy
increased, the average total being about 3 per centimeter per second.
During one 30-day period in November and December, the low-energy
counter saw only two small increases in radiation intensity. At this
time, the mean velocity of the solar wind was considerably lower than
during September and October. This might suggest that high-velocity
plasma and low-energy cosmic rays might both originate from the same
solar source.
A MAGNETIC FIELD?
Prior to the Mariner II mission, no conclusive evidence had ever been
presented concerning a Venusian magnetic field and nothing was known
about possible fluid motions in a molten core or other hypotheses
concerning the interior of the planet.
Scientists assumed that Venus had a field somewhat similar to the
Earth’s, although possibly reduced in magnitude because of the
apparently slow rate of rotation and the pressure of solar plasma. Many
questions had also been raised concerning the nature of the atmosphere,
charged particles in the vicinity of the planet, magnetic storms, and
aurorae. Good magnetometer data from Mariner II would help solve some of
these problems.
Mariner’s magnetometer experiment also sought verification of the
existence and nature of a steady magnetic field in interplanetary space.
This would be important in understanding the charged particle balance of
the inner solar system. Other objectives of the experiment were to
establish both the direction and the magnitude of long-period
fluctuations in the interplanetary magnetic field and to study solar
disturbances and such problems in magnetohydrodynamics (the study of the
motion of charged particles and their surrounding magnetic fields) as
the existence and effect of magnetized and charged plasmas in space.
The strength of a planet’s field is thought to be closely related to its
rate of rotation—the slower the rotation, the weaker the field. As a
consequence, if Venus’ field is simple in structure like the Earth’s,
the surface field should be 5 to 10% that of the Earth. If the structure
of the field is complex, the surface field in places might exceed the
Earth’s without increasing the field along Mariner’s trajectory to
observable values.
Most of the phenomena associated with the Earth’s magnetic field are
likely to be significantly modified or completely absent in and around
Venus. Auroral displays and the trapping of charged particles in
radiation belts such as our Van Allen would be missing. The field of the
Earth keeps low- and moderate-energy cosmic rays away from the top of
the atmosphere, except in the polar regions. The cosmic ray flux at the
top of Venus’ atmosphere is likely to correspond everywhere to the high
level found at the Earth’s poles.
[Illustration: _As it encountered Venus, Mariner II made three scans
of the planet._]
SUN
DIRECTION OF SCAN
DATA READINGS (18 TOTAL)
In contrast to Venus, Jupiter, which is ten times larger in mass and
volume and rotates twice as fast as the Earth, has a field considerably
stronger than the Earth’s. The Moon has a field on the sunlit side
(according to Russian measurements) which, because of the Moon’s slow
rotation rate, is less than ⅓ of 1% of the Earth’s at the Equator. Thus,
a planet’s rotation, if at a less rapid rate than the Earth’s, seems to
produce smaller magnetic fields. This theory is consistent with the idea
of a planetary magnetic field resulting from the dynamo action inside
the molten core of a rotating planet.
The Sun, on the whole, has a fairly regular dipole field. Superimposed
on this are some very large fields associated with disturbed regions
such as spots or flares, which produce fields of very great intensities.
These solar fields are drawn out into space by plasma flow. Although
relatively small in magnitude, these fields are an important influence
on the propagation of particles. And the areas in question are very
large—something on the order of an astronomical unit.
Mariner II seemed to show that, in space, a generally quiet
magnetic-field condition was found to exist, measuring something less
than 10 gamma and fluctuating over periods of 1 second to 1 minute.
As Mariner made its closest approach to Venus, the magnetometer saw no
significant change, a condition also noted by the radiation and solar
plasma detectors. The magnetic field data looked essentially as they had
in interplanetary space, without either fluctuations or smooth changes.
The encounter produced no slow changes, nor was there a continuous
fluctuation as in the interplanetary regions. There was no indication of
trapped particles or near-Venus modification in the flow of solar
plasma.
On the Earth’s sunny side, a definite magnetic field exists out to
40,000 miles, and on the side away from the Sun considerably farther. If
Venus’ field had been similar to the Earth’s, a reading of 100 to 200
gamma, a large cosmic-ray count, and an absence of solar plasma should
have been shown, but none of these phenomena were noted by Mariner.
These results do not prove that Venus definitely has no magnetic field,
but only that it was not measurable at Mariner’s 21,598-mile point of
closest approach. The slow rotation rate and the pressure of the solar
winds probably combine to limit the field to a value one tenth of the
Earth’s. Since Mariner passed Venus on the sunlit side, readings are
required on the dark side in order to confirm the condition of the
magnetic field on that side of the planet, which normally should be
considerably extended.
THE SURFACE: HOW HOT?
Before Mariner, scientists had offered two main theories about the
surface of Venus: It had either an electrically charged ionosphere
causing false high-temperature readings on Earth instruments despite a
cool surface, or a hot surface with clouds becoming increasingly colder
with altitude.
The cool-surface theory supposed an ionosphere with a layer of electrons
having a density thousands of times that of the Earth’s upper
atmosphere. Microwave radiations from this electrical layer would cause
misleading readings on Earth instruments. As a space probe scanned
across such an atmosphere, it would see the least amount of charged
ionosphere when looking straight down, and the most concentrated amount
while scanning the limb or edge. In the latter case, it would be at an
angle and would show essentially a thickening effect of the atmosphere
because of the curvature of the planet.
As the probe approached the edge, the phenomenon known as “limb
brightening” would occur, since the instruments would see more of the
electron-charged ionosphere and little if any of the cooler surface. The
temperature readings would, therefore, be correspondingly higher at the
limbs.
The other theory, held by most scientists, visualized a hot surface on
Venus, with no heavy concentration of electrons in the atmosphere, but
with cooler clouds at higher altitudes. Thus, the spacecraft would look
at a very hot planet from space, covered by colder, thick clouds.
Straight down, the microwave radiometer would see the hot surface
through the clouds. When approaching the limb, the radiations would
encounter a thicker concentration of atmosphere and might not see any of
the hot surface. This condition, “limb darkening,” would be
characterized by temperatures decreasing as the edges of the planet were
approached.
An instrument capability or resolution much higher than that available
from the Earth was required to resolve the limb-brightening or
limb-darkening controversy. Mariner’s radiometer would be able to
provide something like one hundred times better resolution than the
Earth-based measurements.
At 11:59 a.m., PST, on December 14, 1962, Mariner’s radiometers began to
scan the planet Venus in a nodding motion at a rate of 0.1 degree per
second and reaching an angular sweep of nominally 120 degrees. The
radiometers had been switched on 6½ hours before the encounter with
Venus and they continued to operate for another hour afterward.
The microwave radiometer looked at Venus at a wavelength of 13.5
millimeters and 19 millimeters. The 13.5-millimeter region was the
location of a microwave water absorption band within the electromagnetic
spectrum, but it was not anticipated that it would detect any water
vapor on Venus. These measurements would allow determination of
atmospheric radiation, averaging the hot temperatures near the surface,
the warmer clouds at lower levels, and the lower temperatures found in
the high atmosphere. If the atmosphere were a strong absorber of
microwave energy at 13.5 millimeters, only the temperature of the upper
layers would be reported.
Unaffected by water vapor, 19-millimeter radiations could be detected
from deeper down into the cloud cover, perhaps from near or at the
planet’s surface. Large temperature differences between the 19- and 13.5
millimeter readings would indicate the relative amount of water vapor
present in the atmosphere. The 19-millimeter radiations would also test
the limb-brightening theory.
During its scanning operation, Mariner telemetered back to Earth about
18 digital data points, represented as voltage fluctuations in relation
to time. The first scan was on the dark side, going up on the planet:
the distance from the surface was 16,479 miles at midscan, and the
brightness temperature was 369 degrees F. The second scan nearly
paralleled the terminator (junction of light and dark sides) but crossed
it going down; it was made from 14,957 miles at midscan and showed a
temperature of 566 degrees F. The final scan, 13,776 miles at midpoint,
showed 261 degrees F as it swept across the sunlit side of Venus in an
upward direction.
The brightness temperature recorded by Mariner’s radiometer is not the
true temperature of the surface. It is derived from the amount of light
or radio energy reflected or emitted by an object. If the object is not
a perfect light emitter, as most are not, then the light and radio
energy will be some fraction of that returned from a 100% efficient
body, and the object is really hotter than the brightness measurement
shows. Thus, the brightness temperature is a minimum reading and in this
case, was lower than the actual surface temperature.
Mariner’s microwave radiometer showed no significant difference between
the light and dark sides of Venus and, importantly, higher temperatures
along the terminator or night-and-day line of the planet. These results
would indicate no ionosphere supercharged with electrons, but a definite
limb-darkening effect, since the edges were cooler than the center of
the planet.
Therefore, considering the absorption characteristics of the atmosphere
and the emissivity factor derived from earlier JPL radar experiments, a
fairly uniform 800 degrees F was estimated as a preliminary temperature
figure for the entire surface.
Venus is, indeed, a very hot planet.
CLOUD TEMPERATURES: THE INFRARED READINGS
Mariner II took a close look at Venus’ clouds with its infrared
radiometer during its 35-minute encounter with the planet. This
instrument was firmly attached to the microwave radiometer so the two
devices would scan the same areas of Venus at the same rate and the data
would be closely correlated. This arrangement was necessary to produce
in effect a stereoscopic view of the planet from two different regions
of the spectrum.
Because astronomers have long conjectured about the irregular dark spots
discernible on the surface of Venus’ atmosphere, data to resolve these
questions would be of great scientific interest. If the spots were
indeed breaks in the clouds, they would stand out with much better
definition in the infrared spectrum. If the radiation came from the
cloud tops, there would be no breaks and the temperatures at both
frequencies measured by the infrared radiometer would follow essentially
the same pattern.
The Venusian atmosphere is transparent to the 8-micron region of the
spectrum except for clouds. In the 10-micron range, the lower atmosphere
would be hidden by carbon dioxide. If cloud breaks existed, the 8-micron
emissions would come from a much lower point, since the lower atmosphere
is fairly transparent at this wavelength. If increasing temperatures
were shown in this region, it might mean that some radiation was coming
up from the surface.
As a result of the Mariner II mission, scientists have hypothecated that
the cold cloud cover could be about 15 miles thick, with the lower base
beginning about 45 miles above the surface, and the top occurring at 60
miles. In this case, the bottom of the cloud layer could be
approximately 200 degrees F; at the top, the readings vary from about
minus 30 degrees F in the center of the planet to temperatures of
perhaps minus 60 degrees to minus 70 degrees F along the edges. This
temperature gradient would verify the limb-darkening effect seen by the
microwave radiometer.
At the center of Venus, the radiometer saw a thicker, brighter, hotter
part of the cloud layer; at the limbs, it could not see so deeply and
the colder upper layers were visible. Furthermore, the temperatures
along the cloud tops were approximately equally distributed, indicating
that both 8- and 10-micron “channels” penetrated to the same depth and
that both were looking at thick, dense clouds quite opaque to infrared
radiation.
Both channels detected a curious feature along the lower portion of the
terminator, or the center line between the night and day sides of the
planet. In that region, a spot was shown that was apparently about 20
degrees F colder than the rest of the cloud layer. Such an anomaly could
result from higher or more opaque clouds, or from such an irregularity
as a hidden surface feature. A mountain could force the clouds upward,
thus cooling them further, but it would have to be extremely high.
The data allow scientists to deduce that not enough carbon dioxide was
present above the clouds for appreciable absorption in the 10-micron
region. This effect would seem to indicate that the clouds are thick and
that there is little radiation coming up from the surface. And, if
present, water vapor content might be 1/1,000 of that in the Earth’s
atmosphere.
Since the cloud base is apparently at a very high temperature, neither
carbon dioxide nor water is likely to be present in quantity. Rather,
the base of the clouds must contain some component that will condense in
small quantities and not be spectroscopically detected.
As a result of the two radiometer experiments, the region below the
clouds and the surface itself take on better definition. Certainly,
heat-trapping of infrared radiation, or a “greenhouse” effect, must be
expected to support the 800 degree F surface temperature estimated from
the microwave radiometer data. Thus, a considerable amount of
energy-blanketing carbon dioxide must be present below the cloud base.
It is thought that some of the near-infrared sunlight might filter
through the clouds in small amounts, so that the sky would not be
entirely black, at least to human eyes, on the sunlit side of Venus.
There also may be some very small content of oxygen below the clouds,
and perhaps considerable amounts of nitrogen.
The atmospheric pressure on the surface might be very high, about 20
times the Earth’s atmosphere or more (equivalent to about 600 inches of
mercury, compared with our 30 inches). The surface, despite the high
temperature, is not likely to be molten because of the roughness index
seen in the earlier radar experiments, and other indicators. However,
the possibility of small molten metal lakes cannot be totally ignored.
The dense, high-pressure atmosphere and the heat-capturing greenhouse
effect could combine over long periods of time to carry the extremely
high temperature around to the dark side of Venus, despite the slow rate
of rotation, possibly accounting for the relatively uniform surface
temperatures apparently found by Mariner II.
THE RADAR PROFILE: MEASUREMENTS FROM EARTH
In 1961, the Jet Propulsion Laboratory conducted a series of experiments
from its Goldstone, California, DSIF Station, successfully bouncing
radar signals off the planet Venus and receiving the return signal after
it had travelled 70 million miles in 6½ minutes.
In order to complement the Mariner mission to Venus, the radar
experiments were repeated from October to December, 1962 (during the
Mariner mission), using improved equipment and refined techniques. As in
1961, the experiments were directed by W. K. Victor and R. Stevens.
The 1961 experiments used two 85-foot antennas, one transmitting 13
kilowatts of power at 2,388 megacycles, the other receiving the return
signal after the round trip to and from Venus. The most important result
was the refinement of the astronomical unit—the mean distance from the
Earth to the Sun—to a value of 92,956,200 ±300 miles.
Around 1910, the astronomical unit, plotted by classical optical
methods, was uncertain to 250,000 miles. Before the introduction of
radar astronomy techniques such as those used at Goldstone, scientists
believed that the astronomical unit was known to within 60,000 miles,
but even this factor of uncertainty would be intolerable for planetary
exploration.
In radar astronomy, the transit time of a radio signal moving at the
speed of light (186,000 miles per second) is measured as it travels to a
planet and back. In conjunction with the angular measurement techniques
used by earlier investigators, this method permits a more precise
calculation of the astronomical unit.
Optical and radar measurements of the astronomical unit differ by 50,000
miles. Further refinement of both techniques should lessen the
discrepancy between the two values.
The 1961 tests also established that Venus rotates at a very slow rate,
possibly keeping the same face toward the Sun at all times. The
reflection coefficient of the planet was estimated at 12%, a bright
value similar to that of the Earth and contrasted with the Moon’s 2%.
The average dielectric constant (conductivity factor) of the surface
material seemed to be close to that of sand or dust, and the surface was
reported to be rough at a wavelength of 6 inches.
The surface roughness was confirmed in 1962. Since it is known that a
rough surface will scatter a signal, the radar tests were observed for
such indications. Venus had a scattering effect on the radar waves
similar to the Moon’s, probably establishing the roughness of the
Venusian surface as more or less similar to the lunar terrain.
Some of the most interesting work was done in reference to the rotation
rate of Venus. A radar signal will spread in frequency on return from a
target planet that is rotating and rough enough to reflect from a
considerable area of its surface. The spread of 5 to 10 cycles per
second noted on the Venus echo would suggest a very slow rotation rate,
perhaps keeping the same face toward the Sun, or possibly even in a
retrograde direction, opposite to the Earth’s.
In the Goldstone 1962 experiments, Venus was in effect divided into
observation zones and the doppler effect or change in the returned
signal from these zones was studied. The rate of rotation was derived
from three months of sampling with this radar mapping technique. Also,
the clear, sharp tone characteristic of the transmitted radar signal was
altered on return from Venus into a fuzzy, indistinct sound. This effect
seemed to confirm the slow retrograde rotation (as compared with the
Earth) indicated by the radar mapping and frequency change method.
In addition to these methods of deducing the slow rotation rate, two
other phenomena seemed to verify it: a slowly fluctuating signal
strength, and the apparent progression of a bright radar spot across
from the center of Venus toward the outside edge.
As a result, JPL scientists revised their 1961 estimate of an equal
Venusian day and year consisting of 225 Earth days. The new value for
Venus’ rotation rate around its axis is 230 Earth days plus or minus 40
to 50 days, and in a retrograde direction (opposite to synchronous or
Sun-facing), assuming that Venus rotates on an axis perpendicular to the
plane of its orbit.
Thus, on Venus the Sun would appear to rise in the west and cross to the
east about once each Venusian year. If the period were exactly 225 days
retrograde, the stars would remain stationary in the sky and Venus would
always face a given star rather than the Sun.
A space traveller hovering several million miles directly above the Sun
would thus see Venus as almost stopped in its rotation and possibly
turning very slowly clockwise. All the other planets of our system
including the Earth, rotate counterclockwise, except Uranus, whose axis
is almost parallel to the plane of its orbit, making it seem to roll
around the Sun on its side. The rotation direction of distant Pluto is
unknown.
The Goldstone experiments also studied what is known as the Faraday
rotation of the plane of polarization of a radio wave. The results
indicated that the ionization and magnetic field around Venus are very
low. These data tend to confirm those gathered by Mariner’s experiments
close to the planet.
The mass of Venus was another value that had never been precisely
established. The mass of planetary bodies is determined by their
gravitational effect on other bodies, such as satellites. Since Venus
has no known natural satellite or moon, Mariner, approaching closely
enough to “feel” its gravity, would provide the first opportunity for
close measurement.
The distortion caused by Venus on Mariner’s trajectory as the spacecraft
passed the planet enabled scientists to calculate the mass with an error
probability of 0.015%. The value arrived at is 0.81485 of the mass of
the Earth, which is known to be approximately 13.173 septillion
(13,173,000,000,000,000,000,000,000) pounds. Thus, the mass of Venus is
approximately 10.729 septillion (10,729,408,500,000,000,000,000,000)
pounds.
In addition to these measurements, the extremely precise tracking system
used on Mariner proved the feasibility of long-range tracking in space,
particularly in radial velocity, which was measured to within 1/10 of an
inch per second at a distance of about 54 million miles.
As the mission progressed, the trajectory was corrected with respect to
Venus to within 10 miles at encounter. An interesting result was the
very precise location of the Goldstone and overseas tracking stations of
the DSIF. Before Mariner II, these locations were known to within 100
yards. After all the data have been analyzed, these locations will be
redefined or “relocated” to within an error of only 20 yards.
Mariner not only made the first successful journey to Venus—it also
helped pinpoint spots in the Californian and Australian deserts and the
South African veldt with an accuracy never before achieved.