Jet Propulsion Laboratory

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?¹ 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.

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.

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 ? 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.