AS MARINER 10 BORE DOWN on the planet Venus, the brilliant jewel scintillated in the clear sky of the Mojave desert, where the Goldstone antenna pointed eastward to pick up signals from the spacecraft. When Mariner 10 was acquired by the 64-m (210-ft) radio antenna, it was about 45 million km (28 million mi) from Earth, approaching Venus from the dark side, its cameras unable yet to photograph the cloud-shrouded planet. At 9:21 a.m. PDT on February 5, 1974, Mariner started to take photographs, but its cameras were still pointed toward space; the first pictures displayed on the screen at JPL were blank.
About 8000 km (5000 mi) from Venus, Mariner 10's television cameras took the first picture of the planet, and shortly after 9:50 a.m. PDT this picture was displayed on the monitor screens. The photo showed the lighted cusp of Venus at the north pole (Fig. 6-1) just 12 min before Mariner 10 made its closest approach of about 5790 km (3600 mi) above the surface of the planet. The scanning sequence of the cameras sent more and more high-resolution pictures of Venus back to Earth, pictures that straddled the terminator boundary between night and day and would have shown detail there if it were present, pictures that curled across the limb of the planet like caterpillars side by side. All showed equally featureless clouds. However, the pictures obtained as Mariner laid tracks across the limb of Venus....
....showed definite haze structure above the limb (Fig. 6-2). Two distinct layers were apparent with definite structure above the limb. This was the type of information that Mariner scientists had hoped for.
Michael J. Belton of Kitt Peak National Observatory, a member of the TV science team,....
....took time off from inspecting the new pictures of Venus to discuss them with science reporters from the national press and overseas. He said the pictures seemed to be getting better as the spacecraft moved away from the planet.
Mariner 10 made its closest approach of 5794 km (3600 mi) at 10:01 a.m. PDT, within one minute of the time scheduled before launch. Then, six minutes later, the spacecraft went behind the planet, and radio signals began to fade as they passed deeper and deeper into the atmosphere. To keep the signals coming back to Earth as long as possible and thus dip as deeply as possible through the atmosphere of Venus, the high-gain antenna on the spacecraft was programmed to turn slightly and direct the signals so that when bent by the planet's atmosphere they would still be received at Earth (Fig. 6-3).
This program was most successful. If Venus had been airless like the Moon, the signals from the spacecraft would have been cut off abruptly at 10:07 a.m. PDT. As it was, the signals continued for several minutes, and scientists were satisfied that they had obtained a completely new probe to great depths of the Venus atmosphere. Since these data were collected simultaneously at two radio frequencies, they were expected to be much better than any earlier radio penetration of the Venus atmosphere.
Four minutes after the tracking station lost lock on the spacecraft signal, the antenna started to search for the signal again as it came around the other side of the planet. Again the signal was picked up and Mariner was tracked to its full emergence from behind the planet. While behind Venus, Mariner had continued taking pictures, which it stored on tape together with infrared data on the temperatures across the night and day hemisphere, fields and particles observations, and scans across the limb in ultraviolet.
Prior to the encounter, the main action was concerned with preparing all the instruments and making sure that the spacecraft followed a precise....
....path to and beyond the target planet. But when the encounter was successful, the accent changed. With data deluging back to Earth about Venus and its environs, action transferred to the teams of scientists who were literally snatching hold of the output from the computers to interpret this wealth of new information from another world. Did it fit the earlier theories? Did it show anything unexpected? Excitement mounted rapidly as team members struggled with the data records to find answers to these and other questions. Teams assembled from scientists of many different disciplines worked toward common goals, rushing to each other with new items of information to fill gaps in the puzzle.
Meanwhile, the spacecraft emerged from occultation, heading out from Venus toward Mercury. The TV pictures had changed from blue- and yellow-filtered to ultraviolet. In late afternoon, the few members of the press remaining in the von Karman auditorium at JPL, where pictures of Venus were being relayed in real-time for the news media-most of the journalists had left, disappointed at the lack of detail in the first images-were treated to a completely new view of the cloud-shrouded planet. The first ultraviolet pictures displayed on the screens showed intricate cloud patterns (Fig. 6-4). Excitement mounted as scientists identified these markings as close-ups of the indistinct ultraviolet markings recorded on Earth-based photographs (Fig. 6-5).
The best telescopic photographs of Venus from Earth only hint at the cloud patterns revealed in....
 ....ultraviolet light. Robert Strom of the University of Arizona's Lunar and Planetary Laboratory compared a handful of photographs of Venus taken by Earth-based telescopes with the new Mariner 10 pictures. "These Mariner pictures exceed our greatest expectations," he exclaimed, and then added that the new pictures would let astronomers view the Earth-based pictures from an entirely different standpoint. "Now we will be better able to understand what it is we see from Earth," he said.
In the Video Analysis Facility, Verner E. Suomi, a specialist in satellite meteorology of the Earth, peered through stereo viewers at the cloud pictures of Venus, seeking the three-dimensional effects that would enable him to measure cloud velocities. A major question was why the atmosphere of Venus, as observed in ultraviolet light, rotates so fast compared with the planet itself: in four days compared with 243 days for the planet.
One suggestion which quickly arose was that solar heating of equatorial regions produces a local wave in the atmosphere that gives rise to a circulating equatorial current. And since hot equatorial air will also tend to move to cooler regions, there is a spiraling speedup of the atmospheric currents at higher planetary latitudes. Transverse bands could be seen across the Venus cloud streams, which Dr. Suomi likened to bands across streaks of cirrus clouds in Earth's skies, but on a much larger scale. He pointed to cellular structures in the Venus clouds, each some 200 to 300 km (125 to 185 mi) across.
Although the spacecraft had performed well, Gene Giberson, Mariner Project Manager, admitted to several anxious moments when interviewed just after the encounter. Twenty minutes of finger crossing occurred when Mariner 10 passed closest to Venus and was aligned in space by the star sensor locked on Canopus. At any moment, glare from the brilliant Venus might have caused the spacecraft to turn around, thereby swinging the cameras and other instruments away from Venus at this critical time. Giberson explained that this calculated risk had to be taken since project management could not risk a potentially disastrous gyro malfunction during the flyby of Venus.
The decision to make the encounter with the star sensor in control had paid off; Mariner 10 kept its lock on Canopus and provided a very steady platform for the photographs and other experiments. The final picture of Venus was taken on February 13, 1974, bringing the grand total to 4165 images of the cloud-shrouded planet. Much had been learned during the encounter to supplement earlier observations of Venus from spacecraft and from the Earth.
As Mariner 10 sped toward Venus from the planet's night side, the spacecraft's instruments observed how Venus disturbs the magnetic field in interplanetary space and the flow of charged particles-electrons and protons-from the Sun, known as the solar wind. Venus causes a tail-like disturbance in the solar wind's charged particles, stretching behind the planet away from the Sun. At the same time, Mariner's magnetometer found that the magnetic field in space was twisted by the presence of Venus so that it pointed toward the planet along the tail of charged particles.
But Venus's magnetic field, which is less than one-twentieth of one percent of Earth's field, is insufficient to deflect the solar wind as Earth's field does. This very small and irregular magnetic field is insufficient also to trap stable populations of particles such as found in the radiation belts of Earth and Jupiter. Yet Mariner 10 showed that the solar wind is greatly modified by the presence of Venus. This effect was particularly noticeable because, for at least three days prior to and during the encounter with Venus, general conditions in interplanetary space were unusually quiet.
Mariner 10 confirmed the earlier findings of Mariner 5 and Venera 4, which had discovered a bow shock-a wave in front of the planet like the bow wave of a ship in water. Somehow the ionosphere of Venus forms this bow shock in the solar wind and stops the wind from plunging directly into the atmosphere of the planet. How and why this bow shock occurs is not fully understood. The charged particle experiment did not detect any high-energy protons or electrons within the bow shock, up to several Venus radii downstream.
Certainly the effect is very different from that on Earth, Moon, Mars, and Jupiter. The Venus bow shock might be a direct interaction of the solar wind with the atmosphere of Venus, or with just the ionosphere. It may alternatively arise because the solar wind induces magnetic fields and produces thereby a pseudo, or false, magnetopause, as though Venus had a magnetic field like the Earth.
The temperature of the clouds of Venus was measured from infrared radiation emitted by  them, using the radiometer carried by Mariner. As expected from other measurements, there was no detectable difference in the 250 K (-9°F) temperature of the cloud tops between day and night. Reduced infrared radiation near the edge of the visible disc of Venus confirmed that the atmosphere is very opaque.
The UV airglow spectrometer measured the amount of ultraviolet emitted by Venus's upper atmosphere to seek important gases there. One of these gases, hydrogen, is believed to control the chemistry of the planet's atmosphere, forming sulfuric acid clouds and water vapor droplets. Were Venus to lose its hydrogen, the dense, heatgathering atmosphere of carbon dioxide might be rapidly dissociated by sunlight into carbon monoxide and oxygen, with significant changes to the planet's heat balance.
Mariner 10 confirmed the presence of hydrogen and, even more important, obtained evidence that indicates that it originates at the Sun. If the hydrogen originated from the chance impact of a comet with Venus, as might have happened, deuterium (heavy hydrogen) would also be expected, but because Mariner 10 found no deuterium on Venus, scientists conclude that the hydrogen comes from the solar wind, which has virtually no deuterium. So the hydrogen on Venus will be replenished as long as the solar wind blows.
Mariner 10 also detected small quantities of helium on Venus, but 10 times as much atomic oxygen as on Mars. This high concentration of atomic oxygen suggests that, contrary to conditions at Mars, the upper atmosphere of Venus, where sunlight splits oxygen molecules into atoms, does not mix with lower layers. This lack of mixing seems also to be evidenced by the limb photographs, which show a distinct flat-topped atmosphere of clouds surmounted by several tenuous horizontal layers (Fig. 6-6).
By observing radio signals coming from Mariner 10, scientists determined how the gravity of Venus pulled the spacecraft, and hence they were able to clarify some of the physical properties of Venus. They found that Venus is 100 times closer to being a perfect sphere than is Earth. Radio waves passing through the Venus atmosphere as the spacecraft went behind the planet showed that a lower cloud layer which rises from 35 to 52 km (22 to 32 mi) above the planet's surface consists of quite different clouds from the higher....
....cloud deck. The highest deck extends 60 km (37 mi) above ground level. This upper layer is thin, broken, and rapidly moving, as contrasted with the thick and probably unbroken lower deck. Four distinct temperature inversions, i.e., places where the temperature increases for a short distance with increasing height, were observed by Mariner 10 at altitudes of 56, 61, 63 and 81 km (35, 38, 39 and 50 ml). They are possibly associated with specific cloud layers.
Mariner also found that the electrically charged particles making up the ionosphere of Venus peak into nighttime layers at 120 and 140 km (75 and 87 ml), whereas a stronger ionosphere in the daytime peaks into a higher layer at 145 km (90 ml). Earth's ionospheric layers by contrast have more layers in daytime than at night.
As mentioned earlier, photographs returned from Venus were at first very disappointing. They showed about as much detail as the top of a thick fog bank. Yet these photos were valuable in that they proved that Venus does have a structureless, hazy, visible surface of clouds down to a resolution of 100 m (300 ft). As Mariner 10 sped from Venus, a special sequence of ultraviolet photographs revealed a complex atmospheric pattern. This pattern had been photographed in....
Fig. 6-7. Over the next few days, series of mosaics were constructed showing a wealth of detail in the ultraviolet markings of the planet. The relatively quick rotation of the markings was confirmed (a) and a picture built up of the cloud pattern around the entire planet (b). On the right side of (b) the pictures were taken from greater distances, so detail is lacking compared with the left side of the picture.
....gross detail from Earth. Now Mariner 10 revealed its intricacies (Fig. 6-7).
At the point on Venus where the Sun shines from directly overhead, rising cells of air take on polygonal shapes. Larger cells have dark edges and intricate internal structure. They cause an area of planetary disturbance surrounded by great waves of atmospheric ripples, like those from a stone thrown into a pond, but on a scale of many hundreds of miles (Fig. 6-8).
 Along Venus's equatorial zone are fine streams of clouds-faint but quite distinct (Fig. 6-9). Y-and C-shaped markings, prominent on Earthbased ultraviolet photographs, are revealed as consistent markings, a spreading pattern of clouds opening in the direction of rotation. Their motions are clearly shown in time-lapse motion pictures made from the individual photographs obtained from Mariner showing several planetary rotations of the cloud patterns. Both polar regions have hoods of clouds with spiral patterns between the hoods and the equatorial regions. These cloud patterns, which would be quite invisible to the eye of an astronaut orbiting Venus because they are only visible in ultraviolet light, can be interpreted by two extreme theories. One is that solar heating develops cloud patterns without large-scale motions of the atmosphere itself. The other is that solar heating actually drives large masses of air from the equator to the poles, accompanied by undercurrents back from the poles to the equator. Which theory is closer to the truth requires further studies of the photographs, probably assisted by results from a later Pioneer Venus mission to the cloud-shrouded planet planned by NASA several years after Mariner 10.