Chapter 3:
In the Time of the Spacecraft: Descent Into Hell

Chapter Breakdown: [ COMPARATIVE PLANETOLOGY - THE CATHEDRALS OF JANUS - RADAR AND ROCKETS - LUCIFER FALLS AGAIN - STORMING THE HEAVENS OF HELL - "SEND IN A CLASS ONE PROBE!": THE VENUS PIONEERS - BRIEFING FROM A DESCENT INTO HELL - MEANWHILE, BACK IN ORBIT... - STRANGE BREW: WHAT'S IN THE AIR DOWN THERE? - WHY CHEMISTRY? - WHO CARES?: WATER ABUNDANCE - WAS VENUS WET? - TRACES OF ANCIENT NOBILITY - AN OCEAN IN THE SKY - THINK GLOBALLY, PROBE LOCALLY - NEW WINDOWS INTO THE DARK SIDE OF VENUS ]

Comparative Planetology

	"Rocket number 9 takes off for the planet... To the planet...VENUS!" -- Sun Ra
	"Where is the sun, oh clouds above me? Don't take the dreams to which I cling!" -- Billy Holiday
	"You are not expected to complete the work in your lifetime. Nor must you refuse to do your unique part." --Talmud
	"After such knowledge, what forgiveness?" --T.S. Eliot

Aliens have invaded our solar system. Beginning in 1961, a fleet of giant metal insects left the third planet and wandered inquisitively throughout this planetary system. We made them and sent them on their way.

Astronomy has been called the oldest science, and Medicine the youngest, but younger still is Astronomy's progeny, the fledgling field of Planetary Science. The planets have been the subject of careful observation and myth for millennia, and the subject of telescopic studies for centuries. Yet it is only in the last few decades that we have been able to send spacecraft, small mechanical extensions of our own senses, sailing through the solar system. As reports stream homeward at the speed of light, visions of a plurality of worlds, of a universe full of planets identical to our own and populated with beings just like us, are being rapidly displaced by new images of real worlds stranger and more diverse than we could have imagined. This newfound ensemble forms the subject matter of a new field (Planetary Science), and a new approach (Comparative Planetology) which have only come into their own in the age of spacecraft.

Historically, planets were part of the turf of Astronomy and, as long as telescopes were the primary tools of discovery, they fit there comfortably enough, a roving subset of all the points of light in the sky. But as spacecraft began returning detailed pictures of alien landscapes and other troves of information, the techniques and concerns of astronomers were no longer adequate for the task of making sense of the planets. What was needed was the expertise of those who had thought in detail about the one planet that had thus far generated any detailed science, the Earth. Who do you consult when trying to understand volcanic flows on the Moon, Mars or Venus? Why a volcanologist of course. And if you want to understand the weather on Mars or the greenhouse effect of Venus, you don't start from scratch. You take an existing Earth theory or model and modify it, changing physical parameters to match those of the world you are studying.

In the 1960's Planetary Science started employing the techniques of, and seducing a few scientists from, the Earth Sciences (geology, geophysics, geochemistry, meteorology, and others). Both groups of scientists benefited enormously from the cross-talk. Planetary explorers got pre-existing frameworks and techniques for studying what would otherwise be a bewildering abundance of new information and phenomena. They inherited the necessary expertise to take on the problems of planetary origins and evolution. Earth scientists got to judge how good their models were by stretching them to encompass whole new worlds, generating tremendous insights about the Earth, and--in those cases where the models bent but did not break--new confidence in the results. We now have something to "compare and contrast", as the test questions require. And we can see our planet more clearly, mirrored in the freshly unmasked faces of our dancing partners as together we circle 'round the sun. If we can successfully explain the differences between Earth's climate, torrid Venus, and frigid Mars, then we have more reason to trust our predictions of climate change here, predictions loaded with significance for humans and the other species with which we share this world. Thus we all stand to gain in very practical ways from this exchange. Not to mention the historical, spiritual quest to know our universe, and thus ourselves, better.

The danger in having so much of Planetary Science grow out of Earth Science is that we may be overly predisposed towards "geocentric" interpretations: we sometimes assume Earth to be the standard against which other planets are measured, rather than simply one among many possible outcomes of planetary evolution. How could we possibly avoid some such bias?¹ Yet, the growth of Planetary Science has also produced an enlarged, less provincial perspective on the Earth.

First generation planetary scientists, bringing with them the expertise, habits and languages of many fields have created an interesting cultural amalgam, with lots of communication and no small amount of mis-communication between people used to thinking differently from one another.

Most of the first generation are still around, actively pursuing research and training the next generation of explorers. My own experience illustrates how new this field is. When I started graduate school at the Planetary Sciences Department of the University of Arizona, where I worked on my doctorate from 1982 to 1989, none of our professors had degrees in Planetary Science, because when they were in grad school there was no such thing. Their doctorates were in Geology, Geochemistry, Physics, Meteorology and Astronomy. Now the faculty of Planetary Sciences departments have been partially infiltrated by people, like myself, with degrees in Planetary Science itself. I guess that makes us the second generation.

*******

In addition to studying its primary target, Venus, Mariner 2 measured for the first time the properties of the unknown, unexplored environment of interplanetary space, including magnetic fields, the solar wind, and cosmic dust. En route to Venus, Mariner 2 was hit by exactly one speck of dust large enough to detect, showing that the inner solar system was relatively free of a micrometeorite threat.

On board was an instrument called a microwave radiometer. This was designed to observe how the mysterious microwaves varied in intensity across the face of Venus, and resolve the loaded question of whether they came from a sizzling hot surface, or some strange atmospheric phenomenon. As Mariner 2 approached Venus, scientists were of two minds, with two clearly different predictions for the pattern that this radiation would make, depending on where it was coming from. If a hot surface was the source, as the spacecraft flew by it should record a peak coming from the center of the planet, where radiation from the surface would be least blocked by the atmosphere. In this case there would be less of a microwave glow observed at the edges, or limbs, of the planet, where it is filtered through a greater thickness of atmosphere. This phenomenon is known as "limb darkening". If the radiation was being produced in the atmosphere (which would allow for a cool, wet surface), the opposite effect was predicted: a "limb-brightening" of the microwaves, with the most intense radiation at the edges.

Allow me a nerdly illustration: Consider your view from above the head of a man with a crew cut. You see more scalp, peeking through his short hair, at the center of his head. The scalp is "limb-darkened" because in the center you have a more direct view of it, whereas at the sides, due to the shallower angle of your view, there is more chance for hairy obscuration. However, if it's hair you are looking for, the opposite is the case. The hair is "limb-brightened", meaning you see more of it at the edges, and less at the center. Mariner 2's microwave radiometer was designed to see whether the great microwave brightness was coming from the skin or the hair of Venus.

Microwave limb darkening means a hot dry surface. Limb brightening means that oceans and all they imply are still possible. It is rare in planetary science that a single observation is made with such a clear choice of predictions that can instantly resolve between two radically different views of a planet's nature. Much more often the gains are incremental and the implications of new observations take years to hash out. But on December 14, 1962, Mariner II flew past Venus at a distance of 21,000 miles, and the microwave radiometer worked perfectly. The results showed a clear pattern of limb-darkening. (There was more microwave radiation coming from the center of the planet than the edges.) This meant that the source of the intense microwaves was at the surface. And that proved that it must be scorching hot down there.

In fact the surface temperature is 735 Kelvin, or nearly 900 Fahrenheit. You could fry an egg on the sidewalk, but you'd have to do it quickly, before the sidewalk melted. It's hot enough there to melt many spacecraft materials and fry electronics.² Any water on the scalding surface would boil instantly. So much for our "twin" planet's global oceans. Alas, no giant jellyfish, tree ferns or dinosaurs. No telepathic frogs, at least not carbon-based ones, and things suddenly looked a great deal more dubious for those "intelligent creatures with musical tongues". Life, or at least "life as we know it", based on carbon molecules dissolved in water, is clearly impossible at the surface of Venus. If we were put on the surface of Venus without a very good climate-controlled suit, we would very quickly and quite literally be dead meat. The water in each of our cells would instantly boil off, and our proteins would quickly shred into tiny fragments which would react violently with the surrounding gasses. Venus is no place to raise the kids. In fact it's hot as hell.

Another important tool for remotely probing the composition of our planetary neighbors is spectroscopy, which starts with the common-sense notion that different substances are different colors, and extends into realms beyond our senses. The radiation³ we call visible is a tiny portion of the much larger electromagnetic spectrum, which extends beyond the violet limit of our eyes out to ultraviolet light, and x-rays, and beyond the red to infrared, microwaves and radio. The Sun shines mostly in visible light. This is no coincidence. We have evolved orbiting a moderately hot star and our eyes have adapted to make good use of the radiation from a star at this temperature. Just beyond the red edge of the visible lies the infrared, light with wavelength too long and energy too low for our eyes to see.

Whereas atmospheric gases tend to be conveniently transparent in the visible range (allowing us to see the sun and the stars, and everything else by reflected sunlight) many gases absorb infrared light. Each has a distinct pattern of absorption at specific wavelengths--a unique molecular fingerprint. With infrared spectroscopy you analyze the infrared color spectrum in great detail, note which wavelengths are being absorbed, and thus decipher the chemical composition of distant atmospheres. But first you must carry your telescope above as much of the Earth's atmosphere as possible. In particular, if you want to observe the absorption signature of water on Venus, you have to get up to a height where it is so cold that most of the Earth's obscuring water vapor has frozen out. Thus a high, cold mountain top is a good choice. The first spectroscopic observations of Venus, made in 1932 showed no for signs of water, but the spectral signature of carbon dioxide was clear. Increasingly refined observations throughout the 40's and 50's also showed evidence only for CO2.

So where was the water on this cloud-covered world? In the early 1960's observers started to carry telescopes equipped with steadily improving spectrometers up in balloons and aircraft that provided colder and clearer vantage points than any mountain top. At last these hi-tech dowsers achieved their goal: they found the infrared fingerprints of water vapor in the air above the clouds of Venus. They were also able to measure the amount of water, but these results were strange. For one thing, several of the different water measurements conflicted widely. For another, they all suggested that it was way too dry for air sitting on top of water clouds. Typical measured values suggested only one or two parts per million of water vapor above the clouds. If this were right, it seemed to rule out clouds made of water or ice.

Mariner 2 had shown that the surface of Venus is hotter than an oven. Spectroscopic observations made during the same time period, and later direct measurements by American⁴ and Soviet entry probes, revealed that the atmosphere of Venus is also bone dry, casting further doubt on the widespread notion that the clouds of Venus were Earth-style water clouds. Remember, these clouds were all that we had ever actually seen of Venus. They give Venus its myth-making brightness in our sky. They enshroud the entire planet, and they were largely responsible for the sister planet fantasy (clouds = water = oceans = life = civilization (?)), and for its persistence (because you can't easily dismiss what you can never see). But with such a minuscule amount of water in the Venusian air, the clouds couldn't be made of water droplets. So what were they?

The quest to identify the mysterious cloud-forming substance took another decade after Mariner 2. Numerous observations and rampant speculation paraded a menagerie of exotic candidates through the scientific literature before we found the answer. These included formaldehyde, plastic polymers, rock dust, oil droplets or other organic compounds, and salt. The answer, when we finally found it in 1973 from a combination of spectroscopy and polarimetry (studying how the cloud droplets polarize light), was at least as strange as any of these: the clouds are made of highly concentrated sulfuric acid, commonly known as battery acid.

Thus we found our "sister planet" to be chemically alien, as well as hot and dry to quite un-earthly extremes. With these revelations, the "twin sister" imagery quickly disappeared, and the notion that "Venus is Hell" took hold. Once again, Lucifer had fallen.

This was a time of furiously paced space exploration, aggressive and optimistic. Less than a decade after Mariner 2, in 1971, Mariner 9 revealed ancient water-carved channels on Mars, and Mars became our next best hope for finding earth-style life, or at least fossils of past life, elsewhere in our Solar system. If we could no longer imagine teeming oceans obscured by the clouds of Venus, we could at least imagine oceans in a more illustrious past on Mars, obscured instead by time, with the ancient, dusty, dried-up river valleys revealed by Mariner and Viking perhaps being all that remains of a mighty Martian hydrological (and biological?) system. Current articles and scientific papers speculating on a more Earthlike past for Mars, and ancient or even extant life there are commonplace. The 300-year vision of Venus as Earth's twin has faded to a quaint relic of Victorian-era romanticism.

With the clarity of hindsight, it is easy to discount the "twin sister" vision of Venus as so much wishful thinking based on rather flimsy evidence, although this is rather unkind to some of the best observers and thinkers of the last 300 years. Our telescopes gave us just enough information to fuel this fantasy, and not enough to dispel it. Maybe we became a bit too comfortable with, and comforted by, this vision of another Earth, perhaps an Eden, so nearby. It was a fantasy created by our own desires and supported by science. We put Venus on a pedestal, and set ourselves up to be disappointed, in a way that is not quite fair to Venus or ourselves. When our fly-bys, orbiters and landers started to pry, poke and peek beneath the clouds, it was against this pre-conceived notion that Venus was judged. Once it became clear that the environment on Venus was not as familiar and friendly (to us) as had been imagined, the language and imagery used to describe our neighbor-world transformed radically and rapidly.

The new truth was "the grim truth", and Venus, as if she had betrayed us by not living up to our expectations, was judged harshly. "Venus is Hell" became a common expression, and words like "harsh", "hostile", "inhospitable", "errant twin" , "poisonous", "catastrophe", "noxious" and "tortured" filled the pages of books and articles describing our closest sibling. Nearly every book, chapter or article written about Venus since the 1960's contains some version of the statement: "The planet named for the Goddess of Love turned out to have a closer resemblance to Dante's Hell."

In Arthur C. Clarke's visionary 1953 science fiction novel "Childhood's End", an advanced race of aliens visits Earth. But they must conceal their physical appearance from humans because of their uncanny resemblance to mythic images of the devil, complete with horns and pointed tails. Venus may suffer similar guilt by association with biblical and literary images of hell, at least as far as being deathly hot, dry and full of sulfur vapors.⁵ But after this point has been made, isn't it awfully Earth Chauvinist to keep hammering the "Venus is Hell" theme? It's true that life based on organic molecules would not fare well there: Sulfuric acid and 900 degree heat are not healthy for carbon based children and other organic living things. But, neither is any other environment we have discovered elsewhere in the solar system, or anywhere else. It is natural to be extremely attached to the environmental conditions we've become accustomed to after living, and evolving, here for 4 billion years. But if we are to harshly condemn every environment where we cannot live comfortably, the universe will seem a lonely, sterile and hostile place. And, especially when we see the great complexity, beauty and familiarity of Venus as revealed later by Magellan and other more recent developments, it seems that "Venus is Hell" may have been in part an over-reaction to the disillusionment of learning that "Earth's Twin" was a naive dream.⁶

Science fiction writers were quick to update their Venus imagery. Larry Niven's "Becalmed in Hell", published in 1965, depicts a troubled cyborg ship stalled in the sweltering Venusian depths, its terrified pilots expecting to be cooked. Niven assumed that at the surface it's "as black as it ever gets in the solar system". As we will see, things down there are not quite that dim.⁷

******

Storming the Heavens of Hell

"Your mysterious mountains I wish to see closer. May I land my kinky machine"? -- Jimi Hendrix, Third Stone From the Sun

After Mariner II the people of Earth, or at least the two nations caught up in the cold war, began sending probes to Venus in steadily increasing numbers and sophistication. We often refer to this period wistfully as the "golden age" of planetary exploration. It was a time of relative affluence and optimism in the United States. A popular young president had declared that we would put men on the moon by decade's end, leading to an ambitious and lightning-fast, by today's standards, program of rocket design and testing. And of course there was that cold-war rivalry giving both sides a sense of urgency in maintaining a technological edge and achieving "firsts" throughout the solar system. For all of these reasons it was a lot easier then, than it is now, to get a new planetary mission approved, funded and flown. Each new spacecraft was larger and carried more scientific instruments than the previous one--very nearly the opposite of recent trends.⁸

Thirteen hundred years ago Mayan astronomer-priests eagerly anticipated and accurately predicted the short disappearance interval when Venus would vanish from his evening apparition and re-emerge, Eight days later on average, from the underworld as morning star. They knew that every 19 months this drama, with subtle variations that they charted and cherished, would repeat. Today, we see this time of disappearance and re-emergence as "inferior conjunction", the time when Venus in its orbit passes between us and the sun. Now this configuration is eagerly anticipated by spacecraft engineers and planetary scientists, because it represents a new kind of opportunity in the continuing quest for human connection with the lights in the sky. For the Mayans these events were times of spiritual renewal, sacrifice and battle. For us, these are launch windows. On these occasions, at 19 month intervals, the two planets swing closest and this means that a spacecraft launched near that time requires least energy (least fuel) to reach Venus. It also makes for shorter travel times, which means less chance of something going wrong along the way, and a shorter radio communication distance from spacecraft to Earth, allowing for maximum data retrieval.

Table 3.1 on page 73 shows the dates of Venus inferior conjunctions from the time of Mariner II in 1962, to the launch of Magellan in 1989, along with the spacecraft we launched at each of these opportunities, and the date of launch and Venus encounter for each. You can see that inferior conjunction always occurs between launch and encounter. This is because we achieve the minimum energy trip if we launch a spacecraft from an orbital position about 60 degrees ahead of Venus as it closes on us in the inside lane. The spacecraft is always en route on the actual date of inferior conjunction, so we launch towards "evening star", and arrive at "morning star". [table of Venus mission dates, with dates of inferior conjunction coming soon!] [drawing of transfer orbit from Earth to Venus coming soon!]

This table indicates some interesting differences between the U.S.A. and the U.S.S.R. in space exploration style. Notice that, beginning with the unsuccessful 1961 mission Venera 1 (which would have been the first mission to Venus had it made it there), the Soviets took advantage of at least every other launch window from the 60's through the 80's. They had many failures, particularly early on, but at nearly every opportunity they would throw whatever they had ready at Venus. The difference between one spacecraft and the next, in the Soviet Venera series, tended to be incremental, with each year's model closely resembling the last one but incorporating whatever improvements in instrumentation had become available since the last launch. The American launches were fewer and farther between but more sophisticated technologically, and they had a higher success rate.

These differences may reflect the different political systems of the two space-faring superpowers. In the U.S.S.R., with its highly centralized government, a small number of people made decisions about space exploration. It was easier to get a mission approved there than in the U.S.A. where every space mission must navigate countless NASA committees and then survive the perils of congressional funding cycles--a journey at least as treacherous as that through interplanetary space. Our missions are often threatened with extinction every year until launch, and some have been cancelled and restarted several times. There is a mis-match between the timescales of conceiving, designing and flying planetary missions and the pace of change in American politics. A mission that receives a green light from one presidential administration and congress may get canned a few years later when the winds change in Washington.

In the U.S.S.R programs, once started, took on lives of their own. But, consequently, there were fewer incentives toward perfection and fewer checks on the system, no bidding between contractors eager to outperform each other, and no congressional oversight committees. So the American missions which did make it through to launch, although less frequent, were more carefully planned, more likely to succeed, and each tended to represent a major technological advance over the previous one. They had a "five year plan"; we had tremendous "Yankee Ingenuity", but little ability to stick to any plan. The two styles complemented each other, and throughout the 60's, 70's and 80's our understanding of Venus slowly increased in depth and sophistication. But still the clouds did not clear, and major mysteries remained.

The early Soviet efforts first produced a string of failures, that nonetheless provided valuable lessons in spacecraft design and navigation. Venera 3 was designed to be the first spacecraft to land on another planet, measure surface conditions and, of course, carry commemorative Soviet emblems to the surface of Venus. It actually reached Venus only to crash into the planet on the first of March, 1966, returning no scientific data.⁹ The Soviet's first successful mission, Venera 4, reached Venus in October 1967, and marked a major achievement: it was the first planetary probe to enter the atmosphere of another planet, do direct experiments and radio home the results. Venera 4 was protected from the intense heat of high-speed entry into the upper atmosphere of Venus by a heat shield developed through the testing of nuclear warhead re-entry capsules for intercontinental ballistic missiles. During its brief (94 minute) descent by parachute, the spacecraft measured conditions in and below the clouds, confirming that the atmosphere of Venus is mostly carbon dioxide and recording increasing temperatures and pressures until it was crushed when it reached a level where the pressure is 18 atmospheres (18 times Earth's atmospheric pressure), and the temperature is about 500 degrees Fahrenheit. Initially, Soviet scientists thought that Venera 4 had reached the surface. Later they realized that the spacecraft had failed at an altitude of more than 16 miles, meaning that the surface temperature and pressure must be considerably higher still. Evidently, future entry probes would have to be built to withstand higher pressures, if we wanted to reach the surface. But how high? Would we ever find solid ground?

Conducted by the incessant 5:8 orbital polyrhythm of Earth and Venus, the "space race" contestants danced to a slow 19 month beat, with spacecraft launches concentrated into the regularly spaced launch opportunities which assumed this same rhythm. Nearly every time Venus swung near to Earth, a small instrumented probe (or several) would leap off Earth and try to hit Venus. Russian and American spacecraft often literally raced each other to Venus and arrived within days of one another. Two days after the launch of Venera 4, the Americans launched Mariner 5 from Cape Kennedy. As the Summer of Love turned to autumn back home, these two little machines from Earth raced towards Venus. Venera 4 made its historic entry into the atmosphere of Venus on October 18, 1967, and the very next day Mariner 5 flew past Venus at a distance of only 2100 miles with a battery of sophisticated new instruments trained on the atmosphere. Radio telescopes on Earth made precise measurements of its signal as it gradually disappeared behind the planet and reappered on the other side. The Mariner data produced a much clearer profile of the atmosphere, revealing a surface temperature and pressure around 980 degrees Fahrenheit and 100 atmospheres!

American and Soviet scientists shared information with each other quite freely, and used each other's results to help interpret their own. The data from Venera 4 and Mariner 5, when analyzed together, created a more complete picture of the atmosphere than either could have provided separately. Also, the Soviets, having had previous missions fail due to communication problems with their spacecraft, received permission to use the large radiotelescope at the British Jodrell Bank Observatory to monitor the signals from Venera 4 as it plunged into the atmosphere of Venus. In these ways the space race acted as its own antidote to the aggressive, competitive forces that had spawned it. Planetary exploration does not recognize national borders. Planetary Science is inherently a collective endeavor that at some level demands a planetary, not national, identity of those who practice it, as scientists from Earth try to understand our near neighbors.¹⁰

These first missions helped to confirm and refine the general picture of the Venus atmosphere that Mariner 2 had sketched, a hot dense envelope composed mostly of carbon dioxide. But many more intriguing questions were raised: what are the clouds made of and to what depth do they extend? Does the upper atmosphere really whip around the globe, rotating many times faster than the solid planet (as had been suggested by ground-based telescopic observations at ultraviolet wavelengths)? What other gases are in the atmosphere? Why doesn't there seem to be any water or oxygen? And what about the surface? What's it really like down there? Is it hot everywhere, or are there cool places at the poles or on mountaintops? Is the surface alive with active volcanoes, rugged young mountain ranges and rapid erosion like the Earth, or is it old, dead and covered with impact craters like the Moon, Mercury and the southern half of Mars? Was there ever water there?

Inquiring minds wanted to know, and they knew how to find out: U.S. scientists drew up ambitious plans for further Venus missions in the '70s, including multiple entry probes, orbiters and a lander. However, the momentum got lost in a series of delays, budget cuts and political shifts. Post-Sputnik hysteria, which had helped launch Apollo to the moon and fueled early American planetary exploration, was subsiding. The bills from Vietnam and the cold-war military build-up were starting to come due, and the relative prosperity of the '60s was receding. The planners and designers of all subsequent American missions, up to the present day, have had to squander huge amounts of time and energy jumping through ever-shifting hoops of bureaucracy and fiscal readjustment.

Meanwhile, the Soviets launched successful entry probes at each of the next three launch opportunities, Veneras 5 and 6 in '69, 7 in '70, and 8 in '72. The latter two were the first Earth machines to land gently on the torrid surface of Venus. Designing and flying a spacecraft that can survive the descent through the ever increasing heat and pressure to the surface of Venus and operate there even for a short while before succumbing is no small engineering challenge. The Soviets failed several times before succeeding. Venera 7 made it all the way down only to fall over upon impact with the ground. This made it hard to receive the signals because the antennae was pointed the wrong way. The operators on Earth heard Venera scream "I made it!", but did not retrieve any data from the surface.

They finally did it with Venera 8, which landed on July 22, 1972 and functioned on the ground for almost an hour, making the first direct measurements of the temperature, pressure illumination level, and rock properties at the surface. As Venera 8 descended towards the daytime surface it recorded the decreasing amount of sunlight. A great deal of light was absorbed above an altitude of 25 miles, correctly attributed to clouds at these heights. Below about 19 miles, the air was very clear. Measurements showed that about 2 to 3 percent of the Sun's light makes it down to illuminate the surface. This is really plenty of light to see by, equivalent to a deeply overcast day on Earth. It would be possible to take pictures at the surface, and the Soviets immediately adjusted their plans to include cameras on future landers. Wind speeds were found to decrease steadily with altitude, from over a hundred miles per hour in the clouds, to just a few feet per second at the surface. The mixture of gases composing the air was found to be about 95% carbon dioxide, no more than 5 % nitrogen and less than 0.4% oxygen. The air on Earth is made mostly of these last two gases. The lack of oxygen on this nearby world is of particular interest to we who breathe it.

Although American exploration of Venus was stalled, at least temporarily, after Mariner 5, Venus scientists lucked out and got a free ride on Mariner 10, whose primary destination was the planet Mercury. Mariner 10 needed to swing close by Venus to get a "gravity assist", slowing down by grabbing on to Venus' strong gravitational pull which would drop it toward Mercury. Launched in October 1973, Mariner 10 experienced a multitude of technical difficulties on the way to Venus, including flaky antennas, gyros and power supplies. Several of these came perilously close to dooming the mission but the American flight controllers, using their now legendary ability to apply on-the-fly ingenuity to fix spacecraft millions of miles out in space, kept the cranky craft running. It flew by Venus at a distance of 3600 miles in February, 1974 and its cameras (designed to photograph Mercury) sent home the first close-up photographs of Venus from space.

At first these pictures revealed... nothing much, really. Seen from close-by at visible wavelengths the planet-wide clouds appear basically bright and featureless, just as they do through a telescope. Photos of the limb (edge) of the planet did reveal several distinct layers of haze above the clouds in the planet's upper atmosphere, silhouetted against the blackness of space. But the real surprises came when Mariner took photographs through an ultraviolet filter; a dynamic and volatile new face of Venus suddenly appeared. Observers on Earth had previously seen some vague features in ultraviolet images taken with telescopes so the researchers expected some kind of markings. But what we actually saw far exceeded anyone's expectations: a complex swirl of high-contrast features, ranging from tiny, detailed splotches, to huge planet-wide streaks. And the stuff moves around like crazy.

The identity of this material, so dark in the ultraviolet that it is responsible for nearly half the solar energy absorbed by Venus, is still not known, one of the great mysteries of Venus. Whatever this "unknown ultraviolet absorber" is, its motions allow us to trace the atmospheric currents in the cloud-top region of Venus. Like ink drops in water, or smoke in a wind tunnel¹¹, the ultraviolet markings suddenly rendered the air currents visible. Meteorologists now had something to sink their teeth into, a new pattern of atmospheric circulation to model and compare to those seen on Earth, Mars and Jupiter. Huge C- and sideways Y-shaped dark markings, symmetrical across the equator, were seen forming and dissolving. These all rushing by at a speed of nearly 200 miles per hour, circling the whole planet in 4 days. This pattern of rapid planet-wide rotation was dubbed the "superrotation". Finding its cause presented a major challenge for comparative planetology. Can our models of atmospheric circulation, developed on Earth, but based on (hopefully) universal laws of physics, also be used to explain such an extremely different pattern of atmospheric motions on this nearby world?: another major mystery that persists to this day.

Other unusual atmospheric features seen in Mariner 10's ultraviolet pictures of Venus, grist for the mill of the meteorological modellers, were bright hoods over both poles with spiral patterns extending from them, and a series of polygonal shapes which seemed to follow the point on the planet where the sun was directly overhead, probably due to rising air from convection caused by solar heating. During its brief fly-by of Venus, Mariner 10's other instruments gathered additional data that helped build a preliminary picture of the composition and thermal structure of the upper atmosphere and its interaction with the solar wind. Then, having received just the right gravitational kick from Venus, Mariner-10 went on to Mercury where it became the first and only spacecraft (so far) to visit the small innermost planet, successfully taking detailed pictures of half of its ancient, cracked and pock-marked surface.

Unlike their American brethren who were tangled up in a draining game of "red light, green light" with the whims of successive administrations and congresses, Soviet scientists and engineers did not have to wait to start planning their next Venus missions. At least while the Cold War simmered, they had a mandate for a long-range plan to explore Venus. Soviet Mars exploration had produced nothing but failures. Once they started having a winning streak on Venus, the "reds" gave up on the Red Planet and concentrated their resources on Venus. This is odd in a way because the environment of Venus presents considerably greater technical challenges than that of Mars, at least for surface landers. It may have come down to luck, but once they set their sights on Venus they were relentless, and eventually remarkably successful.

Emboldened by the success of Venera 8, they set about designing a new, more advanced generation of Venus craft. Each of these included a large, improved lander and an orbiter. The redesigned landers consisted of a spherical shell, eight feet in diameter, designed to withstand high pressures and containing interior layers of thermal insulation and coolant. They had a large disc-shaped drag plate which, mounted on top to slow their descent through the atmosphere, also acted as a communications antenna, and a circular ring of shock absorbers below. Researchers packed a slew of new instruments into each of these landers, including a new camera system, a gamma-ray spectrometer to analyze surface rocks, and several meteorological instruments. The orbiters would allow the first sustained long-term observations of the atmosphere and its interactions with the solar wind. They would also serve as a radio link between the landers and the Earth, eliminating the previous tricky necessity of sending signals directly from the Venusian surface to receivers here. The combined weight of the new Veneras, lander plus orbiter, was over 10,000 pounds, the largest interplanetary craft ever constructed.

Venera 9 and 10 were launched 4 days apart in June 1975, from the Soviet launch complex at Tyuratam. Four months later the landers plunged into the atmosphere of Venus. Each one was equipped with instruments to measured the density of the clouds as they descended. It turned out that the clouds were not very dense and were relatively transparent, so that if you were in them they would seem more like fog or mist than like most clouds on Earth. But these diffuse clouds were also found to be incredibly vast, towering up to an altitude of 44 miles from the cloud base at 33 miles. They were found to be concentrated in three discrete layers, with relatively clear air between, and each layer had a different mixture of droplet sizes, which hinted that they might be composed of multiple materials.

Venera 9 landed safely and photographed the surface of Venus. The first picture ever returned from the alien landscape of another planet¹² (shown here) revealed a rough, sloping surface strewn with flat rocks extending towards a bright sky into the distance. The immediate impression, of an eroded field of volcanic rocks, has stood up under decades of scrutiny. Venera 9 transmitted from the surface for 53 minutes before the signal was lost. Three days later Venera 10 landed, at a site about 1200 miles away, also transmitting photographs of its surroundings, showing a similar landscape, with more rounded rocks and more dark, soil-like material between them. Did this indicate an older, more eroded site? What would erode and transport fine-grained material on a planet with no water and almost no surface winds? Each new observation produced many more unanswered questions.

"Send in a Class One Probe!": The Venus Pioneers

"Poised for flight, wings spread bright, spring from night into the Sun." --Robert Hunter

As the '70s wound on and the Earth spun to a disco pulse, Venus--bright as a mirror ball but fuzzy as a shag rug--orbited sunward of us, presenting himself for Earthly visitation every 19 months. At almost every opportunity the Russians sent whatever they could scrape together. Their investigations proceeded tortoise-like while the American hare slept, dreaming dreams of Venus which turned to nightmares of congressional funding battles. American scientists went through several episodes of detailed planning and preliminary approval and funding only to be sent back to the drawing board by later eviscerating budget cuts.¹³

Studies for a next-generation Venus mission had begun in 1967, following the success of Mariner 5. Early ideas had included balloons that would float in the atmosphere and snazzy surface landers. Neither of these made the cut. Planners had optimistically assumed that each coming launch window would be used to send spacecraft to Venus. What survived was a more modest plan for two launches during the 1978 launch window. The flip side of all this was that by 1978 when the Americans were able to launch again they were tanned and ready. Each stage of the frustrating, repetitive re-casting (and de-scoping) of the plan had produced improvements in design. The resulting mission, Pioneer Venus, was less grandiose than some of the earlier plans, but it included many design innovations, and the scientists had time to cook up and refine a superb complement of new instruments which built upon the results of previous missions. The whole Pioneer package was streamlined, efficient, extremely well integrated and r

The final configuration included an orbiter, launched in May, and a "multiprobe bus" launched that August. The bus was a cylindrical shell, covered with solar cells for power during the interplanetary journey. It would release 4 atmospheric probes with spherical vessels designed to carry instruments safely through the entire atmosphere in a protected low pressure environment. There was one large probe, weighing 700 pounds, and 3 smaller, 200-pound probes. Each was to sample a different location, latitude and time of day as it descended. Earlier craft had revealed many of the basic characteristics of the atmosphere including amounts of major gases, basic temperature and pressure structure, and placed rough bounds on the location and thickness of the cloud decks. Armed with this information the designers of Pioneer knew what they were getting into, at least more than previous Venus explorers had. They had a basic knowledge of the conditions and physical demands that would greet the spacecraft at Venus

A lot of hopes rode to Venus with the scientific instruments on Pioneer. We expected this spacecraft to provide answers which touch on the grand questions that drive us to study the planets, questions about origin and evolution, the similarities and differences among worlds, and the uniqueness of the world we call home. In particular, the Pioneer scientists wanted to know why conditions on Venus are so very different from those on Earth. In order to crack this puzzle, or at least delve deeper, they needed more precise measurements of the amounts of major gases, and an accurate census of trace gases and their variations with altitude. They needed to know at what levels solar energy was being absorbed, and what was absorbing it. This could help provide the answer to the enigmatic super-rotation. We also needed more precise measurements of how wind-speed, temperature and atmospheric turbulence varied with altitude. And were there horizontal temperature differences deep in the atmosphere that could help

Other questions involved the great towering, diffuse cloud system. Does the layered structure discovered by the Veneras extend uniformly around the entire planet, or does this vary in space and time in response to atmospheric motions? How opaque are the clouds to different forms of radiation? Are they pure sulfuric acid or are there other materials mixed in? Are the different layers made out of different stuff?

Clues were badly needed to answer some important evolutionary questions, as well. Was there ever more water on Venus? Could it have had oceans? Has Venus always been hot as hell or has there been climate change from a more mild (or even hotter) past?

And then of course there was the surface. If not a dripping jungle, what then should we find there? All we had so far were a few intriguing snapshots and some preliminary chemical data provided by the Russians. Was the surface broken up into continents and low basins like the Earth? Were there mountain ranges and volcanoes? Does Venus have continental drift and seismic activity?¹⁴

Pioneer instruments could at least take a stab at each of these questions. The orbiter carried a radar altimeter which would create the first topographic map of the cloud-shrouded planet's surface. This worked by continuously sending radar pulses to the surface and precisely measuring the time delay of the return echoes. When the spacecraft was over a high altitude spot, the echo took less time to bounce back. This all got fed into a computer that turned it into a topo map. The orbiter also carried spectrometers to study the clouds and upper atmosphere.

The one large entry probe contained several instruments designed to sample the atmosphere frequently during descent and make precise measurements of gas composition. Other experiments would simultaneously measure the radiation at several wavelengths and from several directions during descent. These latter experiments needed windows to look out of. But how to build a window that would let light through but hold off the crushing Venusian pressure and resist chemical attack by the acid clouds? No known glass would do, so the probe was sent to the surface of Venus bearing gifts of cut sapphires and diamonds.

The three small atmospheric probes also had devices to measure the changing light during their descents at different locations around the planet. The bus itself, destined to crash and burn in the upper atmosphere, carried a couple of spectrometers for additional sampling on its way in. All four entry probes also carried instruments to take profiles of cloud thickness as well as sensitive accelerometers to record spacecraft motions during descent and reveal subtle changes in atmospheric pressure with depth. This would help fill in the 3-dimensional picture of the atmosphere and clouds. Scientists hoped that this would give them enough data to construct realistic models of the relationships between radiation absorption, chemistry, atmospheric motions and clouds, providing the beginning of an understanding of how Venus works.

As December, 1978 approached, Venus had passed near to the Earth at inferior conjunction and was receding. It was a bright morning star in Earth's pre-dawn sky. The two Pioneer spacecraft neared their rendezvous with Venus, the orbiter and the bus with four entry probes clinging tight to its underbelly like baby possums. And they were not alone. The inner solar system between Earth and Venus was rather crowded that December, as two giant Russian spacecraft, Venera 11 and 12, each equipped with a fly-by craft and a new, improved surface lander, were also making their way inward towards Venus. The Soviets had actually skipped the previous launch opportunity in order to study the large amounts of data acquired by Veneras 9 and 10 and assimilate this into later designs. These next generation Veneras included new, improved cameras for better surface panoramas and new instruments for measuring atmospheric gases.

And so it was that in December of 1978 the heavens of Venus were stormed by an armada of ten spacecraft from Earth.¹⁵ On December 4 the Pioneer Venus orbiter successfully entered orbit around the planet, where it would continue to operate and take data for the next 14 years, far outlasting anyone's expectations. Five days later the four Pioneer entry probes (plus the bus which had carried them) shrieked at 26,000 miles per hour into the upper atmosphere where, having survived the rigors of interplanetary travel, each began its perilous journey down to the surface. Just 12 days after this, on the 21st (winter solstice in the North End of Earth), the Venera-12 lander descended towards the surface of Venus as its companion fly-by craft relayed its signals to Earth. And, four days later, on a freezing Christmas day in Moscow , the Venera-11 lander, following close on the heels of its sister craft, dove at Venus and plunged towards the blistering surface.

All of these spacecraft worked, although all experienced some problems. The measurements made in that busy December 19 years ago still form much of the basis for our current knowledge of the atmosphere of Venus.

******

Eighteen minutes into Venus, the probe, having plunged through the sulfuric acid clouds, frantically gathering data along the way, had descended to 28 miles above the surface. At this point the parachute was abandoned, so the rest of the journey was a free-fall through the increasingly dense atmosphere.¹⁶ The spacecraft continued to plummet, sampling and examining the air every few seconds, slowed only by air resistance, until, 36 minutes later, it smashed into the surface, near the equator during daytime, going about 30 miles per hour. The entry probe ceased functioning, or at least communicating with Earth, immediately upon impact.

The three small probes each entered the atmosphere within 10 minutes after the entry of the large probe. One minute after entry, each began to open its windows and doors to examine the clouds. The small probes were simpler in design and had no parachutes. They were given the names "Day", "North", and "Night" to reflect their entry points, which had been chosen to sample as diverse a range of latitudes and lighting conditions as possible. These three were all expected to die upon impact like their larger companion, but one, "Day" survived and continued to function for 68 minutes on the surface. The dying spacecraft dutifully transmitted its ever-increasing internal temperature until it reached 260o F and finally succumbed to Venusian fever.¹⁷

The feast of information returned by the entry probes raised many questions. One which has never been answered satisfactorily is "What the hell happened at 12.5 kilometers?". Each one of these probes went haywire as it passed through a height of about 12 kilometers, or 7.5 miles, above the surface. The temperature and pressure sensors sent back crazy numbers, power surged throughout the probes and some instruments stopped functioning entirely. Was this due to some still-unknown phenomenon which exists on Venus at this height, or was there some component, common to all four spacecraft, which failed quickly and unexpectedly at this altitude due to some environmental factor, temperature perhaps? To this day we do not have a widely accepted explanation for these nearly-simultaneous freak-outs at the same altitude on four entry probes. Recently, 1n 1993, NASA held a workshop to review the data on, and possible explanations of, these "12.5 kilometer anomalies", but we still don't know what happened.

During its hour of glory, plunging from the frigid vacuum of space to smash down onto the sizzling surface, each Pioneer probe assembled a travelogue of data on the Venusian atmosphere that scientists still pore over today. Somewhere on the surface of Venus (we know the approximate locations) rest the remains of these crashed probes which, like their Russian counterparts, lie melting, frying, corroding and eroding as they are gently buried by the ever so slowly shifting sands of Venus.

Even as the four Pioneer entry probes fell towards the surface, the bus, which had carried and protected them during their seven-month journey across interplanetary space and delivered each to its spectacular entry and demise at Venus, itself closed on the planet. The bus had hung back while the probes raced ahead of it, like eager children who just couldn't wait to get to Venus. Now, having lost its primary raison d'tre as caretaker and transport for the entry probes, the bus made its own kamikaze dive. About a half hour after the last of the entry probes hit the surface, and as the "day" probe still lay dying and frying below, the bus dove into the upper atmosphere. Lacking a heat shield to protect it from the intense heat of an atmospheric entry it burned and disintegrated in about two minutes. As a Klingon Warrior would say, the bus "died with honor". Its instruments radioed home to Earth valuable measurements of the upper atmosphere and ionosphere, a region skipped by all the other probes, during its brief flaming streak through the southern daytime sky of Venus.

Footnotes:

¹ However, the early "Earth Science" people who went into Planetary Science were those who were least likely to fall victim to "terrestrial chauvinism."

² Early Soviet probes carried aluminum busts of Lenin to the surface. The surface temperature is higher than the melting point of Aluminum, so these must have quickly become small revolutionary puddles.

³ "Radiation" and "light" are the same thing.

⁴ I am going to refer to the space exploration missions of the United States as "American" missions. I recognize the limitations of this term, but it flows so much better than "United Statesian"

⁵ The religious association of sulfur vapors with Hell probably comes from identifying volcanic vents as gateways to the underworld.

⁶ Venus may seem like "Hell" to us, but of course we are deeply, profoundly biased in this assessment, perhaps more than we know or can know. Alien tastes and aesthetics may be very different. Who knows, maybe they'd like the smell of microwave popcorn and the sound of fingernails scraping on a blackboard. And the inspiring music of Bach or Bob Marley might make them want to puke, if that's something they do.

⁷ Niven's story has people going to Venus in 1975, reflecting the optimism of extrapolations during the Apollo era. The ship in this story, suffering from psychological problems, is a predecessor to the paranoid computer in Clarke's 2001: A Space Odyssey.

⁹ So it was the first object from Earth to reach another planet, even if it didn't do anything useful there.

¹⁰ Many astronauts have commented on the striking absence of political boundaries on Earth when seen from space. Something of the same perspective is demanded of those of us who are stuck down here looking up, sharing resources and attempting to unravel the story of the planets.

¹¹ Or sneakers in the Pacific Ocean. In 1990 a shipping accident dumped 60,000 athletic shoes into the North Atlantic. Scientists opportunistically tracked and modeled the swarm of floating sneakers to study ocean circulation. A few teenagers were spotted in the Pacific Northwest wearing soggy, unmatched Nikes.

¹² On October 22, 1975.

¹³ The British science writer Eric Burgess, commenting on these difficulties, and the consequent lost opportunities for American space exploration asks: "What had happened to the American spirit of private enterprise and exploration that had so successfully developed a whole continent in less than two centuries and could have gone on to develop the Solar System?". He's got a point. If things had gone differently, by now we might have a McDonalds in Venus orbit, strip malls on the Moon and MTV broadcasting "The Grind" from the canyons of Mars. Damn.

¹⁴ Mango groves, bowling alleys and Quickee Marts could probably already be ruled out on the basis of existing data.

¹⁵ Counting entry probes and orbiters as seperate craft.

¹⁶ The entry probe designers had to walk a thin line between falling too fast through the atmosphere, which would not allow enough time to take measurements during descent, and falling too slowly, which would cause the spacecraft to heat up too much and die before reaching the surface.

¹⁷ A venereal disease common on the love-goddess planet.