Venus, the Planet

Venus

Venus, second planet in order of distance from the sun, behaves in a way similar to Mercury. It swings from side to side of the sun. It appears as an evening star in the evening sky after sun-down. The planet is a morning star in the eastern sky before sunrise. But since its average distance from the sun is about 67 million miles. This distance is roughly twice that of Mercury. It can swing away from the sun some 46 degrees compared with 23 degrees for Mercury. Like Mercury, it goes through a cycle of phases but in longer time since its period of revolution is about 225 days. It certainly has a much greater range in brightness and at maximum is rivalled only by the moon and sun.

Fleecy-white Clouds

Venus owes much of its brilliance to the fact that it is a good reflector of sunlight. Fleecy-white clouds in a dense atmosphere completely hide its surface from view. This makes it impossible to determine the size of the planet by ordinary methods of observation. The value usually given, 7,650 miles, is the estimated distance across its atmosphere. The clouds, being almost uniformly bright, also prevent our watching surface marking to determine the period of rotation. Recent observations with radio-radar telescopes indicate that Venus rotates once in about 250 days in a direction opposite to that of the earth. Other observations, based on an analysis of the light reflected by the clouds, reveal that the planet’s atmosphere above the clouds is fairly rich in carbon dioxide. The amount is sufficient to smother earth-like creatures. However it does not follow that the gas is equally abundant beneath the clouds.

Vens by Adolf Schaller. A cloud of pumice emitted by an erupting volcano floats horizontally in the dense atmosphere of Venus, while far overhead, a perpetual rain of sulphuric acid turns the cloud cover a murky yellow color. A soil sampling probe rises from the deep trench in the foreground, preparing to link up with its mother ship. If life ever existed in Venus` past, fossil remains could be found only in rocks billions of years old – rocks that would be located at the bottom of just such a canyon. New methods of ensuring the survival of probes must be discovered before investigation into conditions on Venus reach the stage portrayed here. Artwork by Adolf Schaller.

A new phase in the study of Venus opened in 1962 when the Mariner II spacecraft flew past the planet and sent back valuable scientific data. The instruments aboard the spacecraft indicated that the temperatures beneath the clouds was in the region of 400 degrees centigrade. Also, that the difference in temperature between dark and bright was only about 45 degrees. The planet’s atmosphere therefore functions like a greenhouse, but at too high a temperature for life as we know it. Venus, like Mercury, is no place for astronauts.

Not too long ago, the planet Venus was thought of as “Earth`s twin” in the Sun`s family. Venus was about the same size and density as Earth, received about the same amount of sunlight on its surface (although closer to the Sun, it reflected more sunlight back in space), had a thick atmosphere, and in general appeared to be an inhabitable planet. Compared with desert Mars and stormy Jupiter, sunbaked Mercury and frozen Pluto, Venus seemed the most likely habitat for extraterrestrial life forms in the solar system. Images of lush jungles and steaming swamps had enormous appeal for astronomers and science fiction writers alike. But the ever-present clouds hit the truth from Earthbound observers over the centuries.

With the advent of modern astronomy and space probes, the planet Venus has finally yielded a few of its secrets. Many tantalizing mysteries remain regarding this brightest closest planet, but one overwhelming and undeniable fact now stands out : Far from being a world hospitable to life, Venus is a contemporary incarnation of medieval visions of Hell.

With scorching temperatures, a crushing, unbreathable atmosphere, and bizarre day-night cycles and optical effects, the planet Venus no longer beckons as a nice place to visit or settle. Instead it has become a more interesting planet; it may even hold part of the solution to some of Earth’s secrets.

On Earth, psychologists and sociologists can study the behaviour of human identical twins raised separately under different circumstances, in an attempt to measure the effects of heredity and environment of human character development.

Planetary geologists face similar questions in regard to Earth and Venus. If the two planets began alike, with similar compositions, what events and environments caused their development to part over the 4- ½ billion years since their formation? Why is Earth covered with water and teeming with life, while Venus stands sterile and hostile? If there are, in fact, millions of planets in the galaxy that have the same original composition as Earth and Venus, have these planets developed into millions of becoming copies of Earth – or millions of forbidding copies of Venus?

Some essential differences must be understood, though the causes of the effects are still not known for certain. Earth has a magnetic field and continental drift, while Venus apparently does not. Why should this be so? What effects would these have on the evolution of the entire planet? Is there any connection with what can only be a freak accident – the possession by Earth of a large natural satellite? If this is related to planetary differences, it has profound implications for life on Earth, on Venus, and elsewhere in the Universe.

After many years of spacecraft exploration and ground based studies, the old mysteries have been replaced by new ones. Today’s astronomers are faced with more subtle enigmas than those encountered by previous generations.

To the ancients, Venus was the brightest star, the “mistress of the heavens”. A puzzle even in those days, many observers did not realize that the morning star (Phosphorus) and the evening star (Hesperus) were really the same object.

With the invention of the telescope, the exploration of Venus began, with immediate, far-reaching effects for astronomical theories. Galileo’s observation of Venusian phases proved that the planets shine with reflected light like the moon and Earth. The phases also disproved (by observation of both gibbous and crescent phases) a variant of the Ptolemaic theory that claimed that Venus only moved between Earth and the Sun, and never behind the Sun. Its orbit was found to be the most circular in the solar system, as well as the third most inclined to the ecliptic (after the eccentric paths of Mercury and Pluto). Its mass was measured by subtle gravitational effects on neighbouring planets and passing asteroids. The existence of an atmosphere was confirmed by observations made during stellar occultation and solar transits.

Drawing analogies from Earth, astronomers assumed that the clouds on Venus, like those on Earth, were composed of water vapor. The chemistry that would allow modern scientists to guess at the violent Venusian atmospheric compounds was not developed until the 19th century.

Measurements of the planet’s temperature, made early in the 20th century, produced low figures – below freezing in many cases. The reasonable assumption (and the correct one) was that the reading was off the tops of the cloud layers. The clouds were surprisingly and suggestively opaque to certain wavelengths of light.

Earthbound visions of Venus began to change radically in the 1950, when measurements in the microwave frequencies (which could penetrate the atmosphere) gave an astonishing result. If all the microwave radiation from Venus was due to heat, the planet would have a surface temperature of more than 500 degrees Fahrenheit. Maintaining this steep temperature gradient between the surface and the upper atmosphere would require atmospheric pressures of 10 to 100 times as great as those on Earth.

The angular resolving power of Earth based microwave radiometers (instruments used to measure energy in one particular wavelength) could not determine where on Venus the microwave radiation was originating. Certain “nonthermal sources” were postulated for the higher-than-expected readings: lightning, ionospheric particles, radiation belts. To settle this question, scientists would have to measure microwaves in the center of the visible disk (looking straight down through the atmosphere) and at its ledges (looking through a much longer pathway of atmosphere). The higher reading would pinpoint the origin of the microwaves – either thermal (on the surface) or nonthermal (in the atmosphere).

By the time astronomers had learned enough to ask these questions and envisage such an experiment, space engineers were able to build a machine to carry it out. In 1962, the first Mariner spacecraft were launched toward Venus, and planetary exploration became “on site” as well as “remote”.

Mariner 2 carried a half dozen instruments which established conclusively that microwaves were coming from the surface and not from the atmosphere (the “limb darkening: effect was clearly observed). This meant that Venus was as hot or hotter than Earth based readings had suggested. Other experiments were also performed. Radio occultation measured the depths of the atmosphere by passing radio signals through it, while tracking of the bending of the probe’s trajectory gave precise new values for the planet’s mass. A magnetic field and radiation belts were sought, but not found.

Later American flyby probes in 1967 and 1974 studied the atmosphere and radiation environment more closely. Of particular interest was the Venusian ionosphere, which is pushed down against the upper atmosphere by the solar wind which is not warded off, as on Earth, by a powerful magnetic field. The transport of energy and matter across this boundary determines the cycle of planetary climate and the long term evolution of its atmosphere. But flyby probes could not be certain about atmospheric conditions, nor could they determine the nature of the surface.

Venus and Earth. Venus and Earth – once thought of as twin planets – are now known to be very different; little besides the two planets’ size and mass are similar. Venus has a dense atmosphere, little water, no magnetic field and extremely high surface temperature. Earth has an atmosphere only 1/100 the density of Venus’, much water, a strong magnetic field, and a temperature comfortable to carbon based life. Source of the photo: ESA

While the United States was probing Venus from a range of a few thousand miles, avoiding the difficult problems of high speed entry and data return to Earth, the Soviet Union concentrated on dropping small instrumented capsules into the atmosphere. Direct measurements of temperature, pressure and deceleration could confirm and embellish the results achieved on Earth and in Space.

The USSR launched two or three probes every 19 months for more than a decade, beginning in 1961. A total of 20 rockets were shot into space as part of the project. Initially plagued by equipment failures and insufficient design, the persistent program ultimately led to the brilliant engineering successes and important scientific discoveries of the epochal Venus 9 and 10 landings in late 1975.

In 1967, after six years of failure, Venus 4 became the first spacecraft to enter a planet’s atmosphere and send data back to Earth. Soviet designers, still skeptical of Mariner 2 data, had built a 900 pound probe to penetrate an atmosphere far thinner and cooler than what lay in wait. Descending two slowly by a larger parachute, the probe was crushed and burned tens of miles above the surface.

The next set of probes had smaller parachutes to allow them to pass through the atmosphere more quickly, before the heat could build up and fry the radios. Venus 5 and Venus 6 did reach lower altitudes, but both were destroyed when the actual conditions were far worse than even the most pessimistic Russian engineers had foreseen.

Space engineers and scientists eventually concluded that high American estimates had, if anything, been too slow. Adding several hundred bounds of insulation and structural strength to a new spacecraft, they sent two more such vehicles into space. One rocket went astray just beyond Earth’s atmosphere, but the second was right on course. In late 1970, Venus 7 reached the surface of the planet, intact and functioning. Before failing only minutes later, it reported a pressure 90 times that of Earth’s sea level, and a temperature of 900 degrees Fahrenheit.

After the engineering problems of survival had been solved, another probe was launched to make more elaborate scientific readings. Landing on the day side of Venus in 1972, Venus 8 sent back data on the light levels and on the chemical and physical properties of the surface rock: it looked like granite rubble.

Three more years were spent in the design and testing of a new generation of unmanned space probes. (The main module had been flown on Mars missions 1971, but the lander was of entirely new design). Weighing four times as much as the first generation of Soviet vehicles, the new spacecraft were comparable with the American Viking Mars probes in cost, booster size and instrumentation. In 1975, two Soviet probes of the new type were launched toward Venus.

As the ambitions of the Soviet space scientists had grown, so had their experience and capabilities. The Venus 9 and 10 probes each carried out an intricate space ballet and crucial series of successive descent stages. Both craft worked perfectly on the first try.

Several days out from Venus, each 3,400 pound lander capsule separated from the four ton mother craft by firing a small rocket. The two vehicles approached the planet at opposite edges – one to the west side and the other to the east. The lander was to whip around the left limb and hit the atmosphere near the subsolar point, while the orbiter passed the right limb at a higher altitude, missed hitting the atmosphere, and fired its main rocket engine to put into orbit around the planet. 

Immediately afterward, it would fly directly over the descending lander probe and receive radio signals for several hours. The landing would take place out of contact with Earth, so the signals recorded by the orbiter would be the only signs of success or failure.

Hitting the outer atmosphere at 25,000 m.p.h., the lander endured up to 50 Gs of deceleration for about one minute. Its speed reduced to several hundred m.p.h., it dropped the heat shield 40 miles up and popped a small stabilizations parachute. Almost at once, a larger parachute deployed and slowed the probe still further.

At an altitude of 30 miles, the genius of the Russian space engineers became apparent when the parachute was cut loose. The probe, slowed only by a metal disk around its upper structure, fell freely toward the surface in a frantic race with the mounting heat. So dense was the atmosphere that even without a parachute the probe hit the ground undamaged – exactly as the Russians had planned.

Protective hatches were dropped from positions in front of sensors, and the readings were immediately transmitted 1,000 miles into space where the mother ship recorded them for subsequent playback to Earth. As the temperature slowly penetrated the thick insulation, the harvest of the scientific data continued. When the orbiter passed out of range, many minutes beyond the designed lifetime of a half hour, the scientific station continued to send data. It probably failed within another hour, and radio contact with Earth was never restored. But the most ambitious and bold planetary voyage to date had been an overwhelming success.

The television pictures returned were sensational. The local lighting (“as bright as a cloudy June day) was far brighter than expected, and the horizon was visible against the black sky”). The camera was only at a height of a foot or two. So the horizon was obviously not far off. But cameras, both in visible light and in infrared, have become promising instruments for the study of the lower atmosphere and the surface of Venus.

The glamor of space probes may distract attention from powerful Earth based astronomical techniques. From observatories atop mountains, in high flying aircraft, and hanging from stratospheric balloons, researchers measure the properties of light reflected off the Venusian clouds. 

At radio telescopes and deep space tracking sites, engineers bounce radar signals off the planet study its hidden surface.

Some of the most spectacular results have been radar images showing Venus with optical resolution as good as the naked eye view of the Moon from Earth. Strange new geologic features, puzzlingly reminiscent of terrestrial structures, are teasing planetary geologists. Combined with news from the Soviet landers, this information has brought scientists to the verge of new discoveries about Venus.

The techniques of radar astronomy are fascinating and straightforward. A microwave beam is direct toward a target, and minutes later a faint echo returns. The frequency and time distortions in the return signal, and interference patterns set up in two widely separated receivers, are raw data for some powerful computer techniques that ultimately produce maps of surface roughness and elevation.

The first surface features on Venus were detected by radar in the 1960ies. A highly reflective (and hence probably very rugged) region named Alpha was chosen as the reference meridian for Venusian longitude. This and other features strongly suggested craters, mountains, impact basins and volcanos.

The results released in 1976 showed three new features with exciting hints of active continental drift on Venus. One discovery is a gigantic “trough” structure more than 1,000 miles long. Strikingly similar to to the rift valleys formed on Earth when tectonism splits land masses, its total length is unknown since it goes beyond the north and south edges of the radar map. Another crater shaped structure almost certainly is a volcano, possible still active. Rows of hills seem to indicate mountain building in yet another image (each view covers a circle along the equator about 1,000 miles in diameter), while certain linear features look like terrestrial faults.

Thus, the relief on Venus may not be nearly as low and monotonous as previously suspected. The radar data must be interpreted carefully, since its resolution is not great enough to detect most high mountains on Earth, and it can view only a strip along the planet’s equator. Nevertheless, some radar scans have shown uplift zones nearly four miles above surrounding plains – comparable to most terrestrial mountain ranges. Perhaps, after all, there is something on Venus similar to Earth’s continental drift, except that there are no oceans to clearly demarcate continental plates.

The evidence, of course, is only preliminary. Other observers see only impact basins and erosional structures. New radar probes will be made during future conjunctions, as other areas on the planet are mapped in for greater detail.

What does all this evidence indicate so far? While Venus remains mysterious on many points, certain features are agreed upon by most scientists. But even for these “solved” problems, here are frustrating facts which still do not fit.

The atmosphere is a good example. With a surface pressure of 90 terrestrial atmospheres (equivalent on Earth to a depths of a half mile of water), there is considerable doubt over whether it oven warrants the name “air”, rather than “ocean”. Almost entirely carbon dioxide, the atmosphere has a small percentage of other compounds that give it some special characteristics.

Venus 14. Having fulfilled its mission, the squat hulk of the Soviet probe Venus 14 sits inert in a desolate Venusian landscape; the scattering of light in the dense atmosphere produces its reddish color. Before its sensitive instruments succumbed to the rigors of 900 degrees temperatures and an atmospheric pressure 90 times that of Earth, the probe returned photographs showing that the planet’s surface was both rockier and more brightly lit by the sun than was thought possible. Artwork by James Hervat.

The light yellow clouds seen from Earth are almost certainly droplets of sulphuric acid, concentrated more strongly than an automobile battery. The actual cause of the yellow tinge, and of the variable ultraviolet absorption, is still unexplained. The top of the clouds is about 37 miles high. As altitude drops and pressure and temperature rise, droplets of sulphuric acid condense and begin to fall as rain for several miles before evaporating in the even greater beat below. This band of sulphuric acid rain is probably several miles thick. Below the rain is a clear band of carbon dioxide; the clouds probably do not reach below 30 miles.

At the surface, winds (or currents) of hot compressed carbon dioxide have been clocked at one to four feet per second. Since wind erosion is really caused by particles carried in the wind, and Venusian air currents do not seem capable of picking up such particles, erosion – in the sense of that found on Earth and Mars – probably does not take place. But the acids in the atmosphere may corrode rather than erode, eating away at exposed rock surfaces. The high temperatures would also encourage the subsurface rocks to creep over millions of years like soft wax, allowing gravity to drag the mountains down, level with the surrounding surfaces.

The rocks on the surface appear to be basalt and granite, sure signs of volcanic activity and of the chemical differentiation associated with the formation of a dense nickel-iron core. This is puzzling since Venus has no detectable magnetic field. Earth’s field is often attributed to the terrestrial nickel-iron core; perhaps, as geologists theorize, rapid rotation is also required.

For hundreds of years astronomers speculated about the length of the Venusian day. Revolving so close to the sun, was the planet locked in to a year-long day which made one side continually sunlit and left the other hemisphere eternally dark? Ok, like Earth, did Venus spin quickly? Was the answer somewhere between the two extremes?

Observation of ultraviolet atmospheric marking in the 1950th began to suggest a two to four day rotation rate. But some astronomers speculated that a retrograde (east to west) spin might even be found. Still others did not speculate at all, but built and tested radar equipment.

The first radar contacts with Venus in 1961 allowed the rate to be approximated. The Soviet published preliminary data indicating a rotation rate of about 11 days. More carefully analyzed American data pointed to the astounding conclusions that the spin took more than 200 days, was longer than the year, and was in the “wrong” direction.

If the spin is a much-reduced remnant of an originally high rate (some current theories of solar system formation suggest that all the inner planets once rotated every 10 to 20 hours)., the planet had somehow been turned upside down! If the original spin had died out due to tidal friction from the sun, how could the reverse spin have started up?

The best current theory calls for a large, off-center asteroid impact late in Venus’ formation phase. 

This presents difficulties: Such an accident could reverse the spin but could not account for the spin axis still being at a near perfect right angle to the plane of the orbit (an extremely unlikely result in a freak collision). If the rapidly spinning Venus. However, had been flattened at the poles due to centrifugal force, a spin-reversing collision could set up nearly any new axis – but this axis would eventually wander back to its old position because of the planet’s oblateness. Such oblateness could have disappeared over the millions of years that passed while the new, slow rotation rate no longer provided sufficient centrifugal force. If this explanation sounds like magic, it’s the best there is. Astronomers remain completely baffled.

Although Venus makes one complete rotation every 243 days, it is possibly misleading to call this period a “Venusian day”. Since Venus’ year is only 225 Earth days long, the combination of rotation rate and orbital motion results in a sunrise to sunrise period of about 117 Earth days. On Venus, the sidereal day is thus about twice the solar day, and the “year” is only two “days” long. Earth’s much faster rotation rate makes the sidereal day differ from the solar day by only four minutes.

As Venus’ rotation rate became more accurately determined during subsequent conjunctions, astronomers noticed a curious resonance in the length of the spin cycle. Is one synodic period (the time from one conjunction to the next; for Earth and Venus it is an average of 584 days) Venus has exactly five solar days. As a result, Venus faces its same side to Earth on every close approach.

Comparisons with Earth’s lock on the moon’s spin come to mind immediately, though the parallel is by no means exact. The lock is not perfect, since figures show that the actual Venusian day is lower than the required rate by a few hours. Furthermore, Venus does continue spinning at a constant rate; by the time it has moved from conjunction to elongation, it has turned its other face toward Earth.

Earth and the Sun exert forces of about equal orders of magnitude on the moon, but Earth grabs the moon’s spin because of its steeper gravitational gradient. The sun attracts Venus with a gravitational force 50,000 times stronger than that of Earth – yet it seems that Earth is “capturing” Venus.

The best explanation for this close resonance (and for the fact that the Venusian year is within a few hours of being exactly 8/13 of Earth’s year) is to appeal to coincidence – an unsatisfactory solution at best. Nagging doubts insist that something vital is missing from the logic involved. For Venus to have been grabbed by any other object through a natural process, its mass must have been unevenly distributed at the time. The size of the necessary asymmetry can be estimated, and it is large enough to have been noticed by its effect on passing spacecraft. But no such gravitational anomaly seems to exist. Perhaps it once did, before the forces of erosion and isostasy smoothed out the plastic-like surface.

Yet another puzzle awaits explanation; the wind velocities in Venus’ atmosphere. The broad sweeping bands in the atmosphere of Venus, photographed by the ultraviolet cameras of Mariner 10, Venus 9 and Venus 10, are well known. Over several days before and after the Mariner flyby, a sequence of pictures was taken. Patched into a 30 second movie, they show the dramatic sweep of Venus’ clouds as they move around the planet once every four days.

Venus rotates at a speed of only 5 m.p.h., at its equator, but these clouds markings are traveling at a velocity 100 times that On Earth, jet streams in the outer atmosphere may occasionally travel 20 or 30 percent faster than Earth’s spin, but the energy source for the Venusian velocity is completely unknown.

More than 10 years ago, observers on Earth had detected for to six days fluctuations in the ultraviolet markings. Doppler shift readings off the right and left limbs of the planet produced results which confirmed atmospheric motion at several hundred m.p.h. How could Venus’ upper atmosphere be propelled around the planet at such disproportionate velocities? Initial meteorological computer simulations could not account for speeds anywhere near as high. The dynamics of this atmosphere, which should have been so simple, were increasingly puzzling. Although computer models have been able to simulate the effects thought to have been seen on Venus, they seem rather strained and artificial. A far simpler solution to the whole problem is fast atmospheric rotation has now been proposed, claiming that the fast rotation is an illusion! The actual cloud may be more in line with the slow surface velocity after all.

Clearly, something does propagate around the planet with a four day period. Perhaps it is an illusion of motion caused by shockwaves which somehow alter the ultraviolet absorptivity of a slowly moving cloud layer. The actual cause of the dark and light stripes remains unknown. But what about the Doppler measurements from Earth? This theory disputes not the actual readings, but their interpretations. The scientists who originally reported the data may have ignored a vital factor which weakens their conclusions.

The redshift of the light from the approaching western limb, both indicate a fast retrograde rotation only under the assumption that sunlight reflected off the clouds does not have any Doppler shift itself. But the sunlight reflected off the edges of Venus is not from the sun’s full disk! Near the terminator (the day/night boundary), the sunlight is coming from only a fraction of the sun’s face; the rest of the sun has already set. That fraction is sending shifted light, since the limb of the sun is moving. The light from Venus received at Earth thus has a real Doppler shift, but a shift indicating velocity on the sun, not in the clouds of Venus!

The actual cloud mass does seem to have a four day pulse, but is a slow up and down motion. Lateral movement on the equator, from day side to dark side, probably takes about 60 days. Clouds on the dark side sink lower because they are not boing boiled by the sun, and in the lower atmosphere, they pick up heat convected by the winds – with the odd result that the cloud-top temperatures are several degrees warmer on the dark side!

Encouraged by the presence of such thick clouds, science fiction writers of the latest generations have vigorously and skillfully exploited the theme of Venusian oceans. So appealing was the idea that it was used in many stories even after new data had indisputably ruled out the existence of liquid water on Venus. Scientific discoveries seemed to have banished these oceans into the realm of myth. New calculations, however, indicate that the oceans of Venus may indeed have been real; they have just been dry for hundreds of millions of years.

If Earth and Venus were initially similar in composition – and there is great debate over this point among astro-chemists – the process of differentiation and volcanism on Venus might have produced as much water as on Earth. This water may have existed in liquid form for more than one or two billion years, rolling beneath a sky of carbon dioxide, nitrogen, ammonia, methane and other gases.

But today Venus is trapped in a vicious temperature cycle called the greenhouse effect. If a rise in temperature frees carbon dioxide from subsurface rocks (where a similar quantity remains on Earth to this day), it would cause the atmosphere to become more opaque to infrared light. Sunlight streaming in through the atmosphere would warm the surface; the resultant infrared rays could not escape through the atmosphere, causing the temperature to climb still further. Once the oceans boiled and the remaining carbon dioxide broke free from the rocks, the lid on outgoing heat would be sealed tight, and the temperature would climb several hundred degrees.

But without this lid, the black body (radiation only) temperature of Venus should not have averaged much above 150 degrees Fahrenheit, and oceans would have remained. If we assume a cloud cover with a high albedo, much of the sunlight would not reach the surface, or (without the greenhouse effect) could be reflected harmlessly back into space. The temperatures of the polar regions on this wet Venus could have been below 100 degrees Fahrenheit, well with the range of terrestrial life forms.

All the terrestrial effects associated with liquid water would probably have been present on such a proto-Venus: hurricanes, mountain glaciers, gentle temperature extremes, rivers washing salts and sediments into the oceans, and the primordial organic soup of amino acids and proteins that laboratory experiments have shown can form anywhere there is liquid water. If current theories about the origin of life are correct, biological processes might have begun in these Venusian oceans. 

Depending on mutation rates and the fumbling, but upward, trend of evolution, multicelled plants and animals might have begun to evolve before the greenhouse effect arrived and the oceans boiled. If this hypothetical image was once true, how could traces of these ancient life forms be found today?

If Venus ever had rivers and oceans, the river beds and ocean basins should still be there. Flat areas reminiscent of lunar maria have indeed been detected by radar. Perhaps, unlike the misnamed “seas” of the moon, the maria of Venus were truly once filled with water. Clearly, more data is needed.

Venus by NASA. The photograph of Venus, obtained by Mariner 10 during its flyby in early February 1974, was taken through an ultraviolet filter and shows the general circulation patterns of Venus’ upper atmospheres. The clouds appear to be rotating with extreme speed, but this is actually an optical illusion. NASA photograph.

Since the oceans would have vaporized only long after the tumultuous era of the great asteroid impacts there billion years ago, there would probably be few meteor craters on these dry ocean floors. 

A small number of craters on a dry ocean is a disadvantage in searching for traces of life; impacts would have torn great deep underground and far in the pas greatly facilitating the search for the only traces of life that could survive the heat and pressure of modern Venus.

This evidence would consist of fossils. Living creatures often reorder rocks and minerals into shapes that testify to the former presence of life, even after all traces of the organic compounds have disintegrated. The shells and skeletons of Venusian life, if it ever existed, might still be around.

Finding such evidence is another matter. If this wet Venus had an appreciable land area and frequent rains, a billion years’ worth of sediments could be miles thick. If Venus was once completely covered with oceans which had worn away all the dry land, the deposit of sediments would be very slow or even non-existent, allowing organic remains to completely decompose without a trace. But if conditions were just right, this biological episode in Venus’ history would have left unmistakable remains for exo-biologists to study.

On Earth, studies of ancient marine fossils can give paleontologists information on ocean temperatures and the length of ancient days. Samples from widely separated points can testify to the extent of ocean currents, thermal gradients, and intervening land masses. The same techniques that have been proved out in the oceans of Earth may find applications in the next century’s studied of the dead oceans of Venus.

If Venus had these oceans in the past, the question remains: Where did all the water go? Water vapor in the atmosphere could be slowly broken apart by sunlight into its constituent elements. The hydrogen would simple have escaped into space over a period of a billion years or so; the oxygen could have recombined with surface rocks. But if Venus ever had as much water as Earth now has, it would require a planetary layer of oxidized materials many miles deep to absorb all the oxygen.

Alternately, of course, the ocean theory may be completely wrong, and Venus may never have had much water. Different theories of solar system formation can give different ratios of Venusian water to terrestrial water. The oceans may never have been there, or the temperatures may always have been too high. (But then another agent for high temperatures would have to be invented).

Meanwhile, the abundances of other atmospheric constituents are not really very different between Venus and Earth. Most of Earth’s carbon dioxide is safely locked up in limestone and seashells; if it were to break free through a rise in the planet’s heat, it would increase the atmospheric mass 70 times.

Nitrogen makes up 75 percent of Earth’s air, and the same amount would remain in the air in such a new atmosphere. However, its proportion would be reduced to less than one percent due to the increase in the amounts of other materials. This figure is probably close to that in Venus’s atmosphere.

The vaporization of all Earth’s water would add another 300 atmospheres, and life on Earth would cease forever. Free oxygen would be gobbled up by the hot minerals covering the barren surface. Carbon dioxide would add to the greenhouse effect. Once started, this runaway process would probably be irreversible.

Such a nightmarish scenario is all the more reason to study Venus in detail. If Venus once did have oceans and life, some unknown factor must have upset the balance and destroyed the ecosphere. The solar constant may have increased ; the cosmic catastrophe which reversed the spin rate may have overheated the surface or released enough carbon dioxide to maintain a temperature climb; or, the biological processes may have upset what might have been a precarious balance too close to the sun.

Seeing how human activity on Earth may be affecting our own ecosphere, scientists would dearly like to know how to avoid turning Earth into another Venus. It might be critically important to find out.

The opposite process of planetary transformation might also someday be possible. Early in the space age, astronomers proposed a project to “terraform” Venus, changing its environment into a suitable one for human habitation. This could not be done with massive air conditioners and importation of oxygen bottles; other techniques would be needed.

Blue-green algae were to have been seeded in the upper atmosphere of Venus, where under good conditions they could have floated, reproduced and spread across the planet. They would have broken down the carbon dioxide, weakening the grip of the greenhouse effect. Ultimately, rains would fall on Venus and the clouds would break.

Unfortunately, the only rain likely to fall on Venus is a rain of sulphuric acid, inimicable to blue green algae and humans alike. There is not nearly enough water left anywhere on the planet or its atmosphere.