Blue Flower

The History of Venus (from Earth)

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Category: History of Astronomy
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Abstract

Being the third brightest object in the sky, Venus would have been a significant influence on astronomy as soon as humans began looking into the heavens (and thinking about what they saw).  Venus has the additional benefit of being close enough that man has been able to examine it closely with a variety of technologies, and send spacecraft to it.  This makes it a body that we know a great deal about, providing a long and eventful timeline of interest and discovery from Earth. Whilst Venus the planet has remained relatively unchanged over the past 10,000 years, its perceptions from Earth have changed remarkably.  The history of astronomy is not about the history of the bodies observed (the period covered is insignificant in the life span of nearly all astronomic objects[i]), but rather the progress in man’s understanding of the universe.  Therefore, the history of a planet like Venus can be examined from two perspectives - what was happening on Venus (very little), and what was happening on Earth.  The project will examine the evolution of the knowledge and perception of the planet Venus from the understanding of earth.

Venus is now quite unfashionable, and gathers nowhere near as much attention as Mars, because its future utility to mankind is limited (you can’t go there).  So we are taking a body whose level of understanding may be approaching its limit.  Working backwards, we can see that interest in Venus was much greater, and that a series of important discoveries took us to our present state of knowledge.  Examining this history provides a much greater appreciation for the planet, and the efforts made to bring us to our current (high) levels of knowledge.  The project will trace the development of the understanding of Venus from the Earth.  It will focus on the key stages of understanding and the events that triggered great leaps in this understanding.

Introduction

Whilst the Sun and Moon are the obvious dominating influences in day and night, closer inspection reveals a myriad of other bright objects.  Over time, the brightest of these is the planet Venus, and its high visibility means that it has been a source of interest to humans for tens of thousands of years.  Over time, our opinion and understanding of Venus have changed and grown. At a minimum distance of 41 million kms from Earth, and never having been visited by humans, our current knowledge of Venus is the result of observation, analysis and speculation.  Whilst the space age has delivered massive amounts of data to scientists, the history of Venus from an Earth-based perspective has always relied on these three elements.  This report will examine the changes and evolutions that have shaped our knowledge and beliefs about this heavenly body, throughout the course of human history, focusing on the history of Venus from Earth. This journey is not a neat continuum, but rather a stepped voyage, where scientific discovery and focus have allowed greats leaps in understanding to be made in relatively short periods.  Three significant events stand out, and mark key phases or phase boundaries in the evolution of human knowledge about Venus.  These three phases are derived from the 3 key observation methods available.

  1. the naked eye, whilst delivering little direct information about the planet, constitutes the bulk of human history, and are therefore the key contributor to its early history
  2. the invention of the telescope brought new perspectives that had never been available, and timed with the general advancement in sciences, brought major advances in our understanding, and dispelled many assumptions and conclusions that had been made previously
  3. the space age brought close-up and direct contact and study of the planet, and a wealth of data and knowledge

In the two gaps between these three phases were periods where little extra direct information about the planet was being gathered, but advances in other areas of society and science were driving speculation about Venus.  Because of their implications for the knowledge of Venus, they will be examined as separate phases.  Current speculation on Venus essentially makes a third interim phase, although we have no concrete ideas of what form the forth observation method will be.  This will be discussed in the conclusion on the future of our knowledge of Venus. The history of Venus is one of convergence, starting with localised mythology, through to the essentially uniform global scientific views that dominate today.  With limited or non-existent interaction, knowledge of Venus grew and developed in isolation from a number of starting points (one’s latitude and localised climate play a significant part in observations).  Therefore, tracing its history will involve starting at a number of threads, and following them until they begin to join.  A neat, sequential history of Venus is as non-producible as a neat sequential history of civilisation in general. A key objective of this report will be to put the history of Venus in the context of the history of humans, rather than simply work backwards from our present position, and assume a sequential flow to a natural end-point.  This report will also strive to avoid judging the understanding of Venus from a position that is both scientifically and culturally greatly removed from most of history.  Finally, this report will remain objective on belief systems and only use current scientific knowledge as a benchmark for comparison.

Background

We will need to jump ahead of ourselves briefly, and discuss some of the basic facts about Venus.  This is important, because it provides a context for the discussion of earlier ideas about Venus, when these facts could not have been known. Venus is the second planet from the Sun, orbiting at an average distance of 108.2 million kms.  It is about 88% of the size of Earth by volume, with an orbital period (sidereal year) of about 224 Earth days[ii].  Because of its close proximity to the Sun, and the fact that its orbit is inside that of Earth (a so-called inferior planet), it is only visible in the early evening, or prior to dawn (the evening and morning ‘stars’), with a maximum elongation angle from the Sun of 47°.  Venus is the third brightest object in the sky (after the Sun and the moon). Being an inferior planet also means that Venus displays phases.  When it is in superior conjunction, its entire surface reflect light.  Conversely, when it is in inferior conjunction, a slight sliver is visible, as most of the light is reflected back to the Sun.  This change in reflection, coupled with the change in distance, means that Venus’s brightness alters considerably over the course of its motions. Venus has an average synodic period (the time it takes to return to the same point in the sky, as viewed from Earth) of 584 days.  This interacts with the Earth’s sidereal period in a ratio of 8:5.  This 2920 day period also contains 99 twenty nine and a half day lunar cycles. When Venus is very close to the Sun, it cannot be resolved from Earth, and thus appears to disappear.  The periods for which Venus is not visible follow a regular pattern (although these number are only averages) – visible 263 days, hidden 8 days, visible 263 days, hidden for 50 days – 584 days in total[iii].

Observing Venus

As discussed, because of Venus’s proximity to the Sun, it is only seen in the pre-dawn and post-Sunset skies.  To an ancient naked-eye astronomer, its position is best plotted at the earliest point that is becomes visible.  The location of this point is then designated against a feature on the horizon, and its height above the horizon also recorded.  As the night progresses, Venus drops below the horizon.  If its initial appearance is only slightly above the horizon, then it will only be visible for a brief period.  Plotted over a number of nights, the ‘path’ of Venus would be similar to that shown in Figure 1.  However, this is one of 5 distinct ‘paths’ that Venus may take through the sky, doing so once each 8 years. venus_image001 Figure 1 - Plotting of one of five paths taken by Venus[v]

Naked-eye history

The starting point for the study of the human understanding of Venus is necessarily vague, and linked to the evolutionary concepts and dilemmas about the emergence of homo sapiens.  As we have no record of this time (estimated to be 2-3 million years ago), the point is fortunately somewhat moot.  We can be fairly sure that humans have been aware of Venus as a bright object in the sky for hundreds of thousands of years.  Our starting point for the history of Venus is limited to those civilisations that have left archaeological records that we can glean details from.  A few details survive in the form of oral history, such as the Australian aborigines:

“Venus, as the Morning Star, known to the aborigines as Barnumbir, was an important sign to a people who rose at dawn to hunt. According the Arnhem Land tradition, Barnumbir was afraid of drowning, so she was attached to a long string held by two old women. The string prevented her from rising high in the sky or drowning in the Milky Way river. At dawn the elderly women pulled Barnumbir back and kept her in a basket during the day.”[iv]

but a concrete starting point is the early civilisations to emerge from the hunter gatherer tribes.   Before exploring this early period, we should note that nearly all cultures had some mythology associated with Venus, and to document them all would not be viable, in the interest of space, but also in the interest of the project aims.  Whilst many cultures will be referred to, those of the Babylonians and the Mayans will be studied in more depth.  The Babylonians were the first major civilisation to keep detailed astronomical records, and much of their work and beliefs influenced the Greeks, making them pivotal in the history of Western science.  The Mayans represent a key society outside the Western influence, and one in which Venus played an integral part. The role of Venus in archaeo-astronomy is a subject of great speculation.  Despite some theories (eg. Postin 1982), no conclusive evidence exists that Stonehenge was aligned with any of the planets, rather focusing on the Sun and the Moon exclusively[vi].  No solid evidence exists linking any other monoliths and ancient cultural practices with Venus, but this may be a result of the limited information available to archaeologists.

Practical considerations

Whilst we are often keen to dismiss early societies as crude in their mythology and science, we often view these concepts from our own perspectives, particularly those of gods[vii].  Early civilisations saw the heavenly bodies as manifestations of the gods that controlled the universe.  However, rather than being aloof creators, these gods played an active role in the functioning of the world and the lives of humans.  More importantly, they could be influenced by the actions of human, and thus humans were very keen to determine the ‘mood’ or intentions of a god, so that an appropriate response could be prepared.  Being such a dominant astronomical body, it was natural that Venus would play an important role in the workings of the gods.

Inanna, Ishtar, Aphrodite, Venus…

Venus is the Roman name given to the goddess of love.  Whilst initially an obscure Italian goddess (whose domain was vegetation and gardens[viii]), the influence of Greek culture saw Venus associated with the Greek god Aphrodite, and over time, assumed a significant role in the Roman pantheon.  In spite of the fact that all planets apart from Uranus[ix] are named after Roman gods, the Roman civilisation made fairly minimal contributions to astronomy, when measured in relation to its size and historical influence.  To trace the roots of the god Venus being associated with the celestial body, we need to look at earlier civilisations. Most mythology is derived from that of earlier societies, and assimilated into existing beliefs through contact or conquest.  So it is with the god Venus.  One of the key early centres of civilisation was Mesopotamia (roughly the land between the Tigris and Euphrates rivers – mostly lying in current day Iraq).  The earliest dominant culture in this region were the Sumerians.  Whilst no astronomical records from the Sumerian time (about 3500-2000 B.C.E.[x]) have been discovered, the cuneiform writings used by the Sumerians have given some insights into their culture, and have allowed some understanding of their mythology.  A key god in the Sumerian pantheon was Inanna, whose legacy was later assimilated into the Babylonian god Ishtar.  Rather than an arbitrary designation, Inanna provides a good starting point to discuss the relationship between gods and the heavens. According to Sumerian mythology,

“Inanna travels to the realm of the dead and claims its ruling. However, her sister Ereshkigal, who rules the place, sentences her to death. With Inanna's death, however, nature died with her and nothing would grow anymore. Through the intervention of the god Enki she could be reborn if another person took her place. She choose her beloved consort Dumuzi, who would from then on rule the underworld every half year.”[xi]

Like most legends from past civilisations, this story has a number of different tellings.  However, the key aspects are Inanna’s decent, the stopping of nature during this descent, and the recommencement upon her ascension.  Whilst appearing fanciful to the sceptical eye, this story links quite neatly with the observations of Venus – the descent into the underworld being akin to the disappearance of Venus below the horizon.  More importantly though, is the belief that the well being of nature depended on the return of Inanna.  To a society holding such a belief system, following the motions of the Venus would be an important activity, as her reappearance would ensure the continued growth of crops.  From such an association, the link between Venus and nature, growth and motherhood emerged.  In fact, Venus is represented as female in the majority of cultures (India and Mayan cultures being notable exceptions).  One interesting aspect of this association is that the average gestation period for humans (255-265 days) coincides with the length of the phases that Venus is visible in. Although the Sumerians contributed the myth of Inanna, upon which Ishtar was modelled, the Babylonians (who came to dominate Mesopotamia after the Sumerians) were the first major society to record detailed astronomical writings (by comparison, the Egyptians, the other key society at this time, made little contribution on this field, their lack of an adequate system of mathematical notation being a key limitation[xii]).  The Assyrians (the other dominant Mesopotamian culture of the time) refined the constellations of the zodiac, however, they placed more importance on the five planets they had identified and their movements into these constellations, reasoning that the planets were gods (or homes of gods)[xiii]. The Babylonians extended the study of Venus’s motions into their astrology, and used its position (along with the relative positions of other heavenly bodies) to derive omens and forecast the future of their kings.  An example of such an omen is: “If Venus appears in the East in the month Airu and the Great and Small Twins surround her, all four of them, and she is dark, then will the King of Elam fall sick and not remain alive.”[xiv] Two key works from this period are the Enuma Anu Enlil (a collection of seventy tablets, with 7,000 omens[xv] dating from before 900 B.C.E[xvi] [xvii]). and the Venus Tables of Ammizaduga.  The second work consists of systematic observations of Venus, and the omen significations of these phases[xviii].  Both works were designed to allow previous omens to be used to interpret the meaning of new omens. venus_image002 Figure 2 - Babylonian tablet recording Venus observations[xix] The Gilgamesh epics, a Sumerian story that is believed to be the pre-cursor to many of the great legends of ancient western culture (including the Noah’s Ark tale from the bible), makes reference to Inanna, and invokes the second of the images associated with her – that of the symbol of intercourse and patron of prostitutes!  Aphrodite was noted for her numerous affairs with both gods and mortals, in spite of her marriage to Hephaestus, and one story of her creation, being formed from the “sea foam” of Uranus’s severed testicles prompts similar imagery. The subject of when the morning and evening star were determined to be one object is heavily clouded, with various theories presented.  Pythagoras was reputedly the first Greek to identify the morning and evening stars as one body (known as ‘Phosphorus’ and ‘Hesperus’ respectively), and the reference to Aphrodite for both objects began after this time.  However, as there are no direct writings of Pythagoras, there is difficulty in verifying this case[xx].  The fact that several cultures that pre-date the Greeks assigned multiple images of one god to the morning and evening planets, indicates that some knowledge of the links between the two objects existed. The difficulty stems from the fact that mythology is neither consistent, linear or in some cases, logical.  Beliefs from one area may differ from those of another, and therefore two or more stories concerning a particular god may exist.  Because we are looking back historically, the notion of which on is ‘correct’ is pointless.  Fortunately for our purposes, we are not seeking a precise history of ancient mythology, but rather seeking to identify how the planet now known as Venus was identified with certain gods, and utilised accordingly. A key factor in the prominence of Venus in ancient mythology is an extension of natural curiosity with the afterlife.  To this day, many people are not comfortable with death, and the uncertainty of what follows, and seek reassurance[xxi].  As the moon, the Sun and the stars disappear beyond the horizon, ancient cultures could only speculate about where they might go, and naturally assumed it was some sort of underworld.  The path taken by Venus (regularly dipping below the horizon) makes it a key player in this underworld, and one that can influence the events that take place there. Whilst the Babylonians matched accurate observations with a well-crafted mythology, this theme occurs regularly in many distinct cultures around the world.

Theoretical Astronomy

Whilst many cultures in this era were engaged in some forms of speculative astronomy, the Babylonians left the greatest legacy for the future, and appeared to have been the most advanced astronomers of there era.  The conquest of Mesopotamia (which at the time was dominated by the Babylonians and the Assyrians) by the Persians, allowed their work to influence other cultures.  Of even greater impact were the conquests of Alexander the Great, which brought Babylonian records into direct contact with Greek scientific thought.  The wealth of astronomical records (as acknowledged by no lesser a figure than Ptolemy (see below)) provided a framework for the Greeks to test their scientific ideas. The focus of the Babylonians was on accurately plotting the movements of the planets, and using this information to allow predictions of future locations (with its according benefits to astrology), the Greeks were less interested in the actual motions, and more interested in the possible mechanism for these motions. Although there were many contributors to Greek astronomy and science in general, three individuals stand out for their establishment of the principles that give description to the Greek understanding of Venus.  The first two contributions were more philosophical than practical, but given the impossibility of obtaining any new physical evidence of the nature of Venus, such philosophical inputs were the key to attempting to understand Venus. Plato (427 – 347 BCE) is adjudged by many as the founder of Western political philosophy[xxii].  His writings were many, and centred around philosophy and ideal government.  He proposed the idea of the Uniform Circular Motion of celestial bodies – that all bodies haves perfect motion in a perfect circle.  Having established this base assumption, he set his students the challenge of defining a mechanism to describe celestial motions, as described by 6th century scholar Simplicius[xxiii]: “Plato having, as Sosigenes says, set it as a problem to all earnest students of this subject to find what are the uniform and ordered movements by the assumption of which the phenomena in relation to the movements of the planets can be saved.”[xxiv] Plato attempted to address this issue in his book Timaeus, however the proofs were fairly crude.[xxv] Aristotle (384-322 BCE) was a student of Plato.  His major contribution to astronomy was the construction of a physical model of the heavens, which contained a series of concentric crystalline spheres.  The crystalline spheres (see-through, allowing outer spheres to be visible) were a physical manifestation of models that had been first proposed by Eudoxus, and later developed by Callippus.  The models used combinations of concentric circles to achieve a description of heavenly motions that roughly correlated with observation and mathematical prediction.  In his published work on this subject De Caelo (“On the Heavens”), Aristotle describes 55 spheres within spheres, surrounding a stable and stationary Earth, unable to move due to its large size.  In accordance with the teachings of Plato, the spheres described all moved in uniform circular motion, with the interaction of these perfect motions giving the retrograde and non-uniform motions that were observed. Whilst a breakthrough in the attempt to define a model of the heavens, the models failed to account for variations in apparent size, variations in orbital velocity and different patterns of retrogressive motion[xxvi].  Even the less rigorous observations of the Greeks highlighted imperfections in the model.  However, with Alexander’s conquest of Babylonia in 330 BCE (it was by then part of the Persian Empire), the detailed observations of the Babylonians showed the models to be in greater error.

Refining the Model

Claudius Ptolemy (c105-165[xxvii]) gives his name to the Ptolemaic view of the universe.  However, this view is essentially a refinement of the Aristotelian universe, and was not the work of Ptolemy.  Ptolemy was a compiler of astronomical knowledge at that time.  In order to conform to Plato’s uniform circular motions, and allow the model to match Babylonian data, and improving Greek observations, Ptolemy provided extended geometrical models to predict future movements and positions of the heavens.  The use of geometrical models by the Greeks marks the key differentiated from Babylonian astronomy, which utilised arithmetic formulae to predict future movements. To overcame the limits of the concentric circle, Ptolemy employed several geometrical notions.  Apollonius and Hipparchus provided the ecentre, epicycle and deferent, essentially supporting circles and offsets to allow greater flexibility of celestial movements.  To this Ptolemy added a further offset called the equant.  The detail of these devices will not be covered here, as they require significant explanation, and are ‘fiendishly difficult[xxviii]’ to understand – one of the key difficulties of the Ptolemaic system.  To account for the proximity of Venus (and Mercury) to the Sun, Ptolemy “align[ed] the centre of their epicycles with the ‘mean Sun’, so that all three had the same period of one year”[xxix]. As well as predicting the motions of these bodies, Ptolemy also assigned a positional order outward from the Earth.  Ptolemy chose to place the Sun inside those of the superior planets and the fixed star, but the positioning of Mercury and Venus (whose longer cycles places them outside the moon) appears to have been an arbitrary one (although relatively correct). All these workings and descriptions (as well as a great deal of other astronomical information, such as a detailed star chart) are contained in Ptolemy’s “Mathematical Compilation”, better known by the Latin translation of its Arabic name (al-majisti – “the greatest”) as “The Almagest”.  Whilst a significant work in its own right, its fame rests on the fact that it is the only detailed document of its type to bridge the gap between ancient and medieval Europe (via a fairly convoluted route). In our discussion on the history of Venus, conventional beliefs ascribed the heavenly body to residing on a sphere that was third from the Earth, beyond the moon and Mercury.  Whilst now assigning a physical location and mechanism to Venus, much of the work on the Aristotelian model was driven by the retrograde motions displayed by the superior planets (particularly Mars and Jupiter), with Venus’s motion being considered more straight forward.  Accordingly, during this time Venus was considered among the planets, but little attention was placed upon her individual nature (certainly when compared to other societies). Following the descent of Europe into the Dark Ages, much of the intervening work in astronomy was performed in the Moslem world.  The use of astronomy was primarily for religious purposes such as the determination of prayer times and the direction to Mecca (Islam was/is very dismissive of astrology).  The Almagest was used extensively in this time, with continual refinements made in an effort to more closely match actual observations.  Observations of Venus and the Sun led some astronomers to reject some Ptolemaic assumptions, and place the orbit of Venus on an epicycle with a constant centre.  However, no additional knowledge about Venus emerged during this time.

Venus in Mesoamerican

Europe may have been in a state of reduced culture and learning, but elsewhere, other cultures were thriving.  In Mesoamerica, the Aztecs and Mayans were building complex civilisations, with their own rich mythology and astronomy.  From our perspective, the Mayans were more Venus focused, and we will discuss their Venus beliefs further. Emerging around the time of the Christian era, they survived until the about 1000 AD and built large cities and a complex social structure.  Unlike the Babylonians and the cultures it influenced, Venus was male, and was called Kukulcan.  His brother was the Sun.  In Mayan legend, he ventures into the underworld (Xibalba) and defeats the evil lords who reside there, thereby allowing the world to avoid much pestilence and misery.  He makes five separate journeys to Xibalba, coinciding with the five separate paths that Venus takes through the sky.  When he appears as the morning star, he is announcing the arrival of his twin brother, and having just returned from the underworld, is able to offer some insight to the world.  Properly appeased with appropriate sacrifice, he may share this with the Mayan people. The Mayans viewed Kukulcan as a malevolent god, demanding sacrifices at critical times.  Of particular concern to Mayans were solar eclipses when Venus becomes visible during the day, whilst his brother the Sun disappears.  The Dresden codex, a key document of Mayan astronomical recordings and predictions primarily related to Venus, warns of pestilence resulting from this eclipse(although no known eclipse matches the period being covered).[xxx].  A number of Mayan buildings are sited towards key points in Venus’s cycle, with the temple Caracol at Chichén Itzá being oddly shaped, having its four corners pointing towards the four extreme points in the eight year Earth-Venus cycle[xxxi]. The Mayans present an interesting example of how ignoring context can prevent us from understanding the validity of unfamiliar beliefs.  In the Mayan territories, Venus was linked to rainfall, which at first given its 584 synodic year, appears unrelated.  However, as Venus consistently disappears below the horizon, a pattern relating to the point at which is dips below the horizon and length of time it stays away for emerges.  By tracking the position in the sky, and measuring the length of the disappearance, the arrival of rains could be predicted.  This gave further reason to study Venus, and ensure appropriate ritual and sacrifices were carried out.  Similarly, the Venus god also influenced the decision and timing of wars in the reason, and Kukulcan was a war god, as well as being related to crops and seasons.

The re-emergence of European astronomy

In a leading text book of the time, summarising Ptolemy’s work, Iohannes de Sacrobosco’s Tractatus de Sphaera (“On the Sphere’s” 1220) locates Venus on one of nine sphere’s:

“Around the elementary region revolves with continuous circular motion the ethereal, which is lucid and immune from all variation in its immutable essence. And it is called "Fifth Essence" by the philosophers. Of which there are nine spheres, as we have just said: namely, of the moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn, the fixed stars, and the last heaven. Each of these spheres incloses its inferior spherically.”[xxxii]

He also defined Venus’s orbital period as 365.25 days, a full Earth year, and in explaining Ptolemaic geometry, that Venus’s deferent and equant was equal to those of the other planets, and that both deferent and equant were eccentric and outside the plane of the ecliptic, but identified Mercury and Venus as being in the same plane.[xxxiii]  This summarises the breadth of knowledge of Venus just prior to the great revolution in astronomy. Tycho Brae (1546-1601), was the last great astronomer before the invention of the telescope, bringing a rigour and discipline to his observations that made the Ptolemaic model, even with its centuries of refinement, more and more tenuous.  He also addressed the prevailing geocentric theory, which failed to account for the changes in brightness of the planets, (which suggested that their distance from Earth was not constant), and the bounded elongations of Venus and Mercury.  Tycho’s model (see Figure 3) was an adaptation of an early Greek model by Heracleides of Pontus (4th century BCE), which maintained the Earth at the centre of the universe, but placed Venus and Mercury in orbit around the Sun (although Heracleides postured that the Earth revolved upon its axis, causing the rotation of the heavens). venus_image003 Figure 3 - Tycho Brae's model of the universe[xxxiv] This period of history in the western world saw the rise of the church in its power, with the domination of the Holy Roman Empire, and the break away Protestant movement triggered by Martin Luther in 1517.  But it was also a time of great growth in science, exploration and the arts (the Renaissance in Italy began in the early 1300’s).  As church philosophers, such as Thomas Aquinas, were formalising church doctrine, allowing the church to expand its influence in the emerging educational system, the spirit of discovery was causing many to actively question conventional wisdom, and seek better answers through study and insight. It should be noted that the bible makes only one reference to Venus, with Isaiah (14, 12) categorising the Babylonian Empire as being under Helal (Lucifer), "son of the morning"[xxxv].  The Jewish calendar is oriented around the lunar cycle, and the contributions of the Judeo-Christian faith to early astronomy are essentially non-existent. Chief among these pioneers was Nicholas Copernicus (1473-1543) who is universally known for his postulating of the heliocentric (Sun centred) model of the universe.  Copernicus was not the first to propose this model, with Aristarchus of Samos putting forward such a model in the 3rd century BCE[xxxvi].  Copernicus’s path to his heliocentric model was arrived at whilst endeavouring to determine the relative distances of Venus and Mercury from the Sun, compared to that of the Earth, utilising comparisons of the angles between the planets and the Sun at the planets longest elongation (furthest distance from the Sun as viewed from Earth).  Copernicus determined that Venus had an orbit 72 percent as large as Earth’s.  However, as Copernicus did not know the length of an astronomical unit, his work only yielded relative orbital sizes. venus_image004 Figure 4 - Copernicus's heliocentric model of the universe[xxxvii] He published his theories in his famous De revolutionibus orbium coelestium (“On the Revolution of the Celestial Spheres”).  Rather than a neat summation of his theory, De revolutionibus contains considerable calculations and evidence from observations to support his model[xxxviii].  The Copernican model (see Figure 4) did not improve the predictions of the positions of the planets, Sun, and Moon in the sky over the old Ptolemaic model, and because of the years of adjustments, was often inferior.  Since the planets did not accord to Plato’s uniform circular motion, Copernicus resorted to some Ptolemaic epicycles to align his predictions of planetary positions with observations. However, like Ptolemy, Copernicus could still not explain variations in the brightness of Venus.

Venus in the Age of the Telescope

Before we move into the greatest revolution in the history of astronomy, it may appear that the Greeks and their early followers, whilst making great contributions to astronomy, particularly from a theoretical perspective, have removed some of the glory in which Venus was held.  What has just preceded us beyond the Babylonians, is a fairly textbox discussion of the history of astronomy (taking the Western Civilisation centric view of history).  Venus has gone from a dominating celestial body to a mere component in a collection of objects.  Specific commentary on Venus in this time is almost non-existent. Whilst not helpful for writers of reports such as this, it identifies the holistic approach adopted by the Greeks, as they sought to understand and describe the operation of the world around them.  The emergence and eventual dominance of the Christian religion, and its mono-theistic principles meant that Venus could never regain its position of worship, and would join the other planets as lesser objects of God’s creation.  With only its celestial path to differentiate it, Venus did not attract much individual attention.  All this, however, was about to change. In 1608 a Dutch spectacle maker produced the first telescope[xxxix].  Italian astronomer Galileo Galilei (1564-1642) was one of many scientists to show interest in this new invention but, importantly for history, he also had diplomatic contacts that enabled him to secure plans for its construction.  He later learned the skills of lens grinding and polishing, and began constructing his own telescopes.  Although a monumental technical breakthrough (it essentially offered the first new perspective on the heavens in the entire history of the human species), Galileo was assisted by societal changes and the work of others (particularly Copernicus, but also Tycho Brae and his assistant Johannes Kepler, a contemporary of Galileo) from which to launch his assault on conventional wisdom. In 1610 Galileo published his Sidereus Nuncius (The Sidereal Messenger), in which he recorded his observations of the Moon and Jupiter in 1609 and 1610.  One of the key features of this book was the discovery of satellites orbiting Jupiter.  This was the first evidence of bodies that did not orbit the Earth, which was a feature of the geocentric theory.  In the fall of 1610, Galileo turned his attention to Venus, which was making its return to the evening skies.  Over the course of the coming months of observation, Galileo identified clear phases of Venus, akin to those of the moon.  Galileo wrote: “I began to observe Venus with the instrument and I saw her in a round shape and very small.  Day by day she increased in size and maintained that round shape until finally, attaining a very great distance from the Sun, the roundness of her eastern part began to diminish, and in a few days she was reduced to a semi-circle.  She maintained this shape for many days, all the while however, growing in size.  At present she is becoming sickle-shaped, and as long as she is observed in the evening, her little horns will continue to become thinner, until she vanishes.  But when she then reappears in the morning, she will appear with very thin horns, again turned away from the Sun, and will grow to a semicircle at her greatest digression”[xl] Galileo's announcement of his observations of the phase change of Venus was concealed as an anagram in a letter to the Tuscan ambassador of Prague. Haec immatura a me iam frustra leguntur o y is rearranged to Cynthiae figuras aemulatur mater amorum, which translates to "the mother of love [Venus] emulates the figures of Cynthia [the Moon]."[xli]  (Interestingly, Kepler solved the anagram as Macula rufa in Jove est gyratur mathem – “There is a red spot in Jupiter which rotates mathematically” [xlii]) Jumping back in time for a moment, the reference to horns raises a curious issue.  There have been some that claim that the phases of Venus are visible to the naked eye under ideal viewing conditions.  Whilst contentious (the astronomer Patrick Moore claims to have never viewed this phenomenon[xliii]), it brings into context the association of Ishtar/Aphrodite with the symbol of the bull, and Mexican written descriptions of Venus being accompanied by a crescent shape[xliv]. The results of his observations began to undermine the cosmological system of Ptolemy (which was already threatened by the work of theories of Copernicus and the observations of Tycho), and Galileo’s observations of Venus were key to replacing the Ptolemaic world view with the heliocentric Copernican model. Significantly also, the application of the telescope to examining the heavens opened the field of astronomy from the tracking and predicting of heavenly motions to the physical examination of the bodies therein. Because the teachings of Copernicus and the supporting observations of Galileo challenged church teachings that placed the Earth at the centre of the universe, the world view of Venus also changed.  Although the church mounted a spirited defence (they finally acknowledged Galileo’s correctness – in 1992![xlv]), the accumulation of further evidence (such as the mathematical formula of Kepler) spelt the death of the geocentric model, and Venus shifted from Earth’s child to its sibling, orbiting the Sun. Just prior to Galileo’s turning the telescope towards the stars, Tycho’s assistant[xlvi] Johannes Kepler, utilising the wealth of data Tycho had gathered, and embracing the Copernican model, determined that planets moved in ellipses with a definable motion.  He published his three famous laws (the first two in 1609 and the third in 1618[xlvii]), which allowed accurate predictions of planetary movements.  This had significant consequences for the study of Venus, as we shall see.

Beyond Galileo – the golden age of optical astronomy

In 1645, Francesco Fontana[xlviii] confirmed Galileo’s observation of the phases, and produced the first know drawings of Venus, and recorded seeing first recorded seeing dusty shadings on the planet’s surface.  Fontana also noted the presence of a small satellite, which was to initiate a controversy that would last for several centuries. In 1665, Giovanni Cassini, whose other contributions to astronomy include the discovery of Saturn’s satellite Rhea and important work on distances within the solar system, focused on the determination of rotational speeds of planets, deriving a rotational period for Venus of 23 hours, 21 minutes in 1667.  Francesco Bianchini produced a map of Venus in 1725, and a globe in 1727 (see Figure 5).  He also determined the parallax of the planet, and estimated the period of rotation at 24 days, 8 hours. venus_image005 Figure 5 - Bianchini's 'map' of Venus[xlix] Whilst Johann Schröter published two estimates for the rotational period of Venus, (the second in 1811 an incredibly precise (as opposed to accurate) 23 hours, 21 minutes and 7.977 seconds), greater interest lay in his claims that he detected not only an atmosphere, but mountains on Venus[l].  In his observations of Venus, he detected markings which he believed to indicate atmospheric absorption.  He also detected ‘horns’ on the crescents (extensions of the light into the unlit portion of the planet), which he believed could only be caused by an atmosphere.  His permanent association with the planet is from his discovery of the discrepancy between the observed and theoretical phases of the planet – now known as the Schröter effect (specifically the delay in seeing the half illumination of Venus after it has reached maximum elongation from the Sun[li]). His belief in the existence of mountains came after twice seeing (in 1790 and 1791) a bright spot inside the line of shadow, which based on his studies of the Moon (on which this is a legitimate phenomenon) he concluded to be the reflection from an exceptionally tall mountain.  Schröter was an enthusiastic supporter of plurality, and this unfortunately influenced some of his observations and conclusions (such as a visible atmosphere on Mars).  A number of other astronomers also reported mountain peak sightings throughout the 1800’s. William Herschel, famous for his discovery of Uranus, was also a keen observer of Venus.  He supported Schröter’s observations of an atmosphere and detectable markings, but was unwilling to associate the spots with a measurable rotational period, believing them to be atmospheric phenomenon. The existence of an atmosphere on Venus was actually first detected by Mikhail Lomonosov, during the 1761 transit of the Sun by Venus.  A transit occurs when Venus is at opposition to the Earth, and in the plane of the ecliptic (i.e. it passes between the Earth and the Sun).  This rare phenomenon (of which we will discuss in detail later) occurs roughly twice every hundred and twenty years or more.  During his observation, Lomonosov observed the disk of Venus as it entered and left the solar disk.  Rather than being clearly defined, as occurs during a solar eclipse, a luminous border surrounded Venus as it entered and exited the solar disk, a result of the planet’s thick atmosphere.  Unfortunately, his revolutionary discovery was not made public outside Russia until 1910[lii], leaving others to deduce this fact by less obvious means. Schröter detection of ‘horns’ in the crescent was support by observations by Trouvelot, Gruithisen, and Bogel and Lohse, leading to theories of polar ice caps, high-altitude plateaus or possibly both.  However, mountains and plateaus were only the start for Joseph Perrotin, who described a series of canals on Venus, similar to those described by Schiaparelli on Mars.  Common to both these observations was the follow-up work done by Percival Lowell, who produced maps of both of these supposed networks (see Figure 6).  Both of these observations were eventually proved incorrect. Whilst Schiaparelli do not support the Venusian canal observations, he did propose a definite rotational period – 224 days, the same as the orbital period, and thus giving synchronous rotation (as displayed by the moon).  This figure had some support up until the beginning of the space age in the 1950’s[liii].  Like rotational period, axial inclination was another characteristic subject to much debate.  Figures ranging from 0 to 80° were postulated, but like rotational period, it would have to wait for radar technology to accurately determine this value. venus_image006 Figure 6 - Lowell's map of the 'Canals of Venus'[liv] In an effort to overcome these optical limitations, the new technique of spectroscopy was applied to Venus.  In 1874, Pietro Tacchini claimed to have detected water vapour in the clouds, but this was refuted by Janssen (1882) and Vogel (1871-73).  Bernard Lyot, however, claimed that polarisation readings were consistent with that of water.  More detailed spectroscopy work by Adams and Dunham confirmed a high proportion of CO2 in the atmosphere, with later tests also revealing sulphuric acid. Whilst the Venusian canals  had only a small following, there were at least 33 observations by 15 different astronomers in the 17th and 18th centuries, of a Venusian satellite, as first described by Fontana.  Cassini wrote in his journal on August 28, 1686: "At 4.14 a.m. while examining Venus with a telescope of 34 feet focal length, I saw at 3/5 of its diameter to the east an ill-defined light, which seemed to imitate the phase of Venus, but its western edge was more flattened. Its diameter was nearly 1/4 that of Venus.  I observed it with attention for a quarter of an hour, when, on quitting the telescope for five minutes, I could not find it again, the dawn being too bright." [lv] Despite Johann Lamberts calculations of the size and orbit of the satellite in 1773, sightings could not be replicated, and no further sightings were reported in later centuries.  Work by M. Paul Stroobant, identified the source of several false readings[lvi], whilst the others are attributed to optical illusions. A more legitimate feature was the so-called Ashen Light, the faint illumination of the dark portion of the visible face.  This was first described by Giovanni Riccioli in 1643, and repeated by numerous other observers.  Research in the 1980’s indicated that the effect was caused by atmospheric electrical activity[lvii], but not before more novel explanations were proffered (see below). Using his newly developed model of the solar system, Kepler correctly predicted a transit of Venus in December 1631[lviii], the year after his death.  His next prediction was over 120 years later.  Jeremiah Horrocks (1619-1641) disagreed with available tables for planetary positions, and sought new data to confirm his notion.  Based on this data, he correctly predicted the second transit of 1639 (Venus transits come in pairs, roughly 8 years apart, separated by the much larger gap), and observed this occurrence directly.  Collecting figures of 31'30" for the Suns diameter, and 1'16" for Venus, Horrocks did not allow for terrestrial atmospheric effects, making this data inaccurate for calculations.  However, his work provided the impetus for thorough observations in 1761 and 1769. The reason for the importance of transits, is that they potentially allowed the calculation of the astronomical unit, if measurements were taken at various points around the globe (and accurate timings are used).  The 1761 observations yielded little success.  The 1769 transit achieved workable results, however the Lomonosov atmospheric observation proved a distinct hindrance to accurate measurement.  Captain Cook’s voyage that ‘discovered’ Australia was primarily focused on recording observations in Tahiti.  Apart from the ability to accurately predict her motions, these observations yielded virtually no new information about Venus.

Theories of Venus

Whilst improvements in telescopes brought increased magnification and resolution to observations of Venus, the nature of its cloud cover meant that there was little more to be revealed via this technique, and it would take the arrival of the space age to make the next leap in observation. This did not stop a number of scientists and lay-persons from speculating about what lay beneath the clouds.  Many of these scenarios were driven by the need to support theories or beliefs of the exponents, but a number of objective attempts to turn available data into possible scenarios also emerged.  With few exceptions these predictions envisaged Venus as a home to flourishing life.  The reason for this is two-fold.  Firstly, it reflects the optimism of the time, as the astronomical revolution begun by Copernicus opened the potential for other worlds, and the doctrine of plurality was embraced by many.  Secondly, tales of Venus as a barren lifeless planet do not attract as much attention, or normally require as much telling. One of the earliest and most amusing speculations was presented by Franz Gruithisen in the mid 1800’s, who noted the 47 year interval between two sightings of the Ashen light phenomenon, and suggested a massive coronation display being held for a new emperor (the shorten Venusian year making the gap actually 76 years by their reckoning – about the life span of a well-lived human ruler).  He later altered this theory, citing burning jungles and migration “so that possible wars would be avoided…”[lix] The conclusions of Julius Scheiner based on spectroscopic analysis[lx] that  “there can be no doubt that the atmosphere of Venus exerts an absorption similar to our own, and hence the nature of the two atmospheres must be similar”[lxi] triggered significant interest in Venus as a potential second Earth, both in scientific thinking, and general popular culture.  Ideas on the nature of Venus formed the basis of several works of fiction (“Carson of Venus” by Edgar Rice Burroughs,1915 and “Perelandra” by C. S. Lewis, 1942 are two notable examples).  But several more scientifically based postulations were also presented. In 1915 CE Housden published “Is Venus Inhabited?”[lxii], where he adopted Schiaparelli’s 225 day rotation period, Lowell’s canals and an intriguing weather system, to support his case for life on Venus[lxiii].  More objective works followed from Svante Arrhenius, who proposed that Venus was similar to a 200 million year old Earth, with an “average temperature of 47°C, …[and] humidity six time the average of that on the Earth”[lxiv].  A swampy landscape with amphibians dominating, and the potential at the cooler poles for the development of culture and more sophisticated life forms were some other features.  Before dismissing this notion, it should be noted that Arrhenius was a Nobel prize winner for chemistry, and supported his case with available scientific data.  Soviet astronomer G.A. Tikhov made similar predictions.  Seth Anderson and Charles St John refuted these ideas, based on their spectroscopic studies, and proposed a dry and hot planet, with clouds of dust and constant dust storms, predictions that began to deviate towards the actual conditions on Venus.[lxv] Right up until the start of the space race[lxvi], alternate descriptions of what the Venusian surface might contain were presented.  Whipple and Menzel acknowledged growing evidence of CO2 in the atmosphere, but clung to the hope of an Earth-like ocean.  Their compromise model featured a planet completely covered by ocean, but with large amounts of CO2 dissolved in it – a so-called ‘seltzer water’ model[lxvii].  Ever optimistic, they proposed highly developed life-forms, although necessarily marine based. Fred Hoyle went even further, and proposed a surface of liquid hydrocarbons absorbing oxygen from the air, causing the breakdown of water molecules, until all water was removed from the planet – essentially an oil-ocean planet.  If water was dominant, then the hydrocarbon-oxygen mixing process would cease one the hydro-carbons had fully absorbed available oxygen, leaving excess water.  Hoyle proposed that this was situation on Earth, and that the differences between Earth and Venus were related to the difference in initial hydrocarbon/water balances.  He further reasoned that the oil would create tidal effects that would slow Venus down, which conformed to emerging rotational speed predictions.

Venus in the Space Age

Getting Close to Venus

At the outset of the space age, Venus was the most attractive planet for research, although the spectroscopic studies that had indicated a high level of carbon dioxide in the atmosphere, removed some of the enthusiasm of her hosting life.  Venus’s was an inferior planet, meaning there were more opportunities for launching probes, and a shorter and less fuel costly orbital path[lxviii]. Initial attempts at sending spacecraft to Venus were conspicuous by their failure.  Zond 1, Venera I, II and III and Mariner 1 all failed to either launch, or maintain radio contact for the length of the journey.  However, with the US Mariner II and later Soviet Venera missions great advances were made in our knowledge of Venus, and the age of speculation was replaced by the age of data. As this report is focusing on the human knowledge of Venus, rather than the history of space flight, the 26 missions (both successes and failures) are detailed in Appendix 1.  All but four reached Venus, some on-route to other destinations[lxix].  From these efforts, a much clearer picture of Venus was uncovered.  The break-throughs can be grouped into three main areas.

Venus Above the Clouds

When Mariner II placed itself into orbit in December 1962, it was equipped with an array of instruments that established a number of key facts about Venus, including:

  • a very high temperature (480 degrees K)
  • no detectable magnetic field
  • continuous cloud cover to a height of about 60 km

and confirmed some Earth-based predictions:

  • a carbon dioxide atmosphere long rotation period (did not measure exactly, although it was able to determine that the rotational period was greater than the orbital period, i.e. the Venus day is longer than the Venus year)
  • retrograde rotation

The mission also improved estimates of Venus's mass, and the value of the astronomical unit[lxx].  It was later determined that although an intrinsic magnetic field was absent, the impact of the solar wind creates a magnetic layer around the planet, performing a similar role to Earth’s magnetic field. The atmospheric constitution was refined by the Pioneer Venus mission in 1978, which determined a 96.5% CO2 – 3.5% nitrogen atmosphere, with trace amounts of other chemicals.  Venera 15 and 16, whilst primarily orbiter missions, detected higher levels of Argon 36, which suggested that Venus retained more of its primordial atmosphere than Earth[lxxi].  Current atmospheric components (ppm) are[lxxii]: Sulphur Dioxide (SO2) – 150 Argon (Ar) – 70 Water (H2O) – 20 Carbon Monoxide (CO) – 17 Helium (He) – 12 Neon (Ne) - 7 Mariner 5 extended this data in a flyby mission that measured “interplanetary and Venusian magnetic fields, charged particles, and plasmas, as well as the radio refractivity and UV emissions of the Venusian atmosphere”[lxxiii].  The Mariner 10 mission, en-route to Mercury[lxxiv], was equipped with a camera, and was the first craft to return close-up images of the planet (see Figure 7 - Photo of Venus from Mariner 5).  The images confirmed the 4 days rotation period for the upper cloud level, and showed that little difference existed between the night and day temperatures of Venus. venus_image007 Figure 7 - Photo of Venus from Mariner 5[lxxv] En-route to intercept Halley’s comet, the Soviet probes Vega 1 and 2 released balloon probes into the atmosphere, which drifted for 56 hours.  These provided new information on wind patterns and atmospheric cells, and confirmed the process by which consistent global surface temperatures are maintained.

Venus Below the Clouds

Whilst the Americans focused on going around Venus, the Soviets turned their attentions to what lay below the clouds.  They reasoned that the best way to determine this was to send craft into the Venusian atmosphere, and ideally land a craft on her surface (also Mariner II got the jump on them, and as this was the era of the Space Race, they needed something bigger and better).  The Venera missions from Venera 4 to Venera 14 were all probes or landing craft.  These missions have been our only physical contact with the planet. Venera 4 was the first craft to enter Venus’s atmosphere, and returned a wealth of atmospheric data.  However, it was crushed by the dense atmosphere at a height of 25km, indicating that heat was not the only problem probes to Venus would encounter.  The probe also detected the presence of highly corrosive sulphuric acid in the atmosphere.  The Soviets therefore needed to design craft to handle the most extreme conditions in the solar system – short of flying into the Sun. Venera 7 was the first lander to successfully reach the planet’s surface[lxxvi], and sent signals for 20 minutes.  It measured a surface temperature of 475°C and a surface pressure of 90 bars (equivalent to an ocean depth of about 2km).  Venera 8 performed a similar mission, although landing on the day side of the planet (and confirming Mariner 5’s conclusion about similar day and night temperatures). Whilst significant progress had been made in understanding Venus, the visual progress being made by the various US Mars missions[lxxvii] increased the perceived sense of understanding of this planet.  This probably marked the point at which Venus declined in the public imagination, and Mars moved to the forefront.  Data from the two planets also showed Mars to be by far the more hospitable of the two, and offering far greater possibilities for life and future exploration – even manned exploration – which was at the forefront of the public’s imagination in the shadows of the Apollo program. The lack of surface visuals was addressed by four Soviet missions, Venera 9,10,13 and 14[lxxviii].  Whilst images were taken (see Figure 8), the operational limitations of the Venusian environment severely limited their scope and quality.  They answered a number of queries that had been raised by scientists.  Firstly, despite the heavy cloud cover, light levels at the surface are quite high – akin to midday on a cloudy winters day in Moscow accordingly to Soviet researchers[lxxix].  Also the atmosphere was not super-refactive  - the particles in the atmosphere do not create optical illusions as described by National Geographic in 1975: "If a man could stand on Venus on a clear day the observer would enjoy one of the strangest experiences of a life time. Because of super refractivity, the acute bending of light rays by the ultra dense atmosphere, one could in theory see all the way around the planet. In effect he would seem to be standing at the bottom of a bowl with the entire planet stretching up endlessly on every side.”[lxxx] venus_image008 Figure 8 - surface picture of Venus taken from Venera 13[lxxxi] From the Venera 9 images, two key facts emerged – the lack of dust on the surface (the lander did not kick up any dust on impact), and the rocky nature of the landscape, with a predominance of sharp-edged boulders averaging about 1 metre across.  Venera 10 showed large slabby outcrops, which contained smoother rocks, and gave the appearance of being a much older landscape[lxxxii]. Wind gauges on these lander missions gave readings of 0.3-1.0 m s-1, which although gentle by Earth’s standards, when combined with the much greater atmospheric density, should have produced much more significant erosional features than the images showed.  Venera 10 also confirmed that the cloud base ends well above the surface, and that a layer of super-heated smog exits between surface level and the start of the cloud base. Venera 13 and 14 produced colour images, as well as performing a number of soil analysis experiments.  From these and earlier missions, scientists began to form a picture of the Venusian surface, being highly alkaline, possessing potassic salts, and being primarily of basaltic composition, consistent with a volcanic landscape.[lxxxiii]

Venus Through the Clouds

The constant cloud cover and hellish surface conditions made any significant visual examination impossible.  The Venera pictures were helpful, but from a global perspective, as useful as taking a single picture from a random point on Earth, and attempting to deduce the entire planet from this location (particularly as that location is likely to be water). Whilst optical examination was out, and efforts in the infra-red spectrum were unsuccessful, the development of radio astronomy during and after the second world gave astronomers a way to see under the clouds.  The longer wavelengths used are not reflected by the cloud layer, and allow readings of surface height and ‘brightness’ (the manner in which it reflect the radio signals).  From this data, scientists can piece together a global image of the planet with increasing degrees of confidence[lxxxiv]. The pioneering work in this arena was done from Earth based radio telescopes (and transmitters), with three locations utilised: Haystack in Massachusetts, Goldstone in California and Arecibo in Puerto Rico.  To achieve the best results, measurements must be done at inferior conjunction (to remove as much solar noise as possible).  Whilst experiments in the 1960’s focused on determining distance and rotational speed, the 1970’s turned their focus to gathering topographic data.  Arecibo produced the best land-based data, but was only able to map 25% of the surface area, and raised more questions than it answered.  The limitations of distance meant that radio measurements from an orbiter were the only way to dramatically improve accuracy.  Several US and Soviet missions addressed this. The Pioneer-Venus mission was both an orbiter and a series of co-ordinated probes to allow multiples location analysis,  The orbiter performed radar mapping, combining physiographic mapping with gravity data.  As well as producing the most detailed surface map of that time (it covered 93% of the surface are between 74°N and 63°S[lxxxv]), it precisely determined the retrograde rotational period at 243 Earth days[lxxxvi]. Pioneer Venus discovered a very flat planet, when compared with Earth, Mars and the moon, comprising three broad categories of landform;

  • extensive upland rolling plans
  • less-extensive lowlands, thinner crust and lower density, akin to the Luna maria
  • three limited regions of highland – Ishtar Terra, Aphrodite Terra and Beta Regio

The highest point on Venus is near Maxwell Montes, some 11km above mean surface level. Having lead the way with surface landers, the Soviets embarked on their own radar mapping program, with the Venera 15 and 16 missions, which operated simultaneously.  Their two key contributions were the mapping of the northern poles that had not been covered by Pioneer Venus (showing extensive plains at mean datum), and a number of new landforms, such as ridge and trough belts, parquet (interlocking) terrain, faults and halos of impact ejecta around craters. The most comprehensive radar mapping effort of Venus has been the Magellan mission, arriving at Venus in 1989[lxxxvii].  It sought  to gather improved radar and gravity information to allow resolution of surface feature questions.  Magellan was also seeking evidence of past water, and possible plate tectonics.  It is the last craft to have visited Venus, crashing into the atmosphere in 1994. Magellan mapped 99% of the surface area (see Figure 10) to a resolution of 120 metres, and performed accurate gravity calculations to determine mass variation.  As well as being smooth, Venus has a unimodal landform distribution (Earth and Mars are bimodal).  This means that the mean surface level is also the most common, whilst Earth and Mars have peaks either side of the mean surface level (see Figure 9).  90% of Venus lies within a 3km height interval. venus_image009 Figure 9 - landform elevation distributions[lxxxviii] Actually, initial descriptions of Venus as flat are somewhat misleading, as it lacks the large even plains found on Earth and Mars, and instead has a slight but consistent slope.  At higher altitudes, the surface is considerably rougher.  The two main landforms are volcanic lava plains (85% of surface area), and tectonic highlands (15%).  The tectonic highlands, however, are do not appear to be caused by plate tectonics, but rather are aligned with crustal fissures (as evidenced by gravitational data).  This gives rise to global mountain and ridge structures, focused along these fissures[lxxxix]. The last key piece of data is the frequency and distribution of impact craters.  Compared with Mars and Mercury, Venus has a low number of craters, evenly distributed across the planet.  This indicates that a major resurfacing event took place in the (geologically) recent past – possibly 500 million to 1 billion years ago.  This is supported by the large number of volcanic features, including fields of volcanos unique to Venus, indicating a long and active volcanic history. venus_image010 Figure 10 - Magellan composite topographical image of  Venus[xc]

Venus in the future

A great deal of progress has been made in our understanding of Venus, by both Soviet and US missions.  The thorough mapping of the surface by Magellan, and the difficulty in achieving extended surface times, indicates that the present limits of our understanding of Venus may be approaching.  However, one future missions to Venus has been planned[xci]. The Planet-C spacecraft is a Venus Orbiter mission is focusing on the atmospheric dynamics, particularly the upper atmosphere, and the super-rotation phenomenon. The probe will also attempt to find evidence of atmospheric lightning  and volcanic activity[xcii].  This is a Japanese initiative, and is scheduled for launch in February 2007. With the majority of space research programs being directed toward Mercury (only one previous mission has visited this planet) or the Jovian planets and their satellites, it appears that we are approaching the limits to what we can learn about Venus.  However, a number of questions still remain, which are likely to form the core objectives of future missions.:  These include[xciii]:

  • The age of the surface, and the likelihood and mechanism for proposed massive resurfacing
  • The possibility of current volcanic activity
  • The distribution of rock types
  • The physical and chemical interaction of surface and atmosphere (particularly in relation to erosional landforms
  • The past existence of water on Venus
  • The mechanism and timing of the runaway greenhouse effect
  • The core structure of the planet

Of particular significance to humans is the study of the runaway greenhouse effect, as this is an effect that will eventually (as the sun heats up over the next 100 million years) be faced by Earth, but possibly on a much shorter time-frame if humans show the ongoing capacity to massively influence their environment.

Conclusion

We have seen throughout this report the endless human capacity to seek to understand Venus with the available tools and concepts.  The history of Venus from Earth in some ways mirrors the history of civilisation, with an initial awareness of the environment, turned into attempts to utilise it in a positive way (Venus based mythology).  As the field of science expands, objects are investigated for what their physical nature may be (rather than just their utility), and the expansion of technology enhances the capacities to understand this physical nature. Whilst the current knowledge of Venus is a testament to the brilliance of modern science, and the deep pockets of their supporting governments, within the context of history, the works of Galileo, Ptolemy or the Babylonian astronomers are no less significant.  What we know of Venus is an indicator of how far we have come as a society, and the colourful routes we have taken to get there.

Appendix 1 – Missions to Venus[xciv]

Mission Country Launch Date Arrival Date Type Encounter Characteristics
 
Venera 1 USSR February 12, 1961 ---- Flyby Now in solar orbit
Mariner 2 USA August 27, 1962 December 14, 1962 Flyby Closest approach: 34,833 km
Zond 1 USSR April 2, 1964 ---- Probe Now in solar orbit
Venera 2 USSR November 12, 1965 ---- Flyby Communications failed just before arrival. Now in solar orbit.
Venera 3 USSR November 16, 1965 ---- Atmospheric Probe Communications failed just before atmosphere entry. Crashed on Venus
Venera 4 USSR June 12, 1967 October 18, 1967 Atmospheric Probe First probe to be placed directly in the atmosphere and to return atmospheric data. It was crushed by the pressure on Venus before it reached the surface.
Mariner 5 USA June 14, 1967 October 19, 1967 Flyby Closest approach: 3900 km
Venera 5 USSR January 5, 1969 May 16, 1969 Atmospheric Probe Burn-up
Venera 6 USSR January 10, 1969 May 17, 1969 Atmospheric Probe Returned data down to within 11 km of the surface before being crushed by the pressure.
Venera 7 USSR August 17, 1970 December 15, 1970 Lander First successful landing of a spacecraft on another planet. Returned 23 minutes of data.
Venera 8 USSR March 27, 1972 July 22, 1972 Lander Returned data for 50 minutes
Mariner 10 USA November 3, 1973 February 5, 1974 Flyby Dual planet mission to Venus and Mercury. Closest approach: 5700 km Images of cloud top
Venera 9 USSR June 8, 1975 October 22, 1975 Orbiter Periapsis: 1560 km Apoapsis: 112,200 km Period: 48 hours, 18 min Inclination: 34* 10' Photographed clouds and looked at the upper atmosphere.
Lander Transmitted first black and white pictures of the planet's surface
Venera 10 USSR June 14, 1975 October 25, 1975 Orbiter Periapsis: 1620 km Apoapsis: 113,900 km Period: 49 hours, 23 min Inclination: 29* 30' Photographed clouds and looked at the upper atmosphere
Lander Transmitted black and white photographs of the terrain.
Pioneer Venus 1 (Pioneer 12) USA May 20, 1978 December 4, 1978 Orbiter Periapsis: 200 km Apoapsis: 66,000 km Period: 24 hours Inclination: 29* 30' Operated until 1992 when contact was lost. First spacecraft to use radar in mapping the planet's surface.
Pioneer Venus 2 (Pioneer 13) USA August 8, 1978 December 9, 1978 Atmospheric Probe 4 probes parachuted through the atmosphere.
Venera 11 USSR September 9, 1978 December 25, 1978 Flyby Closest approach: 25,000 km
Lander Returned data for 95 minutes. Imaging systems failed.
Venera 12 USSR September 14, 1978 December 21, 1978 Flyby Closest approach: 25,000 km
Lander Returned data for 110 minutes. Electrical discharges were recorded.
Venera 13 USSR October 30, 1981 March 1, 1982 Flyby  
Lander First colour panoramic views of the planet's surface. Conducted soil analysis.
Venera 14 USSR November 4, 1981 March 5, 1982 Flyby  
Entry probe Returned both black & white and colour panoramic views of the planet's surface. Conducted soil analysis.
Venera 15 USSR June 2, 1983 October 10, 1983 Orbiter Radar imaging
Venera 16 USSR June 7, 1983 October 14, 1983 Orbiter Radar imaging
Vega 1 USSR December 15, 1984 June 11, 1985 Balloon/Lander Vega 1 dropped off a Venera style lander and a balloon. The lander's soil experiment failed. The balloon floated for about 48 hours. Now in solar orbit.
Vega 2 USSR December 21, 1984 June 15, 1985 Balloon/Lander Vega 2 dropped off a Venera style lander and a balloon. The lander conducted soil experiments. The balloon floated for about 48 hours. Now in solar orbit.
Galileo USA & Europe October 18, 1989 February 10, 1990 Flyby Images and near-infrared data on clouds. Used Venus to pick up speed on its way to Jupiter.
Magellan USA May 4, 1989 August 10, 1990 Orbiter Mapped Venus using synthetic aperture radar. The imaging system produced images at 300 meters resolution.
             

Note: the Cassini Saturn orbiter (launched October 1997) utilised Venus for a gravity-assist, but did not perform any observations of the planet.

Bibliography

Moore, P. & Cattermole, P. “Atlas of Venus”, CambridgeUniversity Press, 1997 Aveni, A. “Conversing with the Planets”, Times Books, 1992 Suppe, F. “Venus Alive: Modelling Scientific Knowledge”, Draft - http://carnap.umd.edu/phil250/venus_alive.html Kolb, E. “Blind Watchers of the Sky”, OxfordUniversity Press, 1999 Hoskin, M.(ed.) “The Cambridge Concise History of Astronomy”, CambridgeUniversity Press, 1999 Ward, P. & Brownlee, D. “Rare Earth”, Copernicus, 2000 Henry, J. “Moving Heaven and Earth”, Icon Books, 2001 Kaufmann, W. & Freedman, R. “Universe (5th Edition)”, W.H. Freeman and Company, 1999

[i] except the ones that happen to explode or crash into large planets [ii] Moore et al, p3 [iii] Aveni, p.30 [iv] http://www.att.virtualclassroom.org/vc99/vc_04/planets/venus/myth_venus.html [v] Aveni, p29 [vi] http://www.tivas.org.uk/stonehenge/stone_ast.html [vii] or the non-existence thereof [viii] http://www.pantheon.org/articles/v/venus.html [ix] The Greek god of the sky. [x] As well as being rough date estimates, the dynamics of Mesopotamia in this era are very complex, with a number of competing internal and external influences.  Therefore, this report will stick to general statements, and acknowledge that this are simplifications of a much richer historical period. [xi] http://inanna.virtualave.net/inanna.html [xii] Hoskin et al, p23 [xiii] http://www.touregypt.net/astro/ [xiv] http://www.accessnewage.com/articles/astro/rhist2.htm [xv] Most of these omens relate to the moon, not Venus (Hoskins et al, p23) [xvi] This report will use the term Before Common Era (BCE) to refer to dates prior to the start of the modern calendar. [xvii] Hoskin et al, p20 [xviii] http://www.accessnewage.com/articles/astro/rhist2.htm [xix] Suppe, p29 [xx] This fact is not conclusively acknowledged in any historical work, see http://www.philosophos.com/knowledge_base/archives_16/philosophy_questions_1624.html [xxi] leading to appalling, exploitive TV shows like “Crossing Over” [xxii] Others nominate Socrates, but this is not relevant to our discussion [xxiii] Simplicius focused on the writing of Aristotle and Plato, and on reconciling their views into a consistent philosophy - http://www-gap.dcs.st-and.ac.uk/~history/Mathematicians/Simplicius.html [xxiv]  http://www.cco.caltech.edu/~deborahe/Simplicius.htm [xxv] http://www.ucl.ac.uk/sts/gregory/326/handouts/h04_gas.doc [xxvi] http://www.ucl.ac.uk/sts/gregory/326/handouts/h04_gas.doc [xxvii] No details of Ptolemy’s life survive, so these figures are estimates based on the periods covered by his work – Hoskins, et al p40 [xxviii] http://www.ucl.ac.uk/sts/gregory/326/handouts/h04_gas.doc [xxix] Hoskins et al, p44 [xxx] Aveni, p77 [xxxi] http://www.civilization.ca/civil/maya/mmc07eng.html [xxxii] http://www.esotericarchives.com/solomon/sphere.htm a 1949 translation by Lynn Thorndike. [xxxiii] http://www.esotericarchives.com/solomon/sphere.htm [xxxiv] http://web.clas.ufl.edu/users/rhatch/pages/03-Sci-Rev/SCI-REV-Home/resource-ref-read/chief-systems/08-0TCHON-WSYS.html [xxxv] http://www.newadvent.org/cathen/02029a.htm [xxxvi] http://www-gap.dcs.st-and.ac.uk/~history/Mathematicians/Aristarchus.html [xxxvii] http://web.clas.ufl.edu/users/rhatch/pages/03-Sci-Rev/SCI-REV-Home/05-SR-TABLE-CONTENTS.html [xxxviii] Consequently, the book was difficult to understand, and may explain the delay in his ideas being more widely embraced [xxxix] Hans Lipperhey filed a patent on 2 October 1608, but several others have claimed they invent to the device [xl] van Helden, A. “Siderius Nunicus or the Sidereal Messenger, Galileo Galilei”, University of Chicago Press, 1990.  Note Galileo’s use of the feminine for to describe Venus. [xli] van Helden, p 107 [xlii] Aveni, p 189 [xliii] Moore et al, p [xliv] Aveni, pp 187-188 [xlv] Moore et al, p7 [xlvi] albeit for a brief 6 months prior to Tycho’s death [xlvii] Kaufmann, et al, p82 [xlviii] As we are dealing with a larger number of individual making smaller contributions, we will omit dates of birth and death from hereon in [xlix] http://www.bo.astro.it/~biblio/Vultus-Uraniae/Globe-of-Venus.html [l] he made similar claims for Mercury [li] http://scienceworld.wolfram.com/astronomy/Dichotomy.html [lii] http://users.aol.com/dlehman111/ObsVenus.html [liii]Moore, et al, p14 [liv] http://www.geocities.com/kev_woodward/Venus/Venuscanals.htm  - incorrectly attributed to Schiaparelli. [lv] http://www16.brinkster.com/phantomvenus/BASTARD.htm [lvi] Suppe, p22 [lvii] Moore et al, p17 [lviii] This was not observed, as it occurred after sunset in Europe [lix] Moore et al, p17 [lx] http://home.europa.com/~telscope/histspec.txt [lxi] Suppe, p29 [lxii] He had previously published a similar work on Mars the previous year. http://www.pennpress.org/mariner10/redplanet/bibliography_science.htm [lxiii] Suppe, p29 [lxiv]Moore, p25 [lxv] Suppe, 33 [lxvi] Officially started with the launch on Sputnik in 1957 [lxvii] Suppe, p 41 [lxviii] Inter-planetary travel is achieved by placing an object in orbit around Earth, and then increasing or decreasing the orbital velocity around the sun, pushing the craft into a new orbit that will eventually match that of the intended destination planet. [lxix] Mariner 10 to Mercury, Vega 1 and 2 to Halley’s comet, Galileo to Jupiter [lxx] Earth-Sun distance [lxxi] This in turn gave possible support to the lunar creation model, whereby a large object impacted with Earth, and created the moon, with some key elements being lost to the Earth system as a result. [lxxii] http://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html [lxxiii] http://nssdc.gsfc.nasa.gov/nmc/tmp/1967-060A.html [lxxiv] Venus was used to provide gravitational correction for the Mercury encounter [lxxv] http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-venus.html [lxxvi] Venera 5 & 6 were atmospheric probes [lxxvii] Mariners 4,6,7,8 & 9.  Unlike Venus, the Soviets had a disastrous track record with Mars – non of their 16 missions achieved its objectives, with limited orbital data from three missions being their only reward. http://www.solarviews.com/eng/craft2.htm#mars [lxxviii] The video equipment on Venera’s 11 and 12 failed. [lxxix]Moore, et al, p35 [lxxx] Weaver, K. “Mariner Unveils Venus and Mercury”, National Geographic Magazine, June 1975 [lxxxi] http://www.solarviews.com/cap/venus/vener13b.htm [lxxxii] Moore et al, p36 [lxxxiii] Moore et al, p35 [lxxxiv] For an excellent coverage of this process, see Suppe, F “Venus Alive!” [lxxxv] Moore et al, p44 [lxxxvi] Moore et al, p44 [lxxxvii] It’s journey there were complicated by the Space Shuttle disaster, and the resulting postponement of NASA missions, eventually doing a full orbit of the sun before reaching Venus. [lxxxviii] Moore et al, p54 [lxxxix] Moore et al, p72 [xc] http://www.solarviews.com/cap/venus/topo.htm © Calvin J. Hamilton [xci] The MESSENGER Mercury orbiter will make two fly-bys of Venus, but has limited research objectives.  The European Space Agency BepiColombo Mercury mission will also include a Venus fly-by. [xcii] http://nssdc.gsfc.nasa.gov/planetary/prop_missions.html#planetc [xciii] http://www.nasm.si.edu/ceps/etp/venus/venus_next.html [xciv] http://www.mindspring.com/~broel/pages/missions_to_venus.htm