Astronomy; The Geology, Climatology and History of Planet Venus
Credit to Nasa
Unless otherwise indicated, all photos on this page are credited to Nasa.
My thanks to Nasa - without these spectacular images, this webpage would not have been possible
Venus is our nearest planetary neighbour in space, and for a long time was thought to be quite similar to our own planet Earth. However it has become increasingly clear in recent decades that this is far from being the case.
More than 30 spacecraft since 1962 have visited the planet, and more than 20 of these missions have sent probes into the atmosphere or to land on the surface. Others have been entered into orbit around Venus or have imaged the planet during fly-bys. And yet despite all this close attention, our knowledge remains limited, as more questions have been raised than answered.
The reason why this is so is due to the violent geological history, and very alien climatology of Venus, and in this page I hope to make clear the sequence of events which may have made Venus so different to the planet so often considered its sister in space - our own planet Earth.
The science behind this is very complex and confusing, (particularly the science relating to magnetic fields and greenhouse gasses), yet my desire is for this page to be readable and quite easy to follow. Therefore, detailed science will be limited, but numbered references will link at the bottom of the page to further, more involved reading. Hopefully the information will be comprehensible, logical and up to date - if not, please let me know, and I will try to amend it accordingly.
This page is the second of two pages about Venus.
Page 2) A more comprehensive study of the geology and climatology.
IMAGES OF VENUS
Few good quality images were taken of the surface of Venus prior to the 1980s. This was because hostile conditions on the surface brought about the very rapid demise of most lander craft soon after touchdown, and the planet's very thick atmosphere was quite impenetrable to conventional photography. Then in the 1980s and 1990s, advancements in radar imaging enabled the clouds of Venus to be penetrated at last, and the pictures on this page are the result of that new technology. It should be pointed out that most of these images cover a vast scale (several hundreds of kilometres) and are taken from altitude. The vertical scale of many photos (height) has been greatly exaggerated to enhance smaller surface features. Colour where applied is false, but is based on photographic images from earlier missions.
THE KNOWN FACTS AND SPECULATIONS ABOUT VENUSIAN GEOLOGY AND ATMOSPHERE
The discussion on this page will comprise two main parts.
1) THE EVIDENCE;
These are the clues which help us to understand how this planet came to be in its current form. The evidence of basic physical characteristics, the evidence of Venus's magnetic field, the evidence of the known physical and chemical characteristics of Venus's atmosphere, the evidence of Venus's topography, the evidence of craters on the planetary surface, and the evidence of plate tectonics.
2) THE HISTORY OF VENUS;
Likely scenarios for the very ancient history and the more recent history of Venus, based upon the evidence.
THE EVIDENCE OF BASIC PHYSICAL CHARACTERISTICS
Before the development of new technology and space probes enabling us to study its atmosphere and surface features in more detail, Venus was frequently portrayed as Earth’s twin. In many ways it still is. Venus is our nearest planetary neighbour, it is of quite similar size, with similar density, and it has a similar gravitational strength to Earth, which all suggests that the basic composition and physics of the planet should be similar. It is believed that Venus, like Earth, should have an iron/nickel based core approximately 6000 km (3,700 miles) in diameter. The next layer, called the mantle is probably about 3,000 km (1860 miles) thick, and the crust of Venus should be between 25 km and 60 km (15-35 miles) thick.
However, four possibly fundamental differences between Venus and Earth could long ago be identified or inferred, before the very first space exploration of the planet, and this section deals with some of the possible implications of these four differences:
- Venus is slightly smaller than Earth.
- Venus has no moon.
- Venus has a slow, retrograde spin
- Venus, being closer to the Sun, would presumably not only be hotter today, but would always have been hotter.
1) Venus is 95% of the size of the Earth. The size discrepancy on the face of it may not seem significant, but this will have some implications for such factors as internal heat generation and core pressure.
2) There is nothing remarkable in the fact that, unlike the Earth, Venus has no moon. Mercury doesn’t either, whilst Mars only has two tiny little moons. However as Venus is a comparably sized planet to our own, one might reasonably wonder whether the absence of the immense gravitatiional pull which a moon like ours exerts, may lead to a difference in the development of the two worlds; a large moon like ours - it is becoming clear - can have a massive influence upon the stability and nature of the planet it orbits. 
3) The lack of a moon also brings us to the next point - the slow retrograde spin of Venus. All planets in the Solar System with the exception of Venus spin in an anti-clockwise direction when viewed from above the north pole - a natural consequence of the angular momentum imparted to the planets during the formation of the Solar System. But Venus spins clockwise, and much more slowly than our Earth. The only conceivable explanation for such a reversal of spin would be a massive trauma, most probably the result of an astronomic collision. Such collisions were not unusual in the early days of the Solar System - our own proto-Earth is believed to have been hit in its early evolution by a collision which broke off a sizeable chunk which later formed our Moon. Was Venus hit by a similar cataclysm, reversing and slowing the planet's rotation? Or did Venus once have a moon in close and diminishing orbit which eventually fell back into the planet? Various senarios have been suggested, but whatever happened, a massive impact on Venus in the past would undoubtedly have had far reaching effects which can only be guessed at today. 
4) If Venus, by virtue of being closer to the Sun, has always been hotter than Earth, this would clearly have had some effect upon the development of the planet. Quite how big an effect could only be speculated upon until recently.
The evidence of the basic physical characteristics of the planet therefore suggests that Venus’s interior should really have a fundamental similarity of composition to the Earth, and if there are any differences, then these need to be explained. The slow retrograde spin suggests a violent past, and this, plus the absence of a moon today, may have had implications for the planet's long term evolution. The question of Venus's presumed higher atmospheric temperature than Earth in the early days of the planet's existence, needs to be always borne in mind for the influence that it may have had on the planet's subsequent development.
At least one, and maybe all of these fundamental issues, must be assumed to be critically important in explaining any differences which developed between the geologies and climatologies of Venus and Earth since the earliest days of the formation of the two worlds.
THE EVIDENCE OF A WEAK MAGNETIC FIELD
Venus must have originally formed very much like our Earth, and should therefore also have a very comparable internal structure (including an iron-rich electrically conducting core). However, data recently collected suggests that one big difference is that Earth has a very powerful magnetic field or magnetosphere, whereas the magnetosphere of Venus is extremely weak - only about one hundred thousandth as strong. Given that the strength of a planet's magnetic field undoubtedly owes much to the internal structure of the planet, including the metallic core, why should this aspect of the planet be so different? 
It is believed that both planets would have had a magnetic field in the early days of the Solar System as a result of the residual thermal heat of creation, and radioactive decay. However these processes could not have maintained the magnetic field throughout the planet‘s long history.  What is required to maintain the field is a dynamo effect - a method of continuous generation of electrical current - and the only process by which this can occur is convection. For convection to occur one needs two factors to exist. First one needs a mechanism to generate a flow of electrically charged matter. Second, one needs a fluid medium through which this flow can take place. Three phenomena are believed to meet these requirements and so may contribute to internal planetary convection currents on Earth:
- First, on Earth there is known to be a differentiated core with a solid inner core and a molten outer core. It is strongly believed that as the Earth continuously very slowly cools, iron today is actively solidifying at the boundary between these two layers. As it solidifies, latent heat is released, and the less dense, heated alloy which remains unsolidified, rises through the molten part of the core, creating a convection current. 
- Secondly, there is movement as a result of a heat flux in the mantle (the upper part of the planet's interior), which occurs if temperature gradients within fluid, molten matter generate the circulation of currents. Hot matter rises, and cooler matter sinks. On Earth there is known to be a huge temperature gradient from the intensely hot core through the mantle, to the mild conditions at the planet's surface. 
- Thirdly, there is movement as a result of planetary rotation, which can generate swirling motions in molten matter within a planetary body. Earth spins on its axis relatively fast, creating such motion.  (Conceivably, tidal effects from the Moon may also create swirling motions of this kind in the mantle on Earth). 
So which of these factors might be key to Venus's almost complete absence of a magnetic field? Let us look at each in turn:
1) The differentiated core. On Earth the core is differentiated into solid and liquid or molten components, and the process of iron solidification and consequent generation of convection currents within the outer core is now widely believed by many to be the primary means by which our magnetosphere is being constantly replenished. It may be that unlike the Earth, Venus's core is solid throughout, or more probably liquid throughout. Why this should be so is unclear, but most likely it would be because of the internal pressure difference brought about by Venus's slightly smaller size. Whatever the truth of that, any homogeneity of the Venusian core, be it solid or liquid, or any cessation of the process of solidification in the past, is believed to be responsible for the inhibition of convection currents and hence the lack of a magnetosphere. 
2) Heat flux in the mantle. We should not ignore the role that heat flux throughout the planet, may play. If, unlike on the Earth, temperature is reasonably uniform throughout Venus's interior (we can assume with confidence it is uniformly hot rather than uniformly cold) there will be very little generation of convection currents in the mantle as well as in the core. Such uniformity of temperature could only occur if there is a lack of heat escaping at the planetary surface; this is because an absence of heat loss at the surface would allow the mantle temperature to increase thereby reducing the transfer of heat from the core, leading to a stagnation of convection currents. Why should there be a lack of heat escaping at the planetary surface? Two factors are known which could prevent heat escape:
- It could be that the atmosphere and surface temperature of Venus are extremely high, comparable to that of the mantle, so little convection and escape of heat occurs for this reason.
- The second possibility is that the Venusian surface - unlike the Earth's - is rigid and unyielding to heat exchange; this could only be the case if Venus lacks the crust and mantle interactions brought about by plate tectonics (continental drift) on Earth.
3) Planetary rotation. The rotation of Venus on its axis as we have seen above is much much slower than that of the Earth, and a counter view to heat flux in the core or the mantle, is that planetary rotation may be a key factor in the generation of magnetosphere-inducing convection currents.  However, calculations suggest that if even with Venus's slow rate of rotation, a more substantial magnetic field should be generated than currently exists.  So it seems likely that planetary rotation is - at best - a contributory factor in generation of convection currents to the solidification process described above.
It is therefore generally believed now that convection processes responsible for the magnetosphere begin in the core, not in the mantle, so the evidence of a weak magnetic field suggests a uniform core without any solidification process to generate convection currents, possibly with Venus's slow rotation rate as a contributary factor.
A high surface temperature, or a lack of variation in temperature throughout the mantle due to an absence of plate tectonics, would reduce convection within the planet.  Whether this would have a significant impact on the generation of a magnetosphere is debatable, but it is mentioned here because certainly it is a factor which may have other implications for the development of Venus, and it is a factor to which we will return in the section on PLATE TECTONICS.
Having considered the possible CAUSES of Venus's very weak magnetosphere, we should also consider the possible EFFECTS of a weak magnetosphere.
Two factors can deprive a planet of its atmosphere. One is weak gravity which allows light gaseous atoms to escape into space. The other is the Solar wind - the stream of highly energetic charged particles constantly being ejected by our Sun. The Solar wind interacts with elements in the planetary atmosphere stripping away electrons and energising the remaining ions to allow them to escape. All four of the terrestrial rocky planets - Mercury, Venus, Earth and Mars - are close enough to the Sun to be exposed to the full force of the Solar wind. However, a strong magnetic field around a planet can effectively shield a planet from the most damaging effects of the Solar wind. 
Mercury and Mars are relatively small planets with both low gravity and weak magnetic fields - as a result these two planets have non-existent (Mercury) or very thin (Mars) atmospheres. By contrast, Venus and Earth both have very sizable gravitational forces enabling them to hold on to all but the lightest atoms such as hydrogen. This should ensure that both planets retain a complex atmosphere.
However the effect of the Solar wind on Venus and Earth's atmospheres will be very different. On Earth, the strong magnetic field deflects much solar radiation. On Venus, absence of a significant magnetosphere will allow much more of this radiation to enter the atmosphere, which may have repercussions for the characteristics of the atmosphere. It may not have removed the atmosphere (far from it in fact - Venus, as we shall see, has an atmosphere which is extremely dense) but it may have had an impact on significant parts of the atmosphere (notably the component atoms of the molecules of water vapour).  This is one aspect which we will consider in the next section.
THE EVIDENCE OF ATMOSPHERE
It has been suggested that just as the physical characteristics of Venus would once have been quite similar to that of Earth, so the primordial atmosphere of Venus would also have been comparable. Assessing the hypothetical temperature of an Earth-like planet at Venus's proximity to the Sun suggests that Venus in the past would always have been hotter than Earth, but maybe not too hot for abundant supplies of water to exist on the planet's surface. And it is reasonable to suggest that the mechanisms which released or supplied water to the surface of Earth (released from internal rocks or supplied through cometary impacts), should also have allowed water to exist on the surface of Venus. With a similar geology, a similar atmosphere, and with hypothetical oceans of water, Venus really would have been Earth's twin.
However, investigations of the planet's atmosphere in the second half of the 20th century have revealed that - whatever Venus was like in the past - today it is extraordinarily different.
Venus has been discovered to have an immensely thick atmosphere which comprises 96.5% carbon dioxide, the density of which exerts a crushing pressure at the surface 92 times greater than on Earth. Nitrogen makes up most of the remaining 3.5%, with small amounts of sulphur compounds and clouds of sulphuric acid in the upper atmosphere. Trace elements include inert gases. Oxygen - and ozone - are sparsely found. One other curious aspect of the elements to be found is that where hydrogen exists in molecular combination, (such as in sulphuric acid) the heavy isotope of deuterium is 100-150 times more abundant than anywhere else in the Solar System.  This will be mentioned again shortly. Wind speed at high altitude is impressive. Speeds of up to 500 kilometres per hour (311 mph) drive the cloud cover round the planet. And one other characteristic of the upper atmosphere is lightning, which must be explained (and will be towards the end of this page).
But it is the temperature at Venus’s surface which is most striking; it has been measured at approximately 480°C (900°F) - this is extraordinary, because it is not merely hotter than the Earth (to be expected), but it is hotter even than the planet Mercury, which is much closer to the Sun, and which receives four times as much heat from the Sun. One immediate consequence of this is clear - such temperatures settle the question of existing water on Venus at the present time, once and for all. Whatever Venus might once have been like, today it is far too hot for water to exist in liquid form or even vapour form. 
But what are the possible explanations for this radical change in Venus's atmosphere and climate from conditions believed to be present on the primordial planet? Well, it seems the key reason is believed to be the loss of water from the planet which in turn led to the super-abundance of carbon dioxide. First therefore, we must briefly mention two factors which may have brought about the loss of water. These are:
- The high initial surface temperatures on Venus. High initial temperatures, whilst permitting water to exist in liquid form for many millennia, would nonetheless have brought about a much higher rate of evaporation on Venus. This would have led to greater concentrations of water vapour - H20 - in the atmosphere, and diminishing amounts of liquid water on the surface.
- The weakening strength of Venus's magnetosphere. The significance of this has been hinted at in the previous section. The increased radiation to which Venus would have been exposed as the magnetic field declined, may bring about dissociation of hydrogen and oxygen atoms - the components of water. Hydrogen atoms, because of their lightness, would tend to escape into space and be lost for ever, and the volume of water in any form on Venus would slowly diminish. (The heavier deuterium isotope offers some evidence for this process, because the greater mass of this isotope would result in less deuterium escaping into space than hydrogen; therefore the proportion of deuterium in the atmosphere would increase - and the proportion of deuterium found today seems consistent with the volume of water believed to have been lost to Venus's surface). 
These two factors therefore will lead to a continuous diminution in the volume of water on the planet's surface and in the atmosphere. Water is one of the major factors involved in the carbon cycle on Earth which effectively locks up carbon in the water, in rocks and in living organisms, rather than in the atmosphere. Without water the carbon on Venus remains and accumulates in the atmosphere leading to the well-known greenhouse effect and ever rising temperatures.
More will be written in explanation about the mechanism of the greenhouse effect later. At the moment however, it will suffice to say that Venus is now known to have an extraordinarily high atmospheric and surface temperature - far higher than that which existed after the initial violent upheavals which the Solar System and Venus underwent. As we shall see in due course, the implications of this may extend not merely to the surface crust, but far below the surface of the planet into the mantle. And it is the crust and the mantle which we will consider next.
THE EVIDENCE OF THE SURFACE TOPOGRAPHY
In recent decades the surface of Venus has been shown to be characterised by features of which some are familiar, and some are very alien, but almost all are related to vulcanism. The surface features are dominated by great volcanoes, vast plains, and extensive upland areas.
Many tens of thousands of volcanoes, including hundreds of giant volcanoes, dot the surface, vast in area, but relatively low in height, and surrounded by lava flows which in one case extends for 7000 km (4300 miles). Some of the volcanoes are known as pancake domes because of their characteristic, flattened shape, less than 1000m high yet many kilometres in diameter.  On Earth, two main types of volcano exist. The typical volcano cone-shape is associated with so-called composite volcanoes such as Vesuvius, Krakatoa, Mount Etna and Mount St.Helens which are to be found at or near the edges of tectonic plates. Subduction of the crust into the mantle and influx of sea water creates a highly viscous lava leading to explosive eruptions. Shield volcanoes are rather different. These are to be found at hot spots over the mantle, and eject magma which is much more fluid; this leads to less explosive eruptions, but more expansive lava flows. The volcanoes of Hawaii are typical shield volcanoes, and the form and distribution of volcanoes on Venus suggests that these are also of the shield type. As such, it seems they are not the result of plate tectonics.
A number of other strange features are to be found on the Venusian surface, some of which are unknown anywhere else in the Solar System, but all are believed to be volcanic in origin, and all may hold clues as to the surface geology and the underlying geology of Venus. Coronae are large ring shaped structures 100–300 kilometres (60–180 miles) across and hundreds of metres high. Other structures include novae, which are characterised by radiating trenches around a central point, and arachnoids, so named because of their similarity to the shape of a spider's web. They are circular-oval features with concentric rings and radial fractures extending outward, between 50 km and 230 km in diameter (30 - 140 miles). Arachnoids are similar to, but generally smaller than, coronae, and may represent a stage in corona formation. 
If the lava flows from Venusian volcanoes are impressive, they are as nothing compared with the Venusian plains, which make up 80% of the entire surface area, and which appear to comprise incredibly massive basaltic lava flows derived from the underlying mantle. Such lava floodplains are unknown on Earth at the present time, but are similar to noted volcanic events recorded in the geological history of our planet. These events were truly big, far far greater in extent than a simple volcanic eruption, and are believed to have originated not at the edges of continental tectonic plates, but rather in a plume of basaltic lava from the mantle breaking through at a hotspot on the planet's crust. About 65 million years ago the Deccan Traps landscape of India were formed from such a process - an outpouring over tens of thousands of years of lava from the Earth's mantle covering an area 500,000 km2 (200,000 mls2). An even greater outpouring of lava in Siberia at the end of the Permian Period 250 million years ago, is believed to have played a role in an extinction event which wiped out 90% of life on Earth. If the Venusian basalt flood plains are similar in appearance, then it is reasonable to assume a similar origin. Again therefore it seems that the Venusian floodplains, like the Venusian volcanoes, are not related to plate tectonics. 
The upland areas of Venus include extensive mountain ranges and plateaus. The highest point is Maxwell Montes which rises some 13 kilometres (8 miles) above the lowest levels of the surface; this is actually considerably less than the variation of altitude of the surface of the Earth, which varies from the deepest ocean trench to the top of Mount Everest - an altitude range of 20 km (12.5 miles). Venus therefore, has a relatively more even topography. The mountainous regions are characterised by a strange complexly deformed, fractured pattern of grooves and ridges known as tesserae. Another strange feature of the mountains is an unusually reflective deposit which occurs in the highest regions. It appears for all the world like snow, but Venus must be too hot for snow. So what could these deposits be? Well, it is thought that the the deposit may indeed be formed exactly like snow, except that instead of frozen water it is an element such as tellurium or a compound such as lead sulphide which at the temperatures on Venus would exist as a gas. At higher altitudes, the temperature is a little lower, so the compound 'freezes'. If the deposit is metallic, then this would explain the high reflectivity. 
Speculation exists as to how these mountainous regions were formed. On Earth, mountain ranges are the result of folding and faulting processes at the edges of tectonic plates. The Himalayas, for example, were created as the Indian continental plate collided with the Asian plate. On Venus, similar processes might also have produced massive folds and faults and upheavals in the crust. But we have already raised the possibility in the section about the magnetosphere that Venus may actually have a rigid surface crust, lacking plate tectonics. And we have already seen in this section that none of the volcanic activity on Venus appears to be related to plate tectonics. If this is the case, then another explanation for mountain building on Venus will be required. This will be discussed further under EVIDENCE OF PLATE TECTONICS.
To reprise - this section on surface topography has shown that the Venusian surface consists of shield volcanoes and other structures which may have volcanic origins, vast lava plains, and extensive highlands.
But these revelations raise more questions than the answers they provide;
- Why are Venus's volcanoes typically of the shield type, and why are they so extensive?
- How do we explain their distribution on the planet's surface?
- What is the origin of the other structures mentioned in this section; the coronae, novae and arachnoids?
- Why does Venus feature such extensive lava floodplains, so rare on Earth today?
- How are the highland regions of Venus formed? By plate tectonics bringing about the folding of the crustal rocks? Or by some other means?
- Why are Venus's mountain ranges so broken and deformed by the strange tesserrae formations?
The answers to all these questions will be speculated upon further in the section 'EVIDENCE OF PLATE TECTONICS' but first we must consider one other group of features of the Venusian surface, which are not created by Venusian geology, but which may hold a strong clue about past processes and present activity on the planet's surface. These features are meteor impact craters.
THE EVIDENCE OF IMPACT CRATERS
Although impact craters derive from the collision of meteors from outer space, they nonetheless can tell us a great deal about the geological activity or climatological activity of the planet. The reason for this is that on a geologically active planet, craters will in due course be obliterated by lava flows, folding and faulting, or sedimentation. Whilst on a climatologically active planet, erosive forces such as rain, rivers, wind or ice, will soon degrade craters.
Three characteristics of Venusian craters appear to be significant:
- On Venus, virtually no craters smaller than 2 km in diameter exist. Craters instead tend to be large, and are often grouped together in a clustered manner.
- In total, Venus has approximately a thousand of these larger craters - many more than on Earth, but far far fewer than on other, possibly comparable, worlds such as Mercury, Mars and our own Moon.
- Most craters seem to be in remarkably good condition.
With these facts in mind, three conclusions may be drawn about Venusian craters;
- Crater size and distribution. The inferences that some geologists have drawn from the size of the craters is that small meteors cannot pass through Venus's atmosphere to reach the surface without burning up, whilst large meteors break up in the atmosphere into smaller pieces before they hit the planet in a cluster formation. Both of these inferences, if true, imply hostile atmospheric conditions which a meteor cannot easily navigate. This of course tallies with what we already know about Venus's atmosphere. 
- The number of craters. The age of a planetary surface can be gauged by the number of craters to be found, because astronomers have a clear idea of the relative rates of meteor impacts both now, and in the distant past. The thousand or so craters to be found today, would be consistent with the rate of impact expected in the past 300 to 800 million years. This allows us therefore to further suggest that all traces of previous impacts (there must have been many, judging by the hundreds of thousands of impacts apparent on Mercury, Mars and our Moon), must have been removed in some kind of a global resurfacing event around that time. 
- The good condition of craters. On worlds without significant volcanicity, or which lack atmospheres with erosive forces, there is no mechanism for crater degradation; this is the reason why our Moon has so many pristine craters which date back billions of years, almost to the birth of the Solar System itself. On other worlds with very significant recent volcanicity and/or climatological erosion, impact craters are relatively quickly destroyed by agents such as water, ice and wind; this is the reason there are so few obvious craters here on Earth - a geologically and climatologically very active planet. On Venus there is clearly a thick atmosphere, and there is clear evidence of past volcanicity, and yet those craters which do exist tend to be in very fine condition, (62% in one study), suggesting there has been relatively little surface erosion in the hundreds of millions of years since the resurfacing event theorised above. 
The evidence of impact craters therefore seems to point to a hostile atmosphere which, as we have already seen, is clearly borne out by analysis of the atmosphere. But also the evidence suggests a global resurfacing event hundreds of millions of years ago, and a relative absence of volcanicity or erosive forces since that time.  How this could have happened will be considered in the next section.
THE EVIDENCE OF PLATE TECTONICS
We already have established a rather confusing picture of Venus' s topography. On the one hand we have described how so much of the planetary surface seems to be volcanic in origin, covered as it is with great volcanoes and extensive lava flood plains. We have also talked about the mountainous regions as being deformed and cracked, as though under great stress. So it seems that the surface of Venus has been the site of great turmoil. And yet we have also said that the majority of existing meteor craters on the surface are in good condition, implying an absence of such extensive geological activity, at least in the time since the craters were made.
All of the above may possibly be explained by the absence of plate tectonics on Venus. On Earth it has long been known how great sections or 'plates' of continental crust effectively 'float' upon the underlying more fluid 'mantle' rocks. As they move against each other, or alongside each other, so continents will drift apart creating depressions filled by oceans and seas, or else collide together creating mountain ranges. And along the edges of the continental plates, are zones of weakness which are characterised by earthquakes and volcanic events. This is why so many of Earth's volcanoes are distributed around the Pacific rim, and in a chain running down the 'spine' of the Americas at the edge of such plates. Away from the edges of the plates, the Earth is relatively rigid and passive. If there were no plate tectonics on Earth, the entire planetary surface would be relatively rigid and relatively inactive - just as Venus appears to be today.
Already we have speculated about the possible absence of plate tectonics on Venus. We have suggested that a rigid, unmoving crust may be responsible for a lack of heat flux through the mantle, reducing convection currents. We have also seen how the distribution of volcanoes on Venus, suggests that they are associated with hot spots on the crust, rather than with zones of weakness at the edges of continental plates.
If there are no plate tectonics, why should this be? On Earth, plate tectonics are largely driven by three or four factors, and as we look at these factors, it will be clear that most or all of these factors have already been mentioned as being absent on Venus:
- Convection driven by heat flux. On Earth, a gradation of heat in the mantle beneath the crust creates convection currents upon which the crust can slide. If the crust is rigid like Venus, heat escape at the surface will be minimal, and temperature variation immediately beneath the crust will be minimal. Convection currents may not be created, so movement of the crust will be minimal or non-existant. 
- Increased density of cooled surface rocks encourages subduction. The cooling of surface rocks on Earth increases the density or weight of the surface rocks and allows subduction of the crust at plate boundaries. If there is no cooling of surface rocks on Venus because of the high atmospheric temperature, this effect will not occur. 
- Water lubricates rock and makes it more fluid. Sea water is abundant on Earth. Water lubricates the movement of rock masses on the crust, and encourages subduction processes at plate margins, which return hydrated rock to the mantle, in turn making the mantle more fluid and more able to flow. A lack of water on Venus may lead to a less fluid mantle and a reduction in the convection forces which drive plate tectonics. 
- The Moon may generate convection currents. One other difference between the Earth and Venus is that the Earth has a very substantial natural satellite - the Moon - the gravity of which not only sets up tidal currents in the water of the oceans. Its gravity effect may also help generate convection currents in the underlying mantle, helping to drive plate tectonics. 
These therefore are possible explanations or CAUSES for the lack of plate tectonics on venus, but equally the absence of plate tectonics may also have significant EFFECTS, and may offer possible solutions to the questions asked in the previous sections on SURFACE TOPOGRAPHY and IMPACT CRATERS.
- Plate tectonics allows movement of the planetary crust so that it regularly releases heat and pressure from the mantle at the edges of the plates. Lack of plate tectonics may therefore allow build-up of heat and immense pressure within the planet's interior. This will suggest mechanisms by which some of the phenomena described above may occur:
1) Hot spots beneath the crust are responsible for the formation of shield volcanoes, and will allow magma to pour out to form the great Venusian floodplains reminiscent of the Deccan and Siberian Traps on Earth. Such hot spots and weak points on the crust may create those other more unusual features of the Venusian landscape. The currently hypothesised method of formation of coronae is via plumes of hot material rising in the mantle, forcing the crust up into a dome shape. Subsequent collapse in the centre and weakening of the rim allows the escape of lava to create the crown-like corona. The characteristic radial or concentric ridges and trenches of structures like novae and arachnoids are believed to result from magma seeping through stress fractures.
2) If the upland regions were not formed through plate tectonic-driven folding and faulting of the continental plates, then how did the uplands of Venus form? Well one possibility is that as a result of immense stresses built up under the rigid plate-free crust, vast pools of magma may force the crust upwards in some parts of the planet.
3) Building of pressure beneath the surface of a rigid planet to create the uplands, will also stretch and expand the crust and produce stress fractures. Release of stress and pressure through magma outpourings would lead to contraction of the crust. Such expansion and contraction is believed to produce the kind of deformed and fractured tesserae, characteristic of the uplands.
4) The evidence of impact craters suggests that pressure release through magma outpourings may not be an on-going event. Rather, it may be that at some time in the past, a massive outpouring of magma and global resurfacing of the planet may have occurred, effectively creating all the lava floodplains which make up 80% of the planetary surface, and obliterating all older features including craters. It would have been like one gigantic planet-wide Deccan or Siberian Traps eruption. Subsequently Venus may have been relatively dormant for hundreds of millions of years, allowing the number of more recent crater-creating impacts to accumulate to the present levels.
The evidence of a lack of plate tectonics therefore may explain all of the geological formations we see on Venus, including the distribution and form of the volcanoes, the presence of other curious features such as the coronae, the great lava plains, the fractured uplands, and also the relatively small number of craters on the surface.
PUTTING ALL THIS EVIDENCE TOGETHER
We have now gathered all the available evidence and looked at the factors of Venus's physical characteristics, the lack of a strong magnetic field, the atmospheric characteristics and the lack of plate tectonics, which may have created Venus’s present day geology, topography and climate.
However, it is now time to try to put this evidence together to create a surmised history of Venus from its origins to the present day. One should make clear that although the key events described are all generally believed to have occurred, the precise sequence, time lines and relative importance of some of the factors must currently be regarded as unproven.
HISTORY OF VENUS - AN EARTH-LIKE PLANET?
Venus, like the other terrestrial (rocky) planets, Mercury, Earth and Mars, would have formed out of swirling masses of gas and dust circa 4,600 billion years ago, during the development of the solar system. It may be that early in its evolution the planet may have had a substantial moon. Whether this is the case or not, this or some other object may have strayed too close to the planet and eventually collided. Whatever, it is believed that a major impact at some stage of Venus's early development may have brought about the slow rotation and retrograde spin which the planet now experiences. 
As Venus began to settle down, it would have developed in a similar way to Earth, albeit with a higher surface temperature due to its closer orbit to the Sun. Venus would presumably have also had a comparable molten iron core and a magnetosphere; a phenomenon initially brought about by convection currents deriving from the remnant thermal energy in the core. This source of convection eventually ended on both Earth and on Venus, after one billion or more years. On Earth, the magnetosphere is believed to have only survived beyond this time as a result of on-going convection processes within the core as the inner core solidified. It seems that Venus lost most, if not all of its magnetic field at this stage partly as a result of its slow rotational spin, but primarily as a result of a cessation of core solidification. This may have happened because of the fact that the planet is slightly smaller than Earth, so internal pressure is slightly less. 
Up until this time, Venus's atmosphere is believed to have been comparable to Earth's. Substantial oceans of liquid water may have existed. Temperatures were higher, so greater evaporation would have occurred, but at this time temperatures are not thought to have been high enough for all water to evaporate, much less boil. Indeed water which did evaporate would have formed clouds which reflect heat, cool the planet a little, and perhaps could have allowed water to be retained on the planetary surface for at least a billion years.
As a little aside at this point, the relatively Earth-like conditions on Venus more than two billion years ago, with water on the surface and copious water vapour containing oxygen and hydrogen in the atmosphere have led many to speculate that life might conceivably have evolved at this time.
But conditions on Venus were changing ...
HISTORY OF VENUS - THE GREENHOUSE EFFECT AND THE LOSS OF VENUS'S WATER
One particular physical entity has kept cropping up throughout this discussion, and that is ‘heat’. There are two sources of heat on a planet; the first is the internal heat of pressure and radiation, but there is no reason why this should be greater on Venus than on Earth. The second is the heat from the Sun, and this would have been higher - but not hugely higher - than on Earth. However, there is a way in which heat gathered from the Sun can be concentrated and escalated, and that is something which can happen if the atmosphere of the planet is of a particular type.
Being closer to the Sun, the upper atmosphere of Venus had the potential to be heavily irradiated by the Solar wind, and without the protective effects of a magnetosphere, this bombardment may have had dire consequences for the atmosphere. in addition to the weak magnetosphere, Venus also lacked a significant ozone layer, which would have contributed to a rise in UV radiation. High evaporation rates on the surface would have led to copious amounts of water vapour in the upper atmosphere, where exposure to all of this radiation would bring about dissociation of hydrogen and oxygen atoms, which did not happen on the magnetic-field shielded, ozone protected Earth. Hydrogen atoms, because of their lightness would tend to escape into space and be lost for ever, leaving the dissociated oxygen atoms. The volume of water on Venus would therefore have slowly diminished. 
At this stage the planetary supply of carbon become significant. Carbon was present within the planet's interior, and it is believed that much of the supply of carbon to the surface on both the early Earth and the early Venus came about through the release of carbon dioxide in volcanic eruptions. On Earth, much carbon became dissolved in the oceans, and bound up in the tissue of living organisms, to be released and recycled as water evaporates, or when organisms die. Very little combines with oxygen in the atmosphere. Any that does enter the atmosphere re-enters the soil when it is washed out in rainfall, or becomes bound up in surface rocks such as limestone. (this is the so-called carbon cycle - clearly inextricably linked to the presence of water).
On Venus, the diminishing 'pool' of water as a result of the two processes of heat evaporation and then subsequent dissociation of water molecules in the atmosphere would have meant that this carbon cycle could not kick off. Instead, carbon atoms would combine with the dissociated oxygen atoms in the atmosphere producing carbon dioxide. Carbon dioxide is a renowned greenhouse gas. It allows heat from the Sun to pass through, but absorbs much of the heat reflected back, leading to an increase in atmospheric temperature. An increase in atmospheric temperature leads to more evaporation, more dissociation of water molecules as a result of the Solar wind effects, and more recombination of carbon with oxygen to further increase the concentration of carbon dioxide in the atmosphere, and therefore even more trapping of heat.
This runaway effect or 'vicious circle' effect leads to escalating concentration and density of carbon dioxide in the atmosphere, continuously rising temperatures and accelerated evaporation of the planet's water (until all available carbon was combined in the atmosphere). As a by-product, the increasing concentration of the dense gas carbon dioxide would also have created the immense pressures which now exist on the planet's surface.
Sulphur, released by volcanic activity common to both Venus and Earth, would further contribute to Venus's hostile conditions. On Earth, sulphur solidifies on the surface or becomes incorporated into surface minerals. On Venus, temperatures would by now have been rising above the level at which sulphur not merely melts, but evaporates to combine with dissociated oxygen atoms to form sulphur dioxide, and clouds of sulphuric acid, and this would explain the concentrations of these compounds in the Venusian atmosphere. Sulphur dioxide can trap certain wavelengths of radiation which CO2 allows through, and thus would contribute further to the greenhouse effect, raising temperatures still further.
Rising temperatures may further exacerbate the situation by creating a more plastic crust and mantle, leading to further volcanic eruptions, and further carbon and sulphur liberated into the atmosphere. 
Ultimately, temperatures reached such a level that no water at all could any longer be sustained on Venus's surface. Water would not evaporate - it would boil.  All water is now estimated to have been gone sometime between two and four billion years ago. 
The absence of a magnetosphere, the development of the carbon dioxide greenhouse atmosphere, and the gradual removal of all water from the surface are considered to have been three of the crucial elements in Venus's descent into a nightmare world.
But worse was to come ...
HISTORY OF VENUS - THE SHUTDOWN OF PLATE TECTONICS AND THE GREAT RESURFACING
Remarkably, high temperatures in the atmosphere and on the surface of Venus would by now not merely have affected a volatile liquid like water; it would even be affecting the rocks of the surface crust and deep into the underlying mantle. On Earth, the mantle is a largely molten layer of rock which extends from the planet's outer core to within about 30 miles of the surface, just below the crust. Plate tectonics on Earth play a major role in recycling the crust and releasing pressures and heat from the mantle, creating the planet we know today. But plate tectonics requires a fine balance of conditions to exist, without which the mechanism may shut down.
And indeed, all of the factors seem now to be in place for such a shut down to occur on Venus. The extraordinary rise in temperature as a result of Venus's greenhouse atmosphere has been noted. But calculations suggest that on Earth, plate tectonics would become unstable even with a rise in surface temperature of just 38ºC (100ºF). Atmospheric temperatures on Venus meant that no significant cooling of the surface rocks could occur. As a result there would be a consequent reduction in heat flux between the mantle and the crust. With a lack of heat flux, (and also a lack of tidal effects because Venus has no moon), convection currents in the mantle would be minimal. Without convection currents, there would be no mechanism for the surface crust to be moved and broken. In addition, the absence of water to lubricate such movement and to facilitate subduction of the crust would also be critical in creating a rigid crust. The mantle would become increasingly viscous and sluggish. 
As plate tectonics allows heat and pressure to be released from the mantle, a lack of plate tectonics means that the internal heat of the core including the decay of radioactive elements, leads to a build up of heat and pressure. Even on a rigid, unmoving crust, there may be hot spots - just as there are in the middle of Earth's continental plates. At these points magma may occasionally have forced its way through to the surface, creating shield volcanoes and other features of the Venusian surface. But the most prominent effect on Venus may have been that pressure from beneath led to a warping of the crust, expanding it into the extensive upland regions of the planet. Such expansion, plus contraction events as pressure is released through vulcanism, may be responsible for the characteristic fracturing, wrinkling and tesserae to be found in the uplands, and the fissures and ridges which accompany so many of the other surface features. But is this expansion and contraction, and occasional release of magma over hot spots, all that ever happened? 
Ultimately, the occasional volcanic events creating shield volcanoes at hot spots on Venus, may not have been sufficient to counter the rising pressure beneath the rigid crust. Without the benefit of the regular, periodic pressure release which plate tectonics allows through volcanic events and earthquakes, build-up of heat and pressure may eventually have become unbearable. Between 300 million and 800 million years ago (based on impact meteor evidence), the rigidity of the crust would finally have been broken as cracks and vents opened across the entire planet. A vast outpouring of lava similar to - but on a scale unimaginably bigger than - the Deccan or Siberian events may have taken place. What's more, outpourings of lava, however massive in scale, would have spread even more extensively over the planet surface as extreme surface temperature would have contributed to a much slower rate of cooling and solidification, creating the ocean-sized floodplains which cover 80% of the planet today. In such an event, it is hardly surprising that all pre-existing impact craters would have been obliterated, as well as other low lying surface features.
This global resurfacing probably took place over a period of tens of millions of years - a short period of time geologically speaking - so it seems likely that virtually all surface features on the planet with the exception of more recent impact craters, and possibly the upland regions, are of effectively the same age. (It has of course not yet been possible to date any Venusian rocks accurately) Ultimately, the release of heat accompanying the global resurfacing would have allowed the surface to cool somewhat, possibly contributing to the fractured appearance of the surface, as the crust once more became rigid under a thick layer of solidified lava, and conditions once more stabilised.
At least for a while ...
HISTORY OF VENUS - RECENT ACTIVITY
What then has happened since this hypothesised great resurfacing of Venus? As noted, of course meteor impacts have been one of the few clear and significant surface deforming events in more recent times. Most of these are in good uneroded condition. From this, it can be assumed that erosive forces of the kind which are so prevalent on Earth are largely absent. These include water and ice (self-evident in the case of Venus) and wind (the strong winds recorded in the upper atmosphere, presumably do not exist at ground level). Volcanic activity can also of course destroy or overlay surface features like craters with earthquakes and lava flows. So the pristine condition of craters some of which must be in excess of 100 million years suggests that Venus today is relatively inactive. 
But perhaps not totally inactive. One study suggested that although more than 60% of all craters are pristine, others have been affected by upheavals of one kind or another, whilst 4% shows signs of lava disruption.  And indeed, recent high resolution radar and infra-red evidence has identified what appear to be recent volcanic eruptions and ejecta from volcanoes  . Remember that in the section about Venus's atmosphere, we mentioned the presence of lightning in Venus's atmosphere? On Earth, lightning is associated with water rainfall. On Venus there is no water rain, but it has been suggested that sulphuric acid rainfall in the presence of volcanic ash can also generate lightning. Recent evidence also shows a significant drop in the levels of sulphur dioxide in the atmosphere between 1978 and 1986, the inference being that previous high levels of sulphur were the result of a volcanic eruption.
One final question remains; was the resurfacing a one-off event, or could it be a cyclical phenomenon?  The obliteration of surface features by the resurfacing several hundred million years ago means that it is impossible to know with any certainty what might have happened before this event. Was the resurfacing unique, or merely the most recent of a series of such events, removing all trace of previous resurfacings? And could another such event occur in the future? Beneath the surface the dormancy of much of the planet’s geological activity may have existed for hundreds of millions of years, but the evidence seems clear that Venus is far from dead. My guess is that at some time in the future, such a global volcanic event may well occur once again.
Inevitably when dealing with the geology and climatology of an alien world which we cannot as yet visit in person, some questions remain unanswered, and some answers seem contradictory. It is impossible to know exactly what goes on at or just below the crust, let alone deep within the core, and we have no working model with which to compare Venus except our own planet Earth.
Just as it's surface is shrouded in a dense carbon dioxide haze, so Venus's history remains shrouded in some mystery. But the driving force behind everything that has ever happened seems to be heat - heat from the interior, heat from the Sun, and above all else, heat trapped by the atmosphere. Following the loss of Venus's magnetic field, all the major events which have shaped the planet's history have been triggered by heat - the loss of the water, the development of a greenhouse effect, and the great volcanic resurfacing several hundred million years ago.
The theme of this story has been how the one-time similarity between Venus and our own Earth has changed so drastically as a result of rather small original differences. Venus was a little bit smaller, Venus lacked a moon and rotated on its axis more slowly, and most importantly Venus was that bit closer to the Sun and hotter. Because of these differences, it seems Venus just could not sustain a potentially habitable environment, and is therefore perhaps the finest example - and a salutary lesson - as to how delicate the balance between planetary life and planetary death really is, and just what could go wrong with a planet like Earth if conditions were a little different to the way they are.
- 1] If We had no Moon
- 2] Why Doesn\'t Venus Have a Moon?
- 3] Venus: Magnetic Field and Magnetosphere
- 4] Geodynamo
- 5] Geomagnetism
- 6] Where is the Water on Venus? - Vanderbilt University
- 7] Venus
- 8] The Environment of Venus
- 9] Venus Geology 2: The Hot Squeeze
- 10] Geology of Venus - Wikipedia, the free encyclopedia
- 11] Flood basalt - Wikipedia, the free encyclopedia
- 12] 'Heavy metal' snow on Venus is lead sulfide
- 13] Geology and Distribution of Impact Craters
- 14] Venusian Impact Craters
- 15] Hot Climate Could Shut Down Plate Tectonics
- 16] New evidence for recent volcanism on Venus
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