Venus is very much in the forefront of interest and excitement in planetary science these days. It's as though it has gone from being the poor relation to being the place everyone wants to get on-board with ... the new planetary El Dorado. Here I'd like to look at why this is so. Why all the recent - and current - excitement ?
If you aren't already aware of just how much excitement Venus is currently generating, look at this compilation of multi-billion dollar missions already financially approved by 4 national and international space agencies, with launch dates spread over the next 10 years. Note that they are not expected to send back any new data from Venus until the mid-2030s. Whatever, these will be the first wave of missions planned to look at all aspects of Venus since NASA's Magellan mission of the early 1990s. Note that the VERITAS Mission funding has been "suspended" by NASA.
I know that some of you are aware of some of the media headliners relating to Venus because I have told you about some of them last year already : see https://www.simonhanmer52.ca/solar-system-science-202122.html
- There was the "life on Venus" story driven by the phosphine molecule in the atmosphere
- ... and the twin notions that both tectonic activity and volcanism on Venus might be very recent - and perhaps even happening today.
I also briefly mentioned this keynote idea. By modelling the evolution of the atmosphere of Venus through time, it's possible to develop a climate scenario where - for ~3 Ga - early Venus may have been cool and wet - even with an ocean - and potentially habitable until an outpouring of huge volumes of lava sometime during the past 1 billion years resurfaced much of the planet and turned it into the dry hothouse it is today, with surface temperatures of ~450 C. However, this is just one model. Folks are still asking the question : when did Venus heat up, or was it always hot ?
An important consequence of Venus being hot and dry is that there has been very little surface erosion during at least the past 500 million to a billion years or so (winds at the surface are not very strong), which means that what we see at the surface pretty much formed at the surface - and, as we'll see - that has major repercussions for when we make comparisons with the Earth.
An important consequence of Venus being hot and dry is that there has been very little surface erosion during at least the past 500 million to a billion years or so (winds at the surface are not very strong), which means that what we see at the surface pretty much formed at the surface - and, as we'll see - that has major repercussions for when we make comparisons with the Earth.
I think it's pretty obvious that before spending huge sums of money on science missions, we should have a pretty good idea of what we already know - and do not know - about the place that we're sending these expensive missions to.You have to know what the next questions to ask are This is fundamental to any scientific project if you want to get the maximum value for the effort and funds expended, especially when we're talking billions of taxpayer dollars. So let's do a whistle-stop tour of what planetary scientists think they already know about the surface of Venus and what they think they need to know more about. Then - for the rest of the presentation we'll look at what I think they don't know, even though they think they do. I'll keep this first part very brief because I've already gone over this in some detail with you back in 2020 https://www.simonhanmer52.ca/venus-magellan-images.html
BTW : Note that all the images of Venus are radar images cos they were taken looking through very thick clouds of mostly CO2.
First : we know that ~80% of the surface of Venus is made of volcanic plains (dark grey), so much of it is relatively flat.The other 20 % includes relatively high-standing plateaux (medium to light grey) that some planetary scientists compare with what they think terrestrial continents might have looked like on the early Earth. Another major feature of Venus is a connected network of huge rift valleys (think Ottawa Valley - or better still the East African Rift - as a terrestrial analogue). These occur where the planetary crust was pulled apart, probably by tectonic movements in the planetary mantle.Volcanoes (white) are present in all shapes and sizes (some as big as plateaux). Large (100-500+ km) round dome-like features - referred to on Venus as Coronae - are very common in the volcanic plains. They are thought to form as mantle upwellings that reach the planetary surface then collapse to form raised rings.
BTW : Note that all the images of Venus are radar images cos they were taken looking through very thick clouds of mostly CO2.
First : we know that ~80% of the surface of Venus is made of volcanic plains (dark grey), so much of it is relatively flat.The other 20 % includes relatively high-standing plateaux (medium to light grey) that some planetary scientists compare with what they think terrestrial continents might have looked like on the early Earth. Another major feature of Venus is a connected network of huge rift valleys (think Ottawa Valley - or better still the East African Rift - as a terrestrial analogue). These occur where the planetary crust was pulled apart, probably by tectonic movements in the planetary mantle.Volcanoes (white) are present in all shapes and sizes (some as big as plateaux). Large (100-500+ km) round dome-like features - referred to on Venus as Coronae - are very common in the volcanic plains. They are thought to form as mantle upwellings that reach the planetary surface then collapse to form raised rings.
The principal arguments advanced by the planetary science community for a massive return to Venus fall into three categories :
- Climate change ... from maybe cool and wet to now dry and hot
- Exoplanets around other stars ... how to avoid confusing an exo-Venus for an exo-Earth
- Surface mapping ... better resolution geological, geochemical and geophysical data
Both of these busy images (each ~100 km or more across) of volcanic plains on Venus show lots of features, but I want to zero in on the long, thin lines. These are what are referred to on Venus as "wrinkle ridges". I don't expect you to see much ... but this is about as good as it gets in terms of resolution !
To see better, we have to turn to the original wrinkle ridges, first described on the Moon (no surprise : they are closer and easier to see). They are long (100s km), sinuous, high-standing ridges made of a broad rise with a "wrinkle" on top of the rise. Their most characteristic feature is that they are mostly asymmetrical, with a steep slope and a shallow slope.
Here we see the lunar example on the left and one of the best images of wrinkle ridges from Venus. The scales are different, but you can see why planetary scientists use the lunar examples as analogues to help interpret ridges on Venus.
Why are wrinkle ridges important on Venus ? Venus has more wrinkle ridges than any other planetary body in the Solar System. The planet-scale map on the right and the detailed blow-up on the left give you an idea of just how widespread they are and how they occur as multiple, vast fields of evenly-spaced (periodic) ridges extending for well over 1000 km both along their trends and across them throughout the volcanic plains of the entire planet.They are such a characteristic feature of Venus that we really do need to understand how they formed. Notice something very important about the detailed map : the wrinkle ridges are drawn as simple lines. Keep this in mind because I'll come back to it shortly.
This is the classical, consensus cross-sectional model for how wrinkle ridges are thought to form - very similar to the rucking-up of a carpet that slides over a rigid floor. In the model the topographical relief of the ridges is due to folding of the upper layers of the volcanic plains. The periodic or even spacing of the ridges is due to the folds occurring in trains (a characteristic feature of folding) and - according to the model - the asymmetry of the ridges is due to formation of a fault that forms beneath the growing fold,.and drives the formation of the fold itself. Note that the fault does not break the surface : hence it is termed "blind" or buried. We cannot directly see the fault ... and therein lies the first major problem.
Now, as I pointed out in a science paper I published earlier this year, it turns out that this model is not a unique explanation for the formation of periodic asymmetrical ridges. Compare the shapes progressively produced with a fault that cuts across the layers (left) and with a fault that slips along the layers (right). The shapes of the ridges are very similar seen from above. This is means that the classical wrinkle ridge model is what's known as a "non-unique solution" ... and therein lies the second major problem.
Worse ... The distribution of wrinkle ridges in vast, uniform fields measuring 1000++ km in every direction presents a structural geological conundrum. Let me show you why it's a major problem.
The Rocky Mountains of Western Canada are built of slabs of rock, separated by faults, that slid over one another ~40 million years ago. Although they look evenly spaced, the two red arrows in the E-W cross-section (right) mark a master fault along which much of the crustal contraction or shortening was taken up. Nature doesn't like to spread work out evenly. If there's a way to get the work localised onto one one weak surface, nature will take it. Geologically, this is called "localisation" of deformation (remember that word) and it is a fundamental principle of the deformation of rocks. Yet it appears to be absent from these vast fields of wrinkle ridges on Venus. This is a third major problem that has so far been ignored in planetary geology !
The Rocky Mountains of Western Canada are built of slabs of rock, separated by faults, that slid over one another ~40 million years ago. Although they look evenly spaced, the two red arrows in the E-W cross-section (right) mark a master fault along which much of the crustal contraction or shortening was taken up. Nature doesn't like to spread work out evenly. If there's a way to get the work localised onto one one weak surface, nature will take it. Geologically, this is called "localisation" of deformation (remember that word) and it is a fundamental principle of the deformation of rocks. Yet it appears to be absent from these vast fields of wrinkle ridges on Venus. This is a third major problem that has so far been ignored in planetary geology !
On the topic of what has been ignored, you may be surprised to learn that we have practically no data regarding the shape of wrinkle ridges on Venus - which would be diagnostic for the mechanisms that might have formed them. Despite the fact that there are lots of geological maps of Venus, the wrinkle ridges on all of the maps are presented as simple lines with no information regarding sense of ridge asymmetry, if any, and therefore no diagnostic information regarding mechanisms of ridge formation. In other words ... the all-important diagnostic wrinkle ridge asymmetry on Venus is an assumption.
So what now ... ? Well, the good news is that Venus offers planetary scientists a chance of a lifetime : an entire planet absolutely riddled with ridges, and all they have to do is determine if the ridges are systematically asymmetrical - or not ! It's a pretty simple question with a Yes or No answer. In the simplistic case where most of the ridges turned out to be symmetrical, there might be no requirement for faults at all, and thereby no requirement for localisation of the deformation of the volcanic plains !
So much for basic structures related to "pushing". Now, what about simple "pulling" For this I'll focus on the rift fabrics of the crustal plateaux that stand proud of the volcanic plains.
Here on the lower left is a satellite's view of a large chunk of a plateau, with thousands of long, thin linear features oriented NS, uniformly developed over 500 km along and 1000++ km across their overall trend at the scale of the plateau. For reasons I won't get into here, we know that these are very long, narrow rifts, so close-spaced that folks talk about a rift fabric. There are two ways to generate this rift fabric : either by stretching the crust in response to externally imposed forces - or by internally inflating the planetary crust. The consensus view is that the Venusian crust has been stretched. The minority view is that it has been inflated. Let me explain ...
At upper left is an inset with the two consensus models for creating these long, skinny rifts by tectonically stretching the crust, essentially by pulling it apart by forces outside of the crust. The details do not matter here. The major problem here is similar to one of the major problems with wrinkle ridges - but much worse. If you pull rocks apart by external forces, they do not break like this/ One or another of the initial rifts is going to grow faster than all the others and turn into a major localised (that word again !) rift as nature tries to economise the work done to achieve the overall stretching. True, we've already seen that major planet-scale rifts do occur on Venus, but they have nothing to do with rift fabrics : they are something altogether different (and very well understood).
At upper left is an inset with the two consensus models for creating these long, skinny rifts by tectonically stretching the crust, essentially by pulling it apart by forces outside of the crust. The details do not matter here. The major problem here is similar to one of the major problems with wrinkle ridges - but much worse. If you pull rocks apart by external forces, they do not break like this/ One or another of the initial rifts is going to grow faster than all the others and turn into a major localised (that word again !) rift as nature tries to economise the work done to achieve the overall stretching. True, we've already seen that major planet-scale rifts do occur on Venus, but they have nothing to do with rift fabrics : they are something altogether different (and very well understood).
So, what's the alternative ? In a science paper published back in 2020, I suggested that Venusian rift fabrics might be the product of inflating the crust from within. How ? To answer that question we have to ask another : Do we see similar extensive, periodically spaced linear features elsewhere in the Solar System ? Well ... yes we do. On Earth we find the remains of Giant Dike Swarms throughout geological time, and all over the planet. These are massive swarms of narrow, vertical sheets of molten rock - magma - that individually can be 1000s of km long. On the left you see an example from northern Canada, and the inset on the right is a detailed line drawing of what part of the swarm looks like close-up. On the right is a radar image of rift fabric from Venus, and a detailed line drawing of the radar image. Pretty similar ... even if the scales are different !!
Before we go any further with this, what exactly are magmatic dikes and what do they look like on Earth? On the right of this slide, you can see swarm of black sheets cutting across a very large outcrop in SW Greenland. This was hot liquid magma that forced its way into the surrounding rocks and pushed them apart to make room thanks to the pressure of the magma itself. In short, the dikes inflated the crust into which they were intruded. To the lower left, you can see similar dikes cutting across a flat plain in NW Canada.
Are there other examples ? Well, if we look at a digital elevation map of the area north of the Tharsis uplift and Olympus Mons on Mars, we see a very similar map-scale pattern of very thin, very long, evenly close-spaced rifts extending over 1000s of km in all directions. The surface images (right) are details of Tractus Catena where you can see pit chains within the rifts that many planetary scientists interpret as collapse pits indicating the presence of the tops of dikes that lie just below the surface under the rifts.
OK ... but I still haven't explained what Giant Dike Swarms might have to do with vast swarms of long, narrow rifts, especially if the dikes are still buried just below the planetary surface. Without getting into the details, it turns out that when a pressurised magmatic dike rises to near the surface, it generates a stress field that pulls the surface rocks apart and pushes the surface up on either side to form a long, skinny rift valley called a "graben". So, since we observe examples of very long, close-spaced features related to underground sheets of intruded magma that inflate the planetary crust over vast areas measured in 1000s of km in every direction on both Earth and Mars, my question was this : does the same process apply to the rift fabrics of Venus ? Remember ... this is a hypothesis : I don't know the answer - yet !
OK - now that we've looked at simple, basic structures related to "pushing" and "pulling" (or possibly crustal "inflation"), what about structures related to sideways sliding on the surface of Venus ? There's no denying that vast volcanic plains on Venus are indeed divided up into large undeformed blocks (dark grey) separated by zones of deformation (light grey), that we can refer to colloquially as rigid lumps separated by squishy zones. However, some planetary scientists claim that the squishy zones are corridors of side-ways slip (technically referred to as shear zones) that allow the rigid blocks to jostle around and slide past each other in a manner analogous to blocks of terrestrial sea-ice, and that this represents both a form of plate tectonics and what our planet might have looked like before modern plate tectonics got established here on Earth. In addition, they suggest that this side-ways slipping might be happening on Venus today, hence the notion of currently active tectonics on the second planet. If correct, this would be revolutionary. However ... there are several major problems with this idea.
First major problem ... If we zoom in a little on the previous slide so that we can read the scale bar, we see that these squishy zones are over 100 km wide. Remember, there's been very little erosion on Venus for at least the past 500 million to a billion years - so how could huge side-ways slipping shear zones be active at the surface of the planet ... any planet ? I'll come back to this in a second ...
Second major problem ... Planetary scientists are not in agreement as to why they think side-ways slip is even present on Venus. Their analyses are universally model dependent ! Here, on the left, is a published analysis of a purported shear zone on Venus. Note the scale-bar : the proposed shear zone is 50 to over 100 km wide ! The arrows on either side of the shear zone (inset) indicate that the planetary scientists think that the motion on this shear zone is left-lateral. However, on the right, a blow-up of same shear zone (red box) is interpreted by other planetary scientists as right-lateral. Without getting into detail here ... if you don't know which way a side-ways slipping shear zone slipped then you really don't know if it is a shear zone at all !
I have spent a major part of my 50 year career as a geologist studying side-ways slipping giant shear zones in northern Canada, especially at Great Slave Lake (NWT). On the right is a simplification of my detailed map of the Great Slave Lake shear zone that formed ~2 billion years ago, which is the widest side-ways slip shear zone known in the history of the Earth. Notice the scale bar - the shear zone is a maximum of 25 km wide. Terrestrial shear zones cannot be wider than 25 km ... but why ?
The different patterns in the Great Slave Lake shear zone map represent rocks deformed at different temperatures and different depths within the Earth's crust. The shear zone was 25 km wide when the rocks we see at the surface today were located at 35-40 km depth within the crust - at temperatures of ~850-1000 C. While the shear zone was active, the hot rocks were progressively unroofed by erosion, rose towards the surface, and cooled to about 450 C at ~10 km depth. Remeber that temperature ! The rocks deformed at those cooler temperatures cut through the once hotter rocks - and are indicated on the map by the narrow black units (called greenschist mylonite).
So, what's the link between hot rocks and wide shear zones ? A shear zone forms where mother Nature wants to localise (that word again !) the slip between two large blocks into a narrow corridor, and shearing can do this because the very act of deforming the rock in a shear zone ("grinding" the rocks down if you like : it's really way more complicated than that !) makes them inherently weaker. So you get a feedback - the more deformation, the weaker the shear zone and the narrower the zone of deformation. This is a mechanical weakening process. However, high temperatures make rocks inherently weaker too. Think of the difference between brittle taffy at room temperature and soft taffy when it's heated in a saucepan ! It's a similar phenomenon. If the rocks are already at ~1000 C (i.e. almost ready to melt, which is as weak as rocks can get) then there's not much advantage to further mechanical weakening. So hot shear zones are wide because the rocks are thermally weakened. 25 km is the maximum width cos any more heating will simply melt the rocks, and they can't get any weaker than that ! But when you cool the rocks to ~450C, mechanical weakening is dominant over thermal weakening, and the shear zone narrows (localises !! ... that word yet again) towards a stable width of just ~1 km.
So, there are just two things to remember here. First ... 450 C is the approximate temperature of the Venusian surface today, and second - structures seen at the surface of Venus must have formed pretty much at the surface cos erosion is so weak. Oh, and BTW, squishy shear zones do not form at all at very shallow depths Instead, they form brittle faults 1-100 metres wide ! In short ... the supposed side-slip shear zones reported by planetary scientists from Venus do not seem to correspond at all to what we know about side-ways slipping shear zones on Earth ... especially if they formed at the surface. So we have to ask ourselves : are they are even shear zones ? If they are not, then what are they ? In truth, I don't know. We may have to wait for the new data in the mid-2030s.
So, what's the link between hot rocks and wide shear zones ? A shear zone forms where mother Nature wants to localise (that word again !) the slip between two large blocks into a narrow corridor, and shearing can do this because the very act of deforming the rock in a shear zone ("grinding" the rocks down if you like : it's really way more complicated than that !) makes them inherently weaker. So you get a feedback - the more deformation, the weaker the shear zone and the narrower the zone of deformation. This is a mechanical weakening process. However, high temperatures make rocks inherently weaker too. Think of the difference between brittle taffy at room temperature and soft taffy when it's heated in a saucepan ! It's a similar phenomenon. If the rocks are already at ~1000 C (i.e. almost ready to melt, which is as weak as rocks can get) then there's not much advantage to further mechanical weakening. So hot shear zones are wide because the rocks are thermally weakened. 25 km is the maximum width cos any more heating will simply melt the rocks, and they can't get any weaker than that ! But when you cool the rocks to ~450C, mechanical weakening is dominant over thermal weakening, and the shear zone narrows (localises !! ... that word yet again) towards a stable width of just ~1 km.
So, there are just two things to remember here. First ... 450 C is the approximate temperature of the Venusian surface today, and second - structures seen at the surface of Venus must have formed pretty much at the surface cos erosion is so weak. Oh, and BTW, squishy shear zones do not form at all at very shallow depths Instead, they form brittle faults 1-100 metres wide ! In short ... the supposed side-slip shear zones reported by planetary scientists from Venus do not seem to correspond at all to what we know about side-ways slipping shear zones on Earth ... especially if they formed at the surface. So we have to ask ourselves : are they are even shear zones ? If they are not, then what are they ? In truth, I don't know. We may have to wait for the new data in the mid-2030s.
So how do we wrap this up ? As I said at the beginning of this presentation, planetary scientists have a good grasp of the often complex, broad-scale aspects of the structure of the crust of Venus and of the volcanology that built it. They even have a reasonable grasp of the planetary atmosphere. However, planetary scientists have not done such a good job with the simple basics of the structural geology of the Venusian surface. They still cannot adequately justify their interpretations of the relatively simple processes of pushing, pulling, - and maybe - the sideways sliding of rocks. Unfortunately, they do not appear to realise this ("they do not know what they do not know"), and I fear it will show up in their detailed mission plans for further geological mapping.
So should planetary scientists just sit on their hands waiting for new data from the Venus missions sometime during the 2030s ? I think not ! I have two principal suggestions.
First, go back to the Magellan Mission data from the early 1990s and high-grade the best quality parts. Some of those data are in stereo ("postage stamps") ... although most are not. Regarding wrinkle ridges, use the best stereo data to test whether or not the wrinkle ridges of Venus really are asymmetrical (as is assumed) or not. If they are not, this fundamentally opens up the paradigm for their formation to re-interpretation. Regarding rift fabrics, if they are really formed above blind magmatic dykes, then at least some of those millions of dykes must have leaked to the surface and we should be looking to see if they have filled the bottoms of any of the rifts.
Second, planetary scientists have ignored terrestrial structural geology for far too long. They should go back to the library and read, and learn and better understand the well established universal laws that govern the behaviour of rocks undergoing simple, basic deformation. Remember, these are the laws of physics and chemistry and they apply to all rocks ... wherever they are found in the universe !
First, go back to the Magellan Mission data from the early 1990s and high-grade the best quality parts. Some of those data are in stereo ("postage stamps") ... although most are not. Regarding wrinkle ridges, use the best stereo data to test whether or not the wrinkle ridges of Venus really are asymmetrical (as is assumed) or not. If they are not, this fundamentally opens up the paradigm for their formation to re-interpretation. Regarding rift fabrics, if they are really formed above blind magmatic dykes, then at least some of those millions of dykes must have leaked to the surface and we should be looking to see if they have filled the bottoms of any of the rifts.
Second, planetary scientists have ignored terrestrial structural geology for far too long. They should go back to the library and read, and learn and better understand the well established universal laws that govern the behaviour of rocks undergoing simple, basic deformation. Remember, these are the laws of physics and chemistry and they apply to all rocks ... wherever they are found in the universe !