Giant ticks on Venus

Venus is probably the only other planet still volcanically active other than Earth, it has lava flows. Venus express has shown sulphur dioxide changes and hot spots on the surface which suggest that it is still volcanically active.

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Heat map of Idunn mons, showing a recent lava flow (ESA/Nasa/JPL)

The volcanoes on Venus should be similar to hotspot volcanoes on Earth like Hawaii. On Venus, they are mostly similar but on much larger scale.

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Pancake domes on venus (Nasa/JPL)

Other volcanoes seen on the planet are called pancake volcanoes. 15 km diameter but only 1 km high these are an artefacts of higher viscosity lavas, which on Earth would build up to steep-walled stratovolcanos  atmospheric pressures (90 times that of Earth’s atmosphere) prevents more viscous lavas from building up vertically and the high surface temperature of Venus which allows the lava to flow for longer before setting forming pancake shapes.

Sometimes these volcanic structures can undergo collapse along the edges of them with radiating valleys around the outside, creating the appearance on an insect with legs. these are known as scalloped margin domes or Tick – Like structures.

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The Tick – a scalloped dome volcano on venus (NASA/JPL)

The volcanoes of Venus show what a difference that the conditions on a planet can make to the shape and style of surface features even if the processes seem similar processes to those on Earth.

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Cryotectonics

Previous posts have looked at the nature of tectonics on the terrestrial planets and how they are very different to Earth. Further out in the solar system, it is not the planets which are of geological interest but the moons, many of these are ice worlds. Cores of rock covered in layers of ice. Some of these have pockets or layers of water in them generated by tidal friction generating heat.

Evidence for this layer of water or at least an icy slush is shown by fountains of ice sprayed into space on both Enceladus and Europa showing that these moons are active places however, there are other telltale signs of a system similar to plate tectonics.

 

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Ice erupting from volcanos on Enceladus (NASA/JPL/Space Science Insitute)

Photos of Europa, an Ice moon of Jupiter, revealed a cracked surface covered in bands. Limited cratering suggests that surface is relatively young (around 40-90 million years). Some of the surface shows repeating bands similar to spreading ridges seen on Earth. Where the surface ice cracks and moves apart and new water rises up and fills the gap with ice.

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Europa as seen by Galileo (Nasa/JPL-Caltech/SETI institute)

These same spreading ridges and associated bands are also clearly seen on Miranda, a moon of Uranus, which has alternating patterns around a central rift, just like is seen on the ocean plates of Earth.

Where tectonics must differ is what happens at the other end; ice is very buoyant making it difficult to find a form of subduction which would allow pulling of the crust down into the water/ slush layers like the plates on Earth. As the surfaces of these moons cannot be ever increasing in diameter, something has to be happening to the ice.

On Europa there no regional scale mountain belts where the crust thickens to accumulate this extra crust. Instead, there is some evidence of complex fault systems. Similar to subduction zones, but which behave differently and have been termed subsumption zones – one section of the ice crust is forced under another, where it rapidly becomes incorporated into the underlying water ice… so rather than continuing to sink like the plates on Earth it rapidly becomes part of the underlying layers, enabling tectonics to continue.

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Miranda as seen my Voyager 2 (Nasa/JPL)

The ice moons of the outer solar system are harsh alien places, but tectonically they may be the most Earth-like of all the bodies in the solar system.

The marks on Mars

Looking at Earth, Venus, and Mercury so far has shown different systems of tectonics. The last terrestrial planet Mars has some familiar features but all is not as simple as it seems.

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Mars Magnetic crust as seen by Mars Global surveyor (Nasa/JPL)

Magnetic images of Mars show band like repetitions of positive and negative anomalies at its southern hemisphere. These bands look similar to magnetic bands seen in Earth’s oceanic crust.

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Bands of magnetic anomalies in the crust of the Atlantic ocean

The bands on Earth’s ocean floors represent the gradual formation of crust over time; as the plates move apart and magma rises up to form new crust it records the magnetic field as it forms, over time the periodic flipping of the Earth’s magnetic field leads to an alternating pattern.

Mars doesn’t currently have an active magnetic field, and crater dating shows that the south is older than the northern 1/3 of the planet which is topographically 3-6 km lower than the southern portion of the planet. This northern lowland does not show magnetic banding. Theories suggested for this lowland include proto-tectonics like that on Earth but which did not occur until after the planet’s magnetic field stopped and/or large impacts causing the substantial topographic difference between the two regions.

Another feature which provides clues about the nature of Martian tectonics is Tharsis Plateau, this is a vast volcanic system close to the equator and includes the 22 km high Olympus Mons, the highest volcano in the solar system. This volcano was able to grow much bigger than the hotspot volcanoes seen on Earth due to a lack of movement, if the plates had been moving then the volcano would not have developed to the same extent. This shows that Mars the plates were not moving unlike on Earth.

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Olympus Mons (JPL)

The huge weight of the plateau put a lot of stress on the crust around it and led to the formation of  Valles Marineris. The extra load on the crust mean that large valleys formed close to the edge of Tharsis Plateau as the weight caused the crust to buckle and break and shear.

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Valles Mariners, a crack caused by the weight of nearby volcanic plateau (NASA)

The magnetic bands suggest that early in Martian history it may have had some form of spreading ridge generating crust in a magnetic field. Both of these processes stopped as Mars cooled. These bands and the north-south topographic divide hint at an early active tectonic history, however, there are few signs of large-scale subduction which would allow recycling of plates so it seems unlikely that Mars ever developed a fully functioning multiple plate tectonic system, though an understanding of its history is far from complete.

 

A massive cover-up

Venus is our closest neighbour both in terms of distance and size. It is also a very alien world with surface temperatures of 46O °C and clouds of sulphuric acid.

To get an idea of what the surface of Venus looks like we have to use radar to see through the thick clouds and build up a topographic model, and what we see is a world with features which are in many ways similar to our own.

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Left the mountain range Akna Montes on Venus (NASA/JPL) right Parra Mountains Mexico (JSC)

Mountain ranges, showing folded and faulted rocks structures surrounded by large flat plateaus, are very similar to the mountain chains on Earth. Rifts, which are cracks in the surface of the planet along which volcanoes generate new crust are found on both planets. Large volcanoes are found all over the surface.

However whilst the features seem similar on the surface, there distribution there are stark differences: on Earth, these are linked to plate tectonics, rifts form where plates move apart or break up. Mountains form as the crust crumples as these plates collide. There are also deep trenches such as the Mariana trench, linked to subduction as plate sinks down back into the mantle as a recycling of the surface. The only major features not caused by plate tectonics are hotspot volcanoes such as Hawaii. Even  these hotspots, show the fingerprints of the movements of plates, which causes them to form chains of volcanoes as the plates move over the source creating lines of islands on the surface of the Earth.

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Hawaii islands, formed by the movement of the plates over a hot-spot (JSC)

On Venus, the volcanoes do not form hotspot chains like those on Earth but instead appear at random all over the surface. Whilst both trenches and rifts are seen they are isolated and discontinuous. In short, just like with Mercury, it is a one plate world without plate tectonics.

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Maat Mons, a 3d topographic model of a volcano on Venus (NASA/JPL)

However, unlike Mercury,  Venus’ surface has very few scars from the harsh environment of the solar system. Earth gets rid of it craters through tectonic processes and water eroding them away. Venus, without water or tectonics, has a very young surface with hardly any craters, which suggest that the majority of the surface is less than 500 million years old (very young in solar system terms).

To understand why this is and why Venus seems to have a universally young surface (even parts of Earth’s surface can be several billion years old) we can look at a side effect of plate tectonics: heat removal. the generation of crust at ridges allows heat to move to the surface and be emitted. Without this plate tectonics, heat builds up under the surface over time. If heat builds up over time, it is possible that a layer of hot rock builds up and the hard crust gets thinner as it warms and softens from the underneath and melt.

It is possible that heat may build up and cause occasional sudden and larger scale volcanism over the majority of the planet, covering much of old surface with a new one, erasing the craters, mountain and rifts and allowing new features to form. Whilst this destruction of features hides the history of Venus, it also provides fascinating insights to our nearest but very different planetary neighbour.

 

The world is getting smaller every day

For the smallest of the planets, Mercury is a surprisingly active place with a complex internal structure and magnetic field, however, it’s tectonics is dominated by its size.

Smaller bodies lose heat a lot faster than bigger bodies, due to a higher ratio of surface area to volume, this means that Mercury has cooled relatively rapidly. As the planet cooled it has shrunk. Estimates for this contraction of its radius range from  ~2 – 7 km over its lifetime, whilst this is only 0.2% of its total radius, this equates to its diameter shrinking   44 km at its equator.

Mercury’s surface is a single tectonic plate, it doesn’t have the subduction and rifting zones which can accommodate strain. As the planet has cooled and contracted the crust at the surface has become compressed as the larger diameter crust is pulled in to fit into a smaller area. Rocks when under compression buckle and eventually break and form faults. In the case of Mercury, shallow angle faults known as thrust faults are created. As one part of the crust rides over another it forms cliffs (called “Rupes”) which can be 100’s of km long and several hundred meters high.

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Carnegie Rupes, 2 km high wall running diagonally across the image created by a thrust fault (NASA/Johns Hopkins University Applied Physis Laboratory/Carnegie Institution of Washington)

Where multiple faults interact they can produce complex structures. The Great Valley is 1000 km long valley formed from thrust faults which make up either side and has been bent downwards in the middle.

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The Great Valley (in Blue) close to Rembrandt basin on Mercury, (Nasa/JHUAPL/CIW/DLR/SI)

Smaller scale scarps have also been identified, whilst smaller (10’s of km long and 10’s of meters high) they are often found cutting across small impact craters and smooth younger planes which means that these features are less than 50 million years old and suggests that Mercury is still tectonically active now.

 

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Smaller scarpes from recent tectonic activity, (NASA/JHUAPL/ Carnegie Institution of Washington/Smithsonian Institution

 

Whilst there are other features such as some structures on its surface which are linked to thicker patches of crust probably linked to mantle structures Mercury is dominated by the side effects of being a small cooling planet which is still slowly shrinking.

Earth’s plates

No body is perfect; forces on both the inside and outside of them push and pull them around. This leads to cracks and wrinkles on surfaces which represent the passage of time. On planetary bodies, this is referred to as tectonics.

Most bodies have tectonic features spread out over their surface. Earth is different. The surface is divided up into interlocking blocks which move around over time, called tectonic plates. These plates are constantly moving: colliding, separating or moving past each other, this movement causes the majority (but not all) of earthquakes and volcanoes on the Earth. The vast majority of tectonic features are related to either current or historical plate boundaries.

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The Earth’s tectonic plates, courtesy of the U.S. Geological Survey

The driving force behind plate tectonics is dominated by the gradual overturning of the mantle as it moves heat from the core to the crust in a process called convection. The plates are also pulled by old cold crust sinking down and pulling the plate with it.

These forces linked to the moving of plates transfer stresses through the crust and lead to the rocks deforming. On a large scale, this leads to several features on the Earth’s surface which can be seen from space. In future posts, we’ll look at some of the features on other planets but first, let’s have a look at a few features formed by plate tectonics on the  Earth:

Faults

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A fault offsets banded rock south of the Tien Shan mountains as seen by Landsat 8 (USGS/NASA)

Faults are breaks in rocks along which movement occurs and are associated with earthquakes. They release strain built up in the crust and are found on all plate margins, including those that slide past each other like the San Andras fault. These are often seen as breaks and offsets in geological patterns.

Rifts

In areas where the crust is under tension it can be pulled apart, the rocks break into a series of faults, which form a staircase-like feature dropping down to form a valley. As the crust gets thinner as it is pulled apart it enables volcanism to start and volcanoes form along the valley (such as in the East African Rift valley).

This rifting can continue and the crust can break into two separate plates. with a volcanic ridge in the middle which generates new crust as the two sides move apart. These features form the seas and the oceans.

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East African rift valley, formed by multiple faults (seen as lines going diagonally across the image) which lead to low lying areas as the plate thins  (NASA/JPL)

Mountains

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Mt Everest and other mountains as seen from STS-066 (NASA, JSC)

In areas where plates collide with each other, the outcomes vary depending on the type of crust colliding.  If it involves a thinner denser crust called oceanic crust (which is the type generated by mid-ocean ridges) subduction occurs – an oceanic plate will be forced down and sink into the mantle. This activity leads to a trough and then a line of volcanoes behind them. Where two pieces of thicker, less dense continental crust collide the parts crash into each other they crumple rather thank sink and form mountain ranges such as the Himalayas.

Earth is unique with its highly active plate tectonic system defining its surface, other planets have been and are tectonically active but in different ways with features spread out and controlled by different tectonic systems.

Scarred faces

The Colorado River slowly cutting its way through rock over millions of years has formed a 450 km long, and 1.8 km deep canyon called the Grand Canyon. Whilst up close and even from space it looks impressive, in reality, it’s fairly small compared to other features which scar the planets across the solar system. Today, we’ll look at the three largest rifts and canyons in the solar system.

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The Grand Canyon in the south if the image at seen from the ISS (NASA)

Valles Marineris on Mars at 4000 km long is 9 times as long as the Grand canyon and stretches 1/4 of the way around the entire planet. There are a few theories about how it formed, the most prominent is that the nearby Tharsis bulge (an area which hosts the tallest volcano in the solar system) caused tension in the crust, which lead to it tearing and pulling apart, in a process known as rifting. Erosion then would have deepened the valley further and has produced outflow channels at the end of it.

 

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Valles Mariners, Mars (NASA)

 

One Venus, Baltis Vallis is even longer at nearly 7000 km long, its ends are covered so its original length is unknown. This feature, like the Grand Canyon, was also likely formed by erosion. Instead of water, high-temperature lava flows would have torn up, melted, and dissolved the surfaces as they flowed over them, slowly wearing them away in the same way rivers do on Earth. Similar features were probably once present on the early Earth, Komatiite lavas, found in Australia also show signs of eroding the rocks beneath them and forming channels as they flow through an area.

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a 600km segment of the 7000 km Balits Valley from Radar data collected by Magellan, the channel can be seen as a meandering line going diagonally across the image arrow to arrow (NASA/JPL_

However, the largest in the solar system is right here on Earth the 10’000 km long and up to 8.5 km deep Atlantic Ocean is a large cut into the Earths Surface. Like Valles Marineris, this was formed by rifting rather than a valley. Tectonic forces pulling and pushing Europe and Africa away from the Americas. Plate tectonics allowed the rift to develop much further and volcanism generated new oceanic crust was generated in between the two.

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Bathymetric image of the Atlantic ocean (NOAA)

This quick overview of the largest valleys and rifts in the solar system highlights two main processes which can form them; erosion and tectonics. Volcanism on Venus and Mars, extensional forces tearing the crusts show these planets have been tectonically active. In the next few posts I’ll examine tectonics work across the solar system.