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Water in the Universe | Part 2

Water in the Universe | Contents | Intro | Part 1 | Part 2 | Part 3

Water and the Formation of the Planets

One characteristic that makes the Earth unique in the Solar System is that it is the only planet that has plate tectonics and that is partly due to its water.  To understand why that is will require some background beginning with the composition of terrestrial planets (Earth-like: relatively small, rocky, and dense compared to the gas and ice giants of the outer Solar System). 

In a quick recap, the material from which terrestrial planets could potentially form in the early Solar System included rock, gas, and ices.  The gas would be hydrogen and helium and the ices would be water ice, methane (CH4) ice, and ammonia (NH3) ice; water would by far be the most abundant ‘ice.’  In the inner Solar System, these ‘ices’ wouldn’t be solids but rather vapors in the vacuum of space.  But because of the early Solar wind, almost all of the gas and ‘ices’ were blown out of the inner Solar System.  Waiting just beyond the asteroid belt, sat a newly forming Jupiter, eagerly gobbling up much of the gas coming from the inner Solar System.  What Jupiter didn’t get, a more distant Saturn intercepted which is why they are gas giants, the largest planets in the Solar System. 

The gas giants certainly picked up a big share of the ices as well but are mostly gas.  The smaller but still giant Uranus and Neptune did pick up some gas but are mostly made up of the ices, hence, ice giants.  All of the gas and ice giants also contain some rocky material which was available in the neighborhood at the time they formed but the rocky material makes up a relatively small portion of the planets.  The terrestrial planets had to make do with mostly just the rocky material which is how they ended up relatively small, dense, and rocky. 

So, what is this rocky material from which the terrestrial planets formed, material which would resist being pushed out of the inner Solar System?  Simply put, most of it was silicates and iron/nickel.  Silicates are combinations of silicon and oxygen with other elements such as aluminum, iron, and magnesium; the simplest silicate is SiO2 of which the mineral quartz is an example (SiO2 dissolved in water is known as silica). 

An earlier article discussed the relative abundance of the elements in the universe and it might be expected that the bodies of the Solar System would reflect that relative abundance with the notable exception of carbon.  This is certainly the case for hydrogen and helium.  Most of the Sun, Jupiter, and Saturn are hydrogen and most of the rest is helium.  Oxygen, the third most abundant element, is the most abundant element by volume in a terrestrial planet but as a gas, you would not expect it to be abundant in the inner Solar System.  However, oxygen readily combines with other elements including another relatively abundant element, silicon, hence, silicates.  Oxygen will bond with iron too but it doesn’t take much heat/pressure to break that bond which is why such compounds are not common on a planetary scale and certainly not in a very hot planetary core.  Silicon and aluminum form much tighter bonds with oxygen; heat alone cannot break those bonds. 

Carbon, number four in universal abundance, might be expected to be a very important component in the composition of the Solar System bodies – but it isn’t.  This is because most of the carbon in the universe is locked up in white dwarf ‘stars’ and not usually available to form planets.  Ironically, the pressure conditions within white dwarfs make their carbon take the form of diamonds:  twinkle, twinkle little star, like a diamond in the sky.  Who knew?  (not very accessible, though)  Probably most of the carbon in the Solar System is in the outer Solar System in the form of methane ice.  Fortunately for life on Earth, the Earth does have some carbon but nothing like the amount suggested by its universal abundance. 

On a gross scale, rocky planets, asteroids, and moons are made up of silicates and iron/nickel.  However, it does include small but important amounts of radioactive elements such as two isotopes of uranium (238U & 235U), 232Th, 40K, and 26Al (now long gone because of it short half-life).  The heat from the decay of these radioactive isotopes plus heat produced by the collision of the particles that formed these rocky bodies built up enough shortly after their formation to turn the bodies molten.  Once that happened, these rocky bodies became differentiated which means that the heavier, denser stuff, the iron/nickel, sank to the center of the body to form an iron/nickel core and the less dense silicates floated above it to form the crust and mantle.  The crust is further subdivided into oceanic and continental crust but both mantle and crust are essentially silicates.  What differentiates them is what kind of silicate. 


The mantle is made up of denser iron and magnesium silicates and the crust of less dense aluminum silicates (feldspars).  Within the crust, oceanic crust is made up of denser calcium/sodium aluminum silicates and continental crust of less dense sodium/potassium aluminum silicates.  This explains why the crust sits above the mantle and why continental crust sits higher than oceanic crust.  However, another question comes up when you compare the amount of crust versus the amount of mantle. 

About 1/3 of the mass of the Earth is iron/nickel core; the mantle is ~ 2/3s of the mass and the crust only about 0.5%.  Why so little crust compared to mantle?  The answer lies in the universal abundance of the elements.  The amount of magnesium and iron is roughly comparable but the amount of aluminum is much, much less, hence, lots of iron and magnesium silicates (mantle) and not much aluminum silicates (crust). 

You might notice that not all of the iron is in the core (although most of it is).  Iron can be found in all layers of the Earth which is a good thing for the industrial revolution.  There is a truism that nothing is ever pure in geology.  The differentiation of the Earth was never complete and later extraterrestrial bombardments after the Earth was mostly differentiated brought in more near-surface iron and nickel. 

Structure of the Earth - Crust Mantle Core

Aside from composition, the structure of a terrestrial planet can be described in another way: what part of it is solid and what part not (at least slightly molten).  Rocky planets are in the process of freezing solid: freezing from the inside out and from the outside in.  Freezing from the inside out refers to the core of the planet.  For most of Earth’s history, its core was completely molten.  About half a billion years ago as the core continued to cool, Conditions at the center of the Earth became such that a solid inner core could begin to form.  This was important for Earth’s water as convection within the molten core generates the Earth’s magnetic field which helps protect The Earth’s atmosphere from the solar wind and cosmic radiation, a protection Mars and Venus lack. 

Some time before the inner core began to form, maybe about half a billion years ago, convection in Earth’s molten core was slowing and the magnetic field significantly weakening.  Formation of the solid inner core rejuvenated convection in the outer core, additional heat coming from the change in state of molten iron to solid iron in the newly developing inner core. That solid inner core has been expanding ever since at the expense of the molten outer core.  In about another billion years or so, the entire core will freeze solid and that will be the end of Earth’s magnetic field with dire consequences for surface life.  For this and several other reasons, one already mentioned (loss of the oceans), surface conditions on Earth will become very unpleasant in a billion years. 

The Earth is also in the process of freezing solid from the outside in beginning with its surface.  That outermost portion of the Earth now frozen includes all of the crust and the uppermost part of the mantle; it is called the lithosphere.  Note that crust and mantle are defined by composition whereas the lithosphere is defined as the outermost frozen (solid) part of the planet.  Over time, the lithosphere of a planet will become thicker and thicker.  Since small things freeze solid faster than big things; you can imagine that the lithosphere of Mars is now considerably thicker than that of the Earth.  A major reason Mars has little evidence of plate tectonics is that its lithosphere became too thick too fast.

One not fully answered question is that if small things freeze solid faster than big things, how is it that so many small objects including some outer Solar System moons, Pluto, and a few large asteroids can still have unfrozen subsurface oceans after more than four billion years?  Tidal interaction between some moons and their planets can explain some of that, making Io the most volcanically active body in the Solar System because of interactions between Saturn and the next big moon out, Europa, but bodies like Ceres and Pluto probably have subsurface oceans and tidal action cannot explain them. 

Why is the Earth the only Planet with Plate Tectonics and What does that have to do with Water?

Plate Tectonics

To answer that question first requires a brief description of how plate tectonics operates on the Earth of today.  Although the lithosphere of a terrestrial planet grows thicker over billions of years as the planet slowly cools and freezes from the outside in, the lithosphere of the Earth is still thin enough that convective movement of the not completely frozen rock beneath the lithosphere (the asthenosphere) breaks the lithosphere into various size pieces known as plates.  Plate tectonics is the story of how these plates interact; they can pull apart (divergent plate boundary), collide (convergent plate boundary), or slide past one another (transform fault boundary).  A given plate could be entirely made up of oceanic crust such as the Pacific plate which is the largest plate the Earth has or it can be a mix of continental and oceanic crust as is the case of the North American plate which includes oceanic crust out to the MidAtlantic Ridge in the middle of the North Atlantic Ocean.

Plate Tectonics

The plate tectonics story began with the recognition in the 1960s that the sea floor could spread; sea floor spreading was the precursor to what became plate tectonics. This also meant that continents themselves could and did move great distances horizontally. When these plates collided, they would produce lofty mountain ranges. The highest mountains on Earth today, the Himalaya Mts., still growing, are the result of the ongoing collision of India and Asia. The Andes Mts. are the result of the ongoing collision of South America with the oceanic Nazca plate. Moving continents have enormous inertia so the collisions, moving at the rate at which your fingernails and hair grow, can drag on for tens of millions of years.

The moment a mountain is created on Earth, water erosion and weathering start to tear it down; the higher the mountain, the faster the erosion.  Once a collision ends, erosion gets the upper hand and can level a mountain range in several hundred million years.  The Ural Mountains, created when Siberia rammed into Europe, were once as lofty as the Himalayas but they are much older than the Himalayas and erosion has greatly humbled them.  Plate collisions are responsible for most of the positive relief (difference between the highest and lowest elevations of an area) of the Earth’s surface).  Tectonics refers to mountain-building, hence, plate tectonics. 

Subduction (an oceanic plate dives down into the mantle) is responsible for areas of great negative relief such as the Marianas Trench which is considerably deeper than Mt. Everest is high.  If the Earth did not have plate tectonics, its surface would be much flatter (less relief) and Earth would be a water world (very little land above water).  For comparison, the relief of Venus is 12 km and that of the Earth is 20 km. 

More than two thirds of the Earth’s surface area is oceanic crust but since Earth’s continental crust is about three times thicker than oceanic crust, Earth has roughly equal amounts of each.  Note, however, that there is not enough of either one to form a complete layer around the Earth.  Note also that each occupies separate areas of the Earth’s surface; there is no oceanic crust beneath continental crust (except where it is subducting beneath continental crust). 

The Earth cools by moving heat from the hotter interior to the cooler exterior.  Heat can move three ways: by radiation, by conduction, and by convection.  Heat cannot radiate through rock.  Some heat can conduct through rock but rock is a horrible conductor of heat.  If you had a foot-long rod of most any kind of rock, you could heat one end of the rod until is was so hot that it melted while holding the other end without any discomfort.  So, unable to radiate and with very little conduction, the heat can build up in the rock until it becomes, at least partly, molten which then allows the heat to move upward through convection of the molten rock.  It is this convection which drives the lithospheric plates apart and which eventually starts continents moving, creating new oceanic crust in between them (sea floor spreading).  Lithospheric plates literally float on the asthenosphere.  The mantle floats on the molten outer core and the solid inner core rotates slightly faster than the rest of the Earth. 

However, creating new oceanic crust in one place means that old oceanic crust somewhere else must be removed by subducting it into the mantle to make room for the new oceanic crust.  Continental crust, being less dense than oceanic crust, doesn’t subduct; it jams up against another plate and creates mountain ranges.  In places where oceanic crust is old and cold enough, it can begin to subduct on its own leading to the question of which is more important, the push of spreading sea floor or the pull of subducting cold oceanic crust.  Either way, without subduction, plate tectonics would be impossible. 

Continental crust is mostly granite which can represent it.  Granite is mostly feldspars (aluminum silicates), specifically orthoclase (potassium) feldspar and sodium-rich plagioclase feldspar, minerals which make up more than 80% of granite.  The other common minerals in granite are quartz [SiO2], two micas: muscovite KAl2(AlSi3O10)(OH)2] and biotite [K(Mg,Fe)3(AlSi3O10)(OH)2], and hornblende [NaCa2(Mg,Fe,Al)5(Si,Al)8O22(OH)2.  All but the quartz are aluminum silicates.  The biotite and hornblende have iron in them, illustrating that all of the iron did not end up in the Earth’s core, that differentiation was never complete.  The two micas and hornblende have OH groups which are derived from water. 

Basalt can represent oceanic crust and Peridotite the uppermost mantle.  Basalt is mostly calcium-rich plagioclase feldspar and pyroxene (an aluminum silicate with Fe, Mg, and others) and peridotite is primarily pyroxene and olivine [Mg,Fe)2SiO4].  The density of peridotite is about 3.85 g/cc, basalt is 3.0 g/cc, and granite 2.75 g/cc.  Incidentally, speaking of minerals, some 80% of all known minerals require water to form.  This does not mean that the mineral necessarily has water in it (this would be shown by •nH2O or (OH)n in its chemical formula).  Consider, for example, calcite and aragonite, both of which are different crystalline versions of CaCO3.  There is no water in their chemical formula; they precipitate from a water solution (in rock form, they would be limestone). 

There is an apparent problem.  The question is how can less dense basalt (oceanic crust) subduct into denser peridotite (upper mantle)?  A clue is that the densities given are the densities of the rocks at room temperature but (older) oceanic crust is not at the same temperature as the upper mantle below it; it is colder. 

Cold oceanic crust is slightly denser than much hotter upper mantle which is what makes subduction and modern plate tectonics possible on Earth.  You would expect that as cold oceanic crust subducts into hot mantle, the oceanic crust would heat up and it does, enough to melt at the edges.  But remember that rock is a horrible conductor of heat so the interior of the subducting oceanic crust remains cold and dense enough to continue to subduct.  If it can subduct far enough into the mantle, the increased pressure will cause the minerals in the basalt to recrystallize into new, denser minerals, changing the basalt into eclogite which has an average density of 3.6 g/cc, helping the altered oceanic crust to continue to subduct deeper into the mantle. 

As the surrounding hot mantle melts the outer portions of the descending oceanic crust, the surrounding mantle cools and becomes less efficient at melting the oceanic crust which, if more oceanic crust continues to subduct in this region, allows the descending slab to penetrate even deeper.  Additional crystallization of minerals into denser forms aids the process and some, now considerably modified, oceanic crust may make it all the way to the bottom of the mantle.  Eventually it will melt and begin to ascend again, completing the convection cycle. 

When subduction occurs on Earth, the descending oceanic crust takes quite a bit of water with it which as the oceanic crust melts, eventually becomes incorporated into the mantle.  This bit of water substantially lowers the melting point of the mantle rock, facilitating mantle convection and extending such convection for billions of years.  Some of the subducted water is recirculated to the surface by volcanism; most of volcanic gas on Earth is steam. 

It is thought that up until about a billion years ago, more water was being released to Earth’s surface by volcanism than was lost to it by subduction.  After that, the surface began suffering a net loss of water to the mantle.  In another billion years from now, the mantle will have become cool enough that, in spite of any water, convection will be greatly reduced and plate tectonics will slowly grind to a halt. 

Making Continental Crust

Let us begin with modern plate tectonics on Earth as it forms small amounts of new continental crust; most of the continental crust the Earth has today formed billions of years ago but the principal is the same and we can use known, modern locations.  The following description of the process is not ideal as it does not begin with undifferentiated crust today but again, we are trying to illustrate the process, perhaps as it might have happened in an early Earth.  The process is also oversimplified in that such mineral separations are inefficient and incomplete and may have to happen a number of times to effect a change to something resembling continental crust. 

Water plays a crucial role in the process as it lowers the melting point of the minerals, facilitating the partial melting.  Consider, for example, quartz (SiO2).  If you look up the melting point of quartz, you will find that it is about 1650 °C.  But in a granitic magma, quartz will be the last mineral to crystallize out at around 600 °C.  The magma, by definition, is below the surface of the Earth where it is under considerable pressure which would, if anything, raise its crystallization temperature (melting point) but a few percent of water in the magma dramatically lowers it. 

Undifferentiated crust would still be molten around 1200 °C but, ideally, as it cooled to about 1000 °C, certain minerals would begin to crystallize out and separate from the magma.  An assemblage of these minerals, principally Ca-rich plagioclase feldspar and pyroxene with a dash of olivine, would be similar to the composition of the rock, basalt, which is the rock most representative of oceanic crust.  This process is known as partial melting which allows for the separation of different mineral compositions according to their crystallization temperatures or melting points. 

Assuming that the magma, now largely depleted of the minerals of those minerals, continued to cool, another assemblage of minerals would begin to crystallize out, mostly plagioclase feldspar richer in Na along with some amphiboles (amphiboles are a mineral family, hornblende being the best known member).  This assemblage of minerals would form a rock, best represented by andesite, which has a composition intermediate between oceanic crust and continental crust. XX

Chart regarding Ca-feldspar Na-feldspar k-feldspar

Intermediate composition magmas which produce andesite are commonly found where an oceanic plate is being subducted underneath another oceanic plate (two oceanic plates are colliding) or where an oceanic plate is colliding with and being subducted under a continental plate.  A classic case of the latter is where the Nazca oceanic plate (a small plate that splintered off of the Pacific plate) is colliding with the western edge of South America.  As the oceanic plate subducts under South America, it partially melts, preferentially creating a magma rich in what will become minerals of intermediate melting points when the magma solidifies; the magma has the composition of andesite.  The magma rises and becomes incorporated into the Andes Mountains created by the plate collision, often fueling active volcanism in the mountains.  The rock composition of the Andes Mountains, then, is partly andesite; not all of South American is actually strictly continental crust. 

Andesite is also generated where two oceanic plates collide.  In such an event, one oceanic plate subducts beneath the other.  The subducting plate undergoes partial melting with produces a magma of intermediate composition which rise, fuels volcanism, and is incorporated in the leading edge of the other plate.  That other plate is crumpled and pushed up, creating a long, semi-arc mountain range whose tallest active volcanic peaks stick up above sea level to create what is known as an island arc. 

The subducting plate creates a deep sea trench parallel to the arc of islands.  Island arcs are common in the oceans of the world, some examples of which are the Aleutian Islands of Alaska, the Philippines, Indonesia, and the Lesser Antilles of the Caribbean.  Do not confuse island arcs with oceanic hot spot island tracks like the one that ends at the big island of Hawaii.  The islands in an island arc are all actively volcanic.  The only active volcanism in an oceanic hot spot island track is at the end of the chain beneath which is the current location of the hot spot.  Also, there wouldn’t be a parallel deep sea trench.  Hot spot islands are dominated by basalt, not andesite.  

Thanks to plate tectonics, continental plates can and do move considerable horizontal distances.  It may happen that such a continental plate may encounter and run over an island arc rich in andesite.  The speed bump represented by the island arc may spur a new round of mountain building at the edge of the colliding continent (an orogeny) and the subducting andesite-rich island arc may produce a partial-melt magma whose composition is similar to that of continental crust – new continental crust. 

An Iron/Potassium Tangent

By now you may be wondering how a magma of intermediate composition can be extracted by partial melting from oceanic crust which is presumably already differentiated or how a granitic magma can be generated from andesite.  The key is that differentiation is never complete.  It sometimes seems that nothing in geology is ever pure.  To illustrate this (and go off on yet another tangent) consider iron (Fe) and potassium (K).  The Earth has an entire core of iron which makes sense since iron is denser than the silicates that make up the rest of the Earth.  And yet, iron can be found throughout the Earth, including the crust.  Most of the iron is, indeed, in the core, comprising some 85% of it but even the crust is about 5% iron.  Recall that the biotite and hornblende in granite contain iron. 

The chemistry and physics of potassium are such that you would expect that most of it should be in the crust and most of that in continental crust (orthoclase feldspar).  Although the Earth has much less potassium than it does iron, potassium is 2% or the crust.  That iron core?  0.1% is potassium; at the temperatures and pressures in the core, it can form an iron alloy.  It might not sound like much but even that little bit turns out to be significant.

The geothermal energy that drives things like plate tectonics, volcanism, and earthquakes comes from the heat produced by the decay of radioactive isotopes, the two most important of which are 238U and 232Th.  The number three most important heat-generating radioisotope in the core is 40K which has a half-life of 1.3 billion years.  Most potassium (mostly 39K) is not radioactive; 40K is about 0.01% of all potassium.  That means that 0.01% x 0.1%  = 0.00001% of the core is 40K and yet it is still the number three heat-producing radioisotope in the core.  40K is important near the surface of the Earth too. 

When 40K decays, 11% of the time it becomes 40Ar (the other 89% of the time it ends up as 40Ca). Argon gas makes up almost 1% of the Earth’s atmosphere today. Guess where almost all of it comes from. Go orthoclase.40K is also the most abundant radioisotope normally present in the human body; 14C is a close second.

Early Continental Crust Generation

If any planet other than Earth should exhibit plate tectonics, it should be Venus, Earth’s near twin – but it doesn’t.  But then, the early Earth didn’t either.  To explain all that, consider the history of the Earth.  Shortly after Earth, Venus, and Mars formed, they became completely molten, allowing the incomplete differentiation of core, crust, and mantle.  What was to become the crust was still molten and, therefore, undifferentiated into oceanic and continental crust.  That time in Earth’s history when it was molten is referred to as the Hadean (Hell) Eon.  

Recent studies suggest that the Hadean didn’t last nearly as long as previously thought, that the Earth’s surface might have solidified more than 4.4 billion years ago (the Earth formed some 4.567 billion years ago).  Very shortly thereafter there is some controversial evidence that there were surface oceans.  Where all that water came from is not completely clear, whether from the interior or from ice-bearing asteroids, but either way, there must have been one impressive rain when the condensation started.  

As the undifferentiated crustal magma cooled on a global scale, at some point you might expect a mineral assemblage resembling oceanic crust to start to crystallize out with a more continental crust-like magma composition above it which would eventually solidify into continental crust.  Such a description is, of course, very much oversimplified but if the general idea is correct and there were enough crust, the planet might be able to form a complete layer of oceanic crust underneath a complete layer of continental crust which, incidentally, would make any subsequent plate tectonics impossible.  

As more material solidified on the bottom of the lithosphere, regional heating of some of that now heavier material by convection of hot mantle from below could cause part of the bottom lithosphere to melt and drip down into and mix with the mantle in a process known as sagduction.  That mantle convection can also add new material to the bottom of the lithosphere in a process known as massive regional relamination.  Such processes probably still operate in Venus.   

This repeated melting and re-solidification might eventually produce some magma of more continental crust composition but how would you get it through the lithosphere and in an area as large as even a microcontinent?

The undifferentiated crustal lithosphere could conceivably be broken up into plates but it would be more like sea ice, blocks of which can jostle one another but because you cannot subduct the ice blocks down into the water in which it floats, it cannot move aside to make room for even small areas of what could become continental crust.  mantle which prevented subduction.  No subduction, no plate tectonics. 

One suggested solution is that the early planets were still undergoing considerable bombardment by various bodies such as asteroids and that an asteroid strike could break up an area of the thin lithosphere, allowing for the formation of a few early microcontinents (cratons).  Slowly, the Earth cooled from the outside in and the lithosphere, possibly still largely undifferentiated crust, formed a largely unbroken lid over a still very hot mantle.  

As the Earth aged, the lithosphere thickened, and the mantle cooled, the amount of continental crust increased, forming the first mini-continents (cratons) and early plate tectonics.  Descending oceanic crustal slabs penetrated ever deeper into the mantle, the lithosphere fractured into plates, and modern plate tectonics slowly developed.  

Another problem is that we don’t know how much water would have been in the early interior of the planet to facilitate the crustal differentiation.  The interior would have been much hotter than it is now which meant that the interior could not hold as much water.  Perhaps the hotter interior would not have required much water to differentiate the crust although what the water might do is not only lower mineral crystallization temperatures but also spread them out over a wider temperature range which would make separation easier and more efficient.  And how critical are extensive oceans and the recycling of water between the surface and the interior to the full development of plate tectonics?

We just don’t know how much early crustal differentiation there might have been or even how much crust a typical terrestrial planet could be expected to have.  The subduction and partial melting part of plate tectonics is so much more efficient at differentiating crustal types but maybe the norm for terrestrial planets is stagnant lid tectonics even after billions of years as is the case with Venus today.  Absent subduction, plates could still jostle one another enough to produce some crumpling but nothing compared to the lofty mountains and deep trenches of modern plate tectonics.  Such crumpling (tesserae) can be seen in some areas of Venus.  With much less relief, the Earth at this time would have been a water world as would Venus and Mars today if they still had oceans.  Venus remains in stagnant lid tectonics while Mars simply froze solid too deeply and too quickly.  

The Theia collision means that the abundance and history of Earth’s crust is probably not typical of terrestrial planets (a bit of irony there).  A better understanding of Venus may be crucial in resolving this issue.  It will be very important to ascertain the amount, type, and distribution of crust on Venus, assuming that Venus would be a more typical terrestrial planet, not having undergone a Theia-like collision (although its very slow backward rotation is suspicious).  More understanding of Mars’ crust would also be a big help.

Hot Spots

Earth is the most volcanically active planet in the Solar System (because of plate tectonics), much more so than Venus or Mars (Io, one of the four big moons of Jupiter, is even more volcanically active but that is another story).  Most of Earth’s volcanism is at plate tectonic boundaries but there is another source of volcanism largely independent of plate tectonics and plate boundaries: hot spots.  Hot spots, plumes of magma from deep within the mantle fueled by heat from the core, could penetrate a stagnant lid and produce a very limited  number of huge volcanoes, two of which could cover the entire state of Pennsylvania.  Venus’ largest volcano is Maxwell Montes and Mars’ is Olympus Mons.  The closest the Earth comes to such huge volcanoes is Mauna Loa on the big island of Hawaii, Earth’s largest volcano.  If measured from the sea floor, Mauna Loa is taller than Mount Everest.  All of these huge volcanoes are caused by hot spots which all three planets have. 

Plate Tectonic Hot Spots

When such a hot spot plume first breaks through to the surface it produces a flood of basalt (flood basalts) which can cover tens of thousands of square miles (large igneous provinces).  Some examples include the Columbia Plateau (parts of Oregon, Washington, and Idaho) and the Deccan Plateau (India).  The plumes continue to periodically erupt through the crust and as the crust moves above them, they burn tracks through it.  A classic hot spot which happens to be erupting through continental crust is the one which created the Columbia Plateau and is now situated under Yellowstone National Park, a supervolcano powering geysers like Old Faithful.  The Hawaiian Islands and the subterranean mountain chain from Midway and beyond leading to them are the track of another hot spot erupting through oceanic crust. 

Strange thing, though.  All of Earth’s volcanoes are comparatively young, on the order of tens of millions of years old or less and most are relatively small.  The Earth is much more volcanically active than Venus and Mars but it is because most of Earth’s volcanoes are located at plate boundaries and are fueled by plate tectonic interactions which in relatively short order destroy the very volcanoes that they create, hence, the young ages of Earth’s volcanoes.  Mars and Venus have far fewer volcanoes but they are extremely old, huge, and they don’t appear to be very active.  Mauna Loa is less than one million years old; Maxwell Montes and Olympus Mons are billions of years old.  The largely unbroken Venetian and Martian lithospheres don’t move.  It doesn’t take much volcanism to produce a very impressive volcano if the eruptions continue intermittently for billions of years at the same place and there is no water erosion or plate movement to destroy them.

Venus and (no) Plate Tectonics

Subduction and plate tectonics on Earth are possible because the Earth has cold oceanic crust.  The crust of Venus is anything but cold primarily because of its atmosphere.  Hot oceanic crust cannot subduct into hot mantle. 

When radar first penetrated Venus’ very thick cloud layers, it revealed what first seemed to be a familiar surface.  Generally, most of its surface is low-lying and rather flat (oceanic crust?) with some mountainous highland areas (continental crust?).  It appeared to be similar to Earth minus oceans.  Then it became apparent that the highland ‘continental’ areas were high, not because they were continental crust and not because of plate tectonics of which Venus seems to have very little or none, but because these were hot spot areas like a Mauna Loas on steroids.  Even the lowland areas looked more like flood basalts.

Venus (and maybe Mars) appear to be still stuck in the stagnant lid tectonics phase through which an early Earth evolved.  That stagnant (it doesn’t move) lid (mostly unbroken except where penetrated by hot spots) makes it much more difficult to transfer water and heat to the surface of Venus or an early Earth.  Even in modern plate tectonics on Earth, heat gradually builds up under the thicker continental areas, especially when there are super continents, ultimately splitting continents which can move horizontally.  Venus can’t do that so great amounts of heat build up under its lithosphere, much more than would be the case on recent Earth, until there is a global outbreak of magma at its surface with a great release of heat and water vapor. 

Such an event has happened at least once in Venus’ relatively recent history.  About 800 million years ago there was a global outbreak of volcanism that resurfaced some 40% of Venus’ surface with (presumably) massive flood basalts.  The Earth also experiences occasional flood basalts from its hot spots but nothing close to the scale of those of Venus.  As on Earth, the volcanism releases volcanic gases, most of which is steam (water vapor) with some carbon dioxide, sulfur dioxide, et al. 

Now here is a bit of speculation.  Venus lost its surface water (if it ever had any) and its atmospheric water vapor eons ago and so you might expect that its atmosphere would be clear, no clouds.  That may have been the case for most of Venus’ history but the massive release of water vapor 800 million years ago into its atmosphere and its interaction with sulfur dioxide created sulfuric acid (H2SO4) whose droplets form the basis of Venus’ thick clouds which persist today.  Perhaps, given enough time, photolysis will thin out those clouds and the Venetian atmosphere will become clear again. 

In an early Venus with a relatively thinner lithosphere, mantle convection below the lithosphere could melt a portion of the lower lithosphere and subduct it downward but the unmelted lithosphere above it would remain largely unbroken (no moving plates).  In a later Venus with a thicker lithosphere and a cooler mantle, mantle convection would be diminished although mantle plumes could continue to push up into the lithosphere, in places lifting it and fueling hot spot volcanoes like Maxwell Montes. 

Mars, Plate Tectonics, and Water

Early Mars had surface oceans or seas, perhaps for as long as a billion years.  Why doesn’t Mars have plate tectonics?  Unlike Venus it certainly was never too hot and at least in its early days it wasn’t too dry.  How much of its crust is continental is a question but one other reason is related to its size.  Mars is considerably smaller than Earth and Venus and small things freeze faster than big things.  The lithosphere of Mars probably became too thick too soon to support much plate tectonics. 

In addition to a few super volcanoes, Mars is noted for Valle Marineris, a huge canyon which may be Mars’ early feeble attempt at plate tectonics.  This gigantic crack in Mars’ lithosphere is often compared to the Grand Canyon which is tiny in comparison.  If moved to Earth, Valle Marineris would stretch from New York City to San Francisco, the length of interstate 80 which connects those two cities.  But this is an example of comparing apples and oranges.

The Grand Canyon is an erosional feature.  It was created when a descending slab of oceanic crust beneath Colorado produced enough pull to delaminate (separate) the mantle part of the lithosphere above it.  The crustal part of the lithosphere, relieved of the weight of its mantle component, rebounded upwards (isostasy) to form the Colorado Plateau which then induced the Colorado River to start cutting downward, eventually producing the Grand Canyon. 

Valle Marineris is not primarily an erosional feature.  It is probably the initial stage of lithosphere splitting.  Its Earthly counterpart would be, not the Grand Canyon, but the Red Sea which is quite comparable in dimensions to the Valle Marineris.  But that was as far as Mars went.  Earth doesn’t stop at a Red Sea but continues to an Atlantic Ocean and beyond. 

Lacking plate tectonics, early Mars was unable to subduct any of its surface water into its mantle although probably most of the water in its interior is still there.  Too small, too cold. 

Colliding Worlds and Moon Formation

One question is which planet, Earth or Venus, is the norm for terrestrial planets and which one is the exception?  Would most terrestrial planets in habitable zones around their stars be water worlds with a few hot spot islands like a cooler Venus or have extensive land areas like Earth? 

Shortly after the planets formed and differentiated (separated into core, mantle, and crust), there is evidence that many of them suffered massive collisions with other bodies.  Mercury appears to have lost most of its crust and mantle, Venus rotates backwards very slowly, Uranus is tipped 98°, and the Earth has a major moon, the only terrestrial planet with a big moon (Mars has two little nothings).  Modern computers are wonderful devices, capable of modeling many scenarios, delineating what is possible, what is likely, and what is impossible.  One question computers have modeled is the origin of the Earth-Moon system. 

Computer models have suggested that the moon originated as the result of the collision of the Earth with a Mars-size body within 50 million years of the formation and differentiation of the Earth, that the colliding body (Theia) hit the Earth off center, blowing off much of Earth’s crust and some mantle, and pretty much demolishing Theia.  Much of the debris ended up orbiting the Earth, giving the Earth a temporary rocky ring. 

Before continuing this scenario, some generalities on planetary rings and their physics are needed.  Consider the rings of Saturn which are made up almost entirely of particles water ice in a very thin plane around the planet.  The rings probably formed relatively recently (because the ice particles are still so highly reflective) when one of Saturn’s icy moons came to close to Saturn (the Roche limit) and tidal forces pulled the moon apart, creating of ring of ice and rock particles. 

Most of the outer Solar System moons are about half water ice and half rock.  The physics of rings are such that denser material tends to fall to the planet and less dense material remains in orbit or even drifts away from the planet.  In Saturn’s case, most of the denser material, the rock, probably plunged into the planet leaving behind the less dense ice particles to form the present rings.  Material in rings may coalesce and form a moon but Saturn is so large that it seems to have prevented this from happening or perhaps it is just that the disruption of an icy moon that strayed too close to Saturn happened not all that long ago.  It is interesting to note that one of the medium-size moons of Saturn, Tethys, is almost entirely made up of ice which suggests that it formed from the disruption of an earlier generation moon. 

Whatever hit Mercury and blasted away most of its crust and mantle may have briefly formed a rocky ring but Mercury is so small and so close to the sun that probably most of that material fell into the sun rather than onto Mercury, leaving Mercury smaller and mostly iron/nickel core. 

In the case of the Earth, much of the mantle and most of the iron/nickel core of Theia ended up falling to Earth along with some of the mantle blasted off of the Earth.  Lighter material including potassium, rare earth elements, and phosphorus (KREEP) were blown out of the rocky ring and away from the Earth.  In fairly short order, the remainder of the material in the ring coalesced into Luna (our moon) which was initially hot enough to differentiate into a tiny core, mantle, and crust depleted of KREEP.   The loss of potassium meant that the moon could not form a continental crust like on Earth.  Earth’s continental crust is mostly orthoclase (potassium) feldspar and sodium-rich (Na) plagioclase feldspar = granite.  The moon’s analogue to continental crust is sodium-rich (Na) plagioclase feldspar which in rock form is known as anorthoclase. 

The front face of the moon is mostly light-colored highlands (the anorthoclase) with dark-colored Maria (‘seas’ of flood basalts).  The back side of the moon is almost entirely highlands anorthoclase.  Our moon, possibly like Venus, seems to have a complete shell of ‘continental’ crust with some ‘oceanic crust’ (the flood basalts of the Maria).  On Venus, the flood basalts come from hot-spot super volcanoes.  On the early moon, the flood basalts were likely released when massive collisions pierced a very thin lithosphere. 

This scenario suggests that the Earth became depleted of crust, especially continental crust, allowing for the segregation of the two crustal types in different surface areas.  Such a segregation would allow for the movement of continents and the subduction of oceanic crust, leading to plate tectonics with extensive land area.  It also means that the Earth is the exception and a cool Venus the norm, that terrestrial planets in the habitable zone would more likely be water worlds and plate tectonics rare.  Perhaps, in order to find an Earth-like planet in another stellar system, we need to find, not only a terrestrial planet in a habitable zone, but one with a large Luna-like moon, not a very likely event.  Are technological, space-faring civilizations likely to develop on water worlds with little dry land?  How important is a large moon as a stepping stone to space? 

But is all this just speculation, an overextension of computer models?  New studies of the interior of the Earth may show some of the remnants of Theia now deep within the mantle of the Earth.  They are two continent-size dense masses known as large, low-shear-velocity provinces (LLSVPs) sitting at the bottom of the mantle below the Pacific Ocean and Africa.  These masses are a bit denser than Earth’s mantle.

Italian Trulli

Animation showing remnants of Theia thought to be embedded inside the Earth. (Source Wiki Commons)

Just How Unique is the Earth?

Up until this last generation, the only planetary system we knew was our own Solar System.  The presumption was usually that other planetary systems most? many? were similar to ours.  Boy, did that turn out to be wrong!  After logging more than 5000 planets in other systems, it has become clear that few of them follow the pattern of our Solar System.  The first hint of that was the discovery of hot Jupiters. 

Big gas giants like Jupiter cannot form close to their stars; the stellar winds clear out the inner system of such light gas.  How, then, could you explain a hot Jupiter which is hot because it is close to its star, at a distance at which it could not have possibly formed?  The answer had to be that the planet formed much farther out and then moved close in.  Planets could and did drastically change their positions relative to their stars. 

This was a major revolution in planetary astronomy, an insight comparable to the revolution in geology when it was realized fifty years ago that continents could and did change their positions.  The geologic insight developed into plate tectonics.  The astronomical insight that planets could and did change their positions became the Grand Tack and the Nice models.  Just as plate tectonics explains so much more of how a dynamic Earth operates, so the two planetary models explain how a dynamic planetary system operates. 

Put simply, the gas giants formed first while there was still a lot of smaller unincorporated rocky material throughout the Solar System.  The gas giants are very big but innumerable interactions with other bodies allowed them to change their positions.  An interaction of a gas giant like Jupiter with a smaller body could result in the smaller body being incorporated into Jupiter, flung farther out of the Solar System, or flung farther into the Solar System, perhaps close enough to the sun to be incorporated into it.  The physics of this process seems to favor a Jupiter-like gas giant flinging other bodies outward with the result that the gas giant would move closer to the sun. 

In some star systems, the process went so far that their Jupiter got closer to its star than Mercury is to the sun, producing a hot Jupiter, the existence of which validates this story because a gas giant could not form anywhere near that close to a star.  In such situations, don’t expect to find terrestrial planets near the star.  What becomes the hot Jupiter would have flung them into their outer planetary system, perhaps beyond the gravitational influence of their star to wander in interstellar space as rogue planets. 

Lucky for us, our Jupiter never got that far.  The models suggest that it may have gotten close to the current orbit of Mars, depleting the inner Solar System of rocky material and stunting the growth of what was to become Mars and any possibility of planet formation in what is now the asteroid belt.  At some point, Jupiter came into a 1:2 resonance with Saturn (Saturn’s year = twice Jupiter’s year) and stopped Jupiter’s inward progression (hooray for Saturn).  Additional interactions eventually broke the resonance and caused Jupiter and Saturn to move back to their current positions. 

Jupiter’s interaction with the asteroid belt sent many of them into the inner Solar System including some icy bodies which probably added water to the surface of the Earth during a time known as the Late Great Bombardment about 4.1 to 3.8 billion years ago.  Other interactions among the giant planets may have flung one or more giant planets far beyond Pluto; one may be lurking out there, yet undiscovered. 

The current positions of the planets are much more stable (although not completely) because there is far less loose material left with which they can interact.  How much the asteroid belt has been depleted can be seen in that if you were to add up all of the mass of the asteroids in the belt today, it would be less than the mass of our own moon.  Contrary to some video games, if you were to stand on an asteroid in the belt, it would be very difficult to see any other asteroids.

Are Most Terrestrial Worlds in Habitable Zones Water Worlds?

If you were to take a gas giant like Jupiter and blow off all of its gas, you would be left with something that would look a lot like a Neptune.  If you were to scrap off all of the ice in a Neptune, you would be left with something that would look a lot like an Earth – except that that ‘Earth’ might be a super Earth, several times bigger than the real Earth.  (The rocky centers of, especially the gas giants, might be rather fuzzy mixes of rock, ice, and gas, making them bigger than if they were only rock)

Now that astronomers have discovered and described many thousands of planets in other star systems, it has become clear that a common type of planet is a super Earth or a mini Neptune.  The terrestrial planets in our Solar System appear to be stunted, likely because of Jupiter.  If super Earths are more abundant than our own Earth, what might such a super Earth be like in a habitable (water can exist in liquid form at its distance from its star) zone?  (look up the Trappist star system on the net, a system with 7 Earth-size planets, at least 3 in the habitable zone, and no super Earths, ice giants, or gas giants)

As a speculation, a super Earth in a habitable zone would have a hotter mantle (big things cool slower than small things) which suggests that it would still be in a pre-plate tectonics stage (stagnant lid tectonics).  Lacking plate tectonics, surface water would not be subducting into its mantle although that might mean that less water would be coming from its interior.  How important is delivery of more surface water from icy asteroids?  Would its system even have a Jupiter that moved enough icy asteroids to deliver such water? 

An important factor, however, might be that super Earths might have too much continental crust, so much that it would be a complete layer around the entire planet, perhaps like our smaller Venus.  A complete continental layer would prevent plate tectonics; continental crust does not subduct and such a complete layer might be the norm for terrestrial planets.  How likely is it that a Mars-size body would blow off much of the crust as happened to the Earth? 

Lacking plate tectonics, the relief of the planet would be considerably lower.  There would be no plate movements to create lofty mountains or deep ocean trenches.  There wouldn’t be Earth-like ocean basins; any oceans would be on relatively low-relief continental crust in such a scenario.  If such a planet had an amount of surface water comparable to that of the Earth, it would be a water world with some Hawaii-like hot-spot islands.  There are probably many terrestrial, if bigger, worlds out there in habitable zones but few of them may have plate tectonics and the balance between oceans and land that the Earth has.  Maybe that is another reason ET has yet to contact us.

Water in the Universe | Contents | Intro | Part 1 | Part 2 | Part 3

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