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Water in the Universe | Part 1

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

Water in the Universe: Water, Water Everywhere

Since the beginning of the universe hydrogen has been and continues to be, by far, by far, the most abundant element in the universe followed by helium. The third most abundant element in the universe, a bit of a surprise, is oxygen followed by carbon. The graph below shows the abundance of some of the lower number elements. Note that the vertical axis is logarithmic which means that increasing the relative abundance by one on the log scale increases the abundance by a factor of ten.

Cosmic Abundance of the Elements

Several trends are immediately obvious.  One such trend is that even-numbered elements are noticeably more abundant than adjacent odd-numbered elements, creating a sawtooth pattern.  This is because most of the main stellar fusion reactions can be thought of as the sum of alpha particles (a) which are the nuclei of  helium-4 atoms (2 protons + 2 neutrons) plus a lot of kinetic energy (they are moving very quickly).  The sum of two alpha particles would be the nucleus of a beryllium atom (4Be8) but it turns out that such a combination is very unstable and it almost immediately splits apart back into two alpha particles (half-life = 7 x 10–17 sec). 

A three-alpha combination is very stable, giving us the most abundant isotope of carbon, 6C12.  In similar fashion, combining four alphas leads to 8O16, five to 10Ne20, six to 12Mg24, and so on.  The odd-number elements are produced by less common fusion reactions or by the decay of unstable heavier isotopes.  This idea that the elements are produced by the successive addition of alpha particles is very simplistic in part because stable combinations of protons and neutrons require more neutrons than protons for the higher elements but it remains important enough to produce the sawtooth pattern.  (If you are a little shaky on some of the terminology like isotopes and half-life, check out the Water Basics - Physics section of the website)

Another obvious trend is that higher elements are less abundant.  Combining these two trends describes most of the pattern of the graph.  There are, however, some interesting anomalies.  The surprising rarity of beryllium was discussed above but oxygen, iron (Fe) and lead (Pb) stand out as being quite abundant for their positions in the graph. 

The extra oxygen abundance is because there is a stage in the evolution of very massive stars that creates extra oxygen.  Such stars eventually explode as supernovas which blow most of the oxygen (and other important elements such as iron) all over the neighborhood in which future generations of stars and planetary systems form.  In contrast, most of the carbon is locked up in the white dwarfs that form from the much more numerous lower-mass stars.  The result is that not only is oxygen more abundant than carbon in the universe but that the oxygen is much more abundant in the material from which planets form; there are a few stellar systems that do have more carbon than oxygen, leading to carbon planets (look it up on the net). 

Iron is so relatively abundant because it is the break-even point between fission and fusion.  There is a net energy release in fusing elements up to iron.  You can fuse elements beyond iron but it would take more energy to do it than the fusion reaction would release for a net energy loss.  This is why when very massive stars reach the fusion stage where they have produced an iron core, they implode and then explode as a supernova.  Some nickel is produced as a kind of overrun. 

Incidentally, even though fusion beyond iron is an energy-losing process, so much energy and so many neutrons are released in a supernova explosion that even elements beyond uranium can be produced (and there are other ways to get beyond iron).  There are no stable elements beyond lead and bismuth and so any isotopes of any elements beyond them decay, most of them becoming some isotope of lead which explains its relatively unusual abundance.  There are a few isotopes of elements beyond lead that, although ultimately unstable, do have very long half-lives, the most important of which are 92U238 (half-life = 4.51 billion yrs) and 90Th232 (half-life = 13.9 billion yrs).  These and other long-lived isotopes are crucial in providing heat in the interior of planets. 

The graph only depicts the relative abundance of elements but elements combine to make compounds and the relative abundance of elements could suggest the relative abundance of compounds.  For instance, consider the composition of the Earth which is divided into an iron-nickel core which makes up about 1/3rd the mass of the planet, and a silicate mantle and crust being the other 2/3rds.  A silicate is a huge group of minerals which are a combination of oxygen, silicon, and lesser amounts of other elements.  Both mantle and crust are made up almost entirely of silicates but while the mantle is about 2/3rds of the mass of the Earth, the crust is only about 0.5% of the mass.  Why such a discrepancy? 

While both mantle and crust are silicates (oxygen + silicon), the mantle is made up of iron and magnesium silicates.  The crust is essentially almost entirely aluminum silicates.  A quick check of the elemental abundance graph shows that there is a lot more magnesium and iron (even-numbered elements) than there is aluminum (an odd-numbered element). 

Finally, what might the graph suggest would be the most abundant compound in the universe (excluding diatomic combinations like H2)? Since hydrogen is the most abundant element, the most abundant compound should be a hydrogen compound. Helium rarely makes compounds so the most abundant compound should be a combination of hydrogen and the third most abundant element, oxygen, which gives us H2O.

Water in the Solar System

Our Solar System has eight planets: four relatively small, dense, rocky terrestrial (earth-like) planets in the inner solar system (Mercury, Venus, Earth, and Mars) and four giant planets in the outer solar system: two huge gas giants (Jupiter and Saturn) and, farther out, two smaller but still very big ice giants (Uranus and Neptune). 

In the vacuum of space, water can exist only as a gas (water vapor) or as solid (ice); the pressure is far too low for water to be liquid.  Close to the newly formed sun, water would have been in the vapor form along with other hydrogen compounds such as methane (CH4) and ammonia (NH3).  These gaseous compounds, along with hydrogen and helium gas, were blown by the early sun’s solar wind out of the inner Solar System (Mercury to Mars).  The main asteroid belt lies between the orbits of Mars and Jupiter and it is in about the middle of the belt that temperatures are low enough in space that water vapor freezes into water ice in what astronomers refer to as the snow line.  Asteroids in the closer half of the belt are essentially devoid of H2O whereas those farther out have substantial quantities of water ice.  Methane and ammonia have lower freezing points in space and so freeze farther out.  They, along with the water ice, make up the three ices of the outer solar system. 

Jupiter formed just beyond the asteroid belt.  Much of the gas and icy material coming from the inner solar system was intercepted by Jupiter which may be why Jupiter is the largest planet in the solar system.  Much of what Jupiter missed was caught by Saturn, making it the second largest planet.  Since most of that material was hydrogen and helium, Jupiter and Saturn became gas giants although they do have substantial amounts of the ‘ices.’  Uranus and Neptune formed largely from the icy material that was abundant at their distance from the sun and became the smaller ice giants.  Both types of giants, however, also incorporate a considerable amount of rocky material similar to that of the terrestrial planets.  

While Earth doesn’t have enough water to make a complete layer around the planet, Uranus and Neptune have layers of mostly water ice hundreds of miles thick which, although very hot, is (sort of) solid because of the enormous pressure.  The rings of Saturn are almost entirely particles of water ice up to boulder size.  The composition of most of the outer solar system moons is about half rock and half water ice.  In the outer solar system, H2O can be thought of as a rock but within the interior of some of the icy moons, it can melt, forming molten ‘ice’ oceans analogous to the magma within the Earth.  This molten ‘ice’ can erupt at the surface and form volcanic-like features in a process known as cryovolcanism. 

The gas giants have a lot of water, methane, and ammonia ice too but most of their mass is hydrogen and helium.  If you could blow most of the hydrogen and helium gas off of a gas giant, what would be left would look a lot like an ice giant.  If you could then scrape off the ice, what would be left would look a lot like a terrestrial planet.  The point of all this is that almost all of the H2O in the solar system is in the outer solar system. 

Water in the Inner Solar System: Why is there any at all?

Venus, Earth, and Mars originally had or still have significant amounts of H2O at or near their surfaces. Most of Earth’s water is in its oceans.  Early Mars very probably had surface seas.  That is now all gone but it does have considerable ice in the ground today and in its polar caps.  Early Venus may have had surface oceans but that water rapidly ended up in its atmosphere (further heating the planet) and was eventually destroyed by photolysis (more on that later).  There are two possible sources of that water: it was brought in from outside after the planet formed and/or it came from the interior of the planet.

Comets are roughly half rock and half ice and could be a possible external source of water.  Icy asteroids are another possible source.  Impacts of comets and icy asteroids could have delivered water to the planetary surfaces, especially in the early solar system when there were many more icy bodies available.  Studies of the deuterium to hydrogen ratios of water from Earth’s oceans, comets, and icy asteroids suggest that the most likely external source of water would be icy asteroids such as can still be found in the outer asteroid belt. 

Incredibly, Mercury and our own moon have some water ice on or close to their surfaces.  Mercury has no axial tilt (its rotational axis is perpendicular to the plane of its orbit around the sun).  This means that craters near its poles can be in perpetual shadow, far below the freezing point of water.  Should an impact bring some ice to Mercury, it would quickly vaporize.  If any of that water vapor should pass into the shadow of one of those polar craters, it would freeze out and remain in the crater; the polar craters are cold traps.  A similar process happens on our moon, resulting in significant deposits of ice in its polar craters which could be very important for any moon colonies. 

There is still argument about the relative importance of those two water sources but the current consensus is that most of the water at or near the Earth’s surface came from the interior of the planet and, presumably, this is also the case for Venus and Mars.  But how could there be water in the interior of the early Earth? 

How is water locked up in rock and minerals?

Any free water vapor in the early Solar System would have been blown out of the inner Solar System by the solar wind.  It is only the water that was bound in certain minerals that would have been stable, incorporated within the rocky material that became the terrestrial planets.  One simple example of a hydrated (it has water) mineral is gypsum, CaSO4•2H2O.  Under the right conditions, calcium (Ca++) and sulphate (SO4=) dissolved in water can precipitate as the mineral gypsum.  The formula clearly shows that water is part of the mineral structure even though gypsum appears to be completely dry.  If the depositional environment happens to be exceptionally hot as in near the shore of the Red Sea or you heat the gypsum, the water can be driven out, converting the gypsum to anhydrite (without water), CaSO4.  If a sedimentary rock contains anhydrite instead of gypsum, it indicates that the climate in which the calcium and sulphate precipitated must have been quite hot. 

A similar example can be seen when dissolved iron oxidizes and precipitates.  Iron can be in several oxidation states.  Uncombined with anything else, it will have an oxidation state of zero (same number of protons and electrons).  In reducing environments (very few electron acceptors; low oxygen) the iron will form compounds in which the iron has an oxidation state of +2, Fe++, otherwise known as ferrous iron.  A good example of that is the iron in pyrite, FeS2.  Since the charges in a compound must balance, each sulfur has an oxidation state of –1 (the sulfurs share one bond with each other which doesn’t count and each sulfur has one bond with the iron) whose sum balances the positive charge of the Fe++.  Pyrite is associated with coal deposits and in a flooded underground coal mine with limited oxygen available in the water, the sulfur in the pyrite will oxidize and dissolve in the water as a sulfate (SO4=) ion while the iron becomes dissolved ferrous iron (Fe++):  2 FeS2 + 7 O2 + 2 H2O ––>  2 Fe++ + 4 SO4= + 4 H+.  The mine water often becomes saturated with dissolved ferrous iron.  Note also that the H+ makes the water acidic ––> acid mine water loaded with dissolved iron. 

Should that mine water come to the surface in a mine outfall where it is exposed to the air, the ferrous iron will quickly oxidize to ferric iron, Fe+++.  As it happens, ferric iron is much less soluble in water and so it immediately combines with water to produce a reddish-yellowish-orange, gelatinous ‘solid’ known as ferric hydroxide, Fe(OH)3 (locally known as yellow boy) which can sometimes be found staining stream beds for miles.  The same thing can happen if your well water happens to contain too much dissolved ferrous iron, leading to colorful stains on your sinks, toilets, and laundry.

The chemical formula for this compound doesn’t show water as such, it is hidden in the hydroxide (–OH) part. Although the formula is Fe(OH)3, it could be rewritten as Fe2O3•3H2O which does show the water (like in the gypsum) although no chemist would write the formula that way. Ferric hydroxide is not a mineral because it is unstable; it is held together by weak hydrogen bonds as shown below. xx

Ferric Hydroxide Limonite Geothite Hematite

Outlined in blue are what could become separate water molecules.  If one of those blue-outlined waters is removed, there would be an oxygen bridge left between the two irons (Fe), creating a strong link between them.  The result would be a mineral generally known as limonite, Fe2O3•2H2O.  Should all of the water be removed, you would have hematite, Fe2O3

Yellowish-brown limonite is stable in temperate climates whereas warmer climates will dewater the compound to the reddish hematite, making them great indicators of past and present climate.  Soils in Pennsylvania are often brownish because of limonite in contrast to the red soils of warmer Georgia.  There are red rocks in Pennsylvania, such as the red shale of the Mauch Chunk Formation, but it is red because at the time the clay that became shale was deposited hundreds of millions of years ago, what is now Pennsylvania was very close to the equator and had a very hot climate.  Now exposed to the current cool climate, over decades of weathering, the red hematite will fade into the brownish limonite which is more stable in the cooler climate.

Upper Left – Fresh Mauch Chunk Shale | Upper Right – Pocono Sandstone (Pennsylvania)
Lower Left - Atlanta, Georgia Subsoil | Lower Right – Pittston, Pennsylvania Subsoil

In a hot climate (Atlanta and the Mauch Chunk Shale at the time of its formation) iron oxidized to red hematite (Fe2O3).  In a cool climate (current weathering of the Pocono Sandstone and Pittston) iron oxidized to yellow-brown limonite (Fe2O3•2H2O).  Note the weathering rim on the Pocono sandstone; the un-weathered interior of the sandstone remains its original gray color; given more time and weathering, the red will fade. 

The first two examples of hydrated minerals, the gypsum and the hydrated iron oxides, are found at or near the surface of the Earth and they can be dewatered at relatively low temperatures.  More complicated minerals hold more tightly onto their hydroxides and require much higher temperatures to ‘dewater.’  One example is a mineral group known as the amphiboles, one common member of which is hornblende, NaCa2(Mg,Fe,Al)5(Si,Al)8O2(OH)2, a mineral in continental crustal rocks like granite. 

The formula is certainly complicated, in part because it allows for considerable variation in the composition.  That first bit in parentheses, for example, just means that it could be magnesium or iron or aluminum in that part of the formula.  The important part for this discussion is the OH at the end.  Under the right conditions (very hot) the OH from one part of the hornblende could combine with the OH from another part of the hornblende to free a water molecule, leaving behind an oxygen bridge. 

The minerals found in oceanic crustal rocks such as basalt crystallize out at higher temperatures, temperatures too high to include an OH group, forming an analogue to the amphiboles known as the pyroxenes, essentially amphiboles without the OH.  Oceanic crustal rocks and upper mantle rocks are rich in pyroxenes. 

Yet other minerals hold even more tightly onto OH groups, minerals such as bridgmanite and ringwoodite which include small amounts of OH.  So tightly do these minerals hang onto their OH groups that it is only the temperature and pressure of the transition from the upper to the lower mantle that are enough to separate out the water.  There are no minerals with OH groups in the lower mantle and it is, indeed, their absence, that partly differentiates the less viscous upper mantle from the more viscous lower mantle.  The water equivalent of the OH groups in the minerals of the upper mantle is so large that it is many times the volume of all of the water on or near the surface of the Earth.  All of this mantle ‘water’ is also very important for plate tectonics on Earth because it substantially lowers the melting point of the mantle rock and makes it less viscous; it makes subduction possible.

Hydrothermal Solutions

Deep within the Earth, the water does not always remain locked up in the rock.  Should conditions become such that the rock becomes molten, it will rise.  That molten rock (magma – if it reaches the surface of the Earth it’s called lava) contains a small amount of water dissolved in the magma.  It may happen that the magma, cooling as it rises, doesn’t make it to the surface, instead crystallizing out various minerals that become intrusive (plutonic) igneous rocks such as granite.  But the story doesn’t end there.  That small amount of water in the magma remains fluid as the silicate minerals crystallize out.  This very hot water under considerable pressure is the hydrothermal (literally hot water) solution that is left after the magma solidifies. 

Because the hydrothermal solution is so hot and under so much pressure, it can contain in solution much greater amounts of dissolved minerals than could the much colder water under much less pressure found at or near the surface of the Earth.  One example is silica, SiO2

Quartz is a large, clear or milky, crystallized version of silica.  Most of glass is a noncrystalline ‘solid’ form of silica.  You might not think that something like quartz or glass is soluble in water but a tiny amount of dissolved silica can be found even in tap water.  The tap water of Wilkes-Barre, Pennsylvania typically contains about 20 ppm dissolved silica which is not unusual.  Hydrothermal solutions contain many orders of magnitude more dissolved silica. 

As the hydrothermal solution rises to the surface, it cools and loses pressure which reduces its capacity to hold in solution things like silica.  Unlike magma which can melt its way through overlying rock, hydrothermal solutions rise through fractures in the rock or through the pore spaces of permeable rocks.  At some point, the hydrothermal solution may become saturated with silica at which time some silica starts to crystallize out to form quartz veins in fractured rock, a very common occurrence. 

Should the hydrothermal solution also contain some dissolved gold, gold may be included in the quartz veins (the mother lode).  Many other economic mineral deposits can be attributed to hydrothermal deposits such as the native copper in the Keweenaw Peninsula of Michigan, the silver of Potosi, Bolivia, and the tin of Cornwall, England.  Should a hydrothermal solution reach the surface at the bottom of an ocean as often happens at oceanic ridges and rises, various minerals may precipitate out and form manganese nodules which can be found littering some ocean floor areas.  These nodules are rich in many important metals in addition to manganese including: iron, cobalt, copper, nickel, and zinc. 

Although it is generally true that the warmer the water the more dissolved something it can hold, there are a few exceptions.  In a reversal and on a much humbler scale, the solubility of calcium in water (Ca++) decreases as the temperature rises, sometimes leading to scale deposits in hot water heaters and pipes. 

The importance of hydrothermal solutions is that in their interaction with rock, they can create economically important concentrations of many metals and even valuable trace elements.  Hydrothermal solutions and the undersea vents from which they emerge may also be important to life. 

Deep within an ocean, hydrothermal vents can provide the energy and nutrients to sustain an ecosystem (tube worms et al) independent of the energy of the sun.  An ever-expanding list of outer Solar System bodies appear to have subsurface oceans which may include systems of hydrothermal vents.  If there is life beyond the Earth, it may be found in the subsurface oceans of bodies like Europa, a major moon of Jupiter, and Enceladus, a medium-sized moon of Saturn.  Astonishingly, even Ceres (the biggest asteroid) and Pluto may have subsurface oceans.

Water in the Atmospheres of Venus, Earth, and Mars: Where did it go?

The early atmospheres of Venus, Earth, and Mars would have largely formed from the gases released from within the interiors of the planets.  After water vapor, the most common gas released in volcanic eruptions is CO2 followed by N2, SO2, and lesser amounts of other gases.  So what happened to all that water vapor?  It certainly was the most abundant gas by far from which to form an atmosphere.  Did it remain in the atmosphere? 

Shown below are the current compositions of the atmospheres of Venus, Earth, and Mars along with Titan, the big moon of Saturn.  Titan is the only moon in the entire solar system with a significant atmosphere and it s atmosphere happens to be considerably thicker than that of the Earth but like the Earth and unlike any other body in the solar system, its atmosphere is dominated by nitrogen gas.

In every case, water vapor makes up a tiny fraction, if anything, of the atmospheres which requires an explanation.

The Destruction of Water Vapor by Photolysis and the Evolution of the Atmospheres

In addition to light, the sun produces ultraviolet radiation as attested by anyone who has gotten a sunburn.  Among other things, that UV radiation can strip loosely-bonded hydrogen off of gaseous hydrogen compounds in an atmosphere (photolysis).  Such compounds include methane (CH4), ammonia (NH3), and, water vapor H2O.  The separated hydrogen gas drifts off into space.  The water vapor becomes oxygen gas (O2), the ammonia becomes nitrogen gas (N2), and the carbon from the methane combines with oxygen to become carbon dioxide (CO2).  None of these products are hydrogen compounds and so they remain untouched by the UV. 

Venus lost all but a trace of water vapor to photolysis.  CO2, the number two gas after water vapor, was then promoted to the number one position in what then became a thinner atmosphere (though still very thick by Earth standards).  Since water vapor is a greenhouse gas, as hot as Venus is today (~ 850 °F), because of its thick CO2 atmosphere, Venus has cooled considerably from when it also had oceans of water vapor in the air. 

Mars, being farther from the sun (141 million miles) than the Earth, has always been colder and so had little water vapor in its atmosphere.  What little there was was subject to destruction by photolysis but most of the H2O would have been in early seas and in extensive ice caps.  Like Venus, its number two gas, CO2, was promoted to number one.  However, although the atmospheric compositions of Mars and Venus are similar, the amount of atmosphere is dramatically different; the thickness of the Venetian atmosphere is more than 10,000 times as thick as that of Mars. 

The loss of so much of the Martian atmosphere is attributed to its lack of a global magnetic field to protect it from stripping (sputtering) by the solar wind (there are other processes that contribute); Mars did have an early magnetic field but quickly lost it.  Simply put, Mars is considerably smaller than Earth and Venus and small things freeze faster than bigger things.  The molten core which generated the Martian magnetic field largely froze solid, ending the magnetic dynamo.  The atmosphere of Mars then became vulnerable to the charged particles of the solar wind which gradually stripped away most of the Martian atmosphere.  Venus, also lacking a protective magnetic field (because it rotates so slowly), nevertheless has held onto much more of its atmosphere because it is much more massive than Mars. 

As the Martian atmosphere was largely stripped away, the decreasing amount of CO2 weakened the greenhouse effect to the point that Mars became so cold and its atmosphere so thin that water could only exist on the Martian surface as ice.  It got so cold that much of the CO2 froze out of the atmosphere too.  Today, the Martian ice caps are about half water ice and half frozen carbon dioxide ice. 

Our moon is too close to the sun to hold on to any atmosphere.  The four big moons of Jupiter (Io, Europa, Ganymede, and Callisto) are also too cold for water vapor and still vulnerable to the solar wind.  Titan is one of seven major moons (including our own) and the only one to have an atmosphere at all but it is far too cold to include any water vapor.  Titan is in the Goldilocks position for an atmosphere, too cold for water vapor and carbon dioxide but just warm enough for some methane gas which is also found in liquid and solid form.  Most of its atmosphere, however, is nitrogen gas which is probably generated by the photolysis of ammonia.  It is still subject to atmospheric loss by the solar wind but may be protected by Jupiter’s magnetic field which itself may strip some atmospheric ions off of Titan.  Titan, however, likely makes up atmospheric losses from releases of methane and ammonia from its interior.  The big moon of Neptune (Triton) is so cold that even nitrogen gas freezes out. 

We all know that most of the water vapor released at Earth’s surface ended up in what became the oceans with only a small fraction in the atmosphere.  As was the case for Venus and Mars, carbon dioxide was promoted to the number one gas when most of the water vapor condensed out of the atmosphere or was otherwise lost but, unlike Venus and Mars, Earth went even farther, eventually removing all but a trace of its atmospheric CO2, promoting the number three, gas, nitrogen to number one in an even thinner atmosphere.  Why did that happen?  Earth had and still has oceans of water in which photosynthesizing life developed very early in its history. 

The reaction for photosynthesis is 6 H2O + 6 CO2 <==> C6H12O6 (carbohydrate) + 6 O2. Note that the reaction goes both ways, the reverse of which is respiration.  Everyone is impressed with the production of free oxygen gas by photosynthesis which is certainly important and which eventually, over billions of years, accumulated to make up today’s 21% of today’s atmosphere.   That 21% represents a dynamic equilibrium; photosynthesis releases O2 into the air and other processes remove it.  If all photosynthesis were to suddenly stop, the O2  levels would probably drop to less than 1% within a century or two.  If we ever find any planets outside our Solar System with high levels of oxygen in their atmospheres, it would suggest the presence of photosynthesizing life forms. 

Equally important is that water vapor and carbon dioxide are removed from the atmosphere.  Over billions of years, the removal of carbon dioxide from the atmosphere gradually lowered its atmospheric concentration to its current (and rising) value of ~ 420 ppm.  The carbon which was once in CO2 became concentrated deposits of coal, oil, and natural gas sequestered within sedimentary rock.  Although seemingly less impressive, small amounts of organic matter in common sedimentary rock such as shale hold even greater amounts of carbon.  There may be less carbon in such rock but there is a lot more shale than coal, oil, and gas and it adds up.  Later in Earth’s history when the oceans became less acidic, the formation of limestone (CaCO3) also contributed to the loss of atmospheric CO2.  None of this could happen on Venus and Mars so they retained their CO2-dominated atmospheres.  Water is also consumed in photosynthesis but the Earth has much, much more water than CO2

With the removal of most of Earth’s atmospheric CO2, the number three gas, nitrogen, was promoted to number one.  The loss of most of its water vapor (it condensed into the oceans) and carbon dioxide (taken out by photosynthesis) greatly thinned its atmosphere which is why the larger Earth has a much thinner atmosphere than smaller Venus (Perhaps the collision which produced our moon also greatly thinned out the atmosphere?).  Mars retained a Venus-like atmospheric composition but lost most of its atmosphere to the solar wind. 

But if Venus lost most of its water vapor to photolysis, why didn’t the Earth?  Being cooler than Venus, Earth would have much less water vapor in its atmosphere; unlike Venus, most of Earth’s water was in the surface oceans which would have been largely protected from photolysis even without an ozone layer.  The little amount of water vapor that was in the early Earth’s atmosphere would have been vulnerable to photolysis but would have been replaced by more from the oceans, perhaps slightly depleted them but photosynthesis slowly came to the rescue.  Over billions of years the free oxygen that photosynthesis produced gradually built up to form an ozone (O3) layer in the stratosphere which blocked most of the solar UV from reaching the water vapor in the lower atmosphere.  To understand how this happens, you need to understand a little about the structure of the Earth’s atmosphere and the distribution of water vapor within it. 

The Earth’s atmosphere currently consists of about 78% nitrogen gas, 21% oxygen gas, 1% of argon gas, and trace amount of other gases such as carbon dioxide.  Those percentages remain the same whether at sea level or high up near the outer edge of the atmosphere.  The amounts of the gases greatly decrease with higher elevation but the percentages do not change.  This is not true of water vapor. 

Water near the surface of the Earth is close to what is known as the triple point.  The temperature and pressure are such that water can exist in all three states: solid, liquid, and gas.  Because of this, the amount of water vapor in the atmosphere can vary greatly both horizontally and vertically.  If it is cloudy and the air is still, water vapor will tend to hug the ground and remain more concentrated near bodies of water (mist and fog). 

The atmosphere of the Earth is largely transparent to the passage of visible light from the sun (aside from clouds and fog).  Some of that light is absorbed by the ground, heating it up.  The heated ground re-emits the energy as infrared radiation which heats the air immediately above it which expands and rises, creating convection (vertical movements of the air).  The lowest layer of the atmosphere, the troposphere where most of Earth’s air is, is defined by how high convection will reach (of course, to balance air going up, there must be air going down to complete the convection cycle).  Eventually the rising warm air will expand and cool until it is no longer warmer than the surrounding air at which point it will stop rising.  That upper end of convection marks the top of the troposphere, a boundary known as the tropopause.  Convection will carry water vapor up to the tropopause but not normally any higher. Above the tropopause is the stratosphere in which there may be very fast horizontal movement of air but virtually no vertical movement of air; the air is stratified (in horizontal layers) and is normally quite dry.  In the stratosphere is the ozone layer.

 A Comparison of the Atmospheric Structure of Earth and Venus

The elevation of the tropopause depends on how much the Earth’s surface is heated.  Above the poles, the tropopause has an elevation of some 5.6 miles compared to 11 miles at the equator.  The ozone layer, with average ozone concentrations of < 10 ppm (compared to ~ 0.3 ppm near the surface) ranges from 9 to 22 miles in elevation.  Water vapor is confined to the troposphere which means very little of it is above the ozone layer which protects it from photolysis.  The ozone layer is not a perfect protector but the amount of photolysis is greatly reduced. 

The absorption of UV by the ozone releases heat which means as you go higher in the stratosphere, the air, although still very cold, gets warmer as can be seen in the image above and it is this which stratifies the air in the stratosphere and prevents convection.  An air layer in the stratosphere cannot rise, creating convection, because the air above it is warmer.  Venus, lacking an ozone layer, has no stratosphere.  Its troposphere extends much higher than does that of the Earth because it is so much thicker and hotter.  By definition, there is convection but it lacks any water vapor.  Its waterless clouds begin at an elevation of ~ 50 miles at which elevation its air pressure is about equal to that of Earth’s at sea level. 

Shortly after the Earth formed, the sun was about 40% less bright than it is today.  Over billions of years the sun has slowly been becoming bigger, brighter, and cooler.  It continues to grow brighter by about 1% every hundred million years.  As it does so, the Earth, receiving more solar radiation, will grow warmer and the tropopause will rise higher into what is now the stratosphere.  In about a billion years, the tropopause will have lifted into and above the ozone layer, exposing water vapor to photolysis which will begin destroying the water vapor molecules.  That water vapor will be replaced by evaporation from the oceans.  Over many more hundreds of millions of years, the oceans will disappear, not because they boil away in a hotter Earth but because they will be lost to photolysis above the ozone layer.  The Earth will then begin to resemble Venus. 

Water, the Greenhouse Gas

Everyone knows that carbon dioxide is a (not particularly efficient) greenhouse gas as is the more efficient methane gas.  What many don’t know is that water vapor is also a greenhouse gas, probably more important than any other greenhouse gas.  Worse yet, water vapor has a strong positive feedback which means that the more water vapor in the air, the warmer the atmosphere gets which means that the air can hold more water vapor which means that the air gets warmer which means it can hold more water vapor which means …

Atmospheric Absorption Bands

The infrared absorption bands for methane look to be less than that of carbon dioxide but that is because there is so much less methane in the atmosphere than carbon dioxide.  Were there equal amounts of methane and carbon dioxide, you could see that the methane would absorb more infrared than the carbon dioxide.  There is a similar story with water vapor.  Up to 4% water vapor in the air is ~ 100 times more water vapor than carbon dioxide.  True, the water vapor is confined to the troposphere but the troposphere is where most of the air is. 

Water in the Universe Continued | Part 2 >

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

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