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 the elements. Note that the vertical axis is logarithmic which means that increasing the relative abundance by one on the log scale means that the abundance increases by ten.
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 (α) or the nucleus of a helium-4 nucleus (2 protons + 2 neutrons). The helium-4 (2He4) atom is an alpha particle + 2 electrons. 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, four alphas lead 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.
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.
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 is released in the 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).
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, about 1/3rd the mass of the planet, and a silicate mantle and crust, 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.
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 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 solar wind out of the inner Solar System (Mercury to Mars). The 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. The methane and ammonia have lower freezing points in space but, along with the water, make up the three ices of the outer solar system.
Jupiter was forming just beyond the asteroid belt. Most of the gas and icy material coming from the inner solar system was intercepted by Jupiter which is why Jupiter is the largest planet in the solar system. Most 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.
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 and that ice is hot but 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 is 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 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.
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. Mars has considerable ice in the ground and at its polar caps. Early Venus probably had oceans of water in its atmosphere. There are two possible sources of that water: it was brought in from outside after the planet formed 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. 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 comes 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?
Any water vapor in the early inner solar system certainly wouldn’t remain there for very long; the solar wind would push it out of the inner solar system. However, water can react with minerals to form hydroxides (they contain one or more –OH groups). Examples of such minerals include: hornblende NaCa2(Mg,Fe,Al)(Si,Al)2O22(OH)2, Staurolite FeAl4Si2O10(OH)2, Epidote Ca2(Al,Fe)2Si3O12(OH) and many more. Once part of a hydrous mineral, the ‘water’ will remain with the rock in the inner solar system. The same thing happens to oxygen which, if it were a gas in space, would be pushed out of the inner solar system by the solar wind but when it combines with silicon, it becomes a silicate rock.
H2O can be produced from hydrous minerals by combining two –OH groups, leaving behind an oxygen bridge, –O–, between whatever the –OH groups were originally connected to. Heat and pressure can do this and it happens all the time within the interior of the Earth. The depth at which a particular mineral is ‘dewatered’ depends on what the mineral is and can happen within the crust down to the transition zone between the upper and lower mantle. The water is then free to rise with magma, sometimes all the way to the surface where it is released as part of volcanic gas in a volcanic eruption. In fact, most of the ‘gas’ in volcanic gas is actually water vapor.
The early atmospheres of Venus, Earth, and Mars would have 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 SO2, N2, 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 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. Although Titan has much more H2O than the terrestrial planets, at its distance from the sun the H2O has always been ice, all of which is in the interior of the moon. An early Venus was probably much cooler than it is now and Mars much warmer which meant that all three planets probably had early oceans; most of the water vapor condensed out of the air and formed oceans.
Venus is closer to the sun (61 million miles) than the Earth is (93 million miles) which would make an early Venus warmer than the Earth, all else being equal. For comparison, the Earth’s average global temperature is about 60 °F. If the Earth were moved to the orbit of Venus, it would have an average temperature at its poles of something on the order of 90 °F. The higher the temperature, the water vapor the air will hold and water vapor happens to be a greenhouse gas. Gradually, Venus became hotter and the hotter it became, the more water vapor the atmosphere could hold and the greater the greenhouse effect until Venus had, literally, oceans of water vapor in the atmosphere. When that happened, Venus, lacking a protective magnetic field, began to lose its water vapor in a process known as photolysis.
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 hydrogen off of gaseous hydrogen compounds in an atmosphere. Such compounds would 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 loses all but a trace of water vapor to photolysis; some of it reacts with SO2 to produce sulfuric acid(H2SO4), droplets of which make up much of Venus’s very thick cloud cover. Sulfuric acid is a hydrogen compound and so over many millions of years, photolysis will slowly destroy it too and the clouds will gradually dissipate. CO2, the number two gas after water vapor, is promoted to the number one gas. Interestingly, as hot as Venus is today because of its thick CO2 greenhouse atmosphere, when it also had oceans of water vapor in the air, it was even hotter.
Mars, being farther from the sun (141 million miles) than the Earth, has always been colder and so had little water vapor left 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 its number one gas. However, although the atmospheric compositions of Mars and Venus are similar, the amount of atmosphere is dramatically different.
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 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 most of its atmosphere.
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.
Titan is one of seven major moons (including our own) and the only one to have an atmosphere at all, much less one of water vapor. Our moon is too close to the sun to have any atmosphere. As small as it is, it couldn’t hold on to an atmosphere, especially in the face of the solar wind. The four big moons of Jupiter (Io, Europa, Ganymede, and Callisto) are too cold for water vapor and still vulnerable to the solar wind. The big moon of Neptune (Triton) is so cold that even nitrogen gas freezes out. 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 generated by the photolysis of ammonia. It is still subject to atmospheric loss by solar wind but has replenishes its atmosphere from releases of methane and ammonia from its interior.
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. However, unlike Venus and Mars, had and still has oceans of water in which photosynthesizing life developed. The net and generalized 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 the atmosphere. That 21% represents a dynamic equilibrium; photosynthesis release 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.
Equally important is that water vapor and carbon dioxide are removed. Over billions of years, the removal of carbon dioxide from the atmosphere gradually lowered its atmospheric concentration to its current value of ~ 400 ppm. Water is also removed but the Earth has oceans of water. With the removal of most of the CO2, the number three gas, nitrogen, was promoted to number one. Some of that N2 probably came from the photolysis of ammonia and some from the breakdown of nitride minerals within the Earth.
When early Venus had oceans of water vapor in its atmosphere, its atmosphere, as thick as it is now (92 times as thick as the Earth’s) was even thicker. The loss of the water vapor to photolysis thinned out its atmosphere. On the Earth, the loss most of its water vapor (it condensed into the oceans) and carbon dioxide (taken out by photosynthesis) thinned its atmosphere even more which is why the larger Earth has a much thinner atmosphere than smaller Venus. 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 to be lost to photolysis. However, water vapor lost to photolysis would be replaced by water vapor which would evaporate from the oceans so over billions of years the Earth should have lost its water too. Photosynthesis comes to the rescue. That free oxygen that photosynthesis produces gradually built up to form an ozone (O3) layer in the upper stratosphere which blocked most of the UV from reaching the water vapor. 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 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.
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 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, is defined by how high convection will reach. 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 end of convection marks the top of the troposphere, a boundary known as the tropopause. 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). In the lower stratosphere is the ozone layer.
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 near the surface) ranges from 9 to 22 miles. 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 is photolysis is insignificant.
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. Its clouds begin at an elevation of ~ 50 miles where its air pressure is about equal to that of Earth 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 molecule. 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.
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 gases. 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 more water vapor the air can hold which means that the air gets warmer which means it can hold more water vapor which means …