Sizes are to scale (but not distances); the Sun is to the left.
Keeping in mind that everything in our solar system is composed from the same raw materials (elements created in a supernova) but with a general outward trend toward lighter elements, we can begin to study how an object's mass, impact history and other features help determine the acquisition and evolution of an atmosphere.
Fortunately our solar system provides us with two planet-size laboratories, Venus and Mars, shown here in this composite image of all four rocky planets:
From left to right are Mercury, Venus, Earth and Mars, with relative sizes shown to scale.
Both Venus and Mars are rocky planets of the inner solar system. Venus (0.7 AU) is closer to the Sun than Earth (1 AU) and Mars (1.5 AU) is farther away. Exploring how they evolved as planets with atmospheres quite different from Earth's atmosphere can help us understand the processes involved in the evolution of our own atmosphere.
The Entire Solar System Formed From A Cloud of Gas and Dust
Let's begin by revisiting how planets and moons form. When the Sun ignited into a ball of nuclear fusion, the dust and gas and larger clumps of matter that had already accreted because of mutual gravitational attraction, the protoplanetary disk in other words, shown below, experienced a variety of forces.
The outward force of the Sun's fusion reaction, technically called radiation pressure, would have been proportional to the cross-sectional area of each particle in the disk. At the same time, the gravitational attraction of the particles to the Sun would be proportional to the mass of each particle. This means that the smallest bits of dust and gas moved away from the Sun into slower outer orbits while conserving their angular momentum.
Angular momentum is the product of the linear momentum of a particle and it position vector. The animation below helps describe this quantity for any object with a fixed mass rotating around a fixed axis:
The little lilac r represents a force called torque. Force is the dark purple F, linear momentum is the small green p, and angular momentum is the light green L. Having a basic sense of angular momentum will help you understand why all the planets orbit the Sun and why they spin. It is always conserved in systems and that is why a skater will spin faster when she draws in her arms, for example.
Larger clumps, moving in closer faster orbits captured this dust and gas with every orbital sweep, building up mass and angular momentum. Eventually rapidly spinning planet and moon-size clumps formed. As these bodies formed, their orbits experienced resonant interference from other orbiting bodies. Gravity tends to push two or more bodies of similar masses into resonant orbits but ejects them if their masses are dissimilar enough. For example, Neptune and Pluto are in a 3:2 orbital resonance with Neptune making 3 orbits for every 2 orbits Pluto makes. Even though their orbits cross, they will never collide, thanks to this kind of stabilization. However, an asteroid coming into a resonant orbit with Jupiter (much more massive) will tend to be quickly shot off. Smaller orbiting bodies in orbits similar to a larger body would eventually be overcome by the faster orbiting larger body with the larger body acquiring the new mass or the smaller bodies being acquired as satellites (moons) of the larger body.
It is the above process that determines the initial elemental composition of each orbiting body, but keep in mind that formation is not yet entirely complete. Planets and moons could have impacted with each other or asteroids or comets and acquired new orbits closer to or further away from the Sun. Thus the composition of the planets today does not necessarily reflect a perfectly smooth progression toward lighter element make-up. The young solar system would have had many more comets and asteroids than it does now, all clumps of material that did not quite yet coalesce into planets or moons. Perturbations of planetary orbits as they interacted with each other as well as gravitational forces from other stars pulled comets into highly elliptical orbits and they regularly crashed into other orbiting bodies. Comets are compositionally diverse, meaning that they originated at different distances from the Sun. Contributions by comets therefore needs to be considered as well when comparing the compositions of the planets. Asteriods tend to be more compositionally similar to each other. They may be clumps that never formed planets as well as fragments from collisions between forming planets. They fall into three classes based on their composition, those rich in carbon, silicon or metals. There is much evidence that the rocky planets such as Earth, Mars and Venus, are composed of the aggregation of these asteroids.
The heat from the young Sun would have also had a huge impact on the composition of the planets. Planets forming close to the Sun would have received a great deal of heat from it. When the Earth was forming the temperature of hot gases and dust populating the solar system near its orbit is estimated to have been between 800°C and 1400°C. Near the orbit of Mercury the temperature would have been far greater. Even the temperature of the asteroid belt, orbiting between Mars and Jupiter, would have been several hundred degrees above zero. The kinds of compounds that could form out of the elemental raw material of the protoplanetary disk would have been highly dependent on temperature. Where Earth formed, only high melting-point metal oxides and silicate minerals could survive. Much of Earth's original volatile compounds and gases would have boiled away into space. However, far from the Sun not only could these minerals, but more volatile carbon compounds and ices as well, could form. There, highly volatile gases like methane could exist without boiling away from the body's surface. This is why there is a great variation in the composition of the solid material making up the planets today. It is also why a planet further from the Sun such as Mars can and does contain lower melting-point minerals such as carbon rich compounds, than Earth does, and why carbon-rich asteroids are more abundant in the outer reaches of the asteroid belt and metal-rich ones dominate the inner reaches.
Newly forming planets were also heated and reheated by countless high-velocity collisions. Not only were the velocities of orbiting asteroids and comets already significantly accentuated by the gravitational pull by the Sun, they were also accelerated by the gravitational pull of the planets themselves as they approached them before impact. These forces would have resulted in spectacular collisions that could partially or even entirely melt a planet.
The young solar system would have also been much more radioactive than it is now. The supernova that created all the gas and dust from which the planets formed created mostly light elements such as helium and hydrogen but it also created heavier elements and even small amounts of massive unstable elements with short half-lives. As these elements decayed into more stable isotopes, they released heat inside the forming planets.
Planets and moons of sufficient mass are also heated by compression, by the gravitational inward pull of their own material in other words. The inner cores of the gas giant planets such as Jupiter are thought to have been intensely hot, as high as 140,000°C. Although they would have cooled since then, much of that original heat remains. Tidal forces, forces exerted by the gravitational pull from other large bodies nearby, also deformed and heated planets. With many collisions and unstable orbits within the young solar system, these forces may have been extreme.
Planets Differentiated as They Formed
When we consider how atmospheres formed on the young planets, we must take into account not only the various sources of their initial elemental make-up but also how they differentiated. The material making up a planet will separate according to density with the densest material sinking into the core and lighter materials floating up to the surface, as long as the planet remains in a sufficiently molten state. The amount of differentiation that occurs depends largely on how thoroughly molten a planet was when it formed as well as how many times it re-melted as a result of subsequent impacts. Earth has the highest density, the highest mass and the fastest rotation of all the rocky planets. It is also probably the most highly differentiated. It remained molten long enough to differentiate into a distinct outer and inner core. The outer core is very low viscosity liquid nickel and iron that surrounds a solid inner iron-nickel core. There is some evidence that the inner core is actually freezing out of the surrounding core as the Earth's interior gradually cools. All the rocky planets have a well-defined crust, mantle and core, but we don't know much about the composition and physical state of Mars' and Venus' cores.
Not all of the lighter gases and compounds making up the just-formed rocky planets boiled away into space. Some remained trapped inside them, dissolved in molten rock, much like gases dissolved in magma inside volcanoes. These gases had a second opportunity to escape while the planets remained in their molten state, through a process called outgassing. Solar wind from the young Sun would have blown most of these gases away as they were released, but the gravity of some of these rocky planets would eventually hold onto heavier gases, such as carbon dioxide, that outgassed from their interiors. Both carbon and oxygen were fairly abundant in the region where the rocky planets formed. Earth, Venus and Mars were probably all initially blanketed in a CO2-rich atmosphere as well as significant amounts of outgassed water vapour. Today, however, CO2 accounts for only 0.035% of Earth's atmosphere whereas it accounts for almost all of Mars' and Venus' atmospheres, 95% and 96% respectively. Why?
Earth's Liquid Surface Water Offers Clues and Questions
No one is entirely sure how Earth acquired a significant amount of surface water soon after it formed. There is some radioisotope evidence that water-rich meteorites (these would have been protoplanets that struck young Earth) contributed significant water to our oceans. The process of photosynthesis creates water as a byproduct and this could have contributed water over time but its contribution would have been well after Earth was blanketed by liquid water. Like the other young planets Earth would have had a significant amount of water in the material that formed it, but most experts believe it is not enough to account for the amount in our oceans today.
There is another mystery associated with Earth. It should have more noble gases in its atmosphere than it does, based on their abundance in the raw material that formed it. Like carbon dioxide and water, some of these gases should have been retained by Earth's gravity as they outgassed from its interior. Their lack suggests a catastrophic impact may have shot them away into space. This may have been an impact with another large (about Mars' size) forming planet that resulted in Earth's re-melting and the formation of the Moon from the orbiting leftover debris. That impact is shown here in this artist's depiction:
The impact, estimated to have occurred around 4.5 billion years ago, would have left a temporary atmosphere consisting of rock vapour, lots of carbon dioxide and light volatiles like hydrogen gas. This atmosphere would have been heavy enough to create a surface pressure sufficient to maintain liquid water oceans even with an estimated surface temperature of 230°C, shortly after impact. Water could have been contributed by the impacting protoplanet. Regardless of the continuing mystery of the ocean's origin, studies of zircon minerals suggest that liquid water existed on Earth's surface as long ago as 4.4 billion years ago, just 600 million years after Earth itself formed. Recent radioisotopic studies of hydrogen isotope ratios in comet minerals make a significant contribution of water from comet impacts unlikely, greatly weakening a long-held theory that water-rich comets contributed to Earth's oceans.
The early existence of surface liquid water played a very important role in Earth's unique atmospheric evolution. Carbon dioxide is very soluble in water. Much of Earth's carbon dioxide would have dissolved in its oceans to react with minerals and form compounds like silicon dioxide and limestone. Much of Earth's carbon dioxide became sequestered in its rocky crust, taking it out of the atmosphere.
A Comparison Between Mars, Earth and Venus
Mars, Venus and Earth are all rocky inner planets that shared similar formation histories, and yet the atmospheres of these three planets are so different from each other. This is an image of the thin atmosphere of Mars taken by the Mars Rover:
Here is an artist's conception of Venus's crushing atmosphere:
Compare the above images to Earth's blue life-giving atmosphere:
Let's examine the origin histories of three atmospheric gases that vary greatly between these planets to get an idea of what happened.
Nitrogen
Earth's atmosphere is composed mostly of nitrogen and oxygen. We know from looking at its evolution that oxygen in Earth's atmosphere is unique. Oxygen is highly reactive and if it were not continuously released into the atmosphere by plants through photosynthesis, this gas would quickly disappear through reactions with metals and other crust elements. It would be sequestered in rock. As mentioned in the atmosphere composition article, Earth's atmosphere is 78% nitrogen. Although no one is quite sure through which processes Earth acquired all that nitrogen, both Venus and Mars appear to have (or had) similar mechanisms for creating nitrogen-rich atmospheres. Nitrogen has an atomic weight similar to oxygen, but it is almost completely nonreactive, which means that it could have simply built up over the eons through outgassing from the planet's surface and, at least on planets with as much gravity as Earth, it would be heavy enough not to be lost to space. Although Venus' atmosphere is almost all carbon dioxide with only a relatively small amount of nitrogen, that small amount is roughly four times more than Earth's. Venus is roughly the same size as Earth but it experiences much more volcanism. It therefore has had a greater source of outgassed material and this could explain why it has much more nitrogen than Earth does. Venus' atmosphere is simply much denser overall than Earth's atmosphere. Mars, on the other hand, has only 3% nitrogen and a far thinner atmosphere than Earth does. The surface air pressure on Mars is less than 1% Earth's surface air pressure. If Mars started out with similar raw ingredients, where did most of its atmosphere go and why is it so low in nitrogen? One clue comes from isotopic studies of the nitrogen on Mars. It contains a higher proportion of heavy isotopes than Earth's nitrogen does, suggesting that many of the light nitrogen isotopes have escaped its atmosphere. Perhaps Mars at one time had a nitrogen content similar to that of Earth but its low gravity (0.38 g) cannot hold onto it.
Water and its Connection To a Magnetosphere
Water was abundant in liquid form on Earth soon after the planet formed, and it continues to be abundant. Mars, however, currently cannot support liquid water on its surface. What little water it has (0.03 % of its atmosphere compared to 0.4% of Earth's atmosphere) is locked up in permafrost and in its polar ice caps. Its extremely low air pressure means that any liquid water would rapidly evaporate (or sublimate as its surface is on average -55°C). There is a great deal of evidence that Mars had significant amounts of surface liquid water in the past, perhaps billions of years ago. There is geographical evidence of ancient lakes and rivers, meaning that it once snowed and rained on the small planet. There is some evidence that highly salty water may currently exist temporarily in small areas before it freezes on the cold surface. And there is also evidence that past volcanic activity on Mars could have released enough water vapour to put Mars on average 120 metres under water and create a carbon dioxide-rich atmosphere twice as thick as Earth's. Such an atmosphere would have been warm enough to support liquid water released from the volcanoes. Mars may have once been much like Earth was billions of years ago, with surface water and a warm atmosphere.
Unlike Earth, Mars had some factors working against the retention of its atmosphere and surface water. It has less gravitational pull on its gases (about 0.38 g) than Earth does. And, unlike Earth, it did not maintain a magnetosphere that protects its atmosphere from solar wind erosion. Like Earth, Mars' atmospheric gases have been regularly replenished through volcanic activity, although eruptions on Mars are thought to be less frequent and more massive than those on Earth, largely due to Mar's lower gravity. With no protective magnetosphere, its volcanic activity was not enough to balance out atmospheric loss by solar wind.
The lack of a magnetic field gives us some clues that Mars differentiated differently than Earth did. Recall that the magnetic fields of the rocky planets are the result of the circulation of liquid metals within their cores. There is evidence that Mars underwent differentiation like Earth did, and current models suggest Mars has an iron/nickel/sulphide core that is partially fluid, with about twice the concentration of lighter elements in it than Earth does. Although Mars does not have a current magnetic field, parts of its crust have been magnetized in the past, suggesting that it once did have a core dynamo. At some point billions of years ago the dynamo stopped functioning and the magnetic field faded away. Mars' atmosphere was blown away by solar wind, transforming it from a warm wet world into the inhospitable planet it is today. Why Mars' dynamo failed is still a mystery. It was once thought that Mars' core simply solidified, as it is a small planet and therefore it probably cooled more rapidly than Earth did (this implies that Earth's dynamo is too destined to eventually fail as Earth continues to cool). Recent evidence of an at least partly liquid core within Mars complicates this theory. The result, however, demonstrates how important a magnetosphere is to maintaining an atmosphere. Or does it?
Let's examine the magnetosphere of Venus. Venus's atmosphere is incredibly dense (its surface pressure is about 90 times higher) than Earth's. Perhaps Venus has more volcanic activity to pump out more gases and an even more powerful magnetic field to help keep them all in.
Unlike Mars, we do not have seismic data about the internal structure of Venus. However, geologists expect Venus' similarity to Earth in size and density point to a similar internal structure with an at least partially molten iron-rich core that should be capable of sustaining a magnetic dynamo. It doesn't. Like Mars, no one is sure why, but there may be two reasons for this. First, in order to create a dynamo, molten metals must be rotating. The rotation of these metals should be directly linked to the planet's rotation. Earth rotates relatively fast (once every 24 hours) compared to Venus, which rotates only once every 243 Earth days. Second, Venus shows no signs of plate tectonic activity. Plate tectonics is a theory that describes the large-scale movement of planetary crust material. It builds on the concept of continental drift. Venus' rotation is also in the opposite direction compared to all the other planets except Uranus. Its lack of plate tectonics and odd rotation may point to a catastrophic collision some time in Venus' past. This is a fairly old theory that has been weakened by a lack of debris left behind, a moon or two for example, as evidence. However, some theorists are revisiting the impact theory to explain Venus' odd rotation.
Scientists now wonder if plate tectonics may be another requirement for a dynamo set-up. The shifting of crust plates may act to cool the mantle and create a large enough temperature difference between the mantle and the core to drive convection. This convective movement may also contribute to a dynamo.
Mars rotates once every 24.6 hours so this is not likely the reason why it has no dynamo. Still, Mars, like Venus, shows no sign of active plate tectonic movement. There is some evidence of striped patterns of different directions of magnetism in Martian rock and this could have been created by early tectonic activity that has since ceased.
Tectonic activity has been active on Earth since it was formed. There is much evidence for current activity, for example in spreading seafloors. On Earth this movement is possible because Earth's crust has a higher strength and lower density than its underlying mantle. Although there is some debate about what motor drives this movement, most agree that convection in the mantle is at the root of it. Interestingly, this means that mantle convection may be involved in both the cause and effect of plate tectonic movement.
One theory about why Earth continues to undergo plate movement while Venus and Mars do not is that Earth's crust remains soaked in water, and water plays an important role in shear zones, weak surfaces in which crust plates can move along and against each other. Mars and Venus, without surface water, no longer have these weak zones. Because Venus does not have a plate tectonic mechanism, it's mantle releases heat instead through substantial volcanic activity.
The reason Venus lacks an Earth-like dynamo may be because of its very slow rotation, and this is why Venus lacks a protective magnetosphere. The fact that it has the densest atmosphere of all the rocky planets, without a protective magnetosphere, is indeed mysterious. Let's explore this. First of all, Venus does in fact have a weak magnetic field, but it is not created through an internal dynamo. Interaction between ionized gases in its ionosphere and the solar wind induces a magnetosphere, but most researchers believe it is too weak to provide any significant protection to its atmosphere against solar wind. So how has it not only held onto an atmosphere but an incredibly dense on at that?
Venus may have started out as a watery world just like early Earth and Mars did. Scientists still don't have direct evidence that it ever had surface water but recent infrared maps taken by the Venus Express mission of its surface suggest the presence of granite rock. Granite can only be created through both tectonic activity and water acting on basalt rock. Venus' atmosphere contains almost no water vapour (just 20 parts per million). In its outer atmosphere, what little water vapour molecules exist are eventually dissociated by ultraviolet radiation from the Sun into hydrogen and oxygen ions. High-energy impacts with the particles of the solar wind energize some of these ions enough to escape the planet's gravity. This erosion of water led to the loss of most of the planet's water over the billions of years since its formation. A much higher ratio of higher mass deuterium to lower mass hydrogen supports this theory.
Carbon Dioxide
Venus' atmosphere is much hotter and denser than Earth's, 467°C at the surface with a pressure of almost 92 atm. It consists almost entirely of carbon dioxide and nitrogen. It may have started out like Earth's, both planets with large amounts of carbon dioxide outgassed into the atmosphere through volcanic activity, but it is starkly different today. Most researchers think that Venus' atmosphere underwent a runaway greenhouse effect, while Earth's did not. The reason for this difference may be rooted in the carbon cycle. Water may have been present on both young planets so that both could sequester carbon dioxide out of the atmosphere. Earth, however also had another method of sequestering this greenhouse gas, by subducting it deep underground through plate tectonics. This process occurs over a very long geological scale and provides a sink for carbon dioxide, which may not be released again until perhaps billions of years later through volcanic activity. Venus has little evidence of tectonic activity except possibly when it was a very young planet. This weakness in carbon recycling was further enhanced by a positive feedback loop. As Venus' atmosphere began to warm, more of its surface water vapourized into the atmosphere. Water vapour is a potent greenhouse gas as and this would have led to further greenhouse warming, ultimately boiling away Venus' oceans and creating hellishly hot atmospheric temperatures. Along with the water feedback loop, a positive carbon dioxide loop also occurred. The carbon dioxide Venus initially sequestered into rock (through processes similar to those I've described for the young Earth), would have baked back out into the atmosphere as its surface heated. Both carbon dioxide and water, two greenhouse gases, would have continued to increase with no mechanism to stop them. Various carbon sinks in Earth's carbon cycle sequester significant amounts of carbon dioxide out of the atmosphere and help to maintain this greenhouse gas in an equilibrium state in the atmosphere, through negative feedback loops. That is why Earth, though as rich in total carbon dioxide content as Venus, has so little of it (0.4%) in its atmosphere
Venus may have started out with an atmosphere similar in composition to Earth's early atmosphere but solar wind eventually stripped away all but the heaviest gases such as CO2 and nitrogen. Both Venus and Mars have little more than carbon dioxide left in their atmospheres because carbon dioxide is a relatively high-mass molecule. High up in the atmosphere, molecules and atoms have a certain average kinetic energy and this is the temperature of the gas. Individual atoms and molecules in this gas, however, can attain much higher velocities as a result of gaining kinetic energy through collisions with each other or collisions with fast incoming solar wind particles. If an individual atom or molecule gains enough kinetic energy it can attain escape velocity and leave the atmosphere altogether into space. This mechanism of atmospheric loss is called Jeans escape. The more massive a molecule is, the lower its average velocity will be, even if a volume of it is the same temperature as the same volume of a less massive one. Therefore, hydrogen gas molecules will tend to attain escape velocity more frequently than more massive carbon dioxide molecules.
Why is Venus' atmosphere is so much denser than Earth's atmosphere? Mars' atmosphere is almost all carbon dioxide as well, but it is so much thinner, as mentioned. Venus' mass is similar to Earth's mass so gravity alone is not the reason why the atmosphere is so dense. The mass of Venus' 250 km thick atmosphere is 4.8 x 1020 kg, whereas the mass of Earth's 100 km thick atmosphere is 5 x 1018 kg. Venus' atmosphere has 100 times more mass than Earth's and, with a gravity of 0.9 g, this accounts for its 90 times greater atmospheric surface pressure. Almost all of this incredible pressure is contributed by carbon dioxide molecules pressing down on the surface, much like the intense pressure of water acting on a deep-sea diver. All those carbon dioxide molecules that originated from Venus' planetary raw material had nowhere to go except into the atmosphere. On Earth, carbon dioxide had many different routes to take it out of the atmosphere and into minerals, water and rock, as well as into living tissues once life evolved. If it didn't, Earth's atmosphere would be much denser, with carbon dioxide dominating it.
This still does not answer the question of why Venus was able to hold onto an atmosphere while Mars was not. After all, Venus is closer to the Sun and receives more energy from it, so one would expect its atmospheric gases to have left long ago through Jeans escape. Venus' higher gravity may have helped its atmosphere withstand solar wind loss. As well, its atmosphere is so dense it forms a thick protective ionosphere that shields layers of gas underneath from solar stripping. But it has no magnetosphere. A magnetosphere protects an atmosphere from loss by deflecting solar wind and thus eliminating many of the high-energy collisions that can promote escape through the Jeans escape mechanism. It protects the lighter gases, in other words, from escape much more than it protects heavier more massive gases. That is why Venus now has an extremely dense atmosphere composed almost entirely of carbon dioxide.
Conclusion
Researchers have many clues why Earth retained liquid water, maintained a magnetosphere, maintained plate tectonics and grew a life-supporting atmosphere while on Venus and Mars, both having similar initial conditions, all these processes failed. A complicated and interconnected series of events seems to be involved in the making of a planet's atmosphere. Relatively small differences in initial conditions may have resulted in the vastly different atmospheres of Earth, Mars and Venus.
Knowing more about atmospheric evolution on other planets gives us a larger perspective from which we can ask ourselves, "How typical is a planet that reaches a state of atmospheric equilibrium sufficient to allow a stable environment in which molecules will increase in complexity into living organisms?" This is an important philosophical question that lies at the heart of our investigation into the atmospheres of our planet and others, one that we can deepen with scientific study.
Is life on Earth today the result of a series of rather improbable events coming together? Many researchers advocate this perspective, called the Rare Earth hypothesis. Many others instead advocate the mediocrity principle, in which life on Earth depends on a few simple molecules and enough time for them to build complexity. I recommend that you read about these two viewpoints and reflect on where you stand. Perhaps our current understanding of how Earth, Mars and Venus evolved may not yet be sufficient to give one principle more weight than the other. There is much more work we can do to understand the dynamics of not just Earth but of the two planet-sized laboratories "next door." Meanwhile, astronomers are discovering many new planets in other solar systems, and with ever-improving telescope technology, we are beginning to get glimpses of their atmospheres too. In a future article we find out what researchers are looking for in these exoplanets and why.
But first, we are not quite done with our solar system. Next we will take a close look at the atmosphere of Titan, a distant Saturnian moon that has both liquid on its surface and evidence of a dynamic climate. Some researchers go as far as to consider it to be a cold-temperature analogue of early Earth.
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