 Earth present (<em>center</em>) Earth future: 3 billion years from now (<em>right</em>))
Loss of certain gases, especially hydrogen, has transformed Earth. It is one of the reasons that oxygen built up in the atmosphere.
In the future, the depletion of hydrogen will dry out our oceans and all but shut down geologic cycles that stabilize the climate. Life may still be able to hold out in the polar regions. Earth past: 3 billion years ago (left) Earth present (center) Earth future: 3 billion years from now (right)
Image: Alfred T. Kamajian
In Brief
- Many of the gases that make up Earth’s atmosphere and those of the other planets are slowly leaking into space. Hot gases, especially light ones, evaporate away; chemical reactions and particle collisions eject atoms and molecules; and asteroids and comets occasionally blast out chunks of atmosphere.
- This leakage explains many of the solar system’s mysteries. For instance, Mars is red because its water vapor got broken down into hydrogen and oxygen, the hydrogen drifted away, and the surplus oxygen oxidized—in essence, rusted—the rocks. A similar process on Venus let carbon dioxide build up into a thick ocean of air; ironically, Venus’s huge atmosphere is the result of the loss of gases.
One of the most remarkable features of the solar system is the variety of planetary atmospheres. Earth and Venus are of comparable size and mass, yet the surface of Venus bakes at 460 degrees Celsius under an ocean of carbon dioxide that bears down with the weight of a kilometer of water. Callisto and Titan—planet-size moons of Jupiter and Saturn, respectively—are nearly the same size, yet Titan has a nitrogen-rich atmosphere thicker than our own, whereas Callisto is essentially airless. What causes such extremes? If we knew, it would help explain why Earth teems with life while its planetary siblings appear to be dead. Knowing how atmospheres evolve is also essential to determining which planets beyond our solar system might be habitable.
A planet can acquire a gaseous cloak in many ways: it can release vapors from its interior, it can capture volatile materials from comets and asteroids when they strike, and its gravity can pull in gases from interplanetary space. But planetary scientists have begun to appreciate that the escape of gases plays as big a role as the supply. Although Earth’s atmosphere may seem as permanent as the rocks, it gradually leaks back into space. The loss rate is currently tiny, only about three kilograms of hydrogen and 50 grams of helium (the two lightest gases) per second, but even that trickle can be significant over geologic time, and the rate was probably once much higher. As Benjamin Franklin wrote, “A small leak can sink a great ship.” The atmospheres of terrestrial planets and outer-planet satellites we see today are like the ruins of medieval castles—remnants of riches that have been subject to histories of plunder and decay. The atmospheres of smaller bodies are more like crude forts, poorly defended and extremely vulnerable.
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26 Comments
Add CommentExcellent articl on a subject that has long piqued my curiosity! Thank You ! ! !
Reply | Report Abuse | Link to thisI have several questions that after reading it. I wonder if anyone more knowlegable than me could shed some
light?
"Hydrodynamic loss would also explain why astronomers find no large planets much closer to their stars than
HD 209458b is. For planets that orbit within three million kilometers or so of their stars (about half the orbital
radius of HD 209458b), hydrodynamic escape strips away the entire atmosphere within a few billion years,
leaving behind only a scorched remnant."
Pardon my slowness, but if a large planet is stripped of its atmosphere, it is still a large planet, regardless of
its distance to its star, or not? What am I missing? Why not "...leaving behind a [large] scorched remnant
"[orbiting at a close distance]?
"A third nonthermal process known as photochemical escape operates on Mars and possibly on Titan.
Oxygen, nitrogen and carbon monoxide molecules drift into the upper atmosphere, where solar radiation
ionizes them. When the ionized molecules recombine with electrons or collide with one another, the energy
released splits the molecules into atoms with enough speed to escape."
I'm no chemist, so I could use an example, please. Maybe N- combines with N+ and releases enough energy
to eject an N from both the molecule and from the atmosphere? Or likewise for C+ and O-, ejecting a C? Also,
why only on Mars and possibly Titan? Do earth's (for example) atmospheric N2, O2 and CO not occupy the
atmosphere above the exobase?
"The large moons of Jupiter also live in a dangerous neighborhoodnamely, deep in the giant planets
gravitational field, which accelerates incoming asteroids and comets. Impacts would have denuded these
moons of any atmospheres they ever had. In contrast, Titan orbits comparatively far from Saturn, where impact
velocities are slower and an atmosphere can survive."
I wish the author had quantified this a bit. How deep in a gravitational well do Jupiter's moons live in, ditto for
Titan? What are the escape velocities from the planet + moon system in each case?
I also have some questions unrelated to the article: Could earth's atmosphere have been a whole lot denser
early on? How dense? And another couple questions for planetary atmospheric science:
1) what was the atmospheric density about the time life came into being? Any clear clues as to its
composition?
2) Around the time animals first learned to fly, could a) the atmosphere been significanlty denser (say by 5% or
more) and/ or b) could the level of oxygen been higher as well, compared to today?
If so for either, it would seem to make learning to fly a lot easier. Thereafter as the atmosphere moved towards
present conditions, evolution would have let only the better- adapted versions continue to fly (lighter bones,
bigger wings, more efficient use of oxygen, better cooling, etc) and "helped" them continue to evolve to what
we see today. Can this be ruled in or out of the realm of possibility?
Keith Labrecque
Excellent articl on a subject that has long piqued my curiosity! Thank You ! ! !
Reply | Report Abuse | Link to thisI have several questions that after reading it. I wonder if anyone more knowlegable than me could shed some
light?
"Hydrodynamic loss would also explain why astronomers find no large planets much closer to their stars than
HD 209458b is. For planets that orbit within three million kilometers or so of their stars (about half the orbital
radius of HD 209458b), hydrodynamic escape strips away the entire atmosphere within a few billion years,
leaving behind only a scorched remnant."
Pardon my slowness, but if a large planet is stripped of its atmosphere, it is still a large planet, regardless of
its distance to its star, or not? What am I missing? Why not "...leaving behind a [large] scorched remnant
"[orbiting at a close distance]?
"A third nonthermal process known as photochemical escape operates on Mars and possibly on Titan.
Oxygen, nitrogen and carbon monoxide molecules drift into the upper atmosphere, where solar radiation
ionizes them. When the ionized molecules recombine with electrons or collide with one another, the energy
released splits the molecules into atoms with enough speed to escape."
I'm no chemist, so I could use an example, please. Maybe N- combines with N+ and releases enough energy
to eject an N from both the molecule and from the atmosphere? Or likewise for C+ and O-, ejecting a C? Also,
why only on Mars and possibly Titan? Do earth's (for example) atmospheric N2, O2 and CO not occupy the
atmosphere above the exobase?
"The large moons of Jupiter also live in a dangerous neighborhood—namely, deep in the giant planet’s
gravitational field, which accelerates incoming asteroids and comets. Impacts would have denuded these
moons of any atmospheres they ever had. In contrast, Titan orbits comparatively far from Saturn, where impact
velocities are slower and an atmosphere can survive."
I wish the author had quantified this a bit. How deep in a gravitational well do Jupiter's moons live in, ditto for
Titan? What are the escape velocities from the planet + moon system in each case?
I also have some questions unrelated to the article: Could earth's atmosphere have been a whole lot denser
early on? How dense? And another couple questions for planetary atmospheric science:
1) what was the atmospheric density about the time life came into being? Any clear clues as to its
composition?
2) Around the time animals first learned to fly, could a) the atmosphere been significanlty denser (say by 5% or
more) and/ or b) could the level of oxygen been higher as well, compared to today?
If so for either, it would seem to make learning to fly a lot easier. Thereafter as the atmosphere moved towards
present conditions, evolution would have let only the better- adapted versions continue to fly (lighter bones,
bigger wings, more efficient use of oxygen, better cooling, etc) and "helped" them continue to evolve to what
we see today. Can this be ruled in or out of the realm of possibility?
Keith Labrecque
Sorry for the double-post and apparently bad formatting!
Reply | Report Abuse | Link to thisWell in regards to HD 209458b, that planet is a gas giant... ergo, you lose the atmosphere, you lose most of the planet.
Reply | Report Abuse | Link to thisFor the reference of planetoids in Jupiter's gravity well, the need for quantification of distance isn't really necessary, since the distinction between orbital radii of satellites around Jupiter and Saturn is very great. The effect of gravitic acceleration is multiplied by the fact that Jupiter is much more massive as well.
Yes, Earth's atmosphere was much denser in prehistoric times. The author made some reference to this, but specifically there was indeed a much greater amount of most gases which compose our air, notably oxygen and carbon dioxide.
As for the evolution of flight, I have no idea what you're talking about, but bear in mind with much greater densities of air, after a certain point flight would be better described as swimming.
The idea of carbon dioxide sequestration should be reconsidered. For every atom of carbon sequestered, two atoms of oxygen are also sequestered. If carbon dioxide sequestration were to be implemented industrially on a global scale, the depletion of oxygen from the atmosphere would be significant.
Reply | Report Abuse | Link to thisHigher air density would make evolution to flight a much smaller leap of bodily configuration.
Reply | Report Abuse | Link to thisThe density question has to do with lift, and the power required to generate the lift. It varies strongly with air density. At higher density, flight takes a LOT less power, varying as the inverse of the cube of the air density. If the air were twice as dense during, say, the Mesozoic era vs today, flight of a given weight of animal would have taken only about one third of the power. That would be a HUGE advantage. Hey, maybe even I could fly under those conditions!
The explanation:
The basic equation for lift in a fixed wing aircraft at non-sonic speeds (a fair first approximation model for a bird or other flying animal) is L = 1/2(rho)v*v*CL*A.
where:
L = Lift force
rho = density of air
v = velocity, as true airspeed, which must be squared
CL = [C-sub-L] = Coefficient of lift for the wing
A = plan area of the wing i.e. the area of the projection of the wing onto a horizontal plane)
Solving for v we get v = sqrt(2L/((rho)*CL*A)
which shows us that as density rho increases, v falls off as
1/( square root(rho)). For example, if you double rho, the required v is only 0.707 as much.
That doesn't seem like much of a gain, until you look a little further, to the equations for power.
The power required for this flight is
P = F*v
where F = the aerodynamic drag force (drag caused in generating the lift, neglecting viscous skin friction and turbulence)
And F = 1/2(rho)v*v*Cd*A
where Cd = [C-sub-d] = coefficient of drag
So P = (1/2(rho)*v*v*Cd*A)*v --> note that P is proportional to v-cubed.
In our example, doubling rho led to 0.707 for v.
Cubing 0.707 gives 0.35 for the new P compared with the original P. Now THAT's a significant difference!
A given animal evolving towards flight in such a dense atmosphere would not need to have near the power-to-weight ratio found in modern fliers. This implies that a wider range of animal configurations were at least marginally capable of flight, or put another way, stumbling into flight capability would have not been such a long-shot.
As the air got slowly less dense, the flier body configurations would have had to evolve towards the current configurations to maintain flight capability.
Compair apples and oranges and one will poorly support logic or scientific study. Gas loss "escape", has been around for a long, long time. Mars did not lose it's envilope because of CO2. Mars has to be studied for it's geo hist, at best it's geo hist, is going to be vary different from ours, Mars orbit is even opposit to that of earth's, it is also farther from the sun and it has lower gravity.
Reply | Report Abuse | Link to thisNo theory like an old theory, the sky is falling, sorry escaping.
There is nothing but to look for a new tipping piont.
When the data starts to accumulate, we are going to see cooler seasons so, there is a desparate search to find the next eco crime. If Jean's escape can be tied to climte change and more CO2, they could get new grants to study "goasts".
This is a primer on Why is there life on Earth?
True green house gases (water vapor) has made all this poss.
Very interesting primer on the evolution of an atmosphere however the it seems that the basic premis is that all the bodies start with the same basic constituents, a simple erroronious assumptsion.
Reply | Report Abuse | Link to thisThe original bodies make up is set by it's formation position in the nebulia that it originated in. Not all bodies are created equal. The modification and resultant atmosphere may well be as posited in the above article.
In response to the fretting over carbon dioxide locking away oxygen, in reality the oxygen comes from the decomposition of water via photosynthesis. In the Earth's oceans there's the equivalent of 100 times the present oxygen levels and there's not enough free carbon on or under the Earth to burn to lock even the oxygen that's presently in the atmosphere up as carbon dioxide.
Reply | Report Abuse | Link to thisSo, in sum, don't panic. Oxygen is the least of our worries.
In reply to Keith's original questions [my replies in brackets]...
Reply | Report Abuse | Link to thisI wish the author had quantified this a bit. How deep in a gravitational well do Jupiter's moons live in, ditto for
Titan? What are the escape velocities from the planet + moon system in each case? [Callisto and Ganymede orbit 1.883 and 1.07 million km from Jupiter, while Titan orbits 1.222 million km from Saturn. But Jupiter masses 317.83 Earth masses, while Saturn is just 95.17 Earth masses - thus Saturn's mass is 3/10 of Jupiter and the escape energy at Titan is 3/10 what it would be around Jupiter.]
I also have some questions unrelated to the article: Could earth's atmosphere have been a whole lot denser
early on? [Maybe. We know oxygen and carbon dioxide have varied immensely over time. During the Carboniferous oxygen was 35% of the atmosphere. If nitrogen remained at 0.78 bar, then the atmospheric pressure was 1.2 times the present. Nitrogen is the big unknown, but it's less reactive than O2 or CO2 so it's more likely to have been stable.]
How dense? [Unknown. We know the relative amount of oxygen, but not the absolute. Oxygen has varied between 12-35%.]
And another couple questions for planetary atmospheric science:
1) what was the atmospheric density about the time life came into being? [no clear indications, but significant amounts of hydrogen (~40%) and carbon dioxide (~20%) are likely, plus some methane, nitrogen and water vapour. If N2 was near constant, then the pressure was roughly 2-2.5 times the present.]
Any clear clues as to its composition? [Some chemical hints in paleosols (old soils) plus the chemistry of deposits, but often these can be biased by localised processes. Equilibrium chemistry of the original source material of the crust suggests a lot of methane and ammonia, plus hydrogen. Could have been lots of water vapour because some data suggests near-boiling temperatures.]
2) Around the time animals first learned to fly, could a) the atmosphere been significanlty denser (say by 5% or
more) and/ or b) could the level of oxygen been higher as well, compared to today?
[Denser, maybe? But during the Triassic the oxygen levels were lower ~12%, perhaps explaining the air-sac pneumatic system in dinosaurs/birds.]
Keith Labrecqur
[it's possible the air was denser, but presently we can't say. A novelty theory is the idea that pterosaurs and birds learnt to fly by swimming...]
Looking at the general equation of photosynthesis
Reply | Report Abuse | Link to thisCO2 + 2 H2O + photons -> (CH2O)n + H2O + O2
we can see that for every four moles of oxygen that go into the reaction, four moles of oxygen come out as reaction products. Two moles come from CO2 and two moles come from H2O. In the reaction products, one mole is incorporated into plant carbohydrate, one mole is released back into the atmosphere as water, and two moles are released into the atmosphere as O2. For every CO2 that goes into the reaction, one O2 is released back into the atmosphere. This is the principle by which biomass cultivated for fuel is a sustainable and renewable resource; it produces no net change in the amount of CO2 in the atmosphere.
The proposed carbon sequestration is actually carbon dioxide sequestration; CO2 is removed from the atmosphere and locked away in solids. The combustion that produces the CO2 removes O2 from the atmosphere, but the CO2 then sequestered is not available for photosynthesis. So even if sequestation succeeds in reducing the amount of CO2 in the atmosphere, it depletes the amount of O2 in the atmosphere. I calculated the potential amount of oxygen depletion, and it is significant.
Renewable sources of energy are far better than "carbon sequestration," which is actually CO2 sequestration.
Steven, significant? Carbon dioxide is 0.00038 moles of every mole of atmosphere - thus 0.00038 moles of oxygen by your count. Oxygen is currently at 0.209 moles per mole of air... I really don't see "significant" implied by those numbers.
Reply | Report Abuse | Link to thisWhat keeps CO2 at 0.00038 moles per mole of atmosphere is the ongoing process of photosynthesis on a global scale. Huge amounts of O2 is cycled into CO2 and back to O2 again. Now start sequestering all the CO2 being produced on an industrial scale globally, and you are sequestering twice as much oxygen as carbon. The atmosphere holds a finite amount of oxygen, which accumulated to its present amount over billions of years. It is not inexhaustible. Go ahead, crunch the numbers. Take the total mass of the atmosphere and multiply by 0.209 to get the mass of atmospheric oxygen. CO2 is 44 grams per mole and O2 is 32 grams per mole, so 73% of the mass of CO2 is oxygen. Now take the total mass of carbon dioxide produced each year, multiply that by 0.73, and you have the mass of oxygen more or less permanently removed from the atmosphere by sequestration every year. Now multiply by 100 to get the mass of oxygen sequestered in a century, divide that by the total mass of oxygen in the atmosphere, and tell me that is not significant.
Reply | Report Abuse | Link to thisSteven,
Reply | Report Abuse | Link to thisIf there was enough carbon to burn it might make a dent in global oxygen levels, but there's not. Not enough coal, oil or gas - or even gas clathrates - to burn to remove even a fraction of the atmosphere's oxygen. But my main point is that free oxygen produced by photosynthesis is produced from the protons being ripped off water, leaving oxygen. And we have a LOT of water.
qraal,
Reply | Report Abuse | Link to thisWe disagree not only on the idea of carbon dioxide sequestration, but also on the facts of the chemistry involved. I will just have to leave it at that.
We also disagree on the quantities of the reactants involved.
Reply | Report Abuse | Link to thisThis is taking more of my time than I had planned, but here goes. Photosynthesis:
Reply | Report Abuse | Link to thisCO2 + 2 H2O + photons -> CH2O + H2O + O2
The "CH2O" is one fragment of the carbohydrate produced. Look at the ratios. Two atoms of oxygen come from CO2, and two atoms of oxygen come from water. One of those oxygen atoms from water is released again as water. That leaves three atoms of oxygen going into the reaction, two from CO2 and one from H2O. One of those three oxygen atoms goes into the carbohydrate produced, and the other two atoms of oxygen are released as O2.
Now look at combustion of the carbohydrate.
CH2O + O2 -> CO2 + H2O
Two of the three oxygen atoms are released in CO2, and one oxygen atom is released as water. There is no net gain in the amount of atmospheric O2 from water.
Steven,
Reply | Report Abuse | Link to thisRespiration or combustion use oxygen as much as photosynthesis makes it, but the net burial of carbon (not CO2) in the natural (open) cycle means a net gain in oxygen over time, as I hope you can appreciate. Sequestering carbon dioxide from fossil fuel burning would only be a tiny decrement in the global oxygen budget since no realistic estimate of fossil fuels remaining comes close to consuming significant oxygen. And oxygen is produced from the water consumed, not from carbon dioxide, in photosynthesis, due to its role as proton-donor.
"And oxygen is produced from the water consumed, not from carbon dioxide, in photosynthesis, due to its role as proton-donor."
Reply | Report Abuse | Link to thisI'm not arguing that oxygen produced by photosynthesis does not come from water, but the fact that it does does not alter the fact that there is no net gain of atmospheric oxygen from photosynthesis:
CO2 + 2 H2O + photons -> CH2O + H2O + O2
Four atoms of oxygen on the left, four atoms of oxygen on the right. Oxygen atoms are exchanged between CO2 and water, but for every CO2 molecule that goes in, one O2 molecule comes out. To draw an analogy, it is like depositing funds to a bank account from multiple sources of income. If you go to the bank and withdraw one dollar, you cannot say which source of income that dollar came from, and it would be meaningless to do so, because all the dollars in the account are indistinguishable from one another. We disagree, and I don't have the ambition to pursue the argument further.
You're right if you mean total oxygen, but you're wrong if you mean free oxygen. Why confuse the two? Sequestering carbon dioxide from combustion means the return to the ground of carbon removed from it to start with, so it remains in balance. As for the oxygen, more will be produced from the water in its various reservoirs to replace it, given time. Burning all the coal, oil and gas left will (temporarily) remove just 0.25-0.5% of the oxygen. I don't see the drama involved in that loss to the crust. Thus I fail to see your point that it's somehow significant.
Reply | Report Abuse | Link to thisI did appreciate this article for its scientific content but what really got me was the imbedded humor. I could list a few examples but the "Air Blast" description "..an enormous explosion that throws rock, water, dinosaurs and air into space." is going to be hard to beat! I hope you and your fellow science authors continue in this vein - I for one really enjoy it.
Reply | Report Abuse | Link to thisWhat prevents the atmosphere from pouring out into space? Isn't gravity an insufficient answer since these gases do not fall to the earth?
Reply | Report Abuse | Link to thisThe negative pressure of the emptiness of space must be a force trying to attract these gaseous molecules away from earth's orbit?
Is the rotational speed of the earth (about 1600km/hr I believe) relevant in stopping the atmosphere from depleting itself into space? If so, what would the mechanism be?
Are the gaseous molecules sweeping each other along at such a speed that they cannot respond to any anti-gravity force pulling them out into space?
Is there an inter-molecular force generated by the immense rotational velocity of the earth? If so, would this explain why the atmosphere doesn't rapidly escape into space?
How come I cannot get this article in full on the Internet when I already subscribe to the hard copy - must I pay my subscription twice!?
Reply | Report Abuse | Link to thisDavid R.
How come I cannot get this article in full on the Internet when I already subscribe to the hard copy - must I pay my subscription twice!?
Reply | Report Abuse | Link to thisDavid R.
Hi All
Reply | Report Abuse | Link to thisJust for the sake of future browsers I'm answering Deist Pete's question above [my comments in brackets]...
What prevents the atmosphere from pouring out into space? Isn't gravity an insufficient answer since these gases do not fall to the earth?
[the gases are in constant thermal motion and as a result they don't fall down. But they feel gravity just like everything else.]
The negative pressure of the emptiness of space must be a force trying to attract these gaseous molecules away from earth's orbit?
[there is no "negative pressure" as such. The "vacuum of space" doesn't suck, nor do vacuums on Earth 'suck'. Instead the surrounding air is pushing inwards to fill the empty space. Earth's atmosphere doesn't push into space because gravity holds it back.]
Is the rotational speed of the earth (about 1600km/hr I believe) relevant in stopping the atmosphere from depleting itself into space? If so, what would the mechanism be?
[it's not relevant and there is no force.]
Are the gaseous molecules sweeping each other along at such a speed that they cannot respond to any anti-gravity force pulling them out into space?
[No. Rotation reduces felt gravity slightly so it's actually trying to fling air into space. Fortunately it's about 20 times too weak to do so.]
Is there an inter-molecular force generated by the immense rotational velocity of the earth? If so, would this explain why the atmosphere doesn't rapidly escape into space?
[No. Gravity weighs air down, just like it weighs you or me down.]
sensative effects by evolutions reality of instinctions revolute destined futures by truths & wises evolutionizes intelligences why exsaggerate?
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