The Planetary Air Leak As Earth’S Atmosphere Slowly Trickles Away Into Space, Will Our Planet Come to Look Like Venus? by David C

p l a n e t a r y sc i enc e

The Planetary Air Leak As Earth’s atmosphere slowly trickles away into space, will our planet come to look like Venus? By David C. Catling and Kevin J. Zahnle

ne of the most remarkable features of the of riches that have been subject to histories of solar system is the variety of planetary plunder and decay. The atmospheres of smaller Oatmospheres. Earth and Venus are of bodies are more like crude forts, poorly defended comparable size and mass, yet the surface of Ve- and extremely vulnerable. nus bakes at 460 degrees Celsius under an ocean Recognizing the importance of atmospheric KEY CONCEPTS of carbon dioxide that bears down with the escape changes our perspective on the solar sys- ■ ■ Many of the gases that weight of a kilometer of water. Callisto and Ti- tem. For decades, scientists have pondered why make up Earth’s atmo- tan—planet-size moons of Jupiter and Saturn, Mars has such a thin atmosphere, but now we sphere and those of the respectively—are nearly the same size, yet Titan wonder: Why does it have any atmosphere left at other planets are slowly has a nitrogen-rich atmosphere thicker than all? Is the difference between Titan and Callisto leaking into space. Hot our own, whereas Callisto is essentially airless. a consequence of Callisto’s losing its atmosphere, gases, especially light ones, evaporate away; What causes such extremes? If we knew, it would rather than of Titan having been born of airier chemical reactions and help explain why Earth teems with life while its stuff? Was Titan’s atmosphere once even thicker particle collisions eject planetary siblings appear to be dead. Knowing than it is today? How did Venus steadfastly cling atoms and molecules; and how atmospheres evolve is also essential to de- to its nitrogen and carbon dioxide yet thorough- asteroids and comets termining which planets beyond our solar sys- ly lose its water? Did escape of hydrogen help to occasionally blast out tem might be habitable. set the stage for complex life on Earth? Will it one chunks of atmosphere. A planet can acquire a gaseous cloak in many day turn our planet into another Venus?

■ ■ This leakage explains ways: it can release vapors from its interior, it can many of the solar system’s capture volatile materials from comets and aster- When the Heat Is On mysteries. For instance, oids when they strike, and its gravity can pull in A spaceship that reaches escape velocity is mov- Mars is red because its gases from interplanetary space. But planetary ing fast enough to break free of a planet’s grav- water vapor got broken scientists have begun to appreciate that the es- ity. The same is true of atoms and molecules, down into hydrogen and cape of gases plays as big a role as the supply. Al- although they usually reach escape velocity less oxygen, the hydrogen drift- though Earth’s atmosphere may seem as perma- purposefully. In thermal escape, gases get too ed away, and the surplus nent as the rocks, it gradually leaks back into hot to hold on to. In nonthermal processes, oxygen oxidized—in es- space. The loss rate is currently tiny, only about chemical or charged-particle reactions hurl out sence, rusted—the rocks. A similar process on Venus three kilograms of hydrogen and 50 grams of he- atoms and molecules. And in a third process, let carbon dioxide build up lium (the two lightest gases) per second, but even asteroid and comet impacts blast away the air. into a thick ocean of air; that trickle can be significant over geologic time, Thermal escape is, in some ways, the most ironically, Venus’s huge and the rate was probably once much higher. As common and straightforward of the three. All a n

atmosphere is the result of Benjamin Franklin wrote, “A small leak can sink bodies in the solar system are heated by sunlight. ji

the loss of gases. a great ship.” The atmospheres of terrestrial They rid themselves of this heat in two ways: by a m

—The Editors planets and outer-planet satellites we see today emitting infrared radiation and by shedding mat-

are like the ruins of medieval castles—remnants ter. In long-lived bodies such as Earth, the former K T. Alfred

36 Scientific American May 2009 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

EARTH PRESENT

EARTH future: 3 BILLION YEARS from now

www.SciAm.com [escape mechanism #1] planetary teakettle One major cause of air loss is solar heating. Heat can drive out air in one of two ways. AIR EVAPORATES MOLECULE BY MOLECULE HEATED AIR FLOWS OUT IN A WIND In an atmosphere’s uppermost layer, or exosphere, nothing stops the Air heated by sunlight rises, accelerates and attains escape velocity. fastest-moving atoms and molecules from flying off into space. This This process, known as hydrodynamic escape, was particularly impor- process, known as Jeans escape, accounts for much of the leakage of tant on early Earth and Venus—in fact, it may be why Venus became hydrogen from our planet. what it is today.

Molecule

Exosphere

first, called Jeans escape, after James Jeans, the which bodies have lost all their air? English astronomer who described it in the early 20th century, air literally evaporates atom by atom, molecule by molecule, off the top of the atmosphere. At lower altitudes, collisions confine particles, but above a certain altitude, known as Extrasolar the exobase, which on Earth is about 500 kilo- planets meters above the surface, air is so tenuous that gas particles hardly ever collide. Nothing stops an atom or molecule with sufficient velocity from Mercury flying away into space. Venus Moon Earth As the lightest gas, hydrogen is the one that Ganymede Mars most easily overcomes a planet’s gravity. But first Io Callisto Jupiter it must reach the exobase, and on Earth that is a Asteroids Europa Saturn slow process. Hydrogen-bearing molecules tend

Stellar Heating Lapetus Titan not to rise above the lowest layer of atmosphere: Uranus water vapor (H 2O) condenses out and rains back Triton Charon Pluto Neptune down, and methane (CH4) is oxidized to form carbon dioxide (CO2). Some water and methane molecules reach the stratosphere and decompose, Airless Bodies with bodies atmospheres releasing hydrogen, which slowly diffuses upward until it reaches the exobase. A small amount clearly makes it out because ultraviolet images Strength of Gravity reveal a halo of hydrogen atoms surrounding our evidence for thermal escape process prevails; for others, such as comets, the planet [see illustration on opposite page].

comes from considering which latter dominates. Even a body the size of Earth The temperature at Earth’s exobase oscillates )

planets and satellites have can heat up quickly if absorption and radiation but is typically about 1,000 kelvins, implying graph n ( atmospheres and which do not. get out of balance, and its atmosphere—which that hydrogen atoms have an average speed of a ji a

The deciding factor appears to m

typically has very little mass compared with the five kilometers per second. That is less than a be the strength of stellar heat- rest of the planet—can slough off in a cosmic in- Earth’s escape velocity at that altitude, 10.8 ki- ing (vertical axis) relative to the stant. Our solar system is littered with airless lometers per second, but the average conceals a

strength of a body’s gravity ); Alfred T. K bodies, and thermal escape seems to be a com- wide range, so some hydrogen atoms still man- (horizontal axis). Airless worlds have strong heating and weak mon culprit. Airless bodies stand out as those age to break free of our planet’s gravity. This loss illustration ( gravity (left of line). Bodies where solar heating exceeds a certain threshold, of particles from the energetic tail of the speed k sec t with atmospheres have weak which depends on the strength of the body’s distribution explains about 10 to 40 percent of e e R heating and strong gravity gravity [see illustration above]. Earth’s hydrogen loss today. Jeans escape also g

(right of line). Thermal escape occurs in two ways. In the partly explains why our moon is airless. Gases Geor

38 Scientific American May 2009 released from the lunar surface easily evaporate dances of their different isotopes would be similar off into space. to their original values, which in turn are similar A second type of thermal escape is far more to that of the sun, given their common origin in dramatic. Whereas Jeans escape occurs when a the solar nebula. Yet the abundances differ. gas evaporates molecule by molecule, heated air Second, youthful stars are strong sources of can also flow en masse. The upper atmosphere ultraviolet light, and our sun was probably no can absorb ultraviolet sunlight, warm up and ex- exception. This radiation could have driven hy- pand, pushing air upward. As the air rises, it ac- drodynamic escape. celerates smoothly through the speed of sound Third, the early terrestrial planets may have and then attains the escape velocity. This form of had hydrogen-rich atmospheres. The hydrogen thermal escape is called hydrodynamic escape or, could have come from chemical reactions of wa- more evocatively, the planetary wind—the latter ter with iron, from nebular gases or from water by analogy to the solar wind, the stream of molecules broken apart by solar ultraviolet radi- charged particles blown from the sun into inter- ation. In those primeval days, asteroids and com- planetary space. ets hit more frequently, and whenever they smacked into an ocean, they filled the atmosphere Dust in the Wind with steam. Over thousands of years the steam Atmospheres rich with hydrogen are the most condensed and rained back onto the surface, but vulnerable to hydrodynamic escape. As hydro- Venus is close enough to the sun that water vapor gen flows outward, it can pick up and drag along may have persisted in the atmosphere, where so- heavier molecules and atoms with it. Much as lar radiation could break it down. the desert wind blows dust across an ocean and Under such conditions, hydrodynamic escape sand grains from dune to dune, while leaving would readily operate. In the 1980s James F. cobbles and boulders behind, the hydrogen wind Kasting, now at Pennsylvania State University, carries off molecules and atoms at a rate that showed that hydrodynamic escape on Venus diminishes with their weight. Thus, the present could have carried away an ocean’s worth of hy- composition of an atmosphere can reveal wheth- drogen within a few tens of millions of years [see er this process has ever occurred. “How Climate Evolved on the Terrestrial Plan- In fact, astronomers have seen the telltale ets,” by James F. Kasting, Owen B. Toon and signs of hydrodynamic escape outside the solar James B. Pollack; Scientific American, Feb- system, on the Jupiter-like planet HD 209458b. ruary 1988]. Kasting and one of us (Zahnle) sub- Using the Hubble Space Telescope, Alfred Vidal- sequently showed that escaping hydrogen would Madjar of the Paris Astrophysics Institute and have dragged along much of the oxygen but left his colleagues reported in 2003 that the planet carbon dioxide behind. Without water to medi- has a puffed-up atmosphere of hydrogen. Subse- ate the chemical reactions that turn carbon di- quent measurements discovered carbon and ox- oxide into carbonate minerals such as limestone, ygen in this inflated atmosphere. These atoms the carbon dioxide built up in the atmosphere are too heavy to escape on their own, so they and created the hellish Venus we see today. must have been dragged there by hydrogen. Hy- To a lesser degree, Mars and Earth, too, ap- drodynamic loss would also explain why astron- pear to have suffered hydrodynamic losses. The omers find no large planets much closer to their telltale signature is a deficit of lighter isotopes, stars than HD 209458b is. For planets that orbit which are more easily lost. In the atmospheres of within three million kilometers or so of their Earth and Mars, the ratio of neon 20 to neon 22 stars (about half the orbital radius of HD is 25 percent smaller than the solar ratio. On 209458b), hydrodynamic escape strips away the Mars, argon 36 is similarly depleted relative to entire atmosphere within a few billion years, argon 38. Even the isotopes of xenon—the heavi- leaving behind only a scorched remnant. est gas in Earth’s atmosphere apart from pollut- This evidence for planetary winds lends cre- ants—show the imprint of hydrodynamic es- leaking hydrogen atoms give dence to ideas put forth in the 1980s about hy- cape. If hydrodynamic escape were vigorous off a red glow in this ultraviolet

a drodynamic escape from ancient Venus, Earth enough to sweep up xenon, why did it not sweep image of Earth’s night side, ow and Mars. Three clues suggest this process once up everything else in the atmosphere along with taken by NASA’s Dynamic Explor- y of I t operated on these worlds. The first concerns no- er I satellite in 1982. Oxygen it? To solve this puzzle, we may need to construct ble gases. Were it not for escape, chemically un- and nitrogen account for the a different history for xenon than for the other reactive gases such as neon or argon would re- gases now in the atmosphere.

A/Universi band around the North Pole and S A

N main in an atmosphere indefinitely. The abun- the wisps in the tropics. Hydrodynamic escape may have stripped Ti-

www.SciAm.com SCIENTIFIC AMERICAN 39 tan of much of its air, too. When it descended which gases can escape? through Titan’s atmosphere in 2005, the Euro- Earth Saturn pean Space Agency’s Huygens probe found that Jupiter Uranus the ratio of nitrogen 14 to nitrogen 15 is 70 per- Neptune cent of that on Earth. That is a huge disparity given that the two isotopes differ only slightly in Mercury their tendency to escape. If Titan’s atmosphere Mars started with the same nitrogen isotopic composition as Earth’s, it must have lost a huge amount Moon Venus of nitrogen—several times the substantial amount it currently has—to bring the ratio down Titan to its present value. In short, Titan’s atmosphere might once have been even thicker than it is to-

day, which only heightens its mystery. Temperature Hydrogen Helium Better Escaping through Chemistry Water Pluto On some planets, including modern Earth, ther- Oxygen mal escape is less important than nonthermal Carbon dioxide escape. In nonthermal escape, chemical reactions or particle-particle collisions catapult atoms to Strength of Gravity escape velocity. What nonthermal escape mechanisms have in common is that an atom or mol- light gases such as hydrogen are more footloose than heavier ones such as oxygen. Their susceptibility to Jeans escape depends on the temperature at the top of a body’s ecule reaches a very high velocity as the outcome atmosphere or, for airless bodies such as the moon, at its surface (vertical axis) and on of a single event that takes place above the the strength of its gravity (horizontal axis). If a body lies to the right of the line for a exobase, so that bumping into something does gas, it holds on to the gas; to the left, it loses the gas. For example, Mars loses hydro- not thwart the escapee. Many types of nonther- gen and helium, retains oxygen and carbon dioxide, and barely retains water. mal escape involve ions. Ordinarily these charged particles are tethered to a planet by its magnetic heavier ions with them. This process may ex- field, either the global (internally generated) mag- plain the xenon puzzle: if the polar wind was netic field —if there is one —or the localized fields more vigorous in the past, it could have dragged induced by the passage of the solar wind. But out xenon ions. One piece of evidence is that they find ways to slip out. krypton does not have the same isotopic pattern In one type of event, known as charge ex- as xenon does, even though it is a lighter gas and, change, a fast hydrogen ion collides with a neu- all else being equal, ought to be more prone to tral hydrogen atom and captures its electron. escape. The difference is that krypton, unlike xe- The result is a fast neutral atom, which is im- non, resists ionization, so even a strong polar mune to the magnetic field. This process ac- [The Authors] wind would have left it unaffected. counts for 60 to 90 percent of the present loss of A third nonthermal process known as photo- Planetary scientist David C. hydrogen from Earth and most of the hydrogen Catling studies the coupled evolu- chemical escape operates on Mars and possibly loss from Venus. tion of planetary surfaces and on Titan. Oxygen, nitrogen and carbon monox- Another way out exploits a weak spot—dare atmospheres. Formerly at the NASA ide molecules drift into the upper atmosphere, Ames Research Center, he joined we say a loophole—in the planet’s magnetic trap. where solar radiation ionizes them. When the the faculty at the University of Most magnetic field lines loop from one magnet- ionized molecules recombine with electrons or Washington in 2001. He is a co- ic pole to the other, but the widest field lines are investigator for NASA’s Phoenix collide with one another, the energy released ) dragged outward by the solar wind and do not lander, which completed its splits the molecules into atoms with enough

loop back; they remain open to interplanetary mission last December. Kevin J. speed to escape. illustration ( Zahnle has been a research scien- k

space. Through this opening, ions can escape. Mars, Titan and Venus lack global magnetic sec t

tist at the NASA Ames center since e

To be sure, the ions must still overcome gravity, fields, so they are also vulnerable to a fourth e R

1989. Even by the eclectic stan- g and only the lightest ions such as hydrogen and dards of planetary science, he has nonthermal process known as sputtering. With- helium make it. The resulting stream of charged an unusually wide range of inter- out a planetary field to shield it, the upper atmo- ); Geor particles, called the polar wind (not to be con- ests, from planetary interiors to sphere of each of these worlds is exposed to the graph

surfaces to atmospheres. In 1996 ( a n fused with the planetary wind), accounts for 10 full brunt of the solar wind. The wind picks up ji Zahnle received the NASA Excep- to 15 percent of Earth’s hydrogen loss and al- ions, which then undergo charge exchange and a m tional Achievement Medal for his most its entire helium leak. work on the impact of Comet escape. Mars’s atmosphere is enriched in heavy

In some cases, these light ions can sweep up Shoemaker-Levy 9 into Jupiter. nitrogen and carbon isotopes, suggesting that it K T. Alfred

40 Scientific American May 2009 [escape mechanism #2] The Houdini Particles The second broad way that air escapes is through charged particle reactions. Electric fields readily accelerate ions to escape velocity. The planet’s magnetic field traps them, but they have various tricks to slip out.

STEAL AN ELECTRON AND MAKE A GETAWAY One way for an ion to break free of the magnetic field is to bump into an uncharged atom and abscond with its electron, thereby becoming electrically neutral.

Ion Magnetic field line

Ion Electron

SNEAK OUT ALONG OPEN FIELD LINES Another escape route is along magnetic field Magnetic lines at high latitudes, which do not loop back field line to the ground but instead link up with interplanetary fields.

GET WHISKED AWAY BY SOLAR WIND A third process known as sputtering operates on worlds that lack magnetic fields of their own. The solar wind, which is magnetized, picks up and Ion spirits off ions.

Solar wind Mars

www.SciAm.com SCIENTIFIC AMERICAN 41 [escape mechanism #3] the cone of atmosphere ejected. For the asteroid air blast that killed off the dinosaurs 65 million years ago, When a comet or asteroid strikes a planet, it creates an enormous explosion that the cone was about 80 degrees wide from the ver- throws rock, water, dinosaurs and air into space. tical and contained a hundred-thousandth of the atmosphere. An even more energetic impact can carry away the entire atmosphere above a plane that is tangent to the planet. Another factor determining the width of the cone is the atmospheric density. The thinner the air, the greater the fraction of the atmosphere that gets lost. The implication is gloomy: once a vulnerable atmosphere starts wearing away, impact erosion becomes ever easier until the atmosphere vanishes altogether. Unfortunately, Mars spent its youth in a bad neighborhood near the asteroid belt and, being small, was especially susceptible. Given the expected size distribution of impactors early in a solar system’s history, the planet should have been stripped of its entire atmosphere in less than 100 million years. The large moons of Jupiter also live in a dan- impact erosion is most which bodies are vulnerable? severe when a body has gerous neighborhood—namely, deep in the gi- weak gravity (horizon- ant planet’s gravitational field, which accelerates tal axis) and when the Airless incoming asteroids and comets. Impacts would bodies Extrasolar incoming asteroids or planets have denuded these moons of any atmospheres comets smack at high they ever had. In contrast, Titan orbits compar- speed (vertical axis). atively far from Saturn, where impact velocities Ganymede Jupiter Airless bodies tend to are slower and an atmosphere can survive. Io Mercury lie toward the upper Saturn In all these ways, escape accounts for much left of the graph, Europa Venus Uranus of the diversity of atmospheres, from the lack of where erosion is worst. Callisto Earth Moon Mars air on Callisto and Ganymede to the absence of (The strength of gravity Titan Triton Neptune water on Venus. A more subtle consequence is

sets a minimum impact Velocity Impact Asteroids Bodies with that escape tends to oxidize planets, because hy- velocity, so the greenish atmospheres area represents a range Charon Pluto drogen is lost more easily than oxygen. Hydro- of velocities that can gen escape is the ultimate reason why Mars, Ve- never arise in nature.) nus and even Earth are red. Most people do not Strength of Gravity think of Earth as a red planet, but much of the continental crust is red. Soil and vegetation hide has lost as much as 90 percent of an earlier at- this native hue. All three worlds started out the mosphere. Sputtering and photochemical escape gray-black color of volcanic rock and reddened are the most likely culprits. In 2013 NASA plans as the original minerals oxidized to iron oxides to launch the Mars Atmosphere and Volatile (similar to rust). To account for its color, Mars EvolutioN (MAVEN) mission to measure escap- must have lost an ocean of water equivalent to a

ing ions and neutral atoms and reconstruct the global layer meters to tens of meters deep. )

planet’s atmospheric history. On Earth, most researchers attribute the graph n (

accumulation of oxygen 2.4 billion years ago to a ji a m

Inescapable Consequences photosynthetic organisms, but in 2001 we sug- a Both thermal and nonthermal escape are like tiny gested that the escape of hydrogen also played trickles compared with the huge splash when an important role. Microbes break apart water comets or asteroids crash into planets. If projec- molecules in photosynthesis, and the hydrogen ); Alfred T. K tiles are sufficiently big and fast, they vaporize can pass like a baton from organic matter to illustration (

both themselves and a similar mass of the sur- methane and eventually reach space. The ex- k sec t

face. The ensuing hot gas plume can expand fast- pected amount of hydrogen loss matches the e e R

er than the escape velocity and drive off the over- net excess of oxidized material on Earth today. g

lying air. The larger the impact energy, the wider Escape helps to solve the mystery of why Geor

42 Scientific American May 2009 [which mechanisms apply where] A Litany of Losses The three escape processes operate to different degrees on different planets and at different points in their history.

THERMAL NONTHERMAL IMPACT ind ic w m ical ttering u ydro olar harge hoto x change p eans P m dyna P escape H e S che C BODY PERIOD KEY GASES LOST J Now Hydrogen ✓ ✓ ✓ Earth Helium ✓ ✓ Primordial Hydrogen, neon ✓ Now Hydrogen, helium ✓ ✓ Venus Primordial Hydrogen, oxygen ✓ Now Hydrogen ✓ Carbon, oxygen, nitrogen, argon ✓ ✓ Mars Primordial All gases ✓ Hydrogen, carbon dioxide ✓ Jupiter’s satellites Primordial All gases ✓ ✓ Now Hydrogen ✓ ✓ Titan Methane, nitrogen ? ✓ ✓ Primordial Hydrogen, methane, nitrogen ✓ Pluto Now Hydrogen, methane, nitrogen ? HD 209458b Now Hydrogen, carbon, oxygen ✓

Mars has such a thin atmosphere. Scientists have survived underground and leaked out after have long hypothesized that chemical reactions the bombardment had subsided. among water, carbon dioxide and rock turned Although Earth seems comparatively un- the original thick atmosphere into carbonate scathed by escape, that will change. Today hy- minerals. The carbonates were never recycled drogen escape is limited to a trickle because the back into carbon dioxide gas because Mars, be- principal hydrogen-bearing gas, water vapor, ing so small, cooled quickly and its volcanoes condenses in the lower atmosphere and rains stopped erupting. The trouble with this scenar- back to the surface. But our sun is slowly bright- io is that spacecraft have so far found only a ening at about 10 percent every billion years. ➥ More To single small area on Mars with carbonate rock, That is imperceptibly slow on a human timescale Explore and this outcrop probably formed in warm sub- but will be devastating over geologic time. As the Origins of Atmospheres. K. J. Zahnle surface waters. Moreover, the carbonate theory sun brightens and our atmosphere warms, the in Origins. Edited by C. E. Woodward, offers no explanation for why Mars has so little atmosphere will get wetter, and the trickle of hy- J. M. Shull and H. A. Thronson. Astro- nomical Society of nitrogen or noble gases. Escape provides a bet- drogen escape will become a torrent. the Pacific Conference Series, ter answer. The atmosphere did not get locked This process is expected to become important Vol. 148, 1998. away as rock; it dissipated into space. when the sun is 10 percent brighter—that is, in a A nagging problem is that impact erosion billion years—and it will take another billion An Extended Upper Atmosphere ought to have removed Mars’s atmosphere alto- years or so to desiccate our planet’s oceans. around the Extrasolar Planet HD209458b. Alfred Vidal-Madjar gether. What stopped it? One answer is simple Earth will become a desert planet, with at most et al. in Nature, Vol. 422, pages chance. Large impacts are inherently rare, and a shrunken polar cap and only traces of precious 143–146; March 13, 2003.

) their frequency fell off rapidly about 3.8 billion liquid. After another two billion years, the sun icons years ago, so Mars may have been spared the fi- will beat down on our planet so mercilessly even Planetary Atmospheres and Life. n ( a D. C. Catling and J. F. Kasting in ji

a nal devastating blow. A large impact of an icy as- the polar oases will fail, the last liquid water will m Planets and Life: The Emerging a teroid or comet could have deposited more vola- evaporate and the greenhouse effect will grow Science of Astrobiology. Edited by tiles than subsequent impacts could remove. Al- strong enough to melt rock. Earth will have fol- W. T. Sullivan and J. A. Baross.

Alfred T. K ternatively, remnants of Mars’s atmosphere may lowed Venus into a barren lifelessness. ■ Cambridge University Press, 2007.