California Association of Chemistry Teachers
W. T. Huntress, Jr. Jet Propulsion Laboratory Califomla lnst~tuteof Technology 4800 Oak Grove Drwe Pasadena. 91 103
The Chemistry of 'Planetary kmospheres
The atmosphere of the earth is one of the major components which makes life possible on our home celestial body. We are intimately familiar with our own atmosphere, and in the previous two centuries have determined its hulk chemical composition. Only recently have we become aware of how important also are the trace components present in the atmosphere and how delicate is the balance between the earth's atmosphere, its oceans, and its biological and technological activity. Besides the earth's atmosphere, astronomers and scientists have always been curious about the atmospheres which exist on the other planets of the solar system and whether, for example, these alien atmospheres could support our own or other forms of life. With the dawn of the space age has come a new awareness of planetary science, and scientific vehicles have become available for the close-up examination of these new unexplored worlds. In the last two decades, a great deal has been learned about the origin and evolution of the planets and their atmospheres, using data obtained from both ground-based and spacecraft observations. In order to describe the chemistrv. of nlanetarv atmos~heres.we therefore begin with an outline of our present kkwledge and the theories which attemnt to e x ~ l a i nthese data. The theories presented here in a favorable light are not unique, and it is stronelv suegested that the reader refer to the more extensive discussions and presentations of alternatives given in the literature cited and in the references contained therein.
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--
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A Survey of Present Knowledge
The density, or thickness, of the atmosphere that a planet may possess, and the gases which will be present in that atmosphere, both depend to a large extent on the magnitude of the gravitational field of the planet and on the temperature a t the top of the atmosphere. The larger the planet. the lighter the molecules it can Drevent from escaping . . into space. The cooler the temperature a t the top of the atmosohere. the less mobable it is that gas may escape the gravitational field of the planet. Given &nilarinput~fluxes of gases from the interior, a larger planet with a cooler uppermost atmosphere will tend to accumulate a more massive atmomhere. Figure i shows the relative physical sizes of the planets. It is easy to see that the earth is by no means the most majestic of the planets in our solar system. We are a member of the smallest class of planets, which after our own example is named the "terrestrial" group and is composed of Mercury, Venus, Earth, and Mars. The group of giant planets, composed of Jupiter, Saturn, Uranus, and Neptune, is sometimes called the Jovian group, after its largest member. Over one thousand earths could easily be fit into the volume of Jupiter. This division of the planets into two groups based on size is also natural on the basis of atmo-
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Presented at the meeting of the California Association of Chemistty Teachers, California State University, Fullerton, California. 204 /
Journal of Chemical Education
0
PLUTO
NEPTUNE
Figure 1 . Sires of the planets
spheric chemistry. Only the giant planets have sufficiently large gravitational fields to prevent hydrogen from escaping their atmospheres. Listed in Table 1 are the gross atmospheric compositions of the planets as they are now known. The terrestrial planets have highly oxidized atmospheres (COz, Oz), while the giant planets have highly reduced atmospheres (Hz, CH4, NHa). This difference is primarily the result of the escape of hvdroeen from the atmos~heresof the terrestrial planets. he earth must be considered a special case among the terrestrial g r o w for two reasons: first. the earth has oceans. and seconi, it'has life. These two evolutionary develop: ments have radically altered the earth's atmosphere from what would be expected otherwise. For example, Venus is onlv . slightlv - smaller than the earth (7.700 miles in diameter versus 1,900 miles) and, therefore, has nearly the same gravitational field as does the earth. Venus is our nearest neighbor in space and has sometimes been termed the earth's sister planet. Yet the atmosphere of Venus contains very little oxygen (less than 0.001%), and is composed primarily of carbon dioxide (about 97%). The surface atmospheric pressure is close to 90 times that on the earth's snrface, and the surface temperature is about 900°F. For two planets of so nearly the same size, and located so close to each other in space, why such drastic differences? The difference between the atmospheres of earth and Venus may be due to the fact that Venus condensed as a planetary body at a position much closer to the sun, and Tabla 1. Atmospheric Com~ositianr
therefore a t a much higher temperature, than did the earth. As a result, Venus either rapidly lost its water from a hot steaming atmosphere early in its history ( I ) , or may never have contained much water in comparison with the earth (2). Without extensive oceans of liquid water on the surface, all the COz outgassed from the interior of Venus in its early history has remained in its atmosphere. If all of the COz locked up in the earth's oceanic sedimentary deposits (and presently dissolved in the oceans) were to he released into the earth's atmosphere, our planet would have an atmosphere very much like Venus'. In addition to the strong buffering of atmospheric COz by the oceans, the balance of oxygen and nitrogen in the earth's atmosphere is maintained by biological activity on the earth's surface. Nitrogen and oxygen are thermodynamically unstable in the nresence of water and should slowlv he converted to surface nitrates, hut are continually replenished by hiological action. Bacteria are mainlv for both nitrogen for- resaonsihle . mation from nitrates and nitrogen fixation from atmospheric Nz. Plants are mainly responsible for oxygen production, although photodissociation of atmospheric water vapor may also contribute to 0 2 formation. The various interactions and balances between the atmosphere, hydrosahere, and biosohere are not com~letelvcataloged as vet, bur the subject has heen exrensivei). re\.iewed h,, ~ e a d k v s ( . < I and hv .Maraulis and Lovelock (?). The atmosp6eres of the giant planets are composed mainly of hydrogen and helium, with various percentages of the hydrides of the other elements such as CH4, NH3, and HzO. Their atmospheres are extremely thick and deep, and these planets are sometimes called the gas giants since it is uncertain whether they have any solid surfaces to speak of at all. These planets are composed almost entirely of gas, becoming more dense and increasingly hotter towards the center. The composition of the atmosphere of Jupiter and indeed of the entire planet itself, is very close to what is called "cosmic composition." This is the composition of the elements as they are observed spectroscopically in the sun, in the other stars of our galaxy, and even in the stars of other galaxies. The abundance of the elements in the various stellar condensations of matter in the universe is remarkably uniform, and is given in Tahle 2. As can he seen, the universe is composed mainly of hydrogen and helium atoms, with small quantities of C, N, 0 , and other trace quantity atoms. If an assemblage of this cosmic material were formed into a planet (with no escape of material allowed) the overwhelming quantity of H atoms would lead to formation of molecular hydrogen and the hydrides of C, N, and 0 atoms. This is exactly the case observed for Jupiter; in fact, the ratios of CH4 and NH3 to Hz observed in the atmosphere of Jupiter are just what would be expected on the basis of condensation from the material with the atomic composition given in the left-hand column of Tahle 2. The difference between the atmosnheric comnosition of the giant planets and that of the terrestrial group is in the ahilitv to hold hvdroeen and in the deeree of volatile retention during the Lnitiz formation of theuplanets. Origin and Evolution of Planetary Atmospheres The Terrestrial Planets
One of the principal clues to the evolution of the earth's atmosphere is the observation that, on the earth a t least, the so-called rare gases are indeed rare. In fact, they are not rare in the universe. and as Tahle 2 shows thev are rather abundant. The earth is quite capable of retaining neon in its atmosahere. hut i t is aresent onlv in amounts l O P of what wokd be expected'on cosmic'ahundance grounds. This implies that when the earth accreted, it incorporated only relatively nonvolatile dust (with some occluded or chemically combined gas) and "left" most highly volatile components, such as rare gases, in the original cloud to he
Table 2.
Stable Volatile Comporitianr in Planetary Atmospheres volatile Retention
Cosmic Abundancer
volafile Elcaw
Table 3. Primordial Rock Gar Composition Released Above 1ZOO0 K N Chondrite
~ a r f hBasalt
~ q u nheo or^
S o l ~ b i l i t yEquilibrium Limits ~ o t a Amount3 l Releared
dissipated to space. Additional evidence of this is that the isotopic ratios in the rare gases are entirely different from the solar ratios. For example, 36Ar is far more ahundant than 'OAr in the sun, yet almost all the argon in the earth's atmosphere is 'OAr, arising from the decay of 40K in the earth's interior. These facts lead to the hypothesis that the earth's atmosphere and oceans were degassed from the solid material of the earth and that some degassing of its interior has continued to the present. However, planetary outgassing could have heen catastrophic, occurring very earlv in the historv of the earth (. 5.)..or i t might have been more gradual with almost certainly a much higher rate shortlv after the formation of the earth than a t the aresent. This &ly secondary atmosphere was then modified suhstantially by the escape of much of the hydrogen to space. This overall scenario is helieved generally applicable to all of the planets in 'the terrestrial group, whereas in the earth's case modification by biology was important as well. The composition of the early terrestrial atmosphere following planet-wide outgassing is believed to have been very reducing in nature. This evidence comes from the study of meteorites, of very old rocks, and from theoretical calculations. Tahle 3 shows the composition of gases released by the most primitive class of meteorites and by earth hasalts a t temperatures above 1200°K (6). The third column in Table 3 shows what would be expected on the basis of chemical equilibrium between the rock melt and gas at 1200°K. Hydrogen, carbon dioxide, and water are the major components of the gas released, and if this mixture is cooled unon reachine the surface of a alanet. out of contact with the hot rock, then the chemical equilibrium shifts stronelv to methane and water.. esaeciallv if the water is re" . moved by condensation to form oceans
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4H2
+ ContCHI + 2H10 1
The initial secondary atmospheres of the terrestrial group planets were, therefore, probably quite reducing in nature, consisting primarily of HZ, CH4, H20, some CO and COz. Oxidation of methane in the atmosphere by thermal processes, solar photolysis, and other energetic mechanisms was then favored by escape of the resulting hydrogen into space. CHI + 2H20 + COz + 4H2 t The length of time the planet was able t o retain its initially reducing atmosphere then depends on the relative rates for outgassing of Hz, and of escape of Hz from the atmosphere. Venus, because of its high condensation temperature so near to the sun., mav have condensed from materi-nossiblv . a1 containing much less of the hydrated silicates than the material from which the earth condensed (2). ConsequentVolume 53.Number 4, April 1976 / 205
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ly, Venus would never have contained much water in comparison with the earth. Alternatively, or a t the same time, the higher temperature at Venus may have resulted in water remainine" in the atmos~here.rather than formine primordial oceans. Several workers ( I ) have shown that in this case, Venus would have developed a "runaway greenhouse" effect to raise the surface temperature to a very hieh level. followed bv. . photodissociation of water to hvdrogen and okygen, loss of hydrogen to space, and loss of oxygen hy oxidation of the hot surface rocks. In either of these scenarios, Venus did not initially have nor develop much of a reducing atmosphere. The earth, in contrast to Venus, retained considerable quantities of hydrated silicate minerals, did not develop a runaway greenhouse effect, and therefore probably expelled considerable quantities of water and hydrogen into the atmosphere in its early infancy. This outgassing process formed extensive oceans and a methane-rich, reducing a t mosphere. This earlv atmosphere could have lasted from 10 nt escaped to to 1b00 million years b e f ~ r ~ s u f f i c i ehydrogen turn the atmosphere into an oxidized one. Large quantities of atmospheric carbon dioxide were dissolved in the oceans and deposited as carbonate sediments. The appearance of biota in the oceans and on the land would s e n strongly modify the remaining gases in the earth's atmosphere. Initially, such hiota may have subsisted on the raw organic materials formed in the reducing atmosphere and oceans. This would accelerate the oxidation of the atmosphere and (as methane was incontrovertably oxidized into COz) photosynthesis must have then developed to allow further evolution of biota. As animal life appeared (subsisting on the Oz waste product of the photosynthetic mechanism in plants), the 02/C02 balance was slowly established and the nitrogen content in the atmosphere regulated by bacteria. The chemistry and mineralogy of some earth sediments suggests the atmosphere was much less oxidized as recently as 2.7 hilliou years ago. Mars also condensed at a sufficientlv cool temperature to cmtain l n r p qutlnriti~suf hydrated minerals, and probaI,lv nlw exnelled canriderahle uuanriries of water and hvdrigen i n t i its primordial atmodphere. In the case of ~ a r s , however, the low zravitational field allows for rapid escape of hydrogen; hence, any initially reducing atmosphere was probably converted to an oxidizing one in 10,000-100,000 ;ears (6). Mars has very little watkr in its atmosphere at present, enough to produce an ocean only about three thousandths of a centimeter deep if condensed on the surface. However, because of the thin C02 atmosphere (surface pressure about 11150 that of the earth) and the low temperature at Mars, this quantity of water in the atmosphere corresponds to very high humidity near the saturation point. Mars is not a desert planet, but a tropical one! There is evidence that considerable water mav he nresent adsorbed in the soil, as subsurface permafrost, and in the residual summer wlar cam . (7). . . The maior comDonent of the extensive winter polar caps is COz frozen out from the atmosphere. Liquid water cannot exist on Mars a t present because of the low atmospheric pressure and temperature. However, surface features resembling ancient river beds have been observed on Mars which may indicate that Mars did have some quantity of liquid water on its surface at least a t one time during its history. However, alternative causes of these landforms have also been put forth. The question "Did life ever originate on Mars?", therefore, critically rests on whether Mars was able to hold its reducing atmosphere for a sufficient length of time and whether any significantly large bodies of liquid water ever existed on the surface during its early history.
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The Jovian Planets
The origin and evolution of the atmospheres of the Jovian planets, at least as we now understand them, are 206 / Journal of Chemical Education
somewhat easier to describe than those of the terrestrial group. Essentially, it is believed that these planets have retained all of the volatile material from which thev condensed. Not even atomic hydrogen can escape the atmospheres of Jupiter, Saturn. Uranus, or Neptune in eeoloeically significant times. As a consequence,the atmospheres of these planets have remained essentially as they were a t their origin, highly reducing and containing very large quantities of molecular hydrogen. Very little (if any) of the original volatile material in the vicinities of these planets was able to escape during planetary condensation, and their atmospheres prohahly closely reflect cosmic ahundances. The mean densities of the planets Jupiter and Saturn suggest that they are made almost entirely of cosmic ahundance material, whereas the smaller planets in the Jovian group (Uranus and Neptune) have densities which suggest that either the heavier gases such as helium, methane, etc., are enhanced relative to hydrogen, or that these planets may have relatively larger terrestrial-like rocky cores than do Jupiter and Saturn. Whether or not Jupiter and Saturn have small rocky cores, or cores of mainly metallic hydrogen with the heavier elements dispersed throughout, is an issue not yet settled. The main differences noted in the relative ahundances of the gases in the atmospheres of the Jovian planets may he attributable to temperature differences. In addition to hydrogen, both methane and ammonia have been detected in the atmospheres of Jupiter and Saturn. Small quantities of Hz0, CzHz and CzHs have also been detected recently at Jupiter. At Jupiter, the relative ahundance ratios of these gases appear to he consistent with the CIH and N/H cosmic abundance ratios. For Uranus and Neptune, no ammonia has been detected. This is most likely due to the very low atmospheric temperatures of Uranus and Neptune where the NH3 is condensed in the atmosphere to form clouds, thus removing it from spectroscopic view. Chemical Processes in Planetary Atmospheres The most visible evidence of chemical and physical processes in planetary atmospheres is cloud formation. Even in the thin atmosphere of Mars, clouds of varvine - " tvoes .. have been observed similar to those common and uncommon in the earth's atmosphere. On Mars, clouds of various comoosition have been ohserved which are composed of dust particles, water vapor condensations, or C0z dry ice particles ( 8 ) .On any given Martian day, the extent of cloud coverage over the Martian globe is quite small compared to that of Earth. On Venus, however, the planet is entirely shrouded by 100% cloud cover all the time, and it is not possible to see down to the surface. The spectrum and pola>ization of sunlight reflected from these clouds are highly suggestive of a concentrated (75-90%) solution of sulfuric acid f91! . . In-fact, HCI and HF have both been detected in the~venusian atmosphere in less than part per million quantities above the cloud tops. The atmospheres of the Jovian planets are also completely enshrouded in clouds. For Jupiter, Saturn, and Uranus, banded structure is observed in visible light, the hands running parallel to the equatorial rotation of the planet. At Jupiter and Saturn, these hands range in color from white through yellow to deep red in color. The bands are different shades of green a t Uranus, and Neptune exhibits a bright green color also. In the latter cases, the green color is most prohahly due to methane absorptions. For Jupiter and Saturn, a variety of chromophores have been proposed, including solutions of metals in ammonia, colored organic compounds, polymerized HCN, polymeric sulfur compounds, and red phosphorus. The coloring agents have yet to be identified (10). Figure 2 gives the gas-liquid P I T stability diagrams of the cosmic composition materials likely to he present in the atmospheres of the Jovian planets (11). Superimposed are
URANUS ---Ar NEPTUNE
SATURN NH4SH
----__ --
200
,.,
-
JUPITER
100
H20----
HZO ICE
2 0
--
---
300
e 60
100 c ,, , "
500
,t
aooC 0.1
0.01
10
1 6
7
10'
m T A L PRESSURE, bar%
Figure 2. Pressure-temperature diagram for the simple hydrides (dashed lines). Superimposed are pressure-temperature curves (solid lines) for model atmospheres of the outer planets. Clouds are expected where the dashed and solid lines cross (from Lewis (2)).
model pressure-temperature profiles for the atmospheres of the Jovian planets, temperature and pressure both decreasing with increasing altitude. In these models, various cloud types are expected to he formed at various altitudes corres~ondineto particular noints on the PIT diaeram. withthe iowest cloud level, the first cloud deck is expected to he com~osedof an NHn-HIO solution. followed ~ for ~ u b i t e and r b i ~ ice, ~ NH~SH, 0 and, finally, N H ice Saturn. The location and cloud mass versus height of these clouds in a cosmic composition Jupiter-model atmosphere are shown in Figure 3 (11). For Uranus, CH4 clouds are possible a t upper levels in the atmosphere, and for Neptune even argon rare gas clouds may he possible. One of the most intriguing aspects of atmospheric chemistry concerning the Jovian planets is the question of organic synthesis and evolution in these "cosmic" reducing atmospheres. This aspect was opened by the classic experiment of Stanley Miller (121, who subjected a mixture of methane, ammonia, and water to a discharge. As a result of the discharge, a red tar-like substance was deposited on the sides of the reaction vessel which, on hydrolysis, revealed significant quantities of amino acids, the hasic building blocks of organic life. The initial gas-phase products were shown to be HCN and aldehydes. Many similar experiments using various irradiation sources, including high energy electrons,. y-rays. . . , X-rays. nhotons from the xuv to the visible, various types of elekrical discharges, and thermal heating- have been renorted since the orieinal Miller-Urev experiment. In general, the gas phase experiments produce organic precursors such as HCN, formaldehyde and aldehydes;a-amino nitriles and both saturated i n d unsaturated hydrocarbons (13). In solution, these precursors apparently react further to produce amino acids, adenine, guanine, sugars, even polypeptides, and nucleosides and nucleotides if certain phosphorus compounds are present (13). This same process may very likely be taking place now in the atmospheres of the Jovian planets, Jupiter and Saturn in particular because of the strong resemblance of the colors observed in the handed clouds with those ohserved in the lahoratory experiments. All the necessary materials are present in the atmos~heresof these nlanets. While oceans may not he present, there is probably an extensive water cloud laver in the atmos~heresof these plan. the temperature and ets. There are levels i ~ i 3)~ where pressure are very congenial for organic (and perhaps biological!) evolution. The sources of energy for chemical reactions leading to the synthesis of urganic material in the atmospheres of the outer planets are several: (1)solar ultraviolet radiation in the upper levels of the atmosphere, (2) electrical discharges
Figure 3. Claud layers in the atmosphere of Jupiter based on a solar-composition m d e l (hom Lewis (2)).
in the extensive cloud layers a t lower levels in the atmosphere, (3) cosmic ray bombardment, and (4) thermal energy at warm levels in the lower atmosphere. At high levels in the atmosphere, high-energy, short-wavelength solar uv light is absorbed to produce ions. For example: hu+CH4-CHzt+H+e-
These ions and electrons can react further to produce new species, such as the process shown below for synthesis of HCN CH3+
+ NH3
CH2NH2+
+ e-
-
CHzNHz++ Hz
+
HCN + Hp H
At lower levels in the atmosphere, longer wavelength uv light is ahsorhed to produce free radicals. For example
-
hu + CH4 CH2 + H2 These radicals may also undergo further reactions. For example
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CHz + CH4 [C*Hs*] + HP
[CzHfi*]
C2Hfi
+ Hpi
where the stabilizing collision is with the major component of the atmosphere-molecular hydrogen. At still lower heights, longer wavelength light in the visihle region of the spectrum is reflected fairly readily from cloud layers, whereas infrared light from the sun can excite vibrational and rotational motions of the molecules to heat up the atmosphere. In addition to solar-photun-initiated reactions, these same species may be produced, and the same reactions may occur, in electrical discharges in the cloudy regions of the atmosphere or by cosmic-ray-induced ionization and dissociation processes. A great deal of work is presently being carried out in the laboratory to elucidate the individual chemical processes and reactions involved in the evolution of organic compounds in planetary atmospheres. in comets. and in interstellar ipaci, Sume pruyri-si has brtn made, and a numbrr oi r e n r t i o n s haw been identified whirh lead to the s\mthesis of simple urganic compounds from the atoms and molecules in primordial gases of cosmic composition. This work points to the inevitability of the synthesis of organic compounds in all of these situations and the pervasiveness of organic chemistry throughout the universe. This evidence directs us even further toward the conclusion that organic chemical evolution is not limited to our own castaway planet a t the edge of one galaxy, but that it is a common occurrence in the universe and that the possibility of life elsewhere in the universe is extremely high indeed. Literature Cited I11 Rxerampie. EPe Waker. J. C.G . J . Alm. Sri,32.1248 (1975). 121 Lewis. J . S . . A n n . Re" Phy. Chem 24,339 119731. (2) ~ e a d o w s , ~ inner. . Spoee sci., 21,1467 (19731.
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Volume 53,Number 4, April 1976 / 207
(4) Msrgulis, L. and Lovdoek. J. E..l c m w 21.471 119741. 15) Fana1e.F. P.. Chem. G m l , 8,79(1971). ( 6 ) Fans1e.F. P., Icorus. 15.279 11971). 17) Fana1e.F. P.andCannon, W.A.,J. Ceophys.Rss.. 79.3397 (1974). (81 Hsrtmenn, W. K., and Raper, 0.."The New Mars: National Aeronfis and Spa= Administration. Washington. D.C.. 1974.
208 / Journal of Chemical Education
(9) Young,A.T.. J. ALm Sci,32.1125 (1975). (101 Fora review see Wolfe,J. H.,Sei.Amen. 233.119(1975).
In1 Lewis, J. S.,Space Sci R e u . 11.401 (1973). 112) Mil1er.S. L.,Science,117.528(1953).
113) Punnamperurna. C.,andGabol, N.W..Spacr L i f e S ~ i . 1.64 , 11968).
