by Ben Patrusky

Conditions in the early atmosphere set the stage for life on earth. All that is needed now is to know what those conditions were.

In 1952 Stanley Miller, a young gradu­ate student at the University of Chi­cago, devised an experiment that

would gain him instant and lasting fame. Using a model provided by his mentor, Nobel Prize winner Harold C. Urey, Miller, who is now at the Univer­sity of California at San Diego, mixed molecular hydrogen, methane , am­monia, and water vapor in a flask. He bombarded the gases with continuous electrical discharges and at the end of a week found he had produced a rich ar­ray of organic substances, many inti­mately involved in life processes.

Other investigators repeated the ex-periments with similar results, and the idea took hold that the earth's early, pre-biotic atmosphere must have been strongly reducing, made up largely of hydrogen and hydrogen-rich gases. Such a primordial atmosphere, by the Urey-Miller construct, would have grown out of captured remnants of the solar nebula, the interstellar cloud of gas and dust that gave birth to the solar system some 4.5 billion years ago.

In recent years, though, researchers have become much less certain about this idea, and an alternative view of the earth's early atmosphere has emerged. Many specialists now believe that the prebiotic atmosphere was far less hydro­gen-rich, far less reducing. Its chief con­stituents, they suggest, were carbon dioxide rather than methane, molecular nitrogen rather than ammonia, plus water vapor and small amounts of free hydrogen. In all, this would have been an atmosphere rather like today's, with the exception of free oxygen, which was produced in abundance only after the advent of photosynthesis . This new view presupposes that the hydrogen-poor, only mildly reducing early at­mosphere was a by-product of events occurring subsequent to the earth's for­mation and owed its origin principally to the release of gases trapped in the planet's interior.

Strongly reducing

The Urey-Miller model devolved from speculations first made in the 1920s by a Russian scientist, A. I. Oparin, and sub­sequently by J. B. S. Haldane of Great

Britain. According to what has come to be called the Oparin-Haldane hypoth­esis, living organisms arose naturally on the primitive earth through a lengthy process of chemical evolution that began in the early atmosphere.

In this process energy sources, in­cluding lightning, shock waves pro­duced by thunder, meteorites and comets, and ultraviolet radiation from the young sun, provided the energy that transformed simple atmospheric molecules into more complex organic compounds. These compounds then rained out into ponds, shallow seas, and oceans. There, through chemical re­actions In watery solutions, they even­tually became the precursors of pro­teins, nucleic acids, sugars, and other molecules vital to life.

The prerequisite to all this activity, however, was presumed to be a strongly reducing atmosphere, one rich in hy­drogen and hydrogen compounds and devoid of free, or uncombined, oxygen. Had oxygen been present, the organic molecules would have been destroyed by oxidation.

Harold Urey provided a powerful the­oretical framework, a model for the for­mation of the earth that provided a natural way to generate the conditions for genesis laid down by Oparin. Urey described this model in 1952 in The Plan­ets, Their Origin and Development.

According to Urey, planetesimals (meteorite-sized bodies) accreted slowly out of the solar nebula. The at­mosphere, a product of that accretion, contained ammonia, methane, hydro­gen, water, and other components . These were present in their cosmic abundances, the proportions In which they are found In Interstellar space. Urey suggested that the atmosphere of Jupiter, which spectrographic analysis reveals to be laden with hydrogen, am­monia, and methane, is made up of the very stuff that had condensed to form the solar system. Stanley Miller's 1952 simulation experiments seemed to provide convincing evidence that just such a Jovian atmosphere had once en­folded the earth.

Not everyone agreed. In 1955 William W. Rubey, a geochemlst at the U.S. Geo­

logical Survey, argued that the putative existence of a dense, strongly reducing early atmosphere was not borne out in any way by the geologic record. Calling on that record, he argued that the at­mosphere had accumulated over time as a result of outgassing from the earth's interior. He saw no evidence for the ex­istence of large amounts of hydrogen gas, methane, or ammonia on the early earth. He proposed Instead that life originated in an atmosphere resembling that of present-day earth, though with­out molecular oxygen.

Geology's argument

In 1962 Helnrlch D. Holland, then a geochemist at Princeton University and now at Harvard University, attempted to reconcile these contrasting views. At the time he was convinced by Urey's theory about accretion, but he could not altogether ignore Rubey's arguments about the evidence, or lack of it, in the oceans and rocks. Holland developed a two-stage theory that allowed for the ex­istence of two consecutive environ­ments, one the product of accretion, the other the product of outgassing.

Urey's hypothesis had the earth form­ing cold and the planetesimals that built it accreting slowly and adding no exces­sive heat to the growing planet. Holland proposed that dur ing this stage the earth's core had not yet formed and me­tallic iron was distributed more or less homogeneously throughout the emerg­ing planetary body. This iron, he ar­gued, served as a reducing agent, stripping oxygen from the water vapor percolating from the earth's Interior.

As a result of this interaction, Holland offered, iron oxide was left behind while free hydrogen was released to the at­mosphere. Similarly some fraction of volcanically outgassed carbon dioxide was reduced to carbon monoxide. Once out, the gases, primarily water vapor, hydrogen, nitrogen, carbon monoxide, and carbon dioxide, reacted to produce a methane-rich atmosphere containing an abundance of ammonia.

During the second stage, Holland suggested, long-lived radioactive ele­ments in the earth's Interior gradually heated the planet. The heat caused the

MOSAIC 11

iron to melt, migrate to the center, and form the core. With most of the metallic iron removed from the earth's upper layers, the gaseous output to the at­mosphere eventually became less and less reducing.

Mildly reducing

Within the last decade, Urey's idea of an initially cold earth has fallen into dis­favor among earth scientists. The con­sensus, supported by lunar observa­tions and recent theoretical studies related to accretion, is that the earth formed in a matter of tens of millions of years rather than a billion or more. Fur­thermore, it formed hot, as a result of torrential planetesimal bombardment. Moreover, as David Stevenson of the

Patrusky is a frequent contributor to Mosaic.

California Institute of Technology ex­plains, it now looks as though the iron core formed at the time of accretion. Un­der these circumstances it is possible that the gases released from the earth were mildly reducing from the start.

That would not necessarily mean the earth never had a strongly reducing at­mosphere. There is always the pos­sibility that such an atmosphere existed at the earth's outset, as a result of accre­tion in the presence of a dense con­centration of gas from the solar nebula. The likelihood, however, is that such a primary atmosphere would not have lasted very long. The hydrogen would have escaped the earth's gravitational pull and drifted into space, and the other light gases making up the at­mosphere would have been blown off rather quickly by the powerful ultra­violet flux from the sun. Alternatively,

certain theoretical considerations sug­gest, the planet might have accreted after the solar nebula gas had already disappeared. In that event the earth would never have had a significant, nebula-derived atmosphere.

Reconstruction ists

Perhaps the most compelling argu­ments against a strongly reducing early atmosphere come from the theoretical modeling done by atmospheric chem­ists. These investigators are actually rel­ative newcomers to prebiotic research. Their entrance into this arena was prompted at least initially by a need to test their understanding of the present-day atmosphere.

As James C. G. Walker, an at­mospheric chemist at the University of Michigan, says, "Our theoretical models fit the present atmosphere because we fitted them to the present atmosphere. There may be errors in the models, er­rors we are unable to detect. The only way to be sure is to test the models un­der changed conditions in order to see if they produce reliable predictions under these circumstances as well. The early atmosphere of the earth offers one such testing ground. If our models are right, then theoretically we should be able to reconstruct the evolution of the at­mosphere from its origin to the present and beyond."

The approach taken by the atmo­spheric chemists is essentially unifor-mitarian. It assumes that the early atmosphere had much the same com­position (with the exception of free oxy­gen) and essentially the same sources and sinks as the atmosphere does today.

Early in their involvement, several at­mospheric scientists put their models to work to assess the probable fate of methane and ammonia if those gases had been present in the prebiotic at­mosphere at levels like today's. The cal­culations led to the conclusion that both gases would have been quickly de­stroyed photochemically. In one series of computer simulations done in 1979, for example, William R. Kuhn and S. K. Atreya of the University of Michigan showed that solar ultraviolet would have broken atmospheric ammonia down to nitrogen and hydrogen In un­der 40 years.

In 1980 Joel S. Levine at the National Aeronautics and Space Administration's Langley Research Center took matters one step further: Recognizing that am-

12 MOSAIC

monia is highly water soluble, Levine sought to investigate the effect of rain-out on ammonia's atmospheric lifetime by comparing calculations to current measurements. He learned that am­monia would have washed out of the atmosphere in all of about ten days.

In subsequent studies Levine deter­mined that methane resided in the at­mosphere for no more than about 50 years. Today both methane and am­monia are constantly being added to the atmosphere by biological processes. On the prebiotic earth, no significant sources of replenishment were at hand. In the absence of such sources, the in­ventory of these strongly reducing agents central to the Urey-Miller hy­pothesis would have been depleted in a geological blink.

Temperature effects

These calculations had important bearing on the so-called cool-sun prob­lem, a conundrum posed in 1972 by Carl Sagan and George Mullen of Cornell University and others. They pointed out that like all stars, the sun has grown brighter and hotter as it has aged; today the sun is perhaps 30 percent more lu­minous than It was at Its birth.

As a consequence, if the atmosphere had much the same composition as It does today, the surface temperature of the early earth would have been consid­erably colder than it is now. It would have been too cold to sustain liquid water on the surface of the planet. The geologic record, however, shows run­ning water throughout the earth's re­vealed history, all the way back to the earliest rock samples, the 3.8-billion-year-old Isua deposits In Greenland. (See 'Tectonics in an Archean Earth7' in this Mosaic.)

Sagan and Mullen presumed that a greenhouse effect had come into play and spared the earth an icy fate. They thought water vapor in the atmosphere might have provided some, but not all, of the prerequisite Insulation, with something else required to stem the loss of sun-derived heat to space. Sagan and Mullen proposed atmospheric am­monia. Together, water and ammonia would have absorbed heat radiating from the earth's surface, and would have redirected it back in sufficient quantities to keep the planet from freez­ing. Subsequent computer-modeling studies, however, have shown ammonia to be far too unstable, too short-lived, to have served any such function.

In 1979 Tobias Owen and Robert Cess at the State University of New York at Stony Brook, in collaboration with Veerabhadran Ramanathan at the Na­tional Center for Atmospheric Research in Boulder, proposed an alternative heat trap: carbon dioxide. They showed that the greenhouse effect of carbon dioxide and water together was great enough to solve the cool-sun problem.

Carbon dioxide concentration In the early atmosphere, however, would have to have been up to 1,000 times today's value. Was such a condition likely? There is no reason not to think so, says James Walker, noting that carbon diox­ide is second only to water in abun­dance among the gases Issuing from volcanoes today. According to an esti­mate made In 1978 by Harvard's Heinrich Holland, the present-day car­bon dioxide emissions from volcanoes are great enough to double the amount of carbon in the atmosphere and oceans in roughly 400,000 years. That not all of the gas remains In the atmosphere pre­vents the runaway greenhouse effect that may have befallen Venus.

In 1981 Walker, In conjunction with Paul B. Hays of Michigan and James F.

Kasting, who was then at the National Center for Atmospheric Research and is now at NASA's Ames Research Center, developed a hypothesis to explain a greater abundance of carbon dioxide in the early atmosphere. They also pro­posed ways for atmospheric levels to have dwindled as the sun grew warmer. In their model the earth's surface was never much hotter than It Is today.

Walker, Hays, and Kasting proposed that the earth's atmospheric budget for carbon dioxide, the balance between supply and removal, is temperature de­pendent. The scavenging process in­volves the reaction of carbon dioxide with silicate minerals to produce carbo­nate rocks; with rising temperatures, the rate of carbon consumption by sili­cate weathering would have increased.

Walker says there Is both experimen­tal and observational evidence to lend support to the hypothesis. Laboratory studies, for example, have shown that carbon dioxide reacts more quickly with feldspars as temperature rises. Field in­vestigations have determined that trop­ical soils today are typically more chemically weathered, more deficient In all but those minerals most resistant to carbon dioxide attack, than the soils at higher, cooler latitudes. At the other ex­treme, glacial deposits have been found to contain many of the minerals most susceptible to chemical weathering.

Walker's idea about the removal of carbon dioxide by weathering depends on the presence of significant amounts of exposed landmass. Other scientists, however, believe the primitive earth was almost entirely covered by water.

Simpler compounds

As evidence has accumulated to sup­port the case for a mildly reducing paleoatmosphere, there has grown with it concern as to whether such an at­mosphere could have served as the cru­cible for the synthesis of chemical compounds essential to the ultimate evolution of life. There was reason to think so. In 1966, for example, Philip Abelson of the Geophysical Laboratory of the Carnegie Institution of Wash­ington, and later long-time editor of Sci­ence, succeeded in producing amino acids from mixtures of nitrogen gas, hy­drogen, and carbon monoxide.

In carbon monoxide, carbon is more reduced, less oxidized, than it is in car­bon dioxide. Abelson selected it for geophysical and geochemical reasons

MOSAIC 13

Free oxygen

Before photosynthesis the earth's only significant source of free oxygen was photodissociation, the break­down of water vapor by ultraviolet radiation. By any reck­oning the molecular oxygen content of the earth's early atmosphere was negligible. Negligible, though, does not necessarily mean nil. Estimates of the amount vary con­siderably, and therein lies a tale riddled with paradox.

Most investigators concerned with the makeup of the prebiotic atmosphere and the origin of life belong to what could be called the 02 minimalist school. They hold to the view that chemical reactions in the atmosphere gave rise to the complex molecules vital to the evolution of life. They point out that no one has yet succeeded in producing these molecules in anything but a reducing environment, an atmosphere with a surfeit of hydrogen atoms and a deficiency in molecular oxygen. Moreover, free oxygen destroys any organic molecules in the making.

Nonetheless many forms of life depend on a layer of atmospheric ozone, O3, to shield them from the sun's oth­erwise lethal ultraviolet radiation. However, there can be no ozone without precursor molecular oxygen. Therein lies the paradox: Life may have had to unfold under what would appear to be circumstances requiring both the pres­ence and absence of molecular oxygen. Scientists sub­scribing to a reducing paleoatmosphere, whether strong or mild, are looking for ozone surrogates.

Kenneth M. Towe, a geologist-cum-paleobiologist at the Smithsonian Institution, thinks they may be on the wrong track, "hitched to the wrong paradigm." Towe posits the existence of an oxidizing paleoatmosphere, one rich enough in free oxygen to produce an ozone screen.

Such an atmosphere, however, would preclude the earthly formation of organics. Instead it would demand delivery of these requisite life-forming compounds by ex­traterrestrial messengers, such as carbonaceous chon­drites, comets, and other organic-laden meteorites forged out of the solar nebula late in the planetary accretion stage.

Towe says his view evolved as a result of his participa­tion in the creation of a new museum exhibit on the histo­ry of life. In reading extensively in the scientific literature, he found himself unable to weave a continuous story line from chemical evolution to biological evolution. As he sees it, scientists trying to define the nature the paleoen-vironment may have gotten their priorities mixed up "in that they concerned themselves with making organic mol­ecules without worrying about their destruction."

Two sides The body of evidence, which admittedly is speculative,

extrapolative, and subject to a range of Interpretations, seems to cut both ways. The theoretical calculations of the levels of oxygen in the prebiological paleoatmosphere have ranged from 25 percent to one-billionth of one per­cent of the levels of oxygen found In the present-day at­mosphere. According to a study in 1980 by James F. Kasting, then at the National Center for Atmospheric Re­

search and now at NASA's Ames Research Center, and by Thomas M. Donahue of the University of Michigan, there would have been enough ozone to provide a biological shield when oxygen reached one-tenth the present level.

The first serious at tempt to ascertain oxygen con­centrations In the early atmosphere was made in 1965 by Lloyd V. Berkner of the Southwest Center for Advanced Studies In Dallas and Lauriston C. Marshall of Southern Illinois University at Carbondale. They calculated the amount of molecular oxygen resulting from the pho­tolysis, or ultraviolet dissociation, of water vapor at rates assumed for the paleoatmosphere. They came up with a figure of 10^3, or one-thousandth, of the present-day lev­els. Four years later Robert T. Brinkmann at the California Institute of Technology raised that level to one-quarter the present concentrations.

A better understanding

In the late 1970s a theoretical assessment by James C. G. Walker of the University of Michigan, based on a new, more detailed understanding of atmospheric physics and chemistry, invalidated these earlier studies. The inves­tigators, Walker pointed out, had neglected to consider all the oxygenic sinks, the various ways in which molecular oxygen could be lost from the atmosphere. Berkner and Marshall had failed to realize, for example, that not all the hydrogen from water molecules escapes Into space, that the rapidity with which the oxygen and hydrogen recom-bine proves to be significantly greater than the rate at which the hydrogen escapes.

Nor did the Investigators factor in the loss of free oxy­gen through reaction with hydrogen and carbon monox­ide gases released during volcanic action. When Walker recalculated oxygen levels, he came up with con­centrations equivalent to less than one ten-billionth of the current levels. A more detailed evaluation by Kasting and his colleagues produced very nearly the same result.

In 1981 J. H. Carver, a researcher at the Australian Na­tional University in Canberra, took Issue with the Walker-Kasting estimates. In a paper in Nature, Carver argued on the basis both of theoretical considerations and measure­ments of ancient rock samples that paleoatmospheric tem­peratures were higher than had previously been sup­posed. Because water vapor concentration increases with temperature, he suggested that previous investigators might have significantly underestimated the amount of atmospheric water vapor. In his assessment the levels of prebiological oxygen approach one-tenth the present lev­els, enough to produce an effective ozone screen.

A year later Joel S. Levine of NASA's Langley Research Center in collaboration with Vittorio Canuto, an astro­physicist at NASA's Goddard Institute for Space Studies, used his computer model to investigate the effect of great­ly intensified ultraviolet radiation upon the composition of the paleoatmosphere.

The Impetus for this work came from data collected by Catherine L. Imhoff, an astronomer at NASA's Goddard

14 MOSAIC

Space Flight Center, from instruments aboard the Interna­tional Ultraviolet Explorer satellite. Imhoff, whose interest lay in the evolution of stars, had set out to study seven young stellar objects, known as T-Tauri stars, in the con­stellations Taurus and Orion, among others. Her data sug­gested that these stars, which ranged in age from 100,000 years to under 10 million years, were radiating ultraviolet light at intensities as much as 10,000 times those of today's sun. Levine and Canuto realized that the sun, in its youth, had probably behaved in much the same way, so that at about the time the earth formed, the ultraviolet radiation bombarding the planet may have been 100 to 1,000 times as intense as it is today.

Levlne's computations indicated that these larger ultra­violet fluxes could have increased the oxygen levels in the paleoatmosphere as much as a millionfold. These calcula­tions, however, were predicated on his assumption that the ancient atmosphere was only mildly reducing. In ab­solute terms, therefore, the newly revised estimate, even with all this extra ultraviolet reckoned in, did not amount to much of an increase in molecular oxygen. Levine put oxygen levels at one-billionth the present-day con­centrations, not nearly enough to create an adequate ozone screen.

Both those scientists favoring a reducing paleoat­mosphere and those favoring an oxidizing paleoat­mosphere invoke some of the same geologic samples to support their position. Case in point: the banded-iron for­mations seen in the earliest known rocks, the 3.8-billion-year-old Isua deposits in Greenland. To Kenneth To we these iron residues, a product of oxidation, hint at the existence of a reasonable supply of atmospheric oxygen. Other explanations have been proffered, however. One is that oxygenic photosynthesis had evolved by Isua time, and that photosynthesis rather than photodissociation was the source of the oxygen that oxidized the Iron In these ancient formations.

Another explanation, first suggested by Heinrich Hol­land of Harvard University, was that hydrogen peroxide, a strongly oxidizing agent, may have accounted for some of this early oxidation. Computer-modeling studies by James Kasting seem to bear this out. Kasting found that a mildly reducing atmosphere, rich in carbon dioxide, could have produced a fair amount of hydrogen peroxide, together with several other potent oxidants.

Sunscreen

For those scientists convinced that the paleoatmosphere was a reducing one, the problem remains: How could the evolution of organic substances, let alone life itself, have come about In the absence of an ozone screen? If life was forming near the surface of the oceans, how might It have been protected from Instantaneous destruction by the sun's powerful and unshielded ultraviolet radiation? Levine believes some earliest forms might have found habitable niches deep In the water, well away from the lethal reach of sunlight.

Those forms closer to the surface may have been kept out of harm's way by ocean-surface turbidity, a suggestion first made by Lynn Margulis and Mitchell B. Rambler of Boston University. They thought that nitrate dissolved in water could have served to scatter the ultraviolet. This action would have created a protective umbrella for organ­isms and organic compounds just a few feet below the surface. Margulis and Rambler abandoned the Idea when it appeared that an abundance of molecular oxygen was required to produce the nitrates.

Levine has now resurrected the Margulis-Rambler pro­posal, following lightning-simulation studies conducted In 1982 that showed that nitrates in significant amount could be produced in a mildly reducing, oxygen-deficient atmosphere composed of nitrogen, carbon dioxide, and water vapor.

Levine thinks the atmosphere Itself might have also provided sufficient protection by way of an ozone surro­gate, an ultraviolet-absorbing haze akin to smog. The haze, Levine suggests, could have been produced by for­maldehyde. This possibility was introduced by Julian Heicklen, a chemist at Pennsylvania State University, who first explained the photochemical origins of present-day smog. According to Heicklen, three molecules of for­maldehyde could combine under certain conditions to form a particle with radiation-blocking characteristics like those of ozone. Levine Is now planning to enter these conditions into his computer model to see if a haze layer of formaldehyde materializes on the computer printout.

Moist atmosphere

Kenneth Towe does not question any of the Levine, Walker, or Kasting calculations, but he does question their assumptions. Towe believes they have been much too par­simonious with the water content of their paleoat-mospheres. In their models, outgassed water vapor condensed quickly to form the oceans. Towe sees matters differently. He conjures an early earth surrounded by a moist greenhouse laden with carbon dioxide and an abun­dance of water vapor. In Towe's construct, no oceans as such existed at first, and the area of land exposed to the early atmosphere exceeded that covered by liquid water. Weathering of silicate rocks, therefore, could proceed rapidly to remove carbon dioxide, thereby preventing a runaway greenhouse effect.

The moist atmosphere provided the necessary condi­tions for the photolytic production of oxygen and the de­velopment of the critical ozone screen. As Towe says, "DNA and ozone have much the same ultraviolet absorp­tion profile. It might be pure coincidence, but I don't think so. I think It's Darwinian, an adaptive phenomenon. I think DNA originated with ozone in place." Towe under­stands that his model atmosphere would not have allowed for the production of life-essential organic compounds. The necessary raw material, he says, could have been de­livered ready-made by meteorites and comets rich In re­duced substances. •

MOSAIC 15

Greenhouse effect. The effect on surface temperature of raising carbon dioxide levels by a factor of 1,000 today (solid curve, left scale) and at the earth's beginning (dashed curve, right scale). The left temperature scale starts where the right one leaves off. Prebiotic earth, this model says, would have been frozen over if today's concentration of carbon dioxide were present in the air; but at 1,000 times the concentration, temperatures would nearly match today's temperatures despite the 30 percent fainter, young sun.

that had also led him to argue against Urey's residual, nebula-derived at­mosphere. Abelson's yield of organlcs, however, was significantly smaller than the lush fallout seen in simulations of strongly reducing atmospheres.

Currently many scientists do not be­lieve that it was necessary for the paleo-atmosphere to rain out organic com­pounds as complex as amino acids for life to have begun. Perhaps more impor­tant, they say, is the potential for such atmospheres to produce two relatively simple inorganic molecules: formalde­hyde and hydrogen cyanide. Formalde­hyde consists of hydrogen, carbon, and oxygen; hydrogen cyanide consists of hydrogen, carbon, and nitrogen. Rained out into bodies of standing water, molecules of formaldehyde and hydrogen cyanide could have acted as the building blocks of amino acids, nu­cleic acids, sugars, and other fundamen­tal constituents of living matter.

Production of formaldehyde does not seem to pose major problems. In 1980 Akiva Bar-Nun of Tel Aviv University and Sherwood Chang of NASA's Ames Research Center performed experi­ments in a chamber containing the Abelson mix: water, carbon monoxide, and nitrogen. The addition of spark en­ergy produced significant quantities of formaldehyde, along with a rich profu­sion of other simple organic molecules, including methane.

That same year, 1980, Joseph Pinto of the Goddard Institute for Space Studies in New York and Randall Gladstone and Yuk Yung of the California Institute of Technology put their computer model to work to calculate .the photochemical yield of formaldehyde to be expected from an even more mildly reducing at­mosphere. They found that an at­mosphere composed of present-day amounts of Abelson's candidate gases would indeed produce formaldehyde, although in very small amounts . However, had the early atmosphere been much richer In carbon dioxide, as suggested by the researchers seeking to resolve the cool-sun problem, then In just 10 million years formaldehyde con­centrations could have risen to levels sufficient for the synthesis of complex organic molecules to begin.

Rainout

It is a far different story for hydrogen cyanide, however. This compound Is difficult to synthesize In the absence of

16 MOSAIC

ammonia. Atomic nitrogen must be available to form hydrogen cyanide, but the dominant form of nitrogen in the atmosphere is N2—molecular, or gas­eous, ni trogen—and the molecular bonds between that molecule's nitrogen atoms are not easily broken.

According to NASA's Joel Levine, the solar radiation required to break nitro­gen molecules apart is available only in the upper regions of the atmosphere. Hydrogen cyanide formed at such high altitudes, however, would be destroyed before it reached the planetary surface.

Low-altitude energy sources were suggested, one of them being lightning. However, a modeling study undertaken in 1981 by Michigan's Walker, in collab­oration with W. L. Chameides of the Georgia Institute of Technology, re­vealed that lightning would spark the formation of only paltry amounts of the compound. They concluded that the rate at which hydrogen cyanide would be produced by lightning would have been too low to provide a reservoir ade­quate for the manufacture of organic compounds in the volumes required by evolution-of-life models.

If the problem of generating a suffi­cient quantity of hydrogen cyanide per­sists, scientists who favor a mildly reducing prebiotic environment may ul­timately have to shift their thinking rad­ically. They may be forced to abandon the premise that atmospheric chemistry and rainout were central to the man­ufacture of the ingredients essential to early biological evolution. This would sever their last remaining link to the Oparin-Haldane hypothesis.

Alternatives have been proposed. One holds that the necessary chemistry went on at the ocean bottom, near hot vents through which the earth belched forth its interior gases. Recently Har­mon Craig of the Scripps Institute of Oceanography uncovered some evi­dence to support such an alternative. Craig found methane and ammonia being produced near one of the black smokers, or vents, in the mid-ocean range known as the East Pacific Rise. The gases appear as products of the re­action of cold seawater, containing car­bon dioxide and nitrogen, with basalt at the vent temperature of 350 degrees Cel­sius. (See "Vulcan's Chimneys," by Hen­ry Lansford, Mosaic, Volume 12, Number 2.)

Another proposed alternative is that carbonaceous chondri tes , meteorites

rich in organic compounds, brought to earth the ensemble of substances critical to the origin of life.

Frozen solutions

Harvard's Heinrich Holland suggests that a worldwide hydrogen cyanide def­icit need not strip the earth's at­mosphere of its focal role in the origin of life. As he explains, the compound's production rate has been assessed in terms of global concentrations, the quantities required for the oceans as a whole. On this scale the calculated yields prove to be far below the required levels. Nevertheless, this fact does not rule out localized microenvironments in which the supply was more than ade­quate for chemical evolution to proceed. One such promising locale consists of glaciers, as proposed in 1983 by Alan W. Schwartz and Matty Goverde of the Uni­versity of Nijmegen in the Netherlands.

Schwartz and Goverde began with the realization that because of the dif­ferences in the ways formaldehyde and hydrogen cyanide are made, local dif­ferences in their production rates would be likely. Formaldehyde, for instance, can be produced through the action of either electrical discharge or sunlight, while hydrogen cyanide would seem to need electrical storm areas.

In any region of mixed production, however, the compounds upon pre­cipitation would combine, through a re­action measured in 1973 by Stanley Miller, to produce an agent called glycolonitrile. Schwartz and Goverde added the observation that hydrogen cyanide in glycolonitrile behaves quite differently than does free hydrogen cyanide: It oligomerizes into more com­plex compounds far more readily. More­over, oligomerization is enhanced when glycolonitrile is in frozen solution.

Schwartz thinks hailstones may have served to promote such activity because their melting and evaporation could have generated high concentrations of the products of hydrogen cyanide on the earth's surface. He thinks of glaciers, though, as providing "an even more in­teresting" environment. "It is likely," he says, "that the early earth was charac­terized by a near-global ocean, with the first land system being formed by emer­ging volcanic islands." The surface tem­perature, he suggests, might have been zero Celsius or lower.

These conditions, says Schwartz, would have resulted in partial glaciation

of many such islands "leading to a uniquely favorable environment for chemical evolution; mechanisms for the synthesis, concentration and further re­actions of centrally important bio­chemical precursors would have all been present." Such a dynamic locale "could have driven chemical syntheses that would have been impossible in di­lute, homogeneous solution."

Support for a contrary view notwith­standing, there are scientists who con­tinue to support the notion of a strongly reducing atmosphere. Stanley Miller is in the forefront. He accepts what is writ­ten in the oldest known rocks: There are carbonates in those rocks indicating an atmosphere containing carbon dioxide. Moreover they show no evidence of having being coated with what has been called a primordial oil slick. This term, coined in 1971 by Holland and Antonio C. Lasaga (then at Princeton), describes the organic compounds, the hydrocar­bon tars, that would have rained out of the atmosphere were it strongly reduc­ing. None of this may seem relevant to Miller, who speaks of the Archean rocks from the Isua in sequence Greenland. "The rocks are 3.8 billion years old," he says. "The earth is 4.6 billion years old, and 3.8-billion-year-old rocks cannot tell us what happened between 4.6 and 3.8 billion years ago. Moreover, the rock samples are actually few in number and hardly enough, it seems, to make global judgments."

As for the theoretical arguments about the earth's accretionary and out-gassing history, Miller says they are "tenuous, inconclusive and open to con­siderable interpretation and debate." He points out additionally that the produc­tion of organic substances from a strongly reducing paleoatmosphere is so rapid, so easy, and so copious, that such an atmosphere need not have ex­isted for very long to provide the com­pounds on which life rests. Scientists favoring the mildly reducing at­mosphere do not dismiss Miller's assess­ment. As James Walker says, "At this point, I'm not prepared to say you can't have a highly reducing atmosphere be­fore the rock record." Which leaves the true story of the earth's prebiotic at­mosphere still up In the air. •

The National Science Foundation contributes to the support of research discussed in this article through programs in the earth and atmospheric sciences.

MOSAIC 17