Philip Morrison, Chairman
Massachusetts Institute of Technology
Kitt Peak National Observatory
A. G. W. Cameron
Center for Astrophysics
Harold P. Klein
NASA Ames Research Center
U.S. Geological Survey
University of Houston
NASA Ames Research Center
The panel was introduced by its Chairman, Philip Morrison. He proposed that there should be an opportunity for members of the audience to question the panelists but that, first, the panelists should have the opportunity to make a few individual remarks.
Harold Klein opened by saying that he was a sessile invertebrate. From this viewpoint he had been impressed by the extent of our ignorance, and thought that we should expect the unexpected. He then gave two examples to illustrate this degree of ignorance. In his first example, he recalled that once, when he was young, he had asked Fritz Lipman why, in the proteins of all living things, the amino acids were all L (left-handed) as opposed to D. Lipman answered that he had thought about this for a long time and  believed that L-acid forms of life had "defeated" D-acid forms in a tremendous struggle for survival between competing metabolisms during the first 2 billion years of life's Earthly history. What we see today is but the remnant of a great complexity of organisms no longer extant. Thus the apparent simplicity of this remnant may obscure the existence of many alternate evolutionary paths that might have been viable, given other conditions. In Klein's second example, he recalled that a few years ago a Space Science Board Panel had assessed the probability of life on Mars and designed the life-detection instrumentation for Viking. They decided that if the Viking biology instruments gave positive results, but the gas-chromatography/mass-spectroscopy (GCMS) results were negative, it would mean that there had been an instrumental failure. It was inconceivable, at the time, that the biology experiments could give positive results in the absence of sufficient numbers of living organisms for the GCMS to detect the presence of complex organic molecules. This combination of negative and positive results did in fact occur, but it was not due to instrument failure. Rather it was caused by the existence of strange and unexpected surface chemistry. We should draw a conclusion from these examples: as scientists, we are often successful at explaining what has been observed, but we are notoriously poor at anticipating the existence of phenomena before empirical evidence has been discovered.
James Walker was next to speak. He said that ignorance notwithstanding, we are making progress. He proposed to illustrate this by talking about atmospheric modeling for the purpose of explaining the densities of trace constituents. The great virtue of theoretical models, he claimed, is that they are essentially logical devices which show you that particular results must inexorably follow once specific, well-defined assumptions have been made. If the results of a given model are seen to be in conflict with observation, one then knows with certainty that at least one of the model's assumptions is wrong and must be relaxed. This approach, applied to the environment of primitive Earth, results in significant insights into physical processes affecting biogenesis and could eventually guide us to an understanding of how environment and environmental change affect the evolution of metabolic processes. Walker proposed to illustrate this with three examples.
In the first place, he said, consider the sort of calculation Kasting talked about. Here, if one specifies the abundances of nitrogen, carbon dioxide, and water, as well as the temperature structure, one can calculate precisely the oxygen concentration in the atmosphere. If one is concerned with the origin of life, this claim of precision is not contradicted by the fact that there may be several orders of magnitude uncertainty in the oxygen concentrations. From a biological viewpoint, it does not matter whether one has 105 or 1010 molecules of O2 per cubic centimeter; there is effectively no oxygen in that atmosphere. If, as Towe maintains, there must have been O2 in the  atmosphere before the origin of life, Kasting's calculations tell you what assumptions must be relaxed. One must change either the water vapor mixing ratio in the stratosphere or the rate of release of hydrogen by volcanoes. Thus one has placed constraints on the physical environment.
For his second example, Walker took the possible role of lightning as an energy source for early chemical evolution. He noted that the study of the production of trace atmospheric constituents by lightning has received significant attention in the past few years. It is straightforward to take any assumed composition for a primitive atmosphere and calculate the rate of synthesis of organic compounds in that atmosphere by lightning, provided one also assumes a lightning rate. These assumptions are clearly defined: the lightning rate and the chemical composition. When one considers a wide range of possible compositions, one arrives at the conclusion that lightning, at its presently observed rate, was not adequate to produce significantly large abiotic synthesis of organic compounds. This implies either that there was a lot more lightning in the primitive atmosphere or that lightning was not responsible for the synthesis of organics. And we can conclude this with certainty.
As his third example, Walker said he would like to pose an unsolved problem -the problem of methane consumption by primitive organisms. In the primitive anaerobic ecosystems of early Earth, photosynthetic organisms produced organic material that was, in turn, consumed by fermenting organisms. But the latter must have produced methane. There are no anaerobic organisms that oxidize methane and, in fact, it appears that methane cannot be oxidized or consumed by organisms in an anaerobic environment. Thus the problem is to find a nonbiological process that cycles methane by converting it into consumable form. One can attack this problem by modeling. One can make various sets of assumptions, develop a model based on each, and rule out those that conflict with empirical data. Such modeling has the potential for showing that certain environmental characteristics (here the physical conditions that made possible the consumption of methane) must have been closely linked to the early evolution of life.
Walker was followed by James Pollack. Pollack argued that the important environmental questions concerning the origin and evolution of life are not restricted to the nature of physical processes in the atmosphere of early Earth, but also include details of the evolution of the Sun and even of the evolution of the Universe. In particular, consideration of the processes of star and planetary-system formation and of stellar evolution is essential to determining the size of the continuously habitable zone (ecozone), in which temperatures on a planet are neither too high nor too low for living organisms to thrive. It is not clear whether the situation on Earth is the norm or a fortuitous and unlikely happenstance, One problem is that we cannot yet  define the temperature range outside of which life cannot exist. Nevertheless, Venus and Mars today seem clearly to lie outside the ecozone. However, early in the history of Mars, conditions were sufficiently clement for there to be liquid water on the surface, and this period may have lasted for a long time. Moreover, although evidence one way or the other is presently lacking, Venus may have experienced a temperate climate during the early history of the Solar System, when the Sun's luminosity is believed to have been some tens of percent lower than at present.
Thus the question that must really be addressed is-what is necessary for a planet to sustain clement conditions over at least part of its surface for a period on the order of 5 billion years? It is possible that Mars may have experienced a self-destructive situation in which liquid water dissolved carbon dioxide out of the atmosphere and deposited it in rocks, disastrously reducing the greenhouse effect. The same process may have been balanced fortuitously on Earth by the rising solar luminosity since, due to its initially higher temperature, Earth may not have been as vulnerable as Mars to a lessened greenhouse effect. A second possibility is that the evolution of the Sun may have been different than what we believe because of changes in the physical constants due to cosmological evolution. Yet another factor is the mass of the planet, which affects the length of time over which the lithosphere remains thin enough so that atmospheric gases that are lost to rocks (e.g., CO2 to carbonates) can be eventually recycled due to subduction and metamorphism. Earth is massive enough so that this condition is met, while the opposite is true for Mars. He closed by emphasizing that input from a wide variety of disciplines is crucial if we are ever to answer the question of the size of the ecozone, as well as many other questions concerning the origin and evolution of life.
Al Cameron followed by saying he would like to try to make some people angry by describing, in two lessons, how one becomes a SETI enthusiast. First of all, he summarized the previous two speakers by saying that on the one hand it is very easy to calculate everything about a complicated system provided you know all the rate constants and what assumptions to make, while on the other hand the history of the Solar System is really terribly complicated. This leads to the two lessons: First, since you must estimate the likelihood that extraterrestrial life exists, you write down a string -of probabilities-which, since you are an enthusiast, you have set equal to one -and then you multiply them all together. Second, you assume that all those guys out there that are smarter than you are will maintain a beacon specifically designed to be easy for you to detect. Cameron suggested that this notion about beacons may be a form of cargo cultism. He concluded that a strategy based on eavesdropping may be the only viable approach to SETI , although it may be more difficult since it would allow less specific assumptions about transmitter characteristics.
 Morrison asked if anybody had been provoked.
Walker replied that he had been provoked. He said that we do know all the rate coefficients.
Morrison then asked whether Walker's models didn't also depend on the nature of the radiation field, that is, on whether one has a blackbody spectrum at ultraviolet or x-ray frequencies. Walker replied that yes, one must say what the radiation spectrum is, but, given that, we can do the calculations. Morrison asked about particles. Walker said they were not likely to be important. Morrison replied that that was the usual attitude of theorists. Where this kind of argument goes wrong is that the solution to a particular problem often turns out not to be a matter of choosing between inputs you know about; what hurts you is the input you didn't know was there. It may do little good to make alternate assumptions to see what conclusions they lead to, when one is unaware of factors that render invalid the rationale for choosing those assumptions in the first place. Walker saw Morrison's point, but maintained that his approach was nonetheless useful. He suggested that one can gradually establish constraints on a situation. By eliminating various possibilities, one can tell if one is heading toward a satisfactory solution.
J. William Schopf said he would like to make one observation, and this speaks to the strength of space exploration and the sort of things NASA has done. There is only one court of last resort for scientists-reality. With respect to the origin and evolution of life, reality is ascertained by looking at the rock record. NASA has provided data on the rock record, and the rock record has supplied us with constraints. For example, the rock record has limestones 3.8 billion years old. This implies that there must have been carbon dioxide in the atmosphere. It used to be claimed that there wasn't any carbon dioxide that long ago, but that claim is no longer tenable. Moreover, there must have been liquid water 3.8 billion years ago because rocks of that age have detrital pebbles in them.
Harold Masursky spoke next. He noted that there are some things that can be tested immediately. For example, Mars has very red soils, worse even than those of eastern Brazil which are highly anorganic. But there is also a great canyon on Mars with 7 km of rocks, and we can see that these are of various kinds: light, dark, etc. The opportunity is there to look at the Grand Canyon of Mars and examine the record of past Martian environments. In particular, we can hope to find rocks dating from the eras in which the channels were formed. Much could be learned should this prove to be the case. We already know from crater counts that volcanoes have been active at epochs in early, middle, and recent Martian history, and that the (H2O?) channels have also been produced throughout these periods. Studies of samples from the canyon could allow channel episodes to be correlated with the past climate of Mars, since channel episodes could be dated radiologically from volcanic deposits above and below them. We might find times when the  Martian soil was not anorganic. Moreover, Martian chronology could be compared to Earth 's. Among other things this could tell us how long liquid water must be present before substantial organic developments take place. Other solar-system bodies also offer opportunity for study. Venus has a great plateau 3 to 5 km high. This, too, must give us a rock record and could show whether the Venusian climate was more clement early in its history. Io has real gases and hot springs; these could be favorable environments for life. Lightning in Jupiter's atmosphere could allow us to study Miller-Urey processes there. If we follow Schopf's advice and seek to ascertain reality through data, the Solar System offers us an enormous abundance of it.
The next panel member to speak was John Oro. He said that there are three major obstacles to be surmounted in our quest toward the origin of life. In the first place, we know nothing about the first 750 million years of Earth's history, and it is essential that we obtain this knowledge, although it is not clear how we can do it. In the second place, we do not know anything about other planetary systems, not even if any exist. NASA has an obligation to make special efforts to detect other planetary systems. Third, regardless of terrestrial history, the fundamental principles of organic life are based on the chemistry of carbon, as well as that of hydrogen, oxygen, nitrogen, sulfur, and phosphorus (the six organogenic elements). We have not yet been able to produce a self-replicating system comparable (but not necessarily identical) to the living organisms of today. This question must be addressed primarily by organic chemists and biochemists. Oro then said he thought there ought to be some questions from the audience.
Kenneth Towe noted that certain meteorites (carbonaceous chondrites) contain amino acids and other organic compounds. He asked to what extent seeding by organic materials in the solar nebula or from space instead of synthesis of organics in Earth's atmosphere could have been responsible for the origin of life on Earth.
Oro replied that there are at least three phases in the formation of organic molecules: a cosmic phase, involving formation in stars and the interstellar medium; a solar nebula phase; and a phase of formation on primitive Earth. Only the last phase really matters, provided the organogenic elements were available on early Earth and conditions were favorable for them to react. If those were the circumstances, then whether additional organic compounds arrived on Earth due to meteorite bombardment is irrelevant; to this extent the answer to Towe's question is negative. Four essential ingredients are required for life: (1) a membrane of some sort to surround the organic material, (2) an informational or replicative material, (3) a catalytic molecule, and (4) a molecule that can translate from informational molecules to catalytic molecules. Once you have these four things you have life; meteorites per se do not give you these.
 Morrison asked if Sherwood Chang had anything to add. Chang said that one can take Walker's approach and make assumptions about how many meteorites fell into Earth during the relevant period of Earth's history. Then one can estimate how many of these meteorites would have been carbonaceous, how many would have survived passage to the surface, how efficiently could the amino acids be extracted from a meteorite, and how large were the bodies of water into which the meteorites fell. If one does all these things, he gets an answer. Chang said he would not give exact numbers, but if the estimates required are very conservative, the result is a 10-6 to 10-7 molar solution. This is very dilute, although it is comparable to what many believe would result from a Miller-Urey type of synthesis. Therefore, the important question may not be how you get organic material, but how you concentrate it once it has been produced and transformed from simple molecules to complex ones. He thought that Oro was correct. It does not matter if organic material was brought in by meteorites; they should be regarded as just one of many sources.
Towe disagreed. He said that it is important to distinguish among sources of organic materials. This is because the atmosphere is supposed to have contained no oxygen since organics could not have been synthesized had oxygen been present. But if organic matter originated in meteorites, then there is no need to postulate an oxygen-free atmosphere. If the atmosphere contained oxygen, an ozone shield could have permitted nucleic acids to form since ozone absorbs ultraviolet radiation at the same frequencies as nucleic acids.
Oro said that Whipple had recently calculated that 1025 to 1026 gm of carbon-containing material from comets or comet-like bodies could have accumulated on Earth. Apparently this assumed a dense atmosphere to aid capture. Oro's own work indicated that 1022 to 1023 gm was the appropriate number. He noted that one of the main Apollo results was that 3.9 to 4.1 billion years ago there was a big peak in meteorite impacts; however, it has been suggested that this was not really a peak at all, but just the tail end of many more collisions of loose small bodies left over from the formation of the Solar System, because remains of previous impacts would have been Obliterated by the final showers of 3.9 to 4.1 billion years ago. This implies that 1022 to 1023 gm of carbon-containing material could well have been acquired by Earth. The present biosphere contains only 1018 gm, so even an amount two orders of magnitude smaller would have been sufficient to account for life.
Another member of the audience said he thought we faced a different dilemma. Irrespective of the origin of organic molecules, he thought there as an unresolved time constraint. Earth was formed 4.5 billion years ago, but the earliest fossil life dates from 3.0 to 3.5 billion years ago. This implies  that it took only about 1 billion years to evolve primitive bacteria from simple organic molecules. He felt that this tremendous quantum leap, in so short a time, was not understood.
Oro replied that 1 billion years was a more than ample amount of time He noted that Gustav Arrhenius had said earlier in the day that Earth could have cooled in as little as 10 million years. Beyond this, Oro stated that chemical reactions are easy; they take only minutes to produce something as complex as a self-replicating molecule. Granted, this is still a long way from having an E. coli, but given the speed of chemical processes, there appears to be no difficulty in understanding the observed time scale.
Schopf noted that George Gaylord Simpson had said in his 1949 book that the oldest trilobites were 500 million years old (this turns out not to be correct), and that these must have been preceded by protozoans, back to 1 billion years. Simpson thought that this scenario made good sense because it seemed it should be a larger step from the origin of life to an amoeba-like organism than from an amoeba to anything subsequent. Schopf said that such statements seemed to be nonstatements and nonquestions. Rather evolution proceeds at some rate and we do not yet understand what determines that rate. Biologists do not appreciate the magnitude of geologic time. Seven hundred million years, the time from which the environment became clement to the first fossil record, is a long time -longer than the time from before the first trilobite to man! It is long enough for lots of things to happen. Since we do not understand what determines the rate of evolution, there is no basis at all for saying there is a problem just because bacteria are complex. That complexity could have been produced rapidly; moreover, there was a tremendous amount of time available.
David Usher asked if he could add one point. He said that his favorite example of the time that has been available for evolution is the Grand Canyon. If you just stand on the edge and look down and reflect that it formed in 3 to 5 million years, you acquire some perspective. The Grand Canyon is no small accomplishment, but evolution has had a thousand times longer to achieve its result.
A member (unknown) of the audience responded that he was not challenging this dogma. Rather he merely meant that, given the complexity of the simplest microbial organism, it could be a mistake merely to assume that lots of time was available; evolution of that degree of complexity is not necessarily something that happens easily or quickly. Moreover, he wondered why we did not see a variety of trials and errors in the record. He felt that there was indeed a potential problem with trying to fit everything, from that primeval solar nebula to the appearance of simple organisms, into the time available. He said that it remains to be seen whether we can explain this.
Oro replied that they had not meant to imply that the evolution o early life need not be explained in detail, but only that they thought then  was a good detailed explanation, and that the processes involved were not terribly difficult to set in motion. He said that the audience member was simply expressing our lack of knowledge, but that Usher was right to emphasize that quite a long period of time was available. Oro said that what he would really like would be for Al Cameron or Gustav Arrhenius to say when Earth first became cool enough for processes leading to life to take place.
Karl Turekian asked whether Helmut Abt had something to say about stellar phenomena that might be relevant to the discussion.
Abt said that a key datum is the fraction of stars that have planets. It is clear that a very large fraction (between 60 and 100%) have companions of some sort. If one considers the stars of a given spectral class (e.g., that of the Sun, class G), about 18% have companions approximately the same mass. Another 14% have companions approximately half their mass, 10% have companions 1/4 their mass, 7% have companions 1/8 their mass, and 5% have companions 1/16 their mass. But there is where the observations stop, because we cannot yet detect companions less massive than about 1/16 the mass of the primary. Therefore, all we can say is that approximately 54% of stars have companions with masses greater than 1/16 the mass of the primary. Now it so happens that 1/16 the mass of the Sun () is the mass below which nuclear burning stops, although bodies more massive than 0.01 (10 times the mass of Jupiter) can radiate at stellar luminosities for up to a billion years as a result of energy released by gravitational contraction. If we extrapolate the observed mass distribution, we conclude that 78% of stars have companions more massive than 0.01. However, we do not really know whether this distribution continues. If we really want to tread on thin ice, we can push the extrapolation to planetary masses and conclude that at least some stars must have companions of planetary mass. Abt felt uneasy about extrapolating so far into a region for which we have no data.
Indeed, Abt felt that there were reasons to believe that the observed mass distribution did not extend to masses much smaller than the presently observed limit of 1/16 . And even if it did, he continued, the existence of companions of planetary mass would not imply the existence of solar systems. Rather there are fundamental differences between known multiple star systems and our Solar System, and these raise the question of whether there are two distinct star system formation mechanisms, one leading to stars with close companions, the masses of which are described by the observed distribution, while the other results in solar systems in some degree similar to our own. He noted that Cameron had long been proposing that the Solar System was created from a disk around a single star with relatively low angular momentum, whereas protostars with significantly higher angular momenta undergo fission to give rise to double- or multiple-star systems. The latter mechanism could account for the formation of all close pairs of stars. Abt said that he had in mind two fundamental differences between our Solar  System and multiple-star systems. In the first place, all solar-system objects lie in a plane, but multiple-star systems are spherical (i.e., in systems of three or more stars there is no correlation between the planes of the individual orbits). Second, the orbital periods of solar-system objects obey a Bode's law relationship, such that the periods get longer by a factor of approximately 1.5 as one goes to each more distant planet. In multiple-star systems, though, the factors are much larger. Indeed there is no known triple-star system in which the longer period is significantly less than 10 times greater than the shorter period, and sometimes the difference is as much as a factor of 100.
Robert Harrington said he agreed with most of Abt's remarks, but not with the idea that there are only two classes of systems of primary star plus companions. He said that the double systems Abt was talking about were the very close, spectroscopic binaries. These may well have been formed by a bifurcation mechanism, but there are also wide binaries with separations on the order of tens to hundreds of astronomical units. In this third class of star system, the possibility exists that each of the individual stars might be accompanied by a planetary system. Given such large separations, the systems would be stable once formed. But there remained a question concerning whether planets could form in wide binaries in the first place. Harrington said that Heppenheimer had been arguing that they could not. Did that argument have any validity?
Cameron responded that he disagreed with most of what everybody had said all day about how planets are made. However, he emphasized that it doesn't really matter; we just do not understand. We cannot estimate formation times or cooling times, even to within orders of magnitude. His own belief was that planet formation occurs via massive gravitational instabilities in primitive protostellar nebulae; these instabilities he called "giant gaseous protoplanets." If they are formed in the inner Solar System, they cannot grow very large before the growing central star tidally strips away their envelopes. In the outer Solar System, though, they can contract, lose only a small amount of mass through a planetary wind mechanism, form cores, and become giant planets. The open question (and obvious evidence points to an answer) is whether the giant gaseous protoplanets in the inner Solar System manage to precipitate condensed matter into a central core before they get tidally stripped. If they can, then the cores can become 80% of the mass of terrestrial planets. By this mechanism it may be possible to make the bulk of terrestrial planets in a very short time (105 yr), with a final sweep-up of the debris remaining in space adding a final layer, perhaps over a period of several hundred million years. The early Solar System was probably very complicated, and any other planetary system is likely to have been equally complicated and quite different from our own. Some spectacular discoveries may await us; we probably have no real appreciation yet for what is out there-for the complexity of nature.
 Cameron then said he thought attention should be focused on one more question: To what degree does the evolution of life respond to environmental change? He thought it clear that evolution, at least to some degree, is driven by environmental change. But what happens if environmental change is speeded up? Does the rate of evolution also speed up, or does it slow down? There may be some rate of environmental change that drives evolution fastest. Cameron thought it very important to try to determine this optimum rate of environmental change and suggested that Earth might turn out to be an example of it.
Abt said he wished to say something more about double stars, in response to Harrington. Harrington was indeed correct to point out the distinction between close and wide binaries, but he did not go far enough. There can be all kinds of separations, and in none of these systems are companions of planetary mass precluded. There could be distant planets orbiting both stars of a close system, and there could be nearby planets orbiting one or more of the individual stars of a wide system. However, Abt again emphasized that having one or two companions of planetary mass is not the same as having a solar system, and it is possible that one of these two possibilities will turn out to be the rule, and the other the exception. He then noted that a popular astrophysical topic of recent years has been accretion disks, particularly in close binaries. Theoretical studies have indicated that disks tend to be thin; moreover, infrared observations have discovered disks around stars. Is it possible, he asked, that solar systems originate due to the occurrence of disk phenomena?
Cameron responded that some of the disks probably behave much as one would expect, and that planetary objects probably form from many of these disks. He pointed out that a large number of the observed disks are associated with massive stars that probably do not live long enough for life to evolve.
Cameron then returned to Harrington's question concerning planetary formation in multiple-star systems. He said there was a distinction to be made between an object like Jupiter and the members of observed multiplestar systems, which are all of stellar mass (0.05 ). The latter tend to be essentially of solar composition, but Jupiter, in bulk, is not. To the best of Our knowledge, Jupiter is enriched in heavier elements, most of which are probably condensed into a core. Moreover, there is a difference between the character of Jupiter's orbit and that of a typical binary companion. AIthough the latter might have a semimajor axis of 5 AU (the same as Jupiter's), its orbit would likely be highly eccentric. The orbits of Jupiter and the other planets are nearly circular. In a triple-star system, the orbits do not even share the same plane; in the Solar System they do. Jupiter shows evidence of a formation more complicated than just a gravitational condensation of solar material. It is very possible, therefore, that planets are made by  a different mechanism than binary companions in eccentric orbits. We do not understand these mechanisms. It follows that it is not possible to say whether planets do or do not form in multiple-star systems. But it is clearly dangerous to put much confidence in the extrapolation of the observed distribution of the masses of companions that Abt discussed.
Oro asked whether there was any estimate of the probable abundance of wandering planets, that is, small, nonluminous objects without a companion star. These might generate enough energy for life. Are they plausible?
Morrison said it was a very speculative proposition since we do not understand how stars form. Of course, there could be all sorts of ejections, and therefore there must be at least a limited number of dark wanderers. If they exist, this would support the notion that life occurs elsewhere, but not very strongly. ,
Morrison said that time had run out, and the discussion must be ended. It seemed quite clear to him that we are dealing in matters about which we know only a little, and that we must make great efforts to narrow the enormous range of possibilities, both by theory and by experiment. We see a landscape through mist and fog, but the features are real. We see a structure, but we cannot yet tell whether it is two separate towers or only one, joined at the base. This is the job for the next generation.