Does this carbonaceous earthy material truly contain humus or a trace of other organic compounds? Does this possibly give a hint concerning the presence of organic structures in other planetary bodies?
—Jöns Jakob Berzelius, 1799–1842, Swedish chemist, referring to his analysis of the Alais meteorite, which fell in France in 1802
From Annalen der Physikalischen Chemie 33 (1834)
In biogeochemistry we have to consider that life (living organisms) really exists not on our planet alone, not only in the Earth’s biosphere. It seems to me that this has been established beyond a doubt, so far, for all the so-called terrestrial planets, i.e., for Venus, Earth and Mars. At the Biogeochemical Laboratory of the Academy of Sciences in Moscow … we identified cosmic life as a matter for current scientific study already in 1940. This work was halted due to the war, and will be resumed at the earliest opportunity.
—Vladimir Ivanovich Vernadsky, 1863–1945, Russian mineralogist
From American Scientist 33 (1945)
“But did anyone really expect to find anything?” I ask Geoff, as he shows me the canister that had contained his sample of moon dust from the 1969 Apollo 11 mission. “Well, no,” he replied, “we didn’t think there’d ever been life on the moon. But we didn’t know. We thought there might be organic compounds.”
And why not? People had been finding organic compounds in meteorites for more than a century, and no one was quite sure where they’d come from or how they’d formed. In 1834, the Swedish chemist Jöns Jakob Berzelius noted the high carbon content of a meteorite that had fallen in southern France a couple of decades earlier. Meteor showers in Europe were described as early as 1492, and their extraterrestrial provenance had been documented in 1803, when the distinguished French physicist Jean-Baptiste Biot featured among the scores of citizens who witnessed the stones falling from the sky above the village of l’Alsace. But the source of the carbon compounds Berzelius and others found in meteorites would remain controversial far into the next century.
Another carbonaceous meteorite fell in Hungary in 1857, and the eminent chemist Frederick Wöhler—Berzelius’s student, and the first to show that one could create carbon compounds like those made by organisms from inorganic substances in the lab—found organic compounds that he was convinced were of extraterrestrial biological origin. A decade later, Marcellin Berthelot found what he called “petroleum-like hydrocarbons” in a meteorite that had fallen near Orgueil, France, in 1864. He postulated that the hydrocarbons had formed abiotically from reaction of metal carbides with water, but in the next few years there was a spate of meteorite treatises in which the fossils of an astounding assortment of exotic extraterrestrial creatures were described in minute detail. Louis Pasteur had just presented his famous experiment showing that a protected, sterile medium remained devoid of life ad infinitum and debunked the popular theory that life could burst spontaneously into being from nonliving matter, but now the debate shifted to the possibility that life on Earth had originated with live cells or spores delivered by meteorites from space. Finally, in an attempt to put the debate to rest, Pasteur applied his meticulous analytical techniques to the Orgueil meteorite and found it to be indisputably sterile and incapable of generating life.
Nearly a century later, three well-respected chemists, including Warren Meinschein, stirred things up again. Using the newly developed techniques of gas chromatography and mass spectrometry, they analyzed samples of the Orgueil meteorite from a natural history museum in France, where they had been kept in a sealed jar since 1864. At a meeting of the New York National Academy of Sciences in 1961, the group reported finding distributions of hydrocarbons like those in terrestrial sediments, which they took as evidence of biological activity on the meteorite’s parent asteroid or planet. Microbiologists then weighed in with reports of “organized elements” that resembled algal microfossils and even more assertive claims that they had evidence of life’s existence in regions of the universe beyond Earth. But the work was immediately challenged and ridiculed, and it soon emerged that the hydrocarbons had come from pieces of coal, the “organized elements” were ragweed pollen and rush seeds, and the meteorite had been broken apart and glued back together. Apparently, back in the 1860s, when the debate about spontaneous generation and extraterrestrial life was raging in the scientific and popular press, someone had added “evidence” to the meteorite, never imagining that the sample would be sealed in a jar and the hoax would play out a hundred years in the future.
Hoax notwithstanding, the 1961 endeavor inspired a rekindling of interest in carbonaceous meteorites, whose chemical composition turned out to be radically different from that of the majority of meteorites. But even analyses of virgin pieces of the Orgueil and other meteorites were inconclusive: though they confirmed nineteenth-century chemists’ findings of hydrocarbons, interpretation was still plagued by an inability to distinguish earthly contamination, albeit accidental, from extraterrestrial organic material.
And what about the moon? In 1969 chemists had their first chance to examine extraterrestrial rocks that hadn’t been lying around on Earth for decades or even years, but were, rather, brought here under meticulously controlled conditions of cleanliness. If organic compounds were generated abiotically in meteorites or on the early earth, then couldn’t they have been generated on the moon as well? Or perhaps they’d formed in space, or on other planets, or in asteroids, and then been delivered to the moon by meteorites. One might even speculate that the molecular precursors of life on Earth had arrived in a similar fashion.… Maybe the moon contained traces of abiotic organic compounds like the ones Melvin Calvin had hoped to find preserved in the earth’s oldest rocks. Calvin, of course, was among the first to speculate about such possibilities. Others were more skeptical.
Keith Kvenvolden says that the only scientists who really thought they’d find much in the way of organic compounds on the moon in 1969 were astronomers. They were, it seems, thinking rather simplistically from a chemical standpoint: you’ve got hydrogen, carbon, and energy, so you ought to have hydrocarbons. “They thought the moon was paved in asphalt,” he says, and then adds, with a grin, “Rational people, like geologists, were skeptical.” Spectral analyses of the lunar surface from previous unmanned Apollo missions indicated that it contained little or no carbon. Nonetheless, Kvenvolden, a rational geologist by training, was lured away from his comfortable job at Mobil Oil to work in NASA’s Ames Research Center in California. “There was one chance in history to work on the first lunar samples,” he says, “and I just couldn’t miss it. We could do any experiment and it was world news.”
NASA orchestrated the distribution and reporting of results from the first lunar samples with Hollywood theatricality and suspense. Despite the scientific consensus that the moon was unlikely to harbor life, NASA designed the Lunar Receiving Laboratory, or LRL, in Houston with the express purpose of quarantining the astronauts and samples. One young chemist from Al Burlingame’s Berkeley lab had just read Michael Crichton’s novel The Andromeda Strain, and when he got to Houston to help prepare the LRL and saw the elaborate measures NASA was taking to protect the earth from dangerous extraterrestrial microbes, he got so scared that he abandoned his job and left Houston before Apollo 11 landed. As Kvenvolden points out, the LRL was hooked into the city’s main sewage system, so it’s hard to gauge if it was all just part of the show, or if it was a serious, and genuinely inept, precaution. “With NASA, there was always this undercurrent about the possibility of extraterrestrial life,” Kvenvolden says. “It attracted public interest, and support for the program.”
Cindy Lee, who was a graduate student in Jeff Bada’s then-nascent amino acid laboratory at the Scripps Institution of Oceanography, says it was an exciting time to be a student. Big, brash questions about the chemistry, origin, and extent of life in the solar system were on the table—and that was in large part due to NASA’s funding and support. It was no easy task to sustain public interest and funding for such basic scientific questions, so perhaps NASA’s theatrics can be forgiven in the name of science. From some of the stories the Apollo 11 researchers tell, it would appear that NASA organizers themselves had been reading Crichton’s novel. The principal investigators from each proposal were required to travel to Houston and retrieve their samples in person, and NASA gave them such dire warnings about thieves that they resorted to all sorts of James Bondish antics to transport them home. Lee recalls a Scripps party where a slightly inebriated Caltech physicist, who was en route from picking up his ration of moon, rose from his seat and, with great solemnity, dropped his pants to show how he’d taped the tin of lunar dust to his leg so it couldn’t be stolen.
Flouting the usual rules of scientific discourse and openness, NASA swore all the scientists to secrecy about their results until January 5, 1970, when they were to gather in Houston for the first Lunar Science Conference and report them simultaneously. They were forbidden not only from talking to the press, but also from talking to each other. “We got our samples last,” Geoff says, “so we had a late start and had to work fast. The hardest part was getting the aluminum sample canister open! They’d sealed it so well we had to use a lathe to get the bloody thing open.” By this time, Geoff and James Maxwell had moved to the University of Bristol and set up their Organic Geochemistry Unit, where the meticulous, ultraclean, and sensitive techniques they’d refined for the analysis of ancient rocks were standard procedure. “We were looking for any identifiable organic compounds—amino acids, porphyrins, biomarker hydrocarbons.… Any signs that there’d been life on the moon at some time.”
“And?” I ask, eager to get to the punch line.
“And then there was the Lunar Science Conference in Houston. Science had exclusive rights to all the papers, so Phil Abelson was presiding. It was hair-raising, because we didn’t know what the others had found out. We were worried we’d report we couldn’t find anything, and then someone else would get up and say they’d found something we’d missed. Everyone was petrified of this!”
“So?”
“Of course there was nothing there.”
As suspected, there were no identifiable biomarkers above the low background contamination levels, no organic compounds at all except the smallest hydrocarbon: methane gas was found trapped in the mineral matrix of the dust. John Hayes, a postdoc in the Bristol lab at the time, says he was amazed when they found methane. He’d worked with Kvenvolden at Ames for a year, but was so convinced that the moon would contain no carbon that he hadn’t even wanted to analyze lunar samples. In fact, he says, laughing, he’d applied for the Bristol appointment thinking he could escape the moon hullabaloo, only to be greeted by Geoff with the “wonderful news” that they were about to receive lunar samples. For his part, Geoff was excited, not because he thought they’d find signs of life, but because he was expecting they might find methane. This had occurred to him when he was working at the Lunar Research Laboratory, listening to the physicists talk about the solar wind that constantly bombarded the moon with a stream of high-energy ions—including carbon and hydrogen, which he imagined might become embedded in the lunar minerals and react to form methane. “We were absolutely chuffed when we found all that methane,” Geoff says, still sounding pleased almost 40 years later. And if there was metallic iron on the moon, as one might expect since it was covered with tiny craters from meteorites, and most of the meteorites that had landed on Earth were composed of iron, then the carbon should also react with iron to produce a sort of lunar steel.…
This was all very interesting and led to an exciting new line of research in Bristol, but what about biomarkers for life? Wasn’t that what all the hullabaloo was about?
Well, yes, Geoff admits, that’s what they were supposed to look for, but there just wasn’t anything to be found. “We kept with the Apollo program through Apollo 15, but mostly because of this methane and iron carbide work.”
Sifting through the old papers, I find one by Kvenvolden’s Ames group that describes the identification of porphyrins in lunar dust, like those found in petroleum. “We had this scientist in the lab who could find porphyrins in anything,” he quips, when I ask him about it. “But it turned out to be contamination.” Others reported finding amino acids in some of the Apollo samples, but it turned out that they had been collected from an area near the spaceship, where the dust had been contaminated by traces of rocket exhaust.
While the Apollo program chemists were still busy analyzing their bits of moon and preparing their reports for the first Lunar Science Conference, a much more interesting example of extraterrestrial organic chemistry had made a spectacular but unheralded landing on Earth. On September 28, 1969, a meteorite exploded in the atmosphere over Australia, and fragments rained down around the town of Murchison. In the wake of the Apollo moon drama, the meteorite fall was a bit anticlimactic. But when Kvenvolden received fragments of the stone at the Ames laboratory, he was excited: it was clearly a carbonaceous meteorite like the one from Orgueil; the fragments had been retrieved by residents and sent to NASA almost immediately after their fall, minimizing chances of terrestrial contamination; and the Ames laboratory was perfectly poised for their analysis.
Kvenvolden and his colleagues found a complex suite of hydrocarbons, but it was completely lacking in the organized, enzyme-directed structures and distribution patterns produced by organisms. They also found significant amounts of amino acids, including some of life’s building blocks for proteins and enzymes, but with none of life’s insistence on left-handedness. The mixture was racemic, with D and L configurations present in equal measure, and it was dominated by a number of strange amino acids that had never been found in organisms or, for that matter, anywhere on Earth except a chemistry laboratory. Indeed, the distribution of amino acids in the Murchison fragments bore a remarkable resemblance to the distributions produced during simulations of the abiotic chemical processes hypothesized to have taken place on the early earth. Here, finally, in this wayward bit of space debris, they’d found solid evidence of what chemists since the 1950s had suspected must exist somewhere in the solar system, what Calvin had been hoping to find in the oldest Precambrian rocks: organic compounds that had been produced not by organisms in their natural habitats or by chemists in their laboratories, but by purely abiotic chemical processes somewhere in space.
Thirty years of scrutiny and rescrutiny of the Murchison meteorite has only reinforced the NASA group’s conclusion. More than 70 amino acids have been identified, most of them nonexistent in the earth’s biota. The soluble organic matter consists mostly of polycyclic aromatic hydrocarbons, or PAHs, which are produced in quantity by abiotic reactions in early earth experiments, and appear to be ubiquitous and abundant in the dust and gas clouds of interstellar space. PAHs can also be generated by burning or heating organic matter on earth, and they are found in petroleum and as man-made pollutants in the atmosphere, but they are not known to be biosynthesized directly by any organisms. Next in abundance are the carboxylic acids, some of which are made by organisms. But like all of the groups of organic compounds found in the meteorite, the distribution of acids exhibits none of life’s carefully constructed patterns: for any given number of carbon atoms, all possible isomers are present in at least trace amounts—a structural randomness that enzymatically mediated biosynthesis would never produce. Contamination remains a problem when analyzing meteorite samples, but the presence of “extraterrestrial” amino acids, as well as evidence of abiotic syntheses, helps to distinguish uncontaminated from contaminated samples, and careful analysis of several other carbonaceous meteorites has revealed remarkably similar arrays of compounds.
The carbonaceous meteorites are among the oldest objects in the solar system, dated by radiometric methods as more than 4.5 billion years old. They contain an elemental composition that is little changed since the first accretions of matter from the solar nebula and are thought to be pieces of asteroids produced during collisions with planets or other asteroids. The provenance of the organic compounds in them remains the subject of study and speculation, but now the focus is on the nature of the chemical reactions and conditions that generated them in interstellar space. Such primordial compounds would have been destroyed on the nascent earth, as its surface was covered with molten rock. But as the planet cooled, some 3.8–4.1 billion years ago, a constant hail of tiny meteors and cosmic dust, much more intense than it is today, would have reintroduced them. Regardless of whether life arose from such interstellar organic compounds, or whether it made use of organic compounds that were synthesized on the early earth as it cooled and water condensed to form oceans, one thing is clear: the carbon-based chemistry that laid the foundation for life was available throughout the solar system.
Has any other planet besides the earth ever provided enough shelter to preserve complex organic compounds, as well as the energy sources and water conducive to chemical evolution and the emergence of life? Clearly the moon is lacking. And according to results from the 1975 Viking mission to Mars, the red planet is equally lacking. Along with a number of automated biological experiments designed to detect life, the Viking included a GC-MS—a miniature, automated rendition of the combined gas chromatograph–mass spectrometer that Geoff had acquired for his Glasgow lab—which detected no organic compounds at all in samples taken from the top 10 centimeters of soil on Mars. That the surface of Mars should lack life is not surprising, as current temperatures are too low for liquid water to be present, and it is subject to intense ultraviolet solar radiation. But what about abiotic organic compounds? The planet is continually bombarded by meteors, and has been since its birth. Unlike the moon, where organic compounds in a meteor are most likely destroyed on impact, Mars has a generous enough atmosphere to cushion the meteor’s fall, and many organic compounds should survive, just as they do on Earth. The hypothesis that best explained the Viking results was that high concentrations of strongly oxidizing chemicals such as hydrogen peroxide in the surface soil rapidly destroyed any organic compounds. NASA’s official stance was that the Viking missions “neither proved nor disproved the existence of life on Mars,” but the consensus in the wider scientific community was somewhat less ambivalent: a living cell clearly could not survive in such an environment. Of course, the Viking sampled a few grams of soil from the surface at a single site, and the theoretical oxidizing layer should be restricted to the surface. And the 1970s-era mass spectrometer was not sensitive enough to have detected truly minute traces of organics. All of this left open the possibility that microbial life exists or existed beneath the surface, in some more protected environment, or at some time in the planet’s distant past. In 1996, when a NASA geologist made the sensational claim that he and a group of colleagues had found both chemical and physical fossils in a meteorite from Mars, the news was received with a mixture of excitement and skepticism, not to mention a feeling of déjà vu among many older scientists.
The meteorite had been found lying on the ice in Antarctica in the 1980s, but only recently had geologists established its origin, placing it among a dozen meteorites whose composition linked them with some certainty to Mars. Other evidence indicated that this particular emissary had landed on Earth some 13,000 years ago, that it had apparently been ejected from the surface of Mars by a large asteroid impact about 16 million years ago, and that it was composed of material that was almost as old as the planet itself. The evidence of past life consisted of submicroscopic, rod-shaped mineral deposits and crystals similar to those fabricated by some bacteria, and the presence of possibly biogenic organic matter. What followed was yet another replay of the extraterrestrial life debates of the mid-nineteenth century and the Orgueil meteorite fiasco of the 1960s.
NASA, which had been suffering from budget cuts and was desperately in need of public support for its programs, called a press conference to publicize the results even before the paper was published. Scientists of all stripes were quick to challenge the conclusions, but the public was immediately seduced by the prospect of life on Mars, and politicians weren’t far behind. “I am determined that the American space program will put its full intellectual power and technological prowess behind the search for further evidence of life on Mars,” said President Clinton in a statement shortly after NASA’s press conference. Meanwhile, geologists, biologists, and chemists went to work to test the evidence.
At Scripps in California, Jeff Bada was skeptical of the way results from the organic analyses had been interpreted. The compounds that were construed as evidence of life were PAHs, whose provenance is ambiguous even when they are found in earth rocks. Unlike the molecular fossils that organic geochemists have discovered over the past 40 years, PAHs contain little specific structural information that links them to compounds made by organisms. Some PAHs are found in petroleum and in shales rich in organic matter, and may indeed be the distant transformation products of polycyclic isoprenoids from plants and bacteria. But PAHs can also be formed by a number of abiotic reactions and are a common constituent of carbonaceous meteorites and cosmic dust. Of course, just to complicate matters, they are also ubiquitous atmospheric pollutants, produced by burning anything from forests to gas. Bada, who had spent much of his career studying the isomerization reactions of amino acids in fossils, decided to look for some slightly less ambiguous and more informative sign of past life in the Martian meteorites.
Bada and his colleagues analyzed the amino acids in two meteorites found in Antarctica, including the one the NASA researchers had studied. They found a distinctly earth-like suite of amino acids, predominantly in the L configuration and lacking the “extraterrestrial” amino acids that had been identified in the Murchison or in abiotic syntheses. In fact, the pattern of amino acids in the meteorites looked suspiciously like the one they found when they analyzed ice from the Antarctic landscape where the meteorites had been discovered. Like the pattern of amino acids Kvenvolden had found in his Precambrian rock samples 30 years earlier, it conveyed one not particularly surprising bit of information: during its 13,000 year residence in Antarctica, the meteorite from Mars had become contaminated with earthly substances.
In the years since the 1996 study, the other evidence of life in the Martian meteorite has been similarly revealed as either equivocal or ambiguous: the purported microfossils of bacteria are simply minute fragments of clay, and though the NASA group continues to defend its conclusions about the mineral formations, most scientists who examined them agree that they are more readily explained by simple inorganic chemical reactions. Nevertheless, the controversial paper and the debate it triggered may well have rescued the search for extraterrestrial life from scientific oblivion. The question of whether life could ever have existed on Mars is being revisited, now in the light of irrefutable evidence that water did indeed exist on the surface of the planet at some time during its history. Recent discoveries of bacterial communities thriving in extremely hostile environments on Earth—buried beneath thousands of meters of solid granite bedrock, or deep within the Antarctic ice sheet, or in the harsh chemical milieu of hot springs—have added a new dimension to the query and led many scientists back to the questions they thought the Viking had answered: Did life ever exist on Mars? Might it still? In the late 1990s, the combination of rekindled scientific interest and public excitement translated into renewed funding for Mars science programs, with both NASA and the European Space Agency making plans for new missions on the surface of Mars and the return of samples to Earth.
Researchers working to define sampling strategies for Mars are faced with the same fundamental question that plagued Geoff and Calvin as they planned their analyses of Precambrian rocks 40 years ago: what should we look for? The supposition of biochemical Uniformitarianism that facilitated the initial search for molecular fossils in ancient rocks has turned out to be surprisingly sound: the fundamental building blocks of life have not changed much in more than three billion years of earth history. We know of no reason why these should necessarily be the same in extraterrestrial life, if it exists—biology as yet boasts no universal laws like those of physics and chemistry. But physics and chemistry can nevertheless constrain the search: we can suppose that life anywhere in the solar system would depend on the chemistry of carbon for the same reasons it depends on carbon on Earth; that water is required as the milieu in which that chemistry functions; and that life of any form harvests energy and creates chemical disequilibrium. We can look, then, for simple patterns in the distributions of organic compounds, like Geoff first saw in his leaf wax n-alkanes. And, of necessity, we can make the provisional assumption that Martian life’s primary biochemical pathways of carbon assimilation and metabolism would be similar to the ones on Earth.
Bada, of course, has his hopes set on amino acids. Together with a large team of NASA-funded researchers from several institutions in California, he has been designing miniaturized analytical systems that can both detect and determine the chirality of tiny traces of amino acids. The system, which is many times more sensitive than the Viking’s GC-MS, is scheduled for inclusion on a European Space Agency mission that plans to analyze samples from up to two meters below the surface of Mars, in the hope of detecting what the Viking may have missed—whether it be signs of present or past life, or of the ancient interstellar organic chemistry evident in meteorites. As molecular fossils on Earth, amino acids are not particularly informative, partly because they are so mobile in the sediments and readily consumed by bacteria, and partly because the same set of 20 amino acids is common to virtually all life on Earth. But in the search for extraterrestrial life this universality is precisely what we need: we don’t want to limit our search to a specific form of life. Amino acids are one of the most essential and abundant ingredients of the simplest bacterium. Though they are readily made by abiotic synthesis in interstellar space, their distribution in living things on Earth is distinctive and idiosyncratic. One might expect that it would be equally distinctive, though perhaps different, in Martian life-forms; likewise, if amino acids are to work as building blocks, linked into chains that form the precise sorts of structures that life requires—coils and sheets of a consistent three-dimensional form, enzymes that fit with a specific substrate—then they would have to be all of the same configuration, either predominantly L, as on Earth, or all D. Under the current frigid, dry conditions on Mars, such a chiral preference could persist in fossil Martian organic matter for millions of years and might indeed provide evidence that the planet was once host to organisms of one sort or another. However, if the Martian climate had been warmer and wetter for more than a few million years, amino acids from any organisms that existed before or during that period would be fully racemized, and it would be much more difficult to distinguish them from amino acids that had been created by abiotic chemical processes.
Other researchers are focusing on biomarkers with the distinctive architectural style of the isoprenoids, whose five-carbon isoprene units, linked head to tail, signal a probable biogenic origin and are sturdy enough to resist chemical breakdown for billions of years. Such a focus presumes an even higher level of uniformity between extraterrestrial and terrestrial biochemistry, as most of these compounds require a more sophisticated enzymatic system to fabricate than do amino acids. But they are also essential components of one of life’s fundamental structures, the formation of which some theorists propose was a first step in the origin of life: its container. The cell membrane confined life’s ingredients in proximity, segregated them from their environment, and generally provided the conditions necessary for a system of self-propagating reactions to develop. Long-chain acyclic isoprenoids make up the cell membranes of some of Earth’s most widely adapted microorganisms, and hopanoids, which are more complex cyclic compounds based on the same architecture, comprise integral components of most bacterial membranes. In fact, as a group, isoprenoids are the most abundant organic molecules preserved in earth rocks. So why not on Mars? If there’s anything at all to the presumption of solar-system-wide biochemical Uniformitarianism, then wouldn’t such ubiquitous, persistent molecular fossils also have been preserved on Mars? Indeed. But some microbiologists are coming at the question from the other side, looking for outliers, for aberrations in the Uniformitarian norm: rather than searching on Mars for the most universal and persistent molecular fossils derived from terrestrial biochemistry, they are looking on Earth for environmental analogues to Mars—frigid temperatures, high exposure to damaging ultraviolet light, extreme aridity, caustic soil—and studying the biochemistry of terrestrial bacteria adapted to live under such conditions.
Most, though not all, researchers are still convinced that there is presently no life on Mars … but the more we learn about the adaptability of life here on Earth and about the chemistry of the solar system and beyond, the more plausible it seems that carbon-based life has evolved, or is evolving, or will evolve, somewhere besides Earth. Whether the search for evidence is worth the resources invested is a hotly debated issue in the scientific community, as well as in the political domain—but the temptation to search is almost impossible to resist. The exciting questions about the origin and distribution of life in the universe that chemists laid on the table and began to test experimentally with their first meteorite analyses in the 1860s, and revived in the 1960s, and then again in the 1990s, are destined to return ad infinitum, it seems, until they find some definitive answers.