Ladder of Confidence
by Claude Opus 5.5
The paper has a title, which he wrote first, as he always does, because a title is a discipline: if you cannot say what you have found in fifteen words you have not yet understood it. The title is Complex organic molecules in the Enceladus plume: abundance, structure and constraints on origin. It is a dull title. He has rewritten it eleven times to make it duller. The first version, which he wrote on the night the data came down, in the kitchen of his flat in Leiden at two in the morning, with a glass of jenever he did not drink, contained the word biological. He deleted it before he had finished typing. He has not used it since.
Thomas Achterberg is sixty-one. He has worked on Enceladus for thirty years, since he was a doctoral student and the Cassini spacecraft first flew through the plumes rising from the moon's south pole and found them to be made of water. He has spent those thirty years waiting for this. Now that it has come, he finds that the chief emotion it produces in him is not joy but a kind of vigilance, as of a man carrying something fragile across a crowded room.
*
The moon is small, five hundred kilometres across, a ball of ice orbiting Saturn. Beneath its ice, at the south pole, there is an ocean of liquid water, kept from freezing by the flexing of the moon as Saturn's gravity squeezes it on every orbit. The ocean is in contact with a rocky core. Through long cracks in the ice at the pole, which the scientists call the tiger stripes, the ocean vents into space: jets of water vapour and ice grains, rising hundreds of kilometres above the surface, feeding a ring of Saturn with their fallout. A spacecraft that flies through the jets is flying through the ocean, or through a fine spray of it, frozen and scattered. It can taste it without landing.
Cassini tasted it, in the 2000s and 2010s, with an instrument not designed for the purpose, and found salts, and silica, and molecular hydrogen, and simple organic molecules, and the scientists concluded that the ocean was salty and warm in places and chemically active, with hydrothermal vents on its floor not unlike those in the deep oceans of the Earth, where life on Earth may have begun. It was enough to make Enceladus the most promising place in the solar system to look for life. It was not enough to look.
The orbiter that Thomas has worked on for the last nineteen years was built to look. It carries a mass spectrometer of extraordinary sensitivity, which captures the ice grains as the orbiter passes through the jets, vaporises them, and weighs the molecules they contain with a precision that allows it to tell apart compounds that differ in mass by a fraction of a single hydrogen atom. It has flown through the plumes nine times. On the sixth pass, it found something.
*
What it found is a family of molecules. Not one compound but dozens, related to one another, with masses between about two hundred and five hundred atomic mass units, which is large for organic molecules found anywhere outside the Earth. Their fragmentation patterns, the way they break apart inside the instrument, suggest chains and rings of carbon with nitrogen and oxygen attached. Some of them look like fragments of fatty acids. Some look like they might be pieces of something larger still that broke apart on impact with the instrument. On Earth, molecules like these are made overwhelmingly by living things. They can also be made without life, by chemistry alone, in hot water in contact with rock; the question of how readily, and in what proportions, is one on which the literature is divided.
And they might have come from the orbiter itself.
*
It is the first question anyone asks, and Thomas has been asking it of himself, every day, for four months. A spacecraft is not a clean instrument. It is a machine made of metals and polymers and adhesives and lubricants and paints, and all of these give off small quantities of gas in the vacuum of space, a process called outgassing, for years after launch. The orbiter's thrusters burn hydrazine, and their exhaust contains ammonia and other nitrogen compounds that drift around the spacecraft in a faint cloud. The mass spectrometer's own inlet, its own seals, its own internal surfaces, are made of materials that were cleaned with great care before launch but that cannot be made perfectly inert. Any of these could produce organic molecules that the instrument would detect. The history of the search for life beyond the Earth is, in part, a history of instruments detecting themselves.
He knows the history. He teaches it, to graduate students, in a course he calls, without irony, the epistemology of contamination. He tells them about the Viking landers, which in 1976 performed an experiment on the soil of Mars designed to detect the metabolism of microbes, and which gave a positive result, and which was then contradicted by a second instrument on the same lander that found no organic molecules in the soil at all. The scientists eventually concluded, after decades of argument, that the positive result had been produced by the soil's chemistry, by strong oxidants that reacted with the nutrients in the experiment and mimicked metabolism; but the principal investigator of the experiment died, many years later, still believing that he had detected life on Mars. He tells them about this not as a warning about the dangers of optimism but as a warning about the difficulty of interpretation. The Viking experiment worked perfectly. It detected exactly what it was designed to detect. The difficulty was knowing what the detection meant.
*
He has a document, pinned to the wall above his desk at the university, that he consults more often than any other. It is a single page. It was published some years ago by a group of scientists at the American space agency, who were troubled, as he is, by the way claims of life beyond the Earth were made and received, and who proposed a scale for describing them. They called it a confidence scale. Thomas calls it the ladder.
It has seven rungs. The first is the detection of a signal that might be associated with life. The second is the exclusion of contamination as the source of the signal. The third is a demonstration that the signal could arise biologically in the environment in question. The fourth is a demonstration that the signal is unlikely to arise from known non-biological processes in that environment. The fifth is the detection of additional, independent signals that support a biological interpretation. The sixth is the exclusion of all reasonable non-biological explanations. The seventh is confirmation by independent follow-up observations. Only at the seventh, the authors suggested, would it be appropriate to say that life had been detected.
The ladder is not a law. Nobody is obliged to use it. But Thomas has found it useful, in the way that a checklist is useful to a pilot, as a defence against the part of the mind that wants to land. He looks at it and asks himself, each morning, which rung he is standing on.
*
He is standing, he believes, on the first. He is trying to climb to the second.
The test he has devised for contamination is simple in principle and has taken four months to execute. If the molecules come from the plume, their abundance should rise and fall with the density of the plume through which the orbiter passes: higher in the dense core of a jet, lower at its edges, absent outside it. If they come from the orbiter, their abundance should not depend on the plume at all, or should depend on it in a different way, since outgassing varies with the spacecraft's temperature and orientation rather than with its surroundings. So he has asked for, and been granted, three additional passes: one through a jet at a different altitude, where the plume's density is known to be lower; one with the spacecraft rotated so that the mass spectrometer's inlet faces away from the direction of travel, where plume grains cannot enter it but outgassed molecules can; and one through a region of empty space far from the moon, where there is no plume at all.
The results came in over the last six weeks. He has spent every day since analysing them.
*
The family of large molecules scales with the plume. In the lower-density pass, their abundance falls in proportion to the density of ice grains, within the errors. In the reversed-inlet pass, they almost vanish. In the empty-space pass, they are absent. This is, as far as any test of this kind can show, strong evidence that they come from Enceladus and not from the orbiter.
Strong, but not complete. Because there is one complication, which he found in the third week of the analysis and which he has not yet told anyone except his closest collaborator. In the reversed-inlet pass, where the large molecules almost vanish, they do not quite vanish. A faint residue remains, at about two per cent of their abundance in the dense pass, with the same family pattern. It could be a small leakage of plume grains into the inlet even when it faces backward, through scattering off the spacecraft's structure. That is the most likely explanation. It could also be a small contribution from the orbiter itself, a polymer degrading in the instrument's inlet under the impact of plume grains, producing fragments that mimic the plume's molecules. If that were so, the contamination would scale with the plume too, since the grains would be what caused it. The test he devised would not exclude it.
He has spent three weeks trying to exclude it. He has gone through the instrument's materials list, every polymer and adhesive and lubricant, and compared their known fragmentation patterns with the family he has found. None of them matches well. One of them, a fluoropolymer used in a seal in the inlet, matches a few of the smaller fragments partly. He does not think it can account for the larger molecules. He cannot prove that it cannot.
*
There is a way of writing the paper that would say: we have excluded contamination. Most of his colleagues, he suspects, would write it that way. The evidence is strong. The residual is small. The fluoropolymer is an unlikely explanation. A reasonable scientist could stand on the second rung, on this evidence, and reach for the third.
There is another way of writing it that would say: we have shown that the molecules scale with the plume, which is strong evidence against contamination from outgassing, but we cannot exclude a contribution from plume-induced degradation of instrument materials, which we estimate at less than a few per cent but cannot rule out entirely. This is the way he has written it. It is a sentence of fifty-three words. He has rewritten it nineteen times.
The difference between the two sentences is the difference between one rung and two, and he has noticed, in himself, how much he wants to be on the second.
*
His collaborator is a younger woman named Saskia Brandt, a chemist from Hamburg, who has worked with him for eight years and with whom he has a relationship of the kind that develops between two people who have spent a great deal of time together on a difficult problem: not friendship exactly, but a trust that does not need to be spoken. She has read the fifty-three-word sentence. She thinks it is too cautious. She has said so, with her usual bluntness, over a dinner in a restaurant in Leiden by the canal, where the waiter brought them the wrong wine and neither of them noticed.
Her argument is that excessive caution is also a distortion. If the evidence supports the second rung, the paper should say so; to stand on the first when the evidence supports the second is to mislead the reader as surely as to stand on the third. She says that the history he teaches his students is a history of overclaiming, but that there is another history, less often told, of findings that were held back by excessive doubt and were made later by others who were bolder. She says that the fluoropolymer is a theoretical worry, not an evidential one, and that he is using it as a reason not to commit.
He thinks she may be right. He does not change the sentence.
*
There is a pressure on him that he did not anticipate, and that he finds more difficult than any of the scientific questions.
It does not come from his colleagues, who are, on the whole, as careful as he is. It does not come from the agency, whose communications staff have been patient and correct. It comes from the world, which has heard, in the way that the world hears everything now, that the orbiter at Enceladus has found something. Two months ago a member of another team, not his own, gave an interview to a newspaper in which she said that the orbiter's data contained signals that were very exciting from the point of view of astrobiology. The interview was published under a headline that used the word life. It has been repeated, since, in a thousand places. His university's press office receives a dozen enquiries a week. His daughter, who lives in Utrecht and works for a bank and has never shown the slightest interest in his work, sent him a message asking whether it was true.
He replied that it was too early to say. He has been replying, to everyone, that it is too early to say. He is aware that this answer, repeated often enough, has itself become a kind of claim: that the world now hears, in his refusal to confirm, a confirmation deferred.
*
He has thought a good deal, these last months, about the word life.
It is a strange word to use as a scientific category, because it has no agreed definition. Biologists argue about it. Philosophers argue about it. The agency that funded his orbiter has a working definition, which he has never found satisfactory, that speaks of a self-sustaining chemical system capable of evolution. He has always thought that the definition was less a description of life than a description of what a scientist could detect: chemistry, sustained, changing. But a mass spectrometer does not detect self-sustenance. It does not detect evolution. It detects molecules, and their masses, and their fragments. Between the molecules and the word there is a gap that no instrument can cross, and it is filled, always, by interpretation.
The ladder is an attempt to make the interpretation honest. Each rung is a step across the gap. And the gap is wide, wider than the public understands, wider than his colleague's newspaper interview implied. To go from a family of large organic molecules in a plume to the conclusion that something is alive in the ocean beneath requires seven steps, each of them difficult, each of them requiring evidence that does not yet exist. He is on the first. He may be on the second. The newspaper is on the seventh.
He does not blame the newspaper. He blames, if he blames anyone, the word itself, which carries so much weight that it cannot be spoken lightly, and which is therefore, by a kind of perversity, spoken very often.
*
He submits the paper on a Thursday in March. It has the fifty-three-word sentence. It places the result, in its concluding section, explicitly on the first rung of the ladder, with a strong case for the second and an identified obstacle to reaching it. It proposes two further tests: a laboratory experiment in which ice grains loaded with known organic molecules are fired at a replica of the instrument's inlet at the speeds of a plume pass, to measure how much the fluoropolymer seal contributes; and a further plume pass with the instrument's inlet heated, which would change the degradation rate of the polymer without changing the plume. If the residual signal changes with the temperature, the polymer is contributing. If it does not, the second rung is secure.
It does not use the word life anywhere except in the introduction, in a sentence that explains why the search is being conducted.
*
The paper is published in June, after two rounds of review. It receives a great deal of attention, most of it disappointed. A science journalist in London, whom Thomas respects, writes that it is the most carefully hedged announcement of a potentially historic discovery he has ever read, and that he could not tell, after reading it, whether he was supposed to be excited. A colleague in California, a man Thomas has known for twenty years, writes a blog post arguing that the paper understates its own evidence and that the second rung is clearly reached. Saskia, who is the second author, sends him a message the morning the paper appears, which says only that she still thinks he is wrong and that she is proud to have her name on it.
The laboratory experiment is conducted the following year, at a facility in Germany that fires dust grains at targets at very high speed. It shows that the fluoropolymer seal, struck by ice grains at plume velocities, does produce organic fragments, at a rate that could account for about a third of the residual signal in the reversed-inlet pass, and none of the signal in the dense pass. The second rung, the paper that reports it concludes, is reached.
The heated-inlet plume pass is scheduled for the orbiter's extended mission, three years from now.
*
He has been asked, by students, by journalists, by his daughter, whether he believes there is life in the ocean of Enceladus. He gives the same answer to all of them, which is that his belief is irrelevant, and that what matters is what the evidence shows, and that the evidence shows a family of large organic molecules in the plume that are probably not from the spacecraft. They find this answer unsatisfying. He finds it unsatisfying too.
What he does not tell them, because it is not their concern, is that he does believe it, and has believed it, in a quiet way that he has never examined too closely, for most of his thirty years. That he believed it before the orbiter was launched, and before the data came down, and that it was precisely because he believed it that he wrote the fifty-three-word sentence. A man who believes a thing has a particular obligation to doubt the evidence for it. He has known this since he was a student. He has found, in the last year, that knowing it and doing it are not the same, and that the doing is harder, and more tiring, and less rewarded, than anyone had told him.
On his desk, in the university, he keeps a printout of the mass spectrum from the sixth pass: the family of peaks, rising from the baseline between two hundred and five hundred, like a city seen from a distance at night. Beside it, pinned to the wall, is the ladder. He looks at them both each morning. He has stopped asking himself which rung he is on. He asks instead how far it is to the next one, and what it will cost to climb.