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The Small-Comets Hypothesis

Do snowballs keep falling on our heads?


Explorer I images with putative comets

Explorer frames show Frank's spots, as presented in Reviews of Geophysics.

Louis Frank/John Sigwarth/NASA

It all started with some black dots in a space image. The satellite Dynamics Explorer 1 was launched in 1981 to look at Earth in far ultraviolet light. The pictures it sent down were thrilling—the whole Earth glowed at these wavelengths, the daylit side shining softly in the far-UV light of excited oxygen atoms, the poles crowned with beautiful haloes corresponding to the aurora.

Black Spots in Space

The scientist in charge of the instrument, Prof. Louis A. Frank of the University of Iowa, set one of his physics undergraduate students to work on the images, but John Sigwarth was hampered by black spots. Maybe they were just glitches, camera noise or data transmission errors or something else that could be safely ignored. "But you cannot alter data on a mere assumption," Frank later wrote. "You have to have a reason."

After nearly a year of intense effort, Sigwarth and Frank and John Craven (who built Explorer's instrument) concluded that the black spots were something real out in space. They must be caused by the disintegration of comets, small ones weighing a few tons, the size of a large house. They must be made of water, and there must be thousands of them a day. They must be too small and too fast and too far away to see from the ground. Frank recounts the story in garment-rending detail in this book excerpt.

A Scientific Flamewar, Then a Fadeout

Their work was published in the April 1986 issue of Geophysical Research Letters, known in the trade as GRL. Rarely does a scientific discovery produce the controversy that erupted. During the ensuing year, 11 separate formal Comments were published and the three authors prepared 10 formal Responses. Finally the editor of GRL, Alex Dessler, wrote that "there is a limit to the number of Comments that will be accepted on even the most controversial of Letters."

This scientific flamewar is an example of the refiner's fire of debate that any new idea must face. It strengthened Frank's convictions. In 1990 he published a trade book that, like some 17th-century classic, laid out the whole scope of his thoughts in its title: "The Big Splash: A scientific discovery that revolutionizes the way we view the origin of life, the water we drink, the death of the dinosaurs, the creation of the oceans, the nature of the cosmos, and the very future of the earth itself."

The most thorough treatment of the ice-comet hypothesis is in Reviews of Geophysics, in two articles, the first (August 1991) by Dessler, who had published Frank's GRL article, attacking the theory ferociously, and the second (February 1993) by Frank and Sigwarth responding to each point in what seems like a sensible way. It and all their other articles are available online from the University of Iowa.

In 1999 the two researchers went on to publish evidence from a camera on NASA's Polar satellite, showing more holes. A team in Berkeley argued that the spots in the Polar images arose from the way Frank had erased bright speckles due to space radiation in the camera. Frank has not refuted that argument, as far as I can tell. In 2000 Robert Mutel, a fellow physics professor at Iowa, reported that a telescopic search from the ground had come up empty. Frank then took Mutel's images and found some very faint streaks in them, dimmer than Mutel had looked for. They are so dim that it's hard to credit them as real.

Unnecessary Roughness

I honestly don't know what to make of it. Some problems in science remain indeterminate for a long time. But two very powerful things are evident in this story. First is the human fury that a disruptive idea arouses. Frank has described the wounds to his career and the contempt and shunning he has felt from his peers—even though he continues to publish respected research in other aspects of geophysics.

Frank's theory shows how people deal with a major scientific controversy. Dessler concluded his critique in Reviews of Geophysics with some understated highhandedness: "It seems unlikely that the scientific community will expend much additional effort in investigating either the small-comet hypothesis or its consequences." He will live that down if he's wrong—every scientist is wrong sometimes. But he didn't have to say it.

Nor could Dessler resist a further patronizing dig. "If small comets exist, their presence would be confirmed in due course. If [Frank and coauthors] were correct, they would enjoy fame and glory; nothing their critics said would be remembered, except perhaps as quotations demonstrating the evils of scientific dogmatism." Never mind that posterity "in due course" is small comfort when you feel personally attacked by your peers.

The second thing is the temptation to pursue a beautiful theory because of the answers it promises.

What Could Have Been

When the small-comet hypothesis first came out, it roused a host of new questions. Ideally that is what happens in a scientific advance: not a knockout blow that ends a boxing match but a realignment of the rules, leading the way to a new game. Unfortunately for Louis Frank of the University of Iowa, the dark spots on his ultraviolet space pictures were persuasively refuted by his critics, and Frank stopped publishing on the subject in 2001. But in 1993, in Reviews of Geophysics, he had erected quite a structure of tempting speculation. I'll summarize it as a Q-and-A:

    Q: Why can't we see these snowball comets?

    A: They're quite far away, at the highest fringes of the atmosphere, when they blow up. They break out of their carbon crusts from electrostatic force and quickly puff outward into a cloud of water vapor like a soap bubble. These chemical species, unlike the metal and minerals in meteors, don't glow very much. And by the time this body of vapor hits the atmosphere, there's nothing solid left to burn up from friction. People could just barely see them as 4th-magnitude meteors if they were looking exactly at them.

    Q: Wouldn't they be striking the Moon, too?

    A: Yes, about a hundred times a day, and they'd strike the surface directly. We have seismographs on the Moon that record meteor impacts, but they don't record these comets. Numerical models of impact cratering suggest that impacts involving these fluffy comets put most of their energy into the expanding cloud of vapor from the comet, not the ground. And the Moon's surface is largely a thick pad of dust and broken rock. But no one has done experiments with materials like this. Meanwhile, people are looking for bright flashes on the Moon. They should be as bright as a zero-magnitude star, like Capella, for one-tenth of a second.

    Q: Wouldn't there be a bunch of water vapor, hydrogen, and so forth in interplanetary space from these things popping?

    A: Near as we figure, we can't tell yet. The Galileo spacecraft had instruments that can make some marginally useful measurements, but basically no one has had a reason before to research the hydrogen content of deep space in detail. The small-comets theory can't be disproved with what we DO know.

    Q: Where do they come from?

    A: From beyond Neptune (in the Edgeworth-Kuiper belt), where huge numbers of comets, left over from the formation of the solar system, are thought to live. They fall pretty much straight toward the sun, but Jupiter disrupts enough of them that a fair supply of them have orbits approaching Earth.

    Q: What do they mean for Earth?

    A: Consider that the estimated influx of water, if kept up through geologic time, would account for the water in the ocean. It means that the water cycle suddenly has a big unknown element. Consider that these comets probably don't come at a constant rate, and consider that water vapor, not carbon dioxide, is by far the most important greenhouse gas. A massive comet shower could wreak—could have wrought—some pretty interesting havoc.

    Q: What about the other planets?

    A: Apparently these comets can't survive much closer to the sun than Earth, so Venus and Mercury are unaffected. The big outer planets emit more thermal radiation than they should, so maybe the influx of small comets is part of that. That leaves Mars, and Mars is quite different from Earth—no magnetosphere, for starters. But we already know that water is a very important part of martian geologic history.

And as for geoscience, the success of Frank's hypothesis would have opened vigorous studies of new subjects: the behaviors of low-density ice in impacts, of water-vapor molecular aggregates in vacuum, of faint objects in space and dim lights in the sky, obscure events in the middle atmosphere, exquisitely precise isotopic studies of glaciers and sediments, numerical studies of orbits and space electrostatics. It would have been thrilling to be the wellspring of such a burst of activity. But Frank's vision of a rain of water comets has come to an inconclusive, probably abortive end.

This story is a striking milestone in what I think of as the universalization of Earth, the growing awareness that large forces and influences impinge on our world from the cosmos, not just from underground or from familiar nearby things. That is the biggest shift in Earth science since plate tectonics in the 1960s. Only occasionally, as here, does this slow shift take such dramatic form.

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