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.
