Originally published 19 May 1986
This week in Baltimore, three Iowa scientists will confront skeptical colleagues with evidence for a radical new theory: that the Earth’s upper atmosphere is continually bombarded by small comets, thousands of them every day, plashing like raindrops into a pond.
The comets are essentially big dirty snowballs of fluffy ice. Upon encountering the atmosphere, the new theory contends, they produce a puff of vapor and disperse.
“In the past 3.8 billion years, enough of these comets have fallen into the Earth’s atmosphere to have contributed enough water for all of the Earth’s ocean and icecaps,” said physics professor Louis Frank, one of the three University of Iowa researchers who came up with the theory, in a interview last week. He will present evidence for it this week to the American Geophysical Union, the nation’s largest society of earth and space scientists.
If Frank and colleagues John B. Sigwarth and John Craven are right, the textbooks will have to be rewritten. According to current theories, the Earth’s oceans have been in place since early in the planet’s history. The Iowa scientists believe the early Earth was essentially dry, and that the planet has since acquired most of its water from space in the form of comets.
Transient dark spots
The evidence for the influx of comets was obtained by the high-altitude satellite Dynamics Explorer 1. The satellite is capable of photographing the Earth’s dayglow (light from the sunlit side of the Earth), especially the emissions of oxygen.
In studying images made by the satellite since 1981, the scientists found a spatter of transient dark spots int he otherwise uniform illumination. The spots are 5 to 20 percent less bright than the surrounding atmosphere. They are typically hundreds of square miles in area (about half the size of Rhode Island) and endure for several minutes. An average of 10 spots per minute occur in the upper atmosphere on earth’s dayside.
According to the Iowa team’s hypothesis, the cause of the dark spot is a sudden injection of material into the upper atmosphere that absorbs light from below. The spectral characteristics of the absorption suggest that the material is water vapor.
There is evidence, furthermore, that the water comes from space. The dark spots have the same daily variations in rate of occurrence as incoming meteors detected by radar.
Because the Earth collects meteors the way an automobile collects bugs on the windshield — by running into them — meteors are observed most often in early morning hours, from sites facing in the direction that the Earth is moving through space. The atmospheric dark spots also occur most commonly on the forward-facing part of the Earth, which may mean the Earth is gathering up small watery comets as it moves through its orbit.
The account for the observed absorption of light, the Iowa team calculated the mass of each comet would have to be approximately 100 tons, the equivalent of a railroad tank car full of water. Twenty of these flying snowballs collide with the Earth every minute, Frank thinks, producing a splash of vapor that briefly darkens the dayglow.
The source of the comets may be the so-called Oort cloud, a virtually inexhaustible reservoir of comets that many astronomers believe surrounds the solar system, far out beyond the planets. When the Oort cloud is disturbed (by a passing star, or when the solar system moves through the central plane of the Milky Way), comets are sent cascading into the inner solar system. The new work suggests that staggering numbers of small comets may be falling from the Oort cloud toward the Sun, and that some of them encounter the Earth.
If the dark spots in the Earth’s dayglow are indeed the splash of comets falling into the atmosphere, the discovery is of revolutionary importance. It means that every 10,000 or 20,000 years the Earth acquires from space an amount of water sufficient to raise the level of the oceans by an inch. In a billion years the water would rise by a mile.
Frank and his associates believe that severe showers of comets from space might have been the cause of the ice ages that have affected the Earth throughout history. They also believe that occasional increases in the rate of comets could have modified the climate of the Earth sufficiently to account for episodic extinctions, such as the mysterious calamity 65 million years ago that eliminated the dinosaurs and other species.
The proposed influx of comets would not be unique to Earth. The comets must also fall on the Moon and the outer planets and their satellites. Mercury, and to a lesser extent Venus, would be shielded from bombardment by the heat of the sun, which would evaporate the comets before they reached the planets.
Questions remain
The theory raises many questions, both terrestrial and planetary. A few: Geologists have identified rocks of marine origin that are 3.8 billion years old, or 85 percent as old as the Earth itself. If the early Earth was mostly dry, how were such marine rocks formed? Why have geologists not seen evidence for a dramatic change in sea level in the record of sedimentary rocks? Why doesn’t Mars have an ocean? How has the shower of comets gone undetected for so long?
Such questions account in part for the stiff skepticism the rain of comets theory is meeting among a number of scientists. Some wonder if the apparent dark spots may be subtle artifacts of the imaging instruments. But Frank and his colleagues are confident that the dark spots are not just noise in the data from the satellite; still, the possibility has not yet been ruled out. Even if the spots are real, they could have a less exotic explanation than comets from space.
One prominent scientist who believes they are real is Michael Belton, a comet specialist at the National Optical Astronomy Observatory in Arizona. There is no question the objects are real and no theoretical reason why they could not be comets, he said, but the supposed implications need more study.
“Frank’s work with the data is beautiful,” Belton said, “There is general agreement that the spots are real… Something is doing something to the Earth’s upper atmosphere. Other ideas will emerge in the attempt to explain the spots. What makes Frank’s work controversial is that the implications of such a flux of water-rich objects into the atmosphere has not been fully explored in their paper.”
It has surfaced at a time when the idea of objects from space impacting with the Earth has been a hot topic in science. It may be an idea whose time has come, one of those great unifying ideas that will unravel long-standing riddles of Earth history. Or it may be that the idea has generated more speculation than the data can bear.
The so-called “small comets” theory of Louis Frank and colleagues did not gain acceptance among astronomers as further evidence to support the theory has not been forthcoming. ‑Ed.
