Fig 00. Presidential Lecture 1999
" It is a shocking notion. Radiation-blasted, black-jacketed snowballs the size of houses swarming out of the cosmic void every day, plowing into Earth's atmosphere and pancaking explosively into clouds of vapor that fall as gentle rains."

"Over billions of years these invaders might have filled Earth's oceans and provided the seeds of life."

-- Kathy Sawyer, Astronomy Magazine, October 1998


The beginning of the debate. This is a narrative of an intense debate about the findings of a startlingly large influx of small comets into our atmosphere. This debate is now thirteen years old and is characterized by numerous emotional and intellectual statements to be found in both the scientific literature and the popular press. If the small comets exist, then fundamentally new insight into such questions as the origins of our oceans, of the influx of organic material from space, and, indeed, of our own origins is gained. The scientific debate is very much alive.

The saga began with the launch of three University of Iowa cameras on board the Dynamics Explorer-1 spacecraft in 1981. The scientific objectives of these cameras were to provide the first comprehensive series of global images of one of Earth's natural marvels, the crowns of auroral lights to be found encircling its two poles. The acquisition of these images of the aurora was successful beyond even our considerable expectations, and are widely used in scientific articles, textbooks and the popular press.

However, there was an important feature which was a puzzle from the time of the receipt of the initial images in space. This puzzle was the quite frequent appearance of dark "spots" in the images which inevitably drew the attention of scientists who were attending our talks on the auroral lights. Our response to these early queries as to the cause of these dark "spots" was that they were due either to an artifact of the camera or the way the images were transmitted to Earth from the spacecraft. The task to explain these dark spots as noise was assigned to John Sigwarth, a very promising graduate student of mine. After several years of intensive studies of the cameras and their images we failed to show that the dark spots in the images were due to noise in the camera. The behavior of these spots as functions of spacecraft position and time, as well as their correlation with observations of radar meteors, forced us to the unsettling conclusion that the spots were real.

As we shall see, the implications of the reality of the dark spots in the images were not accommodated by the standard acceptance criteria known as current wisdom. There were two choices available to us, put the results into our desk drawers and lead a relatively peaceful life, or publish the results and suffer the criticism, and the sometimes extreme animosity of colleagues and previous friends. This was only one choice with integrity in this matter, the work had to be submitted for publication.

The two papers on the dark spots were accepted within a remarkably short review period of 48 hours by the editor, Alexander Dessler, of one of the leading scientific journals in geophysics, Geophysical Research Letters. The papers were accepted over the strongly felt objections Fig 1. Cover of GRL of the two reviewers, one of whom stated that "if the contents of the papers are correct then half the physical science books in our libraries will have to be destroyed." The papers were featured on the cover of this journal. This cover is reproduced in Figure 1 [right]. A global picture of Earth taken at extreme ultraviolet wavelengths beyond the capability of our eyes is shown. In fact these wavelengths are dangerous to our eyesight and are shielded from us by our atmosphere. The lights in Figure 1 are located 200 to 300 miles above Earth's surface. The general glow which looks like the illumination of a ball by a light bulb is indeed the atmospheric glow which is due to the illumination of our upper atmosphere by sunlight. The second feature of note in Figure 1 is the "ring" of auroral lights in the upper portion of the image. This remarkable ring of auroral lights encircles the North Pole and was the primary objective of the camera. The catalyst of the great debate is the appearance of the dark spots in the atmosphere, one of which is shown in the inset.

Given the reality of the dark spots, which soon became known as "atmospheric holes" because of their appearance in the images, there is only one explanation which has endured over all these years to present. That is, the holes are due to the shadowing of the atmospheric light by an object above the atmosphere. This object simply cannot be a stony or iron meteor because the holes are very large, tens of miles in diameter. A rock of this size would provide a disastrous impact on the Earth's surface. As it turns out, water vapor is very good at absorbing the atmospheric light and thus appearing as a atmospheric hole in the images taken by the spacecraft camera. The only other step in the interpretation is to note that a cloud of water vapor will have only a brief existence in interplanetary space so that it must be delivered to Earth as a small comet filled with water snow which is disrupted and expands as it impacts into our atmosphere.

Fig 2. Diagram for viewing an atmospheric hole with a spacecraft orbiting Earth A diagram of the camera as it views an atmospheric hole is shown in Figure 2 [left]. In principle the detection of the atmospheric hole above our upper atmosphere is much like observing a fly walking across a television screen. Of course, Earth is much larger, as is the size of the atmospheric holes. The amount of cometary water vapor which is necessary to provide the atmospheric hole is in the range of 20 to 40 tons. The history of the impact of a small comet with this amount of water snow as it arrives from interplanetary space is shown in Figure 3 [below]. The fragile comet is disrupted at altitudes of roughly 800 miles just above our atmosphere. Sunlight is a very efficient energy source for converting the cometary snow into a water cloud. The small comet approaches the Earth at speeds of approximately 35,000 miles per hour. The diameter of a small comet before disruption is typically tens of feet, typical of a small house, but the expansion of the water cloud after its release from the disrupted small comet is rapid. This rapid expansion provides a cometary water cloud of tens of miles in size and the impact with the atmosphere is relatively benign. The water cloud is then slowed and mixed with the upper atmosphere. This cometary water cloud is subsequently deposited at Earth's surface as a gentle "cosmic rain".

Fig 3. Sequence of events for the disruption of a small comet

There is nothing greatly objectionable about the presence of small comets or to the above impact scenario. The objections are directed toward the numbers, the numbers of such objects required to explain the yearly rates of atmospheric holes. A yearly rate of 1 to 10 events in our atmosphere would be generally accepted by the scientific community, 10 to 100 events might be forgiven, but a thousand such impacts raise considerable outcries on the basis of "current wisdom". The proposed rate of 10 million small comets impacting our atmosphere each year was generally greeted with scorn and ridicule. Remarkably, at this rate over the age of Earth, the small comets can provide enough water to fill our oceans.
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