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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
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.
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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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.
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".
