Contributions to our knowledge of Earth, and thus of our origins. An argument used to deny the existence of the small comets relies upon scientific dogmas which explain all facets of our planet and our origins with absolutely no need for a role for the small comets. But over the past ten years or so our advances in technology have revealed exciting new results which are beyond the reach of the dogmas. The discovery of the small comets is one such example of the advancement of our scientific frontiers. We summarize here several of the important implications of this swarm of small, dark comets which are present in the vicinity of our planet.

The origins of our oceans always have been a fascinating mystery. In his classic paper of 1951 William Rubey noted that an inventory of Earth's water gave the following results:

Rubey was puzzled by the large amount of water that was unaccounted for. Even at the time of writing his seminal paper he queried astronomers as to whether it was possible that this water was being supplied by an infall of objects from interplanetary space. The responses of astronomers were negative. The mystery was not resolved.

Figure 12. Diagram showing the motion of a waterbearing oceanic plate into the Earth's mantle.
But our knowledge of the geological dynamics of Earth is currently expanding at a rapid rate. Large amounts of water are now believed to be lost as subduction of continental plates carries oceanic water under the surface. This scenario is depicted in Figure 12. This water was thought to be recycled to the surface by outflow of gases from volcanic activity. This was a natural suggestion in consideration of the dramatic activity of volcanoes. A picture of such a volcano is shown in Figure 13. This is Paricutin, which was born in a Mexican corn field in 1943. In a recent classic paper of 1999 David Deming reports on the amount of water returned to the Earth's surface by volcanic activity and finds that:

"The losses of surface water due to subduction into the mantle are greater by factors of 7 to 20 than the supply given by volcanic activity."
"The rate of a cosmic influx of water to compensate for the water loss to the mantle is similar to that derived by Frank and Sigwarth [1993] from observations of small comets."
The significance of the small comets is obvious: our Earth would be dry and barren without an extraterrestrial influx of water.
Figure 13. Photograph of the Mexican volcano Paricutin in 1946.

But a further conclusion in Deming's paper is unsettling:

"Life on Earth may be balanced precariously between cosmic processes which deliver an intermittent stream of life sustaining volatiles from the outer solar system or beyond, and biological and tectonic processes which remove these same volatiles from the atmosphere by sequestering water and carbon in the crust and mantle."

One of the arguments used against the existence of the small comets was the extreme dryness of our atmosphere above altitudes of 20 to 30 miles. The claim was that, if there was such a large flux of cometary water clouds entering our atmosphere, then surely there must be some evidence of the water vapor at high altitudes. The lack of such water vapor was cast into cartoon form as "falling chickens" as shown in Figure 14 [Not shown. The Ken Brown cartoon spoofs the many impacts of small comets into our atmosphere. Captioned "The effects of rubber chickens on the atmosphere," it shows a scientist recording in a pad the falls of rubber chickens all around him].

However, surprises concerning the amount of water vapor in our high atmosphere were revealed with instruments flown on the Space Shuttle and on the unmanned Upper Atmospheric Research Satellite. These measurements of unexpected quantities of water vapor at altitudes in our mesosphere in the range of 40 miles or so are depicted in Figure 15. Although most of the cometary water will not be stopped in its rapid downward motion until it reaches the stratosphere, a small amount of water vapor is expected to be left at the high altitudes. The findings of such water vapor, a measurement which is a significant technical achievement, quieted many of the objections which were based upon the "very dry upper atmosphere." A further concern now arises if the upper atmosphere is bathed with a gentle cosmic rain. That concern is the existence of high-altitude ice clouds at about 55 miles. A photograph of these clouds which are found at polar latitudes is shown in Figure 16. If these clouds are due to the presence of cometary water vapor and their fluxes were to increase, then these clouds could advance toward lower latitudes. An ice age would ensue.

Even though the small comets were a frequent topic for the press and were quite popular with the public several icons of the American Geophysical Union were successful in preventing any article concerning them in the union's newsletter EOS. This censure continued for over ten years until the confirmations with the Polar cameras.

Figure 15. The recent discovery of an unexplained excess of water vapor at high altitudes in our atmosphere. Figure 16. Icy clouds at high altitudes over the polar regions
Figure 17. Award winner "Tex" Dessler suggesting to his master that he should look to the skies for evidence of small comets.

There was one exception to this censure which was found to be distasteful to many of the society's members. This exception was the dinner award in 1992 to Alexander Dessler's dog for "keen astronomical observations and loyalty to authority." The award photograph is shown in Figure 17. But such ridicule is expected in such a hotly contested issue as small comets. I point out to young scientists in all fields that an advance to the frontier of knowledge can often lead to difficult "rites of passage."

The photograph and caption of Figure 17 does remind one of the mysteries of meteors in our atmosphere. An example of a bright meteor trail is shown in Figure 18. "Conventional wisdom" of the 1970s and 1980s stated that the brightest of the meteors in our atmosphere, or "fireballs," were due to dense rocks impacting our atmosphere, even though very few were found on the ground at the expected ends of their trails. One such search effort was the Prairie Network located in the midwest U. S. which employed simultaneous sightings by several separated observers and determinations of the point of ground impact by triangulation. Only a sparse handful of meteoritic rocks were found with these searches. A debate over the possibility that the fireballs were due to loosely bound dust rather than the usually accepted iron and stony rocks was never fully resolved.

Figure 18. A bright meteor, a "fireball," streaking across our skies

It is quite possible that the loosely bound "dustballs" are in fact small comets. At the peak times for meteor research 20 to 30 years ago the possibility that these loosely bound objects might be small comets was simply not suggested. After all, the numbers of fireballs with brightnesses similar to or greater than brightest Venus were about 10 million per year, and who would have thought that there were that many small comets impacting Earth's atmosphere? Crude estimates of the brightness of small comets which are devoid of brightly glowing dust particles are indeed about the brightness of Venus, and their impact rate is about 10 million per year. It is remarkable that the orbits and speeds calculated for many of the "fireballs" are similar to those for the small comets. It would be of great interest if we would return to a new investigation of fireballs with cameras which are able to photograph these objects as seen looking down on the atmosphere from a satellite in low Earth orbit. This would provide a wonderful opportunity to determine the contents of the infalling objects without the obscuration by the atmosphere and the very limited view of a ground observer.

The Moon offered an active battleground of contention. The central argument for interpretation of the Apollo measurements with seismometers placed on its surface by the astronauts is illustrated by [a 1985 "Fluffy" cartoon by W. B. Park in] Figure 19 [Not shown. "Fluffy", the mascot of several critics of the small comets

Figure 20. A photograph of an Apollo astronaut walking on the Moon's surface. Note the shallow crater to his right.

who insist that the Moon should "ring like a bell"--even if the objects are fluffy small comets--is shown falling through the air like a rock. The caption notes that it doesn't matter whether Fluffy lands on his feet or not.]. That is, no matter what the impacting object on the Moon was, rock or cat or small comet, the disturbances recorded by the seismometers would be the same. Although amusing, this position is not reasonable when one considers the different results of a hand-thrown rock or snowball, each weighing one pound, for example. It was expected that the Moon would be "ringing like a bell" from the large number of meteors which were causing the fireballs in the Earth's atmosphere. Surprisingly the Moon was silent, except for a relatively rare meteor event or the thermal groans of the surface.

The ambiguity presented by the numerous fireballs in our atmosphere and the "silence of the Moon" can be resolved with the existence of small comets. The lunar surface is covered by a dust layer with thicknesses typically in the range of feet. This soil layer is called the regolith. The lunar seismometers could easily detect the impact of a stony object because it penetrated through the soil layer to the bedrock and "rang" the Moon's interior. On the other hand, the impact of a fluffy small comet would produce shallow craters in the lunar soil, not sufficiently deep to penetrate to the bedrock. Because of this shallow penetration and the structural fragility of the small comets, only weak disturbances would be caused by the small comet impacts. Indeed the lunar surface is known to be saturated with shallow craters with diameters of tens of feet which would be expected from numerous impacts by small comets. A photograph of such a shallow crater is shown in Figure 20 at the right-hand side of the Apollo astronaut.

Figure 21. Photographs of the same area of the Moon during the Apollo mission (upper) and later with the Clementine spacecraft with obviously poorer resolution (lower).

 

With the Polar spacecraft confirmations of the existence of small comets in three independent ways, there was a new flurry of activity intended to show that their presence was contrary to the lunar cratering record. This evidence was based upon the comparison of Apollo images of the lunar surface in 1972 and images of the same area of the Moon with the Clementine spacecraft in 1994. The comparison of these images separated in time by 22 years in order to search for new impacts on the surface of the Moon due to small comets was a great idea. Unfortunately the work failed as noted by the images shown on the cover of Geophysical Research Letters of December 15, 1997 and reproduced here as Figure 21. At the top of the figure is shown the high resolution Apollo image. A scale bar of 1 kilometer, or 3300 feet, in the bottom image is valid for both images. The dimensions of the shallow craters from the impact of the small comets are expected to be in the range of a few tens of feet at most. The Apollo images are magnificent and do record craters of these dimensions at their limits. The Clementine image of the same area of the Moon is shown in the lower panel of Figure 21. The resolution of this image is so poor that it takes some examination to convince the viewer that the same surface area of the Moon is being examined. To detect craters of the size associated with small comet impacts is hopeless. To hypothesize huge bright spots as the cometary impacts defies reasonable considerations of the actual impacts. It is disappointing that the Clementine images were too blurry for the comparison. Otherwise the results would have been very exciting.

 

 

 

 

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