No. 126: Nov-Dec 1999
Our thought, expressed in SF#125, that the icy-comet controversy might be winding down was premature. P. Huyghe, coauthor with L. Frank of The Big Splash, responded to SF#125 with three recent articles. Two reply to major criticisms of the icy-comet theory; the third gives geological and geophysical reasons why there must have been icy comets or some other substantial influx of water and carbon to the earth's surface down the geological eons.
No instrumental artifacts. The basis for the 1985 claim of L. Frank et al that small, icy comets continually bombard the earth's upper atmosphere came from photos taken far above the earth from the Dynamics Explorer 1. Large, transient "holes" appeared in the atmosphere. These were attributed to vapor clouds created by small, icy comets. (SF#44) Critics claimed that these "holes" were no more than instrumental errors. L.A. Frank and J.B. Sigwarth have investigated this possibility and have rejected it.
(Frank, J.A., and Sigwarth, J.B.; "Atmospheric Holes: Instrumental and Geophysical Effects," Journal of Geo physical Research, 104:115, 1999. Cr. P. Huyghe)
Navy radar search used incorrect cross sections. A more recent attack on the icy comets came from S. Knowles et al. (SF#125) They claim that their search of the sky with the Naval Space Command Radar would surely have detected the icy comets if they exist. Frank and Sigwarth respond that
Knowles et al used radar cross sections that are significantly different from those typical of icy comets. It is likely that the Navy radar would not have been able to detect the comets.
(Frank, L.A., and Sigwarth, J.B.; "Comment on 'A Search for Small Comets with the Naval Space Command Radar' by S. Knowles et al," Journal of Geophysical Research, 104:22,605, no. A10. Cr. P. Huyghe.) Knowles et al replied that the cross sections were O.K. and their conclusion stands!
Too much water and carbon. Strong, indirect evidence for the steady influx of icy comets comes from the geologists. They find that on and near the surface of the earth there is much more water and carbon than can be ascribed to the weathering of the earth's rocks. For example, the amount of carbon tied up in rocks (carbonates, etc.) is 600 times that now found in the combined atmosphere, hydrosphere, and biosphere. Where did all this extra carbon come from? The same question can be asked about the earth's water inventory.
Geologists have long assumed that this excess water and carbon came from the outgassing of volcanos. But recent quantitative estimates tell us that the volcanic sources are grossly inadequate. So are all other possible terrestrial sources. Therefore, some scientists, such as D. Deming, University of Oklahoma, have been looking spaceward. Deming ventures that extraterrestrial sources of water and carbon may be four or five orders of magnitude greater than suspected.
Obviously, a steady bombardment of icy comets might fulfill Deming's requirements. Down the long eons of geological time, they could have filled the oceans and showered all that excess carbon onto the planet's surface.
Deming ups the stakes in the icy-comet controversy when he links these fluffy snowballs to the well-known vagaries of life on earth.
"The extraterrestrial influx rate may also act as the pacemaker of terrestrial evolution, at times leading to mass extinctions through climatic shifts induced by changes in accretion rates with concommitant disruptions of the carbon and nitrogen cycles. 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."
(Deming, David; "On the Possible Influence of Extraterrestrial Volatiles on Earth's Climate and the Origin of the Oceans," Palaeo, 146:33, 1999. Cr. P. Huyghe)
Comment. Need we mention the book Living Comets, by F. Hoyle and C. Wickramasinghe? Why stop at water and carbon?