Skyward for September 2025
By
David H. Levy
Aka Doveed.
Thirty-two years ago, Carolyn and Gene Shoemaker and I
discovered a comet that was eventually named Shoemaker-Levy 9. It was the ninth periodic comet that we found
together, although there were a few other nonperiodic comets that we also
located, plus the nine other comets I found on my own since I began my comet
search in the fall of 1965. The discovery of this particular comet and its subsequent
collision with Jupiter, coincidentally my favorite planet, were the most
important parts of my professional life, second only to my meeting Wendee.
Sixteen months after our discovery the 21 pieces of this shattered comet
collided with Jupiter, in one of the most decisive science stories of the
twentieth century. I may not have been
aware of how significant this was until, at this year’s Adirondack Astronomy
Retreat, I watched the July 16, 1994 press conference during which Gene,
Carolyn, and I tried to express the significance of this event. I remembered how much smarter I might have
been back then, being able to speak in complete sentences, compared to my
waning personality now. What I was not
aware of back then is that what we were witnessing might have been an example
not only for our own lifetimes but for the vastly larger history of the Earth
we live on.
Sixty-six
million years ago, the Cretaceous period of Earth’s geologic history ended
rather abruptly with the mass extinction of about three quarters of all the
species of life on Earth. The theory
proposed by Luis Alvarez and his son Walter was based on the large amount of
iridium that was found at exposed rock sites all over the world. The discovery in the early 1990s of the
200-mile wide impact crater whose center was near the coastal town of Chicxulub
Pueblo, in present-day Mexico, began a long stretch of evidence that leads most
scientists to conclude that the impact of an asteroid (or less likely a comet)
had a lot to do with the Cretaceous-Paleogene mass extinction.
More
recently, some evidence has emerged that the impact in the Gulf of Mexico was
not the only one that occurred at that time. The 15-mile wide Boltysh crater in
Ukraine, and the 12-mile wile Silverpit crater in the North Sea, not far from
Great Britain, might have been formed at about the same time. These structures, and others that have been
found or speculated, are all between North latitude 20 and 70 degrees.
Could these
structures be impact craters, and if they are, could they have formed in
connection with the Cretaceous–Paleogene extinction? This suggests the
possibility of near-simultaneous multiple impacts. But the operative word has to be
suggests. The evidence is there, but it
is speculative and not strong, that the Chicxulub impactor might have been just
one of a series of impacts. According to
a paper by Krisopher Dekan of the University of Gothenburg, “To conclude that a mass
extinction of this sort is not associated with immense extraterrestrial impact
is to break the rules of a respected scientist.
There is too much evidence in favor of a least two large impacts and no
other factor can explain the (Iridium) anomaly that is globally widespread in
both sides of the paleomagnetism of that time, being normal and reverse near
the K/Pg boundary.”
We will never know what upended the
Earth’s biosphere 66 million years ago, because we were not there. But at this juncture I would like, not to
ignore the methods of modern science, but to take science out for a walk in the
desert. We will never know, but what if a Shoemaker-Levy 9-style multiple
impact is what caused the elimination of most of the species of life on Earth?
What if? I think it is fun to
speculate on this question. From my own
perspective, as I take that fictitious walk in the desert, my decision to begin
hunting for comets when I was a teenager in 1965 might have led to a personal
communion with a major event on the planet that has given me so much pain, and
so much more joy.
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