70 AUSTRALIAN SKY & TELESCOPE April 2019
COSMIC RELIEF by David GrinspoonIN THE 1960s, we took a giant leap
forward in our understanding of the
threat that meteors pose to our planet.
In 1960, Eugene Shoemaker firmly
established that a meteor — and not
a volcanic eruption or other cause —
produced Arizona’s Meteor Crater. A
huge explosion by an iron meteorite
slamming into Earth 50,000 years
ago gouged out the 1.2-kilometre-
wide pit. This confirmation, combined
with detailed lunar photography
that definitively showed that the
Moon’s craters arose from impacts,
represented an important watershed
in our comprehension of the profound
connections between our planet and the
rest of the Solar System.
The father-and-son team of Luis
and Walter Alvarez starkly illustrated
these connections in 1980. That’s the
year they convincingly argued that
an asteroid impact 65 million years
ago triggered the mass extinction
that wiped out the dinosaurs (and
most other species). We realised then
that occasional, monstrously violent
celestial bombardments have repeatedly
influenced biological evolution on Earth.
This ushered in what some called
the “new catastrophism,” whose
exponents proposed impact events as
the cause of many an unexplained and
climactic change in our planet’s history.
The phrase is a nod to the old tension
within geology between catastrophism
and uniformitarianism. In the 19th
century, it was a breakthrough when
we realised that colossal transitions
in Earth history did not requireSudden
impact
The discovery of a huge meteor
crater beneath Greenland’s ice
sheet has reignited a debate over
cosmic influences.catastrophic happenings, just what we
now call deep time. Slow changes acting
over previously unimagined temporal
expanses — millions of years or more —
could raise mountain ranges and carve
deep canyons.
Yet the evidence for impacts indicated
that some important geologic changes
are indeed sudden and catastrophic.
The new catastrophists sometimes went
a bit overboard, however, attempting
to explain everything mysterious with
large impacts. At worst, meteor strikes
became a kind of deus ex machina, an
explanation to invoke when evidence
was lacking.
One example of this, arguably, was
an attempt to assign blame to an impact
for the Younger Dryas cooling episode
around 12,000 years ago. This period
roughly correlates with the extinction
of many large mammal species and the
possible disappearance of the Clovis
people. Critics disputed the evidence,
pointing out that no crater of the right
age had turned up. The debates over this
became quite heated and, regrettably,
sometimes acrimonious.
The recent discovery in northwest
Greenland of a large impact crater,
31 km across and possibly as young as
12,000 years, has predictably rekindledthis controversy, with proponents
of a Younger Dryas impact feeling
vindicated and detractors digging in.
Certainly, it’s a captivating
possibility. If a crater that massive really
is so young, then one would expect
demonstrable effects on human and
natural history at that time, perhaps
matching the alterations that occurred
then in North America. But the age of
the impact is not precisely known. The
12,000-year number is a lower limit for
the age; it could be much older.
The best and perhaps only way to
determine when the impact took place
will be to drill down through the area’s
nearly kilometre-thick ice, retrieve
samples of melted rock from the crater
itself, and determine their ages in a
laboratory using radiometric isotopes.
It will be some time before this
can happen. Until we know when this
impact occurred, the smart attitude is
to reserve judgment. Meantime, it will
be fascinating to watch the debate play
out — from a safe distance.■ DAVID GRINSPOON is an
astrobiologist who, for his PhD
dissertation, modelled the effects of
large impact events on the evolution of
Earth-like planets. K. H. KJÆR ET AL. /SCIENCE ADVANCES2018, CC BY-NC 4.066°W 64°W79°N0m 78°30′N750 mHeight
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