EARTH & ENVIRONMENT
Giant crater found
Meteorite impact’s effect on
an ancient cold snap is unclear
BY CAROLYN GRAMLING
There’s something big lurking beneath
Greenland’s ice. Using ice-penetrating
radar, scientists have discovered a
31-kilometer-wide crater, larger than
the city of Paris, buried under as much
as 930 meters of ice.
The meteorite that the researchers
say hit Earth and formed the pit would
have been about 1. 5 kilometers across.
That’s large enough to have caused environmental damage across the Northern
Hemisphere, glaciologist Kurt Kjaer of the
University of Copenhagen and colleagues
report November 14 in Science Advances.
The crater is the first one found in
Greenland and one of the 25 or so largest
craters yet found on Earth.
Though the crater has not been dated,
data from glacial debris as well as ice-flow
simulations suggest that the impact happened sometime during the Pleistocene
Epoch, 2.6 million to 11,700 years ago.
The discovery could breathe new life into
a controversial hypothesis that an impact
about 13,000 years ago triggered a mysterious 1,000-year cold snap known as the
Younger Dryas (SN: 7/7/18, p. 18).
Members of the research team spotted
a rounded shape at the edge of Hiawatha
Glacier in northwest Greenland during a
scan by NASA’s Operation IceBridge in
2015. The mission uses airborne radar
to map the thickness of polar ice. The
researchers suspected that the shape represented the edge of a crater, Kjaer says.
For a more detailed look, the team
hired an aircraft equipped with ultra-
wideband radar, which can send pulses of
energy toward the ice at a large number
of frequencies. Combining the Operation
IceBridge and the ultra-wideband radar
data, the team mapped the inner and
outer contours of its target.
The object is almost certainly an
impact crater, says team member and
electrical engineer John Paden of the
University of Kansas in Lawrence. “It’s so
conspicuous in the satellite imagery now.
There’s not another good explanation.”
On the ground, the team hunted for
geochemical and geologic signatures
of an impact within nearby sediments.
Sampling from within the crater itself
was impossible, as it is covered by ice.
But just beyond the edge of the ice, melt-
water from the base of the glacier had,
over the years, deposited sediment.
A sediment sample collected from
within that glacial outwash contained
several telltale signs of an impact:
“shocked” quartz grains with deformed
crystal lattices and glassy grains that
may represent flash-melted rock. The
sample also had elevated concentrations
of certain elements, including nickel,
cobalt, platinum and gold, relative to
what’s normally found in Earth’s crust.
That profile points not only to an impact,
the researchers say, but also suggests
that the impactor was an iron meteorite.
Determining when that iron meteorite slammed into Earth is trickier.
Ice-penetrating radar revealed that
the crater bowl itself has several distinct
layers of ice. The topmost layer has a
continuous sequence of smaller layers,
representing the gradual deposits of snow
and ice through the last 11,700 years, a
period known as the Holocene. At the
base of that layer is a distinct, debris-
rich layer that has been seen elsewhere
in Greenland ice cores and is thought to
represent the Younger Dryas cold period
12,800 to 11,700 years ago. Beneath the
Younger Dryas layer is another large
layer — but unlike the smooth Holocene
layer, this one is jumbled and rough.
“You see folding and strong distur-
bances,” says study coauthor Joseph
MacGregor, a glaciologist with Operation
IceBridge. “And below that, we see yet
deeper, complex basal ice.” Radar images
of that bottommost layer within the cra-
ter show several peaks, which MacGregor
says could represent material from the
ground that got incorporated into the ice.
“Putting that all together, what you have
is a snapshot of an ice sheet that looked
fairly normal during the Holocene, but
was quite disturbed before that,” he says.
Those data suggest that the impact
is at least 11,700 years old, Kjaer says.
And the rim of the crater appears to
cut through a preexisting ancient river
channel that must have flowed across the
land before Greenland became covered
with ice about 2.6 million years ago.
Planetary scientist Clark Chapman
of the Southwest Research Institute in
Boulder, Colo., is skeptical that the crater
formed within the last couple of million
years. It’s “quite unlikely,” he says. Such
strikes are rare in general, and asteroids
barreling into Earth are far more likely to
land in the ocean. And, he says, “it would
be at least a hundred times less likely that
it could have happened so recently as to
have affected the Younger Dryas.” s
Hidden bowl Airborne radar data revealed a round depression (green circle, right) buried
beneath almost a kilometer of ice at the edge of Hiawatha Glacier in northwest Greenland (left).
Geochemical clues suggest that the depression is an impact crater.
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