852 30 AUGUST 2019 • VOL 365 ISSUE 6456 sciencemag.org SCIENCECREDITS: (PHOTO) CHRIS BUTLER/SCIENCE SOURCE; (MAP) A. CUADRA/SCIENCEB
arlangi Rock, an ancient hill in the
outback of Western Australia, is dim-
pled by the quarries of Aboriginal peo-
ple who chiseled its fine-grained rocks
into sharp tools. Now, geologists have
added a much deeper layer of history
to those rocks by showing they were forged
2.229 billion years ago, when an asteroid
crashed into our planet. The finding makes
Yarrabubba crater, the 70-kilometer-wide
scar left by the collision, Earth’s oldest.
The geologists who reported the date last
week, here at the Goldschmidt geochemistry
conference, also point out a conspicuous co-
incidence: The impact came at the tail end of
a planetwide deep freeze known as Snowball
Earth. They say the impact may have helped
thaw Earth by vaporizing thick ice sheets
and lofting steam into the stratosphere, cre-
ating a powerful greenhouse effect.
“It’s intriguing to think what a moderate
to large impact event could do in this time
period,” says Timmons Erickson, a geo-
chronologist at NASA’s Johnson Space Center
in Houston, Texas, who led the study. “The
temporal coincidence is striking,” agrees
Eva Stüeken, a geobiologist at the University
of St. Andrews in the United Kingdom. But
she and other researchers are skeptical that
Yarrabubba—which is just one-third the size
of the crater left by the dinosaur-killing im-
pact 66 million years ago—could have had
such a profound effect on the climate. Still,
Stüeken says, paleoclimate studies should
consider the possible role of such violent
collisions. “It forces us to think more about
these impacts and these potential feedbacks.”
Earth likes to cover its tracks. Erosion
from wind and water, as well as the churn
of plate tectonics, mean impact craters are
scarcer the further one goes back in time—even though the cratered surfaces of the
moon and Mars show impacts were actu-
ally more common in the tumultuous early
solar system. Prior to the dating of Yarra-
bubba crater, the oldest known impact was
the Vredefort Dome, a 2.02-billion-year-old
feature in South Africa that, at 300 kilo-
meters wide, is the world’s largest.
Western Australia is a good place to look
for old craters because it contains the Yilgarn
Craton, one of Earth’s oldest surviving pieces
of crust. In 2001, a magnetic survey near Yar-
rabubba revealed circular features in the bed-
rock, although no crater rim can be seen at
the surface. And when Francis Macdonald, a
geologist at the University of California (UC),
Santa Barbara, took a close look at rocks from
the region, he found the signatures of an im-
pact’s shock: microscopic planar patterns in
mineral crystals and shatter cones, horsetailfracture patterns up to 1 meter long. Some of
the melted and recrystallized rocks from be-
neath the crater—including Barlangi Rock—
had also survived. “We’re looking at the roots
of it,” Macdonald says. In a 2003 discovery
paper, he and his colleagues named the crater
after the local sheep shearing station. They
knew the impact was ancient, but could not
give it a firm date.
In 2014, Erickson saw an opportunity while
on his way to field work elsewhere in West-
ern Australia. He camped near Barlangi Rock
and crisscrossed the hill with a sledgeham-
mer, filling a backpack with a dozen chunks
of rock. In a laboratory tub, he zapped the
rocks with 100,000 volts of electricity, break-
ing them up into their component minerals
without damaging delicate textures.
Next, Erickson had to sift for crystals suit-
able for dating. Like a gold prospector, he
used pans to float off less dense quartz and
feldspar, and he extracted other unwanted
minerals with a magnet. Finally, with twee-
zers and a microscope, he picked out several
hundred grains of zircon and monazite, each
smaller than the width of a human hair. “You
need a good podcast or music when you’re
doing that,” he says.
He wanted crystals with rims that had
melted and recrystallized, an assurance that
the impact had reset a clock in which small
amounts of radioactive uranium, trapped
within the crystal, decay into lead. He
mounted some of the best crystals in epoxy,
polished them down to a fresh face, and va-
porized spots on the rims with an ion beam.
A mass spectrometer measured the abun-
dance of uranium and lead in the vapor;The 2.2-billion-year-old Yarrabubba impact
came at the end of a planetwide deep freeze
EARTH SCIENCEOxygen from ancient life may have led to Snowball
Earth (artist’s concept). Did an impact help it thaw?By Eric Hand, in Barcelona, SpainWorld’s oldest impact
crater dated in
Australian outback
AUSTRALIAPerthYarrabubba
crater
Yilgarn
Craton0 1000
KmCoral SeaGreat
Australian
BightCanberraSydneyShock and thaw?
Yarrabubba crater is in the Yilgarn Craton, an ancient
piece of crust. Dust and steam from the impact may
have helped end a global ice age, researchers suggest.Published by AAAS