Science - USA (2018-12-21)

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1346 21 DECEMBER 2018 • VOL 362 ISSUE 6421 sciencemag.org SCIENCE

The asteroid slammed into northwestern
Greenland like a fusillade of nuclear bombs,
instantly vaporizing rock and sending
shock waves across the Arctic. The scar it
left—a 31-kilometer-wide impact crater
called Hiawatha—is big enough to hold
Washington, D.C. Scientists reported the
startling discovery in November, after
aircraft radar revealed the crater lurking
beneath the kilometer-thick ice sheet.
Hiawatha crater is one of the 25 largest
on Earth. Though not as cataclysmic as the
dinosaur-killing Chicxulub impact, which
carved out a 200-kilometer-wide crater in
Mexico 66 million years ago, the Hiawatha
impact could have had a powerful effect

on the global climate. Meltwater from the

impact, pouring into the north Atlantic

Ocean, could have sent temperatures

plunging by halting a conveyor belt

of currents that brings warmth to north-

west Europe.

The radar images suggest Hiawatha is

exceptionally fresh, dating from the past

100,000 years. And a disturbance in

the crater’s deep ice hints that the asteroid

may have struck as recently as 13,

years ago. That would tie the impact to

the Younger Dryas, a thousand-year global

cooling event that began just as the world

was thawing from the last ice age. It would

also vindicate proponents of the controver-

sial Younger Dryas impact theory. A decade

ago, they proposed that extraterrestrial im-

pacts could account for hints of mayhem in

the archaeological and geological record.

But they could never point to a crater.

The timing of this impact is far from

settled. Ice cores elsewhere on Greenland,

which record the past 100,000 years,

contain no signs of impact debris. A firm

answer will depend on painstaking work

to tease dates from the radioactive clocks

in tiny mineral crystals swept from under

the ice.

If they show the Hiawatha impact did

occur 13,000 years ago, it would have

come just as humans were fanning across

a new continent, chasing mastodons

around North America. It is tempting

to imagine their thoughts as they looked

up to see the searing white orb of the

impactor, four times brighter than the sun.

—Eric Hand

Ice age impact

A computer visualization of

asteroid fragments falling

toward Greenland.

IMAGE: NASA SCIENTIFIC VISUALIZATION STUDIO

Published by AAAS

on December 24, 2018^

http://science.sciencemag.org/

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