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Amazing news

Source: article from Science

Oxygen from ancient life may have led to

Snowball Earth (artist's concept).

Did an impact help it thaw?

 CHRIS BUTLER/SCIENCE SOURCE

Shock and thaw? Earth's oldest asteroid

impact may have helped lift the planet

out of a deep freeze

By Eric HandAug. 27, 2019 , 4:35 PM

BARCELONA, SPAIN-Barlangi Rock, an

ancient hill in the outback of Western Australia,

is dimpled by the quarries of Aboriginal people

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 coincidence:

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, creating a po

werful greenhouse effect.

"It's intriguing to think what a moderate to large

impact event could do in this time period," says

Timmons Erickson, a geochronologist 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 impact 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."

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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 actually more common in the tumultuous

early solar system.

Prior to the dating of Yarrabubba crater, the

oldest known impact was the Vredefort Dome,

a 2.02-billion-year-old feature in South Africa

that, at 300 kilometers 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 Yarrabubba

revealed circular features in the bedrock,

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 impact's shock:

microscopic planar patterns in mineral crystals

and shatter cones, horsetail fracture patterns

up to 1 meter long. Some of the melted and

recrystallized rocks from beneath 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.

Breaking the ice

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.

A. CUADRA/SCIENCE

In 2014, Erickson saw an opportunity while on

his way to field work elsewhere in Western Australia.

He camped near Barlangi Rock and crisscrossed

the hill with a sledgehammer, filling a backpack

with a dozen chunks of rock. In a laboratory tub,

he zapped the rocks with 100,000 volts of

electricity, breaking them up into their component

minerals without damaging delicate textures.

Next, Erickson had to sift for crystals suitable 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 tweezers 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

eset 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 vaporized

spots on the rims with an ion beam.

A mass spectrometer measured the abundance of

uranium and lead in the vapor; from the proportions

and the known half-life of the uranium, he and his

colleagues could calculate an age.

They ended up with a date of 2.229 billion years old,

plus or minus 5 million years.

That puts the impact at a turbulent time in Earth's

history.

Life had existed for more than 1 billion years, but

photosynthetic life-cyanobacteria living in shallow

waters-was a recent evolutionary invention, one

that triggered a sharp rise in atmospheric oxygen

about 2.4 billion years ago.

Previously, high levels of methane in the atmosphere

had generated a greenhouse effect that warmed the

planet.

But many scientists think the methane was destroyed

by chemical reactions with Earth's first ozone, produced

when ultraviolet light from the sun struck the oxygen

molecules.

They suspect loss of methane sent Earth crashing into

a set of severe and long-lived ice ages, even at low

latitudes.

Three or maybe four of these icy episodes took

place between 2.45 billion and 2.22 billion years

ago, which means Australia might have been

covered in ice at the time of the Yarrab

ubba impact.

Scientists have assumed that volcanic eruptions

ended the ice ages, by belching carbon dioxide

and warming the planet. But Erickson and his

colleagues speculate that Yarrabubba could have

helped.

They modeled the effect of a 7-kilometer-wide

asteroid striking an ice sheet between 2 and

5 kilometers thick.

They found the impact could have spread dust

thousands of kilometers, darkening ice and

enhancing its ability to absorb heat.

It also would have sent half a trillion tons of

steam into the stratosphere-orders of magnitude

more water vapor than in today's stratosphere-

where it would have trapped heat.

Andrey Bekker, a geologist at UC Riverside,

doubts that the water vapor would have persisted

for the centuries needed to thaw Earth.

"I'm not convinced that by itself it could do this job

," he says.

Christian Koeberl, an impact expert and the director

general of the Natural History Museum in Vienna,

shares those doubts, but says paleoclimate

researchers need to model the efects explicitly.

If the Yarrabubba impact did thaw the planet,

allowing life to reclaim icy continents and oceans,

it wouldn't be the first example of life benefiting

from a cosmic blow, Koeberl says.

Although the public tends to associate impacts

with extinctions, he notes that impacts 4 billion

years ago could have jump-started life.

Asteroids delivered phosphorus, a key nutrient,

and the impacts also created the protected,

energy-rich hydrothermal systems where some

 biologists believe life began.

"Impacts can be bringers of life, impacts can be

destroyers of life," he says.

Posted in: 

• Earth

doi:10.1126/science.aaz2892