Source: article from Science
Oxygen from ancient life may have led toSnowball Earth (artist's concept).Did an impact help it thaw? CHRIS BUTLER/SCIENCE SOURCEShock and thaw? Earth's oldest asteroidimpact may have helped lift the planetout of a deep freezeBy
Eric HandAug. 27, 2019 , 4:35 PMBARCELONA, SPAIN-Barlangi Rock, anancient hill in the outback of Western Australia,is dimpled by the quarries of Aboriginal peoplewho chiseled its fine-grained rocks into sharptools.Now, geologists have added a much deeper layerof history to those rocks by showing they wereforged 2.229 billion years ago, when an asteroidcrashed into our planet.The finding makes Yarrabubba crater, the70-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 planetwidedeep freeze known as Snowball Earth.They say the impact may have helped thaw Earthby vaporizing thick ice sheets and lofting steaminto the stratosphere, creating a powerful greenhouse effect."It's intriguing to think what a moderate to largeimpact event could do in this time period," saysTimmons Erickson, a geochronologist at NASA'sJohnson Space Center in Houston, Texas, who ledthe study. "The temporal coincidence is striking,"agrees Eva Stüeken, a geobiologist at the Universityof St. Andrews in the United Kingdom.But she and other researchers are skeptical that Yarrabubba-which is just one-third the size of thecrater left by the dinosaur-killing impact 66 millionyears ago-could have had such a profound effecton the climate.Still, Stüeken says, paleoclimate studies shouldconsider the possible role of such violent collisions."It forces us to think more about these impactsand these potential feedbacks."SIGN UP FOR OUR DAILY NEWSLETTERGet more great content like this delivered rightto you!Earth likes to cover its tracks. Erosion from windand water, as well as the churn of plate tectonics,mean impact craters are scarcer the further onegoes back in time-even though the crateredsurfaces of the moon and Mars show impactswere actually more common in the tumultuousearly solar system.Prior to the dating of Yarrabubba crater, theoldest known impact was the Vredefort Dome,a 2.02-billion-year-old feature in South Africathat, at 300 kilometers wide, is the world'slargest.Western Australia is a good place to look forold craters because it contains the YilgarnCraton, one of Earth's oldest surviving piecesof crust.In 2001, a magnetic survey near Yarrabubbarevealed circular features in the bedrock,although no crater rim can be seen at the surface.And when Francis Macdonald, a geologist at theUniversity of California (UC), Santa Barbara,took a close look at rocks from the region, hefound the signatures of an impact's shock:microscopic planar patterns in mineral crystalsand shatter cones, horsetail fracture patternsup to 1 meter long. Some of the melted andrecrystallized 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 colleaguesnamed the crater after the local sheep shearingstation.They knew the impact was ancient, but could notgive it a firm date.Breaking the iceYarrabubba crater is in the Yilgarn Craton, anancient piece of crust.Dust and steam from the impact may have helpedend a global ice age, researchers suggest.
A. CUADRA/SCIENCEIn 2014, Erickson saw an opportunity while onhis way to field work elsewhere in Western Australia.He camped near Barlangi Rock and crisscrossedthe hill with a sledgehammer, filling a backpackwith a dozen chunks of rock. In a laboratory tub,he zapped the rocks with 100,000 volts ofelectricity, breaking them up into their componentminerals without damaging delicate textures.Next, Erickson had to sift for crystals suitable fordating.Like a gold prospector, he used pans to float offless dense quartz and feldspar, and he extractedother unwanted minerals with a magnet.Finally, with tweezers and a microscope, he pickedout 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'redoing that," he says.He wanted crystals with rims that had melted andrecrystallized, an assurance that the impact hadeset a clock in which small amounts of radioactiveuranium, trapped within the crystal, decay into lead.He mounted some of the best crystals in epoxy,polished them down to a fresh face, and vaporizedspots on the rims with an ion beam.A mass spectrometer measured the abundance ofuranium and lead in the vapor; from the proportionsand the known half-life of the uranium, he and hiscolleagues 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'shistory.Life had existed for more than 1 billion years, butphotosynthetic life-cyanobacteria living in shallowwaters-was a recent evolutionary invention, onethat triggered a sharp rise in atmospheric oxygenabout 2.4 billion years ago.Previously, high levels of methane in the atmospherehad generated a greenhouse effect that warmed theplanet.But many scientists think the methane was destroyedby chemical reactions with Earth's first ozone, producedwhen ultraviolet light from the sun struck the oxygenmolecules.They suspect loss of methane sent Earth crashing intoa set of severe and long-lived ice ages, even at lowlatitudes.Three or maybe four of these icy episodes tookplace between 2.45 billion and 2.22 billion yearsago, which means Australia might have beencovered in ice at the time of the Yarrabubba impact.Scientists have assumed that volcanic eruptionsended the ice ages, by belching carbon dioxideand warming the planet. But Erickson and hiscolleagues speculate that Yarrabubba could havehelped.They modeled the effect of a 7-kilometer-wideasteroid striking an ice sheet between 2 and5 kilometers thick.They found the impact could have spread dustthousands of kilometers, darkening ice andenhancing its ability to absorb heat.It also would have sent half a trillion tons ofsteam into the stratosphere-orders of magnitudemore 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 persistedfor 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 directorgeneral of the Natural History Museum in Vienna,shares those doubts, but says paleoclimateresearchers 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 benefitingfrom a cosmic blow, Koeberl says.Although the public tends to associate impactswith extinctions, he notes that impacts 4 billionyears 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 bedestroyers of life," he says.Posted in:
Earthdoi:10.1126/science.aaz2892