– Im­pact re­veals how crater ring formed

Sci­en­tists drilling deep be­neath the ocean pull up clues as to how the dust set­tled just af­ter im­pact. BELINDA SMITH and APRIL REESE re­port.

Cosmos - - Digest -

Sci­en­tists have long won­dered why craters cre­ated by as­ter­oids of­ten have a ring of hills around the cen­tre. Th­ese cir­cu­lar ridges, called peak rings, are com­mon in craters through­out the in­ner so­lar sys­tem. They have been found on the Moon, Mars and Mer­cury.

There is only one good place to study peak rings on Earth: the Chicx­u­lub crater in what is now the Yu­catan Penin­sula in Mex­ico. It formed 66 mil­lion years ago when an as­ter­oid slammed into Earth, trig­ger­ing a se­ries of events that wiped out three-quar­ters of all species on the planet, in­clud­ing the non-avian di­nosaurs. New ev­i­dence from the site holds clues to the ori­gins of the crater’s mys­te­ri­ous ring of moun­tains – and oth­ers like them around the so­lar sys­tem.

Though the 180-kilome­tre-wide crater con­tains Earth’s best-pre­served peak ring, study­ing it is com­pli­cated: the ring, along with much of the rest of the crater, is deep be­low the sea in the Gulf of Mex­ico.

Ge­ol­o­gists dis­cov­ered the Chicx­u­lub crater’s peak ring in 2000 us­ing seis­mic sens­ing, which mea­sures how seis­mic waves travel through the Earth’s lay­ers, re­veal­ing their com­po­si­tion. Ever since, re­searchers have wanted to get at the ring and fig­ure out its story.

One ex­pla­na­tion for the ring’s for­ma­tion, known as the “dy­namic col­lapse the­ory”, holds that the im­pact caused an erup­tion of rock from the lower re­gions of the planet’s crust that then fell back to earth. A com­pet­ing the­ory sug­gests the ridges formed from melt­ing in the up­per crust.

In mid-2016, sci­en­tists got the chance to settle the de­bate. An in­ter­na­tional team led by geo­physi­cists Joanna Mor­gan of Im­pe­rial Col­lege Lon­don and Sean Gulick of the Univer­sity of Texas at Austin used a deep-sea drilling rig to bur­row into the site and pull up cylin­dri­cal sec­tions of rock core. At first, they found layer upon layer of lime­stone – as would be ex­pected in ocean sed­i­ments. But then, about 620 me­tres down, bits of pink­ish-white rock ap­peared in the cores: gran­ite. It was far closer to the sur­face than the rock is usu­ally found, mean­ing it must have got there through some sort of dis­rup­tion. The gran­ite was also un­usu­ally weak, in­di­cat­ing it had un­der­gone sig­nif­i­cant stress, and the rocks re­tained the coarse crys­talline struc­ture of deeper ma­te­rial.

All of which sug­gests, the re­searchers re­ported in Science in Novem­ber 2016, that the dy­namic col­lapse the­ory is cor­rect: the pow­er­ful im­pact blasted gran­ite from

THE “DY­NAMIC COL­LAPSE THE­ORY” HOLDS THAT THE IM­PACT CAUSED AN ERUP­TION OF ROCK FROM THE LOWER RE­GIONS OF THE PLANET’S CRUST.

deep in Earth’s crust to the sur­face and made it be­have like a liq­uid, shoot­ing up into the air like a geyser be­fore col­laps­ing into the basin and set­tling out into a cir­cu­lar ridge around the cen­tre.

The study adds to a small but grow­ing body of ev­i­dence in sup­port of the dy­namic col­lapse the­ory of peak ring for­ma­tion in craters. Last Oc­to­ber, sci­en­tists re­ported that the peak rings within Mare Orientale, a crater on the moon, formed in a sim­i­lar way.

Geo­physi­cist Penny Bar­ton from the Univer­sity of Cam­bridge, who was not in­volved in the study, writes in a

Science com­men­tary on the dis­cov­ery that it ap­pears to “val­i­date the col­lapse mod­els” while also pos­ing “many new ques­tions for fur­ther work on th­ese ex­cit­ing sam­ples”.

THE PEAK RINGS WITHIN MARE ORIENTALE, A CRATER ON THE MOON, FORMED IN A SIM­I­LAR WAY.

One of those ques­tions is how the rocks were weak­ened enough to be­have like a fluid. The team is now at­tempt­ing to find out.

How the Chicx­u­lub peak ring formed isn’t the only ques­tion re­searchers in­volved in the US$10 mil­lion core drilling project hope to an­swer. They also aim to glean other in­sights into Earth’s most fa­mous cat­a­clysm and its ef­fect on the planet.

Four labs are test­ing core sam­ples for irid­ium, a metal that as­ter­oids some­times leave be­hind. While the space rock would have va­por­ised on im­pact, bits of it likely set­tled into the crater.

So far, the search for irid­ium in the cores has come up empty, but in De­cem­ber re­searchers told a meet­ing of the Amer­i­can Geo­phys­i­cal Union they had the next best thing: nickel, which be­haves sim­i­larly to irid­ium and serves as a kind of proxy for the pres­ence of as­ter­oid dust.

Re­searchers hope the hun­dreds of me­tres of core sam­ples from the site will also re­veal more about how the crater formed, how the ma­te­rial it churned up was dis­persed and how long it took life to re­bound at the site.

CREDIT: D. VAN RAVENSWAAY / SPL / GETTY IM­AGES

Im­pact craters of­ten dis­play ‘peak rings’ – as this artist’s im­pres­sion of Mex­ico’s Chicx­u­lub shows. A new study ex­plains.

CREDIT: RON­ALDO SCHEMIDT / AFP / GETTY IM­AGES

Chicx­u­lub rock cores sug­gest the im­pact turned the rock into a geyser that cre­ated the cir­cu­lar ridges.

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