Artist's reconstruction of Chicxulub crater soon after impact, 66 million years ago.

DETLEV VAN RAVENSWAAY/SCIENCE SOURCE

Cientistas se preparam para perfurar o 'ground zero' do impacto que matou os dinossauros

By Eric HandMar. 3, 2016 , 2:00 PM

Este mês, uma plataforma de perfuração vai subir no Golfo do México, mas não terá como objetivo o petróleo. Os cientistas tentarão afundar um pouco de ponta de diamante no coração da cratera Chicxulub - o remanescente enterrado do impacto de um asteroide há 66 milhões de anos que matou os dinossauros, junto com a maioria das outras formas de vida no planeta. Eles esperam que os núcleos de rocha recuperados contenham pistas de como a vida voltou na esteira do cataclismo e se a própria cratera poderia ter sido o lar de uma nova vida microbiana. E perfurando uma crista circular dentro da borda da cratera de 180 quilômetros de largura, os cientistas esperam estabelecer idéias sobre como esses "anéis de ponta", marcas das maiores crateras de impacto, tomam forma.

"Chicxulub é a única estrutura preservada com um anel de pico intacto que podemos alcançar", diz o geofísico Sean Gulick, da Universidade do Texas, Austin, co-chefe cientista do projeto de US $ 10 milhões, patrocinado pelo International Ocean Discovery Program (IODP). e o Programa Internacional de Perfuração Científica Continental. "Todos os outros estão em outro planeta, ou foram erodidos." No final de março, um navio especialmente equipado navegará do porto mexicano de Progreso até um ponto a 30 km da costa. 

 

Lá, na água a 17 metros de profundidade, o barco afundará três postes e se elevará acima das ondas, criando uma plataforma estável. Até 1º de abril, a equipe planeja iniciar a perfuração, rapidamente produzindo 500 metros de calcário que foram depositados no fundo do mar desde o impacto. Depois disso, os perfuradores extrairão amostras centrais, em incrementos de 3 metros de comprimento, à medida que se aprofundarem. Por 2 meses, eles trabalharão dia e noite na tentativa de descer um outro quilômetro, procurando mudanças nos tipos de rochas, catalogando microfósseis e coletando amostras de DNA (veja a figura abaixo). "Temos uma chance para tentar reduzir para 1.500 metros", diz David Smith, gerente de operações do IODP no British Geological Survey em Edimburgo, Reino Unido.

(DIAGRAM) V. ALTOUNIAN/SCIENCE; (PHOTOS) CHRIS JENSON, DES; DAVE SMITH, ECORD; MICHAEL RYMER; DAVID KRING; GEOLOGICAL SOCIETY OF AMERICA; ARTOGRAPHY/SHUTTERSTOCK; VILAX/SHUTTERSTOCK; HINRICH BAESEMANN/ALAMY

Embora esta seja a primeira tentativa de perfuração na cratera, os roughnecks enterraram poços em terra - antes mesmo de os cientistas saberem que uma cratera estava lá. Na década de 1950, geólogos da Pemex, a companhia nacional de petróleo do México, conduziram levantamentos gravimétricos e magnéticos da península de Yucatán e ficaram intrigados ao ver estruturas circulares subterrâneas - possíveis armadilhas de petróleo. Eles perfuraram vários poços exploratórios, mas perderam o interesse quando obtiveram rochas vulcânicas em vez de sedimentos com óleo. “Quando encontraram as rochas ígneas, disseram: 'Ah, esse é um centro vulcânico'”, diz Alan Hildebrand, geólogo da Universidade de Calgary, no Canadá.

In 1980, however, Nobel laureate Luis Alvarez and others called attention to a thin layer of iridium—possible material from an asteroid—found all over the world in rocks from the time of the dinosaur extinctions. It was the signature, they said, of a previously unsuspected cause of the extinctions: a giant impact. In 1991 Hildebrand and colleagues fingered the village of Chicxulub as the site of the cataclysm, finding quartz crystals shocked by an impact in samples from the Pemex wells—samples that had sat around for more than a decade. “Some people are a little embarrassed about that these days,” he says.

The data from the Pemex wells were spotty, and so scientists have always wanted to go back for a detailed look at the impact and its aftermath, says co–chief scientist 
Joanna Morgan of Imperial College London. “It seems like a lifetime’s ambition coming true,” says Morgan, who first proposed a scientifically cored well to the IODP in 1998. Although offshore drilling is expensive, she says that working at sea means the team will face fewer hassles with environmental permitting and won’t have to cope with the Yucatán’s poor roads. In 2005, Morgan and Gulick led a $2 million remote-sensing campaign that used small seismic explosions to help illuminate the subterranean structures and pinpoint the best spot to reach the peak ring.

As the drill approaches the crater, 
800 meters down, scientists expect to find fewer species of the shell-producing animals that make up the limestone, because life was just recovering from the impact. Some scientists think that carbon dioxide released by the impact would have acidified the oceans, contributing to the extinctions, so the drill team will look at whether seafloor animals just after the impact were species that tolerate low pH.

Just above the crater lies an impact layer, 100 meters or more thick, that would have been deposited in the weeks after the cataclysm. At its base, scientists expect to find a hodgepodge of chunks of bedrock blasted up by the impact and once-molten rock that fell back into the crater in the minutes after impact. Above that would be sediments, since hardened into rock, that were swept in as the ocean rushed into the vast new depression. The impact layer may be capped by hardened deposits of ash that persisted in the atmosphere for weeks or more before falling out.

(DIAGRAM) V. ALTOUNIAN/SCIENCE; (MAP) SEAN GULICK, UNIVERSITY OF TEXAS

For many of the IODP scientists, the main event will be reaching the peak ring. Peak rings abound on the moon, Mercury, and Mars. But on Earth, there are just two craters larger than Chicxulub that should also have peak rings: the 2-billion-year-old Vredefort crater in South Africa, and the 1.8-billion-year-old Sudbury crater in 
Canada—yet they are so old that the peak rings have eroded away.

The IODP team wants to test a leading model for peak ring formation, in which granite from Earth’s depths rebounds after a major impact, like water struck by stone, to form a central tower, taller than the crater rim. In minutes, the tower would collapse and collide with material slumping in from the rims to form the peak ring. Confirmation for the model could come from finding rocks “out of order”: deep rocks, probably granite, brought up in the central tower, lying atop originally shallower younger rocks. “They’re going to test whether our numerical models are making any sense or not,” says Jay Melosh, a planetary scientist at Purdue University in West Lafayette, Indiana, who helped develop the model.

The team is interested not just in the structure of the peak ring rocks but in what life they might host. Remote sensing has already suggested that the peak ring is less dense than expected for a granite—a sign that the rocks are porous and fractured in places. It is possible that these fractures, in the wake of the impact, were filled with hot fluids. “Those will be preferred spots for microbes to grow, but it depends whether the fractures have energy and nutrients,” says Charles Cockell, an astrobiologist on the IODP team at the University of Edinburgh. When the drill bit encounters mineral veins or other fracture zones in the peak ring, Cockell and his colleagues will take a subcore from the core: a biopsy on the geopsy. They will count and culture any microbes still living in the fractures, and sequence DNA to look for the genes responsible for metabolic pathways.

Those genes might show that peak ring microbes—descendants of those that lived after the impact—derive their energy not from carbon and oxygen, like most microbes, but from iron or sulfur deposited by hot fluids percolating through the fractured rock. And that would mean the impact crater, harbinger of death, was also a habitat for life.

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doi:10.1126/science.aaf4138