There's no shortage of meteorites that have slammed into our planet since its creation. The vast majority of the craters they've left have eroded away or slowly sunk into the Earth through the process of subduction. Still, the Earth Impact Database, the list of confirmed impact craters, maintained at Canada's University of New Brunswick, isn't a short one. It comprises 176 pocks from 50 feet to 186 miles in diameter.
That doesn't mean it's easy to get on the list.
Sankar Chatterjee, a paleontologist at Texas Tech University's Department of Geosciences, has been trying to get a crater on that list since 1992, claiming that he's found the granddaddy of them all: the Shiva Crater off the west coast of India. He dates the crater to 65 million years ago, and says that it participated in the extinction of the dinosaurs, along with Mexico's Chicxulub and Ukraine's Boltysh impacts.
This runs hard against the consensus in the scientific community, which points only to the Chicxulub Crater as the event that most likely wiped out the dinosaurs. In fact, say some scientists, there's no crater at all off India's coast.
The Shiva Crater, if it is one, would be a monster: About 340 miles in diameter, and slightly tear-shaped, says Chatterjee, because of the flat angle at which he thinks the asteroid arrived, traveling from the southwest to the northeast. Twenty-five miles in diameter, and covering half as many miles every second just before it struck, the asteroid packed a load of energy that, Chatterjee says, created the largest impact crater on Earth—almost twice the diameter of the 186-mile-wide Vredefort crater in South Africa.
Them's fightin' words. He presented his latest data on Sunday, October 18, at the Geological Society of America's 2009 meeting in Portland Oregon. On Monday, a couple of scientists at the conference shared their thoughts with us.
"The short answer is that Chatterjeee, and nobody else, has been peddling this story for years, if not decades," says Christian Koeberl, an impact expert who heads the Department of Lithospheric Research at the University of Vienna, Austria. "Never had a proper peer-reviewed paper published, and periodically has abstracts at meetings on this topic. There's a very good reason why this 'crater' is not in the database: there is no crater, and there is no evidence for any crater or ejecta either. Nobody but Chatterjee sees this stuff."
A major problem is the lack of shocked quartz, according to Koeberl, an essential marker left by meteorite strikes. No event other than a nuclear explosion can deform minerals in the same way. Chatterjee insists he's finding it. Many scientists shake their heads. "Claiming that some exists is not the same as demonstrating, to the satisfaction of the relevant community, that such evidence really exists," says Koeberl. "If there were a huge impact in the Indian Ocean, very thick ejecta deposits would exist there. They do not. No evidence for impact-induced tsunamis either, like there is in the Gulf of Mexico."
"The Shiva Crater is like the crater equivalent of Big Foot, or the Yeti if you prefer," said Philippe Claeys, who heads the Earth System Sciences research unit at Belgium's Vrije Universiteit Brussels. "I listened to the talk of Chatterjee yesterday, and was not convinced. The author brought no new evidence supporting the presence of a large impact structure offshore India. Neither did he produce an illustration of the shocked minerals he talked about." Claeys went on to say that such a huge asteroid would make a crater much larger than the one Chatterjee claims to see, and would leave a blanket of debris more than 300 feet thick outside the crater rim. Chatterjee, says Claeys, showed a layer a few centimeters thick in eastern India, some 5o0 miles away from the center of the crater. At that distance, says Claeys, the Chicxulub ejecta is several meters thick. "The geophysical data he presented does not look anything like the observations at other craters, not rounded morphologies, no central peak, etc."
Chatterjee plans to put his research money where his mouth is.
"We hope to get the core samples from the target rocks of the Shiva Basin this year," he says. "If we can document the shock evidence at the basement rock and the timing of the impact, the evidence would be strong."
Don't hold your breath, says Claeys. "Trust me, it is not soon that it will be on the Earth Impact Database."