The asteroid that hit Earth 65 million years ago and wiped out the dinosaur was at least six miles across and left behind a crater over 110 miles across. But that's nothing compared to a possibly newly discovered impact site.
It's not confirmed yet, but a potential crater site in Greenland would be the oldest, biggest impact ever observed on Earth. The original asteroid would have been about 18 miles across - that's about half the length of Rhode Island, which is pretty damn huge by asteroid standards - and the crater it initially created would have been nearly 375 miles wide and over 15 feet miles deep. While we've seen evidence of impacts on that scale elsewhere in the solar system, we've never seen anything like this on Earth.
So why are we only discovering something this big now? Part of it is its immense age - when this asteroid hit, Earth was only a third its current size. Most of the crater has since been worn away by three billion years of erosion, meaning only the rocks from the very bottom of the original impact site are still there. Besides, part of the reason we know so much about the Chicxulub crater is because it killed the dinosaurs - it left behind plenty of clues in the fossil and geological record that there was something there worth searching for, whereas this massive crater smashed into a relatively empty Earth. The fact that this site is in Greenland instead of Mexico also probably doesn't help.
Still, we can't say for certain that this is an impact site - it's hard to be definitive with something so ridiculously old. But after three years of careful review, researchers from the Geological Survey of Denmark and Greenland say they're pretty much ready to confirm the find. New Scientist has more:
The most compelling evidence is the presence of granite-like rocks that are crushed, melted and pulverised in a way that can only be explained by a sudden, massive impact. The deformed granite is spread throughout an area measuring 35 by 50 kilometres, centred on the supposed impact site. Such large-scale deformation of granite could not have happened over such a large area through any known terrestrial geologic process. "You might see something similar in a geologic fault zone, but not in a circle 100 kilometres across," says team member Iain McDonald of Cardiff University.
You can check out the original article for more of the evidence used to confirm the find.
Top image from outside the town of Maniitsoq on Greenland's western coast, near the site of the find. Image by ilovegreenland on Flickr.