Researchers have long puzzled over what created the giant, elliptical hole called the Sudbury Basin in Ontario, Canada. At 62 kilometers long, 30 kilometers wide, and 15 kilometers deep, it’s the second largest impact crater on the planet. Now, researchers studying the chemistry of the crater and its shattered rocks say it was made by a comet that collided with Earth over 1.8 billion years ago.
Discovered in the late 1800s, the elliptical crater is rich in metals like copper, nickel, and palladium. Because of the presence of “shock features,” researchers know that an impact created the crater. But what sort of impact? Some experts argue that it was made by a meteorite -- solid rocks that deposit iron-loving elements (like platinum) to Earth. Others say it was a comet; these are made of dust and frozen gas, and they were carrying with them few heavy elements.
To see if Sudbury Basin was created by a rocky or icy wallop, a trio of researchers led by Joseph Petrus of Laurentian University conducted geochemical analyses on the iron-loving (or siderophile) elements in 69 samples found in and around the crater. Then, using computer models that simulate the impact, they showed that whatever crashed into the planet was almost completely vaporized on entry. A meteorite big enough to create a crater this large would only be partially vaporized -- which suggests that a comet was the culprit. The work was published in Terra Nova this week.
A massive comet filled with rocky bits blasted into the shallow oceans near the continental margins of the supercontinent Nuna, sending debris and rock fragments flying all the way to where Minnesota is today, LiveScience explains. The crater that resulted was about 150 kilometers across, which eroded over time to its current size.
"Our findings provide further evidence that some very large terrestrial impact basins were created by comets, which is important and interesting in the context of the early bombardment of our inner Solar System," study coauthor Balz Kamber of Trinity College Dublin says in a news release. "It might well be that comets were responsible for bringing volatile elements to the young Earth."
There's still a lot that researchers want to know about comets in general. As Petrus tells Huffington Post: "The Rosetta mission that has been in the news lately will (hopefully) provide unprecedented information about the structure and chemistry of comets."
Kamber adds: "Our geochemical evidence supports, in principle, the idea tested by the recent mission to comet 67P. Namely, we now have confidence from the geological record that the Earth was indeed hit, at least once, by a large comet that deposited its water into our hydrosphere."
Image: Vesta via Wikimedia