What blasted out the Sudbury Basin? A new study points towards an icy culprit

Tuesday, November 18, 2014, 6:08 PM -

Nearly 2 billion years ago, something slammed into what is now the Canadian Shield, blasting out a huge crater that completely destroyed the landscape surrounding where the city of Sudbury, Ontario is located today. What that massive impactor was, though - an asteroid or comet - has been debated for years, but a new study may have solved the mystery.

Considered a remnant of the second largest impact crater on the planet, the Sudbury Basin was formed roughly 1.85 billion years ago by a massive object slamming into the planet.

What remains now, a swath of the Canadian Shield roughly 60 km long by 30 km wide, is but a small fraction of the size of the original crater (which is estimated at around 250 kilometres in diameter), but it still holds clues about the original impact and about the object that formed it.

Numerous studies of the area have given us an idea of exactly how big the impact was, given the location of various 'shock formations' and since some of the materials ejected from the crater have been found even as far away as Minnesota. However, exactly what the object was, though, has remained a mystery.

Topographic map of the Sudbury astrobleme. Credit: NASA

It may have been an asteroid - a large chunk of rock and/or metal, probably several kilometres wide and travelling at around 100,000 km/h when it entered the atmosphere. Instead, it could have been an icy comet - certainly less dense than an asteroid, but typically quite a bit bigger and usually travelling a lot faster too. Since you can put together two different scenarios - a small, dense, fast-moving asteroid and a bigger, less-dense, but faster moving comet - to explain any particular crater, you have to dig a bit deeper to figure out which was the cause.

To possibly solve the mystery for the Sudbury Basin, three researchers - Joseph Petrus, a PhD candidate at Sudbury's Laurentian University, Doreen Ames, a research scientist with the Geological Survey of Canada and Balz Kamber, a geology professor at Trinity College Dublin, in Dublin Ireland - did just that. In their study, published in the journal Terra Nova, they examined two specific pieces of information: 1) Rock samples from the Basin, looking for components that would point to an extraterrestrial source, and 2) how different impactors (both asteroids and comets) of different sizes vapourize when they burn through the atmosphere and then smash into the ground.

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Looking at the concentration and distribution of what are known as siderophile (or 'iron-loving') elements, like iridium - which are abundant in all objects in the solar system, but in low concentrations in Earth's crust due to planetary differentiation - the researchers narrowed the possibilities down to two - a 5 km wide chondritic asteroid, and a 15 km wide comet (which typically contain about 1/4 chondritic material). They were able to further narrow it down based on the size of the original impact crater, and fact that the distribution of the materials meant that the object would have been completely vapourized in the impact.

Using models to simulate both an asteroid and a comet hitting Earth along the most common impact angle - 45 degrees - only the comet was both large enough to produce the 250-km wide impact crater, and had the right density to be completely vapourized in the process. This made it the more likely culprit.

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