The\u00a0Chicxulub crater, buried beneath the Yucat\u00e1n Peninsula in Mexico, was identified in 1990 as a giant impact feature \u2013 some 150km across and 20km deep (although by now well filled-in with sediments).<\/p>\n
The calculated formation of this crater coincides with the mass extinction of the dinosaurs around 66 million years ago, and the impactor is believed to have been an asteroid about 10\u201315km in diameter.<\/p>\n
For more on asteroids and meteorites, read our interview with Dr Sarah Crowther on the science of space rocks<\/a> or find out how dogs can help locate meteorites on Earth<\/a>.<\/strong><\/em><\/p>\n The Chicxulub crater is one of the largest impact structures ever found on Earth. But there\u2019s something else exceptional about this impact.<\/p>\n It seems that the crater was created by a carbonaceous chondrite (CC) asteroid \u2013 a dark rock full of organic compounds.<\/p>\n This is surprising because impacts from this sort of object are rare. They make up only 5% of all meteorites collected.<\/p>\n So why is it that one of the largest craters on Earth \u2013 and the impact that wiped out the dinosaurs \u2013 was formed by such an asteroid?<\/p>\n David Nesvorn\u00fd and his colleagues at the Department of Space Studies, Southwest Research Institute<\/a>, in the US, have been trying to get to the bottom of this mystery.<\/p>\n They built a computer simulation of how the orbits of objects in the main asteroid belt<\/a> can be dislodged to become near-Earth asteroids that have the potential to collide with our planet.<\/p>\n Their model included over 42,000 asteroids with widths greater than 5km in the main belt, and how they\u2019re affected by influences such as the radiation pressure of sunlight or Jupiter<\/a>\u2019s gravity.<\/p>\n They calculated that Venus<\/a> and Earth receive about the same number of strikes from asteroids bigger than 5km, with Mars<\/a> being hit about 3 times less.<\/p>\n Interestingly, Nesvorn\u00fd found that about 6% of the simulated Venus impactors had evolved into a retrograde orbit beforehand \u2013 equivalent to cars colliding head-on with a much higher impact speed \u2013 and hit the planet at a staggering 220,000km\/h.<\/span><\/p>\n But what about the composition of the largest impactors, such as the Chicxulub asteroid?<\/p>\n Nesvorn\u00fd and his colleagues found that because of the orbital dynamics involved, smaller impactors (less than a kilometre in diameter) are most likely to have come from the inner edge of the asteroid belt, so have an ordinary, stony composition \u2013 as found with most meteorites discovered.<\/p>\n But the largest impactors hitting Earth are more likely to have originated from the middle or outer asteroid belt, where CC objects are more common.<\/p>\n They conclude that the impactor that triggered the mass extinction 66 million years ago was a main-belt asteroid that quite likely (they calculate around 60% probability) originated from beyond 2.5 AU.<\/p>\nWhat caused the Chicxulub crater?<\/strong><\/h1>\n
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