The Earth is littered with craters from old meteor impacts, many sufficiently erased to be barely recognizable. One of the clearest examples, Australia’s Wolfe Creek Crater, has been found to be considerably younger than previous estimates and suggests the Earth may be hit disturbingly often.
Wolfe Creek Crater, located in Western Australia's Greater Sandy Desert, is 892 meters (3,000 feet) across on average and was made by an incoming object the size of a large house. While tiny compared to the Chicxulub crater, it is the second where fragments of the meteor have been found, hinting at its relatively young age.
Despite this, until Dr Tim Barrows of the University of Portsmouth undertook a dating of the crater, it was thought to be 300,000 years old. Barrows used optically stimulated luminescence, which measures the time since rocks were exposed to sunlight, to produce a new age estimate of 120,000 years in the journal Meteoritics & Planetary Science.
"The crater is located in a fortuitous situation where we can use two different techniques to determine its age,” Barrows said in a statement. “The impact of the meteorite tilted and overturned the rock, exposing rock that was previously shielded from cosmic radiation. The newly formed crater also deflected the local wind field and created a new set of sand dunes. Results from the two dating techniques mutually support each other within the same age range."
Estimating the frequency of crater-causing impacts is difficult because so many of them are hidden under water, forests, or ice sheets, and age estimates are vague even for many we know about. A crater in Greenland could be anything from 11,700 to 2.6 million years old, despite one of the biggest debates in geology being keen on narrowing this immense range down.
Australia's geology means craters are easier to spot than anywhere else in the world, making it the best continent to use for estimating global impact rates. Wolfe Creek's dating brings the number identified as having been made in the last 120,000 years to seven.
That means one meteor large enough to make a dent every 17,000 years. Whatever the fauna may suggest, there is no evidence the universe targets Australians. Allowing for the fact that, as Barrows notes, "the craters are only found in the arid parts of Australia,” the Earth as a whole should average one such strike every 180 years. Even in the deserts, however, sands could have filled in shallower craters, so the true rate could be even higher.
We know of two substantial objects that hit the atmosphere in the last 180 years, but both the Tunguska and Chelyabinsk meteors exploded in the air, and neither (probably) left a visible crater, let alone one this size, but we probably shouldn't count on that continuing.