Wolfe Creek Crater younger than previously thought

Wolfe Creek Crater, one of the world's largest meteorite craters, is much younger than previously thought.

Wolfe Creek Crater is situated on the edge of the Great Sandy Desert in northern Western Australia. It is the second largest crater on Earth from which meteorite fragments have been recovered (the largest is Meteor Crater in Arizona).

It was likely formed by a meteor about 15 metres in diameter, weighing around 14,000 tonnes.

The age of the impact is poorly understood and unpublished data suggests the impact could have occurred around 300,000 years ago.

However, according to a new study led by Dr Tim Barrows from the University of Portsmouth, the most likely age for the impact is 120,000 years ago.

In the study, published in the journal Meteoritics & Planetary Science, researchers from the University of Portsmouth, Australia and the USA calculated the new age of Wolfe Creek Crater using two geochronological dating techniques.

First, the researchers collected samples from around the crater rim and applied exposure dating, which estimates the length of time that a rock has been exposed at the Earth's surface to cosmic radiation.

They were also able to determine the age through optically stimulated luminescence, (a dating technique used to measure how long ago sediment was last exposed to sunlight) on sand buried after the impact.

Dr Barrows said: "The crater is located in a fortuitous situation where we can use two different techniques to determine its age. 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."

The researchers were able to produce a new topographic survey of the crater using aerial photos by Ted Brattstrom, a school teacher from Hawaii. He flew over the crater in a light aircraft in 2007 and took pictures of the crater from all directions.

The resulting 3D model was used to create a digital elevation model of the crater. The researchers calculate that the maximum width of the crater is 946 metres in a NE-SW direction, reflecting the direction of the impact. The average diameter is 892 metres.

They also predict a crater depth of 178 metres and that it is filled by about 120 metres of sediment, mostly sand blown in from the desert.

Wolfe Creek Crater is one of seven sets of impact craters in Australia dating to within the last 120,000 years. From this, the researchers were able to calculate as to how often these crater-producing events occur.

Dr Barrows said: "Although the rate is only one large meteor hitting Australia every 17,000 years, it isn't that simple. The craters are only found in the arid parts of Australia.

"Elsewhere, the craters are destroyed by geomorphic activity like river migration or slope processes in the mountains. Since Australia has an excellent preservation record with dated craters within the arid zone, we can estimate a rate for the whole Earth. Taking into account that arid Australia is only about one per cent of the surface, the rate increases to one hitting the Earth every 180 years or so. There have been two big objects hitting the atmosphere in the last century - Tunguska in 1908 and Chelyabinsk in 2013.

"This is a minimum estimate because some smaller impacts were probably covered by sand during the last ice age. The number of large objects the atmosphere is probably 20 times this number because stony meteorites are far more common but not as many survive the fiery journey through the atmosphere or effectively make craters. Our results give us a better idea of how frequent these events are."

Using the same geochronological dating techniques, the researchers were also able to recalculate the age Meteor Crater. They found it is likely to be 61,000 years old, over 10,000 years older than previously thought.

University of Portsmouth