Hickman Crater in Western Australia
Traces of outer space were the last thing on geologist Arthur Hickman’s mind as he trawled Google Earth’s satellite imagery, and yet, that’s exactly what he found
DR ARTHUR HICKMAN nods towards a passenger jet passing low overhead as he prepares afternoon tea on the rear drop-tray of his four-wheel-drive. The vehicle is heavy with the equipment necessary for fieldwork and survival in the remote Pilbara.
"You know, we're right under the flight path," he says, bemused. His Manchester accent lingers, despite the fact that for four decades he's worked for the Geological Survey of Western Australia (GSWA), in what he describes as some of the world's most geologically stunning landscapes.
"All those planes flying over - you'd think someone would have looked down and noticed this place!"
"This place" is the Hickman Crater. At 260m wide and 30m deep, this almost perfectly circular hole in Western Australia's Hamersley Range is the country's most recently confirmed meteorite impact structure. It was named appropriately, although still unofficially, after Arthur, who discovered it in 2007.
In late 2012, during a collaborative core-drilling project mounted by GSWA and Atlas Iron Ltd (which has prospecting rights here), samples were extracted from immediately above the fractured bedrock, which lies about 50m below the structure's current floor. Testing of that material in late 2012 provided "final confirmation" of its extra-terrestrial origins, says Dr Peter Haines, the senior GSWA geologist overseeing the analyses.
Results have shown that within the crater's 'melt rock', the levels of nickel and other elements related to platinum - such as palladium and iridium - are considerably higher than is typical in Earth rocks. Iridium, a dense silvery white metal that's exceptionally rare in the Earth's crust, but much more common in meteorites, is among the most telling of these elements.
Elevated levels of iridium have been found in 66-million-year-old clays around the world, and are attributed to a truly massive meteorite impact linked to the global disaster that wiped out most of the dinosaurs.
"There was a rubble layer that sat on the original floor of the Hickman Crater. It was really smashed up and it contains little bits of semi-melted rock," Peter says. "And that's where we've found the strongest evidence of 'contamination' by extra-terrestrial material - that is, parts of the original meteorite."
Another major piece of information about the site was revealed in late 2014 when definitive results were returned on the age of the crater. This is being determined by the lengthy process of argon dating, presently being carried out on 'melt glass' created at the site by the heat of the impact. In the meantime, other analyses, such as of the amount of time that rock faces at the site have been exposed to both sunlight and cosmic rays, suggest the impact occurred about 50,000 years ago. But it could have happened up to 100,000 years ago. Argon dating, which takes more than 12 months to complete, will provide a conclusive answer.
Impact craters are a rare phenomenon worldwide
So far, just 184 have been confirmed across the Earth and about one-sixth of them are located on the Australian continent. It makes Australia one of the world's top locations for impact-crater enthusiasts.
Our sites range in size from South Australia's huge 90km-wide Acraman Crater, down to WA's Dalgaranga Crater, which has a diameter of just 24m. Their ages begin at just a few thousand years and extend up to a 2-billion-year estimate for the Yarrabubba Crater in WA. Some, such as WA's famed Wolfe Creek site, are instantly recognisable as craters, while others only betray their secrets to geological experts.
It's not that Australia has been hit by more meteors than other places in the world, but rather that our impact sites tend to be better preserved. This is mostly because our land surface is ancient and has been geologically stable for a long period of time. It's much harder to recognise and read the signs of impact craters in areas recently exposed to glaciers, earthquakes or volcanoes - major destructive landscape forces from which most of Australia has been free for millions of years.
This spotlight on Australia as a hotbed of impact structures makes it all the more surprising that the Hickman Crater wasn't located until Arthur first spotted its distinctive form on Google Earth satellite imagery. He was searching for signs of channel iron deposits, a potentially rich source of iron ore worth mining.
"I was looking from high up at about 20,000ft [6000m] and I saw this bubble-like shape, like a flaw on the image. I zoomed down into it and could see that it wasn't a flaw but a genuine feature," Arthur recalls. These days, Arthur and other GSWA geologists get at least three calls a week from people believing - often mistakenly - they've spotted something similar.
Considering the Hickman Crater's location, it does seem extraordinary that it remained undiscovered for so long. Although the structure is in a relatively rugged location, not far from the western edge of the Little Sandy Desert, the Hamersley Range has been a focus for intensive iron ore exploration and mining since the 1960s. And the thriving iron ore town of Newman, from which more than 440,000 people fly in and out each year, is just 36km south of the crater.
Up close, even to the untrained eye, the Hickman Crater site looks and feels like a vast flat-bottomed rock amphitheatre created by a massive missile falling from a great height. To professional geologists such as Arthur and Dr Andrew Glikson - the Canberra-based impact-crater expert whom Arthur first contacted for a second opinion - there are telltale clues everywhere in the surrounding rocks that suggest a huge impact occurred.
Tracking down an impact crater
Armed with the site's map coordinates, Andrew tracked down and visited the then-uncharted location in August 2007. It was his technical assessment of the site that first saw it added to the list of likely impact structures. Arthur paid the crater a visit eight months later to gather geological information, which he went on to present at an international Earth sciences conference in Perth. He returned in 2013 for another field investigation.
"All the classic features of an impact are here," he says. There is, of course, the site's overall physical appearance - an almost text-book example of the bowl-shaped topography of what's known geologically as a 'simple crater'. This shape is typical of small meteorite craters of up to a few kilometres wide. Larger, 'complex crater' sites can be complicated by many raised rings and central peaks, caused by the bedrock elastically rebounding after being hit by the energy of a massive strike.
"And we've got the 'ejecta' from the explosion," Arthur says, pointing out shards of broken rock around the site's perimeter. "Then there are the hydrothermal veins in the surrounding rock caused by sudden heating when the impact occurred, including fractures in a glassy black rock called pseudotachylite, which is a certain shock effect."
On a more microscopic level, laboratory analyses of local rocks reveal many other classic signs of shock. Quartz grains viewed in thin sections under a microscope, for example, show signs typical of an impact.
Based on the size and structure of the crater, it's estimated that the meteorite that created it would have been about 10m wide, weighed about 4000 tonnes and travelled at least 11km/sec before it entered Earth's atmosphere. The explosion it caused would have been equivalent to about 600,000t of TNT.
Fragments of meteorite exist at the impact crater site
The bulk of the ejecta material thrown out by this massive release of energy lies within about 500m of the south-west of the crater's rim, suggesting the space rock hit at an angle from the opposite direction. Peter Haines believes, however, that the force of the impact would have flung bits of bedrock kilometres out from the site.
The elemental make-up of some extraterrestrial material recovered from the site's surface has also been significant for building the picture of what happened there many millennia ago. Fragments of nickel-iron alloy space-rock fragments suggest the meteorite would have originated in the core of a planet.
"Although, we've kept that under wraps until now because as soon as you announce that you've found meteorite fragments every man and his dog turns up," Arthur says. "And if a site's not been properly searched first, illegal collectors come in and you lose all that scientific data."
Although the site can only be reached by mining roads requiring access permits, and the area has only recently been serviced by bull-dozed tracks, there's been an extraordinary amount of activity at the Hickman Crater in the past few years that has nothing to do with Arthur and his colleagues.
Numerous visitor books left in a mail box set up at the site by a Newman tour operator have been filled with the signatures of hundreds of people who have come to experience a place on Earth touched by outer space. It's likely some were hoping to bag a few fragments to prove it, but in WA meteorite material remains the property of the state museum where it was found - it's illegal to keep it or trade in it.
Around the world, the laws surrounding meteorites differ, and occasionally pieces appear for sale. Some reportedly can fetch almost as much per gram as gold. The attraction, says Arthur, is to own an object from space. So far he hasn't seen anything being offered on the internet from the Hickman site, but it may just be a matter of time. "We'll be watching," he warns.