Australian craters

Australia’s outback is one of the oldest landforms on Earth. The essentially vast plain that stretches west from the line of Broken Hill, NSW, through to Mt Isa, Qld, has changed little in the past billion years, other than for the gradual processes of erosion flattening out the few high points. There has been little sign of uplift, mountain ranges, volcanoes or great river systems. It’s kind of like a place where time has stood still.

That is why it’s also one of the best places on Earth to search for the traces of interstellar visitors. Not for the little green men in metallic discs, but rocky and icy visitors from space.

Sometimes those traces are lumps of rocks or ice that are plainly unearthly in composition. These rocks that we commonly find on Earth are known as differentiated. That means they’ve been churned over and over by volcanic action, much like a stew or a pudding. The lighter material has floated to the top and the heavier material has sunk towards the core of the Earth. Heavier material, such as iron and nickel, mixed with small amounts of other heavy elements such as gold, platinum, and uranium largely make up the underlying layers of our planet’s structure. The deposits of these metals we find on the surface are really just the scummy traces brought to the surface by volcanic action or been trapped by chemical actions.

The rocky chunks zooming about in space are generally undifferentiated, that is, they show a very different set of trace element ratios, with platinum or such rare elements as iridium in much higher levels than is normally found on the Earth’s surface.

Comets and asteroids

Comets are dirty snowballs, composed of frozen gas, rock and dust, which can be up to a hundred kilometres in diameter. As they are heated by the sun they give off gas and dust in a glowing tail that is easily seen from Earth. Comets, also, can collide with Earth and as a general rule, explode in the atmosphere. It’s believed the Tunguska event of 1908, which flattened 2000sq km of forest, was caused by a large comet exploding five to 10 kilometres above the ground.

Had this comet entered the Earth’s atmosphere several seconds later it may well have exploded over the densely packed centre of Europe and could have, for example, wiped out Adolf Hitler, the British royal family or Albert Einstein, changing the face of human history.

The term asteroid refers to micro-planets, generally greater than a metre in diameter that orbit our sun and are remnants from the early solar system’s formation. They orbit in a belt and are occasionally knocked out of their orbit by collisions and can approach the Earth.

Meteors, meteoroids and meteorites

Meteorites fall into the Earth’s gravitational field quite commonly. If you look up into space on a clear, cloudless night you should, on average, expect to see a “shooting star” about once every 20 minutes. These are mostly small chunks of rock known as meteoroids which fall into the Earth’s upper atmosphere, where they heat up and glow brightly enough to produce a brief streak of light, when they become more correctly known as meteors. These are usually about the size of a pea to start with, but soon burn up and the light ceases, leaving just bits of dust to fall to Earth. Any chunk of rock that is big enough to survive the fall to the Earth’s surface (greater than 10cm) is then become known as a meteorite.

The Earth’s gravitational field attracts about 60 tonnes of meteoritic dust every day. That equates with about 22,000 tonnes per year, which falls gently through the atmosphere. It’s no wonder you need to clean the house and wash the car so often.

However, sometimes meteorites can be much bigger, and strike the Earth with monumental force.

The frequency of such larger impacts varies with the size of the meteorite. Objects of one metre diameter enter our atmosphere about once a year; those of around 7m in diameter enter about once every five years and have the same energy as the Hiroshima atomic bomb. Such large asteroids generally explode in the atmosphere through the results of heating and intense pressure on their face.

The meteor which exploded over Chelyabinsk in Russia in 2013 (estimated to be over 20 metres in diameter — a roughly twice in a century event) and injured 1500 people released over 500 kilotons in energy, equivalent to about 30 times that of the Hiroshima atomic bomb. Had it exploded above a dense city area rather than a small regional town the number of injuries would have been much greater.

When meteorites strike Earth they can leave very significant features and have brought about very profound revolutions in the Earth’s history, including the formation of the moon, which was blasted off the Earth by an impact with a very large object about the size of Mars. Meteorite strikes have been credited for the evolution of life, the very start of life on Earth, the origin of water and a number of major mass extinctions, including that most famous one which wiped out the dinosaurs 65 million years ago.

AUSTRALIAN CRATERS

Recently Australian researchers claimed to have found the largest meteorite impact zone in the world with two adjacent impact points creating a disturbed area over 400km in diameter, resulting from two impacts of asteroids more than 10km in diameter when one huge body broke into two. The zone, known as the Warburton Basin, is near the conjunction of the SA, Qld and NT borders but is now buried under 2km of sediments and is dated to around 300 million years.

But the Australian outback — including in the Northern Territory — is pitted with very visible meteorite craters, almost all of which are accessible by 4WD. The best known, thanks to the eponymous horror movie, is the Wolfe Creek crater in northern Western Australia. The crater, about 875 metres in diameter and 60 metres in depth, is located on the Tanami Road, 152km south of Halls Creek on an unsealed road through Carranya Station (composting toilets are available, check road conditions in Halls Creek before travel). It was created about 300,000 years ago but was only discovered by Europeans in 1947.

More easily reached is Gosse Bluff (Tnorala), which lies just to the north of Mereenie Loop Road, the popular tourist route past Kings Canyon and around to Alice Springs (175km to the east) in the southern NT. It is believed to have been formed by a meteor impact about 142 million years ago. The original 22km-wide crater rim has been eroded away but the 5km uplifted central rebound, forming a jagged central crater, is a clear presence in the landscape. Camping and fires are not permitted in the reserve.

To the east lie the Henbury Meteorite Craters, just a short well signposted 10km drive off the Stuart Highway, 145km south of Alice Springs. Here there are 12 craters, ranging in size from 7 to 180 metres in diameter and up to 15 metres in depth, formed when a larger meteorite fragmented and crashed into the desert 4700 years ago. This crater field was used by American astronauts training for the moon landing expeditions in the late 1960s as an example of what they might encounter on the moon. The crater rims of soft shale are now eroded and lack the starkly jagged nature of many more obvious crater structures.

Should I be worried?

Is there any need to worry about meteorite strikes? While there is a strong statistical potential for meteorites and foreign bodies from outer space to enter into the Earth’s gravitational field the chances of it happening on any given day or in any given year are quite small. While the estimates vary widely, it is thought that the odds of dying from are meteorite collision are around 1 in 1.6 million. That’s a lot less than the odds of dying in a car accident, fire, earthquake, cyclone or flood, but is lower than that of being killed by a shark, or winning the Powerball lottery, so don’t worry too much about it.

However, should a meteorite larger than about a kilometre in diameter strike the Earth the side effects could well stop civilisation as we know it and result in huge numbers of deaths on a global scale. It is deemed to be potentially serious enough for governments around the world to sustain programs to watch the skies for near misses from bodies crossing the Earth’s orbit, with thousands having been mapped since research began in the 1990s.

Check out the full feature in issue #124 of Camper Trailer Australia magazine. Subscribe today for all the latest camper trailer news, reviews and travel inspiration.