ScienceWise - Spring 2010

A big impact on climate

Examining a new asteroid crater found in the Timor Sea

As new land-based oil deposits become increasingly scarce, oil companies have turned to the seabed in search of new reserves. The drilling and seismic surveying often find oil, but occasionally they turn up something far more interesting, at least from a scientific perspective.

This was exactly what happened when oil company geologist Dariusz Jablonski of Finder Exploration was conducting seismic surveys in areas straddling the Ashmore Platform and Browse Basin north of Australia. His results led him to suspect the existence of a large impact feature so Dariusz contacted Dr Andrew Glikson at ANU who is a specialist in the study of extraterrestrial impacts. Dr Glikson was asked to study cuttings from the Mount Ashmore-1B well and investigate whether there was indeed evidence for an ancient impact structure.

But how exactly does a scientist go about determining if a 35 million years old structure deep below both rock and sea is indeed impact related?

‘It’s a process of elimination,” Dr Glikson explains, “Essentially we look at all plausible explanations and eliminate them one by one. But the process can be difficult depending on the type of rocks in that particular area.”

When an impact occurs in Igneous or Metamorphic Rocks it’s usually possible to see tell-tale crystallographic fracture planes under the microscope. These particular structures known as planar deformation features (PDFs) only form due to high velocity shock imparted by an impact but can’t form due to volcanic explosition, which makes them a “fingerprint” for impacts. However in the sedimentary rocks that are commonly found under the sea the high concentration of volaltiles – mainly water and carbon dioxide - makes it much less likely that PDFs will form, so scientists have to look for other clues.

Two candidates for non-impact explanations of dome structures are volcanism and salt domes. Salt domes are created when restricted marine basins have a cyclical evaporation causing salt and gypsum to be deposited over many centuries. As time progresses these salt deposits can become covered in layers of sediment. However because the salt is less dense than the sedimentary rock that forms over it, it has a tendency to rise up through the rocks above like a bubble.
“The Mount Ashmore feature extends far too deep into the crust to be a salt Dome and has a basement rise underneath it, “Dr Glikson says, ”In the seismic profiles we can see a number of features deep below that are not consistent with such an explanation. We also see no indication of igneous material in the drill core, which makes a volcanic explanation unlikely. So what we’re left with is an impact explanation.”

“At the time of the impact the ocean would have been a few hundred metres deep” Dr Glikson adds, “But when you’re talking about large impacts, the presence of a relatively small amount of water is not a major factor in attenuating the impact.”

Only very low angle impacts which plunge into deep water may be slowed down.  When a large mass of a hard material like silicate rock or iron hits the Earth the tremendous kinetic energy is converted into thermal energy at the end of its trajectory. This releases a vast amount of heat in a very small time, melting the rocks and vaporising the solid mass into a series of hot gasses such as CO2, water and silicate vapour. The process involves a rapid and enormous increase in volume, perhaps analogous to an underground nuclear detonation.

You can see clear evidence of this impact explosion mechanism for yourself if you look at the moon through a small telescope. The many thousands of asteroids and comets that have hit the moon over the last 4 billion years came in at every possible angle. If you imagine throwing stones into mud, the ones that hit at a grazing angle leave long elliptical indentations and it’s actually quite difficult to create a circular carter. However when you look at the moon, all the craters and basins are essentially perfect circles.

The explanation of this is that when an asteroid hits a planetary sized body, the initial crater (which may be elongated) is wiped out a few milliseconds later by the explosive release of kinetic energy. This explosion creates an essentially spherical impact feature. So all the craters on the moon are perfectly round despite the variety of impact angles.
There is a rule of thumb that says that the diameter of a particular crater sill will be about 10 to 20 times larger than the impactor that caused it. “The minimum size of the Mount Ashmore dome, which represents elastic rebound doming of the Earth’s crust triggered by the impact, is 50 kilometers at the base, but the full size of the impact crater - not yet defined - may be significantly larger” Dr Glikson says. “This would suggest that the asteroid that created the structure was at least 5 kilometers across.”

The impact of such a large asteroid throws up vast amounts of dust and fine particulate matter into the upper atmosphere. This reflects sunlight resulting in a significant, though temporary cooling of the planet. Although a single massive impact could create this effect, a series of smaller ones close together in time may have a bigger and more prolonged effect. Despite the fact that asteroid impacts are very infrequent, clusters of asteroid impacts have occurred several times through the history of Earth.

Relatively small objects like asteroids that orbit the Sun are very strongly influenced by the gravity of massive planets, in particular Jupiter. Astronomers believe that perturbations caused by Jupiter’s gravity either prevented a planet forming from the debris between Mars and Jupiter or even tore it apart. This perturbation coupled with gravitational attraction between different asteroids means that many of them orbit in loose clumps. The implications for the Earth being, that if one member of such a clump hits the Earth many of the others may also do so within a few orbits, creating an impact cluster. Scientists know that there have been a few such clusters of impacts throughout the history of the Earth. There are a number of impact features around the world that, like the Mount Ashmore dome, are about 35 million years old suggesting the newly discovered dome is part of this impact cluster.

“Around the same time as the Mount Ashmore impact, a 100 kilometer wide asteroid impact structure formed in Siberia, and another measuring 85 km in diameter in Chesapeake Bay, off Virginia, in the United States.  Likewise a large field of tektites – molten rock fragments splashed by impact – fell over northeast America. This defined a major impact cluster across the planet,” Dr Glikson says.

This impact cluster would have contributed significantly to a global cooling in so far as it may have triggered the opening of the Drake Passage between Antarctica and South America. The opening of the Drake Passage allowed continuous circulation of the circum-Antarctic ocean current, isolating the Antarctic continent from warm mid-latitude currents and allowing the onset of its large ice sheet, which acts as a ‘thermostat’ for the Earth’s climate.” Dr Glikson explains.
The opening of the Drake Passage and the impact may have occurred around the same time purely by coincidence. However it’s also possible that the disturbance of the crust caused by such massive impacts may have nudged the existing tectonic movements into action. A bit like kicking a boulder on a hill. The kick isn’t enough to move the boulder far but it can give it the tiny extra impetus it needs to start and gravity will do the rest.

“We really don’t know for certain if there’s any causal relationship between the impact cluster and the opening of the Drake Passage,” Dr Glikson says, “But what we do know with certainty is that impact clusters have had and may continue to have, profound implications for life on planet Earth.”

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