Geoff Aimers, of Dunedin, asks :-

Has anyone ever considered the scenario of a meteor the size of the K-T event hitting Antarctica right on the greatest thickness of ice sheet? What would be the effects on global climate?

Duncan Steel (duncansteel.com), a space researcher based in Nelson and author of "Rogue Asteroids and Doomsday Comets" and "Target Earth", responded.

The "K-T event", which is shorthand for the boundary between the Cretaceous and Tertiary geological periods, is the mass extinction occurring 66 million years ago, when the dinosaurs died along with many other species.

Essentially all animals weighing over 25 kilograms became extinct at that time. The cause appears to have been an impact by a large asteroid or comet. A 180-km-wide crater which dates from then has been identified in Mexico, although several impact craters of the same age are suspected, and some scientists think other factors including extensive volcanism were factors in the extinction.

We know that the global environment was severely upset, and the first definitive fingerprint of the cosmic impact to be identified was an anomalous layer of the metal iridium in rock strata in three locations: one in Italy, another in Denmark, and the third in New Zealand (at Woodside Creek, between Kaikoura and Seddon). For material from the impactor to have been spread around the world, clearly a hugely-energetic explosion was involved.

The asteroid or comet that collided with the Earth was more than 10 km across, perhaps 15 km. At the high speed involved (around 20 km per second, or 72,000 kph) the kinetic energy released was equivalent to about a hundred million megatons of TNT. That’s almost ten billion times the energy of the Hiroshima nuclear bomb.

Obviously that’s a huge figure, but it’s only equivalent to the sunlight absorbed by the Earth across 45 days. The difference is that the K-T impactor delivered all its energy at once, in one place. The maximum thickness of the Antarctic ice sheet is almost 5 km. A 10 km asteroid would punch right through that, and deep into the bedrock below, excavating a crater similar to that in Mexico.

When one does the sums, it turns out that the energy delivered by the impactor would only be enough to melt one-twentieth of the Antarctic ice. Therefore only a minority of that ice would be directly affected.

The overall effect of the impact would quite different, as occurred 66 million years ago. The pulverised rock thrown around the globe would stay for an extended time in the atmosphere, reflecting away sunlight and so cooling the Earth. The initial effect would be a short period of severe heating and widespread fires – the rock ejected above the atmosphere would largely re-enter and turn the sky into a gigantic griller – but this would be followed by years of cold.

Paradoxically, such a cosmic impact in the Antarctic would melt much ice immediately, but longer-term would lead to the ice sheet growing.

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