Spotlight

Walking on Water

Tim Higham

Auckland University mathematics lecturer Colin Fox says his students are always surprised when he tells them that his main research interest is in Antarctica.

"Most come to university to study maths with the notion that it is an esoteric arena of mental gymnastics -- a romantic escape from the real world."

In fact much of Fox's work has proven to be immensely practical.

Its value becomes apparent the moment one arrives in Antarctica, when your plane touches down effortlessly and glides to a halt on a runway made from frozen water. Sea ice is Fox's research passion. It is a remarkably stable material close to its melting point, he explains. Steel, a few degrees off melting, would be quite soft and useless. Its properties most closely resemble lead or a ceramic.

The Starlifter door opens and we step out to walk on water. The ice, Fox explains, is a two-metre thick skin stretched over the sea. The buoyancy provided by the water below is what gives it stability.

Fox tells me that his co-worker, Tim Haskell from Industrial Research Ltd, has been working on assessing the strength of beams of ice, partially excavated by chainsaw and "ditchwitch". The ends of the beam are jacked up and down and the mechanical stresses within it measured. Haskell has found that a two metre thick beam of sea ice can be broken by the amount of energy generated by hand turning an egg beater.

Faith in my ability to walk on water begins to wane. Fox reassures me by saying that even huge C5 Galaxy planes can be parked on the runway, though they do depress the porous ice below water level and have to be moved now and again to prevent flooding.

It should be another month before the runway becomes too slushy and unstable to use. Around Christmas wind-driven waves will start to break it up into pack ice and by January McMurdo Sound will be open sea.

Fox's main research interest is in the passage of energy waves through sea ice. When the runway was first developed in 1955-56 to service McMurdo Station and New Zealand's Scott Base it was assessed from a classical engineering viewpoint -- weight of plane, speed of impact, thickness of ice equals...

What the engineers didn't consider was the wave effect created by the landings. Like a stone thrown into a pond, shock waves generated by planes ripple out across the ice.

Fox's colleague, Vernon Squire from Otago University, has found that waves generated by traffic moving across the ice can behave similar to those which build up in front of a jet aircraft approaching the speed of sound. The sound waves become messy, amplify in resonance, and climax in a sonic boom. The findings, Fox assures me, are no reason to shake my recently-restored confidence -- but they have proved valuable in modelling ice behaviour and have since been applied in many engineering situations.

Fox's most recent interest has been in modelling the interaction of ocean waves and sea ice. Coming up with a mathematical formula for such behaviour was a long standing puzzle for physicists until Fox and Squire solved it in 1990.

Already the formula has proved useful in helping predict the best place for ships to anchor off an ice edge in a storm to avoid big waves.

Fox's trip south last summer was aimed at testing the mathematical model by making wave energy measurements at the ice edge. The results should help predict the pattern of sea ice break-up.

Because of the dynamic nature of the ice such work is hazardous and requires researchers to anchor themselves by rope to avoid the risk of a very cold swim. Orcas cruising the ice edge looking for penguins are best observed from distance, not face to face.

Unfortunately a helicopter reconnaissance of McMurdo Sound revealed the sea ice was unsuitable for the experiments. Several large cracks behind the edge blocked snow mobile transport and meant a sudden wind shift could leave the research party sailing north on an ice flow.

Fox concentrated on testing equipment and is philosophical about the decision to postpone the ice edge trials to this coming summer. In Antarctica delays are not uncommon and caution is a good rule of thumb. His work forms part of a five-year, multi-disciplinary study of sea ice supported by the New Zealand Antarctic Programme.

Sea ice, he explains, has a major bearing on the climate of the Southern Hemisphere. Its formation each winter -- the largest seasonal phenomena on the planet -- doubles the size of Antarctica. Each spring sea ice covers 20 million square kilometres: 74 times the size of New Zealand, nearly three times that of Australia, and one-eighth the area of the southern oceans. This vast sheet of white reflects back much of the sun's radiation, modifies the exchange of gases between ocean and atmosphere, and affects the formation of clouds.

Back in Auckland, lecturing a fresh intake of university mathematics students, Fox will show that their career choice is no escape from the world.

Tim Higham is with the New Zealand Antarctic Programme.