Secrets of Silent Flight

Secrets of Silent Flight

Anyone who’s been in an airplane has heard that constant roar of flight. That’s partly the engines—but the noise comes more from air rushing over the wings. Anyone who’s seen an owl fly… hasn’t heard anything. Because unlike an airplane, owls fly almost silently. How do they do this? Aeronautical engineers have examined owl wings and found several features working together: The leading edge of the wing, called the comb, is finely serrated, to break up the air flowing over it. The middle of the wing is covered in soft feathers called velvet, which dampen the high pitch of rushing air that would be audible to rodents and other prey. The trailing edge has a wispy fringe which further silences the moving air. In a technique called biomimicry, engineers design products inspired by successful plant and animal features. We’ve covered several in prior EarthDate episodes. Now they’re looking to adapt the owl’s silent flight to our technology. New designs have shown that adding a sound-absorbing middle section and a porous, flexible trailing edge to an airplane wing can cut its noise by 25%. Adding plastic finlets to the leading edge of a wind turbine blade can cut its noise in half! Owls have other remarkable features: Their eyes are up to 100 times better than ours. And their ears are the most sensitive of any animal ever tested. What else might engineers learn from this wise old bird?

We Moved the North Pole

Water is heavy. When we move enough of it, it can change the tilt of Earth’s axis. It’s that axial tilt that creates our seasons. And the tilt is always changing, due to changes in Earth’s gravity. Natural phenomena like earthquakes and volcanoes can redistribute the weight of continents, affecting Earth’s gravity. But so can the movement of water. Researchers recognized that water melting out of glaciers and ice sheets into the ocean had redistributed the weight of that water from the colder latitudes to the equator, where seawater accumulates in a bulge around Earth. When they calculated the resulting gravitational changes, their numbers didn’t add up. Until they also factored in the weight of the groundwater humans had extracted. We use groundwater mainly for irrigation and in city water systems, which discharge into rivers that eventually lead to the sea. This moves groundwater from where it’s concentrated underground to soils and oceans. Across the globe, humans have pulled more than two trillion tons of water from aquifers over two decades. And, the scientists realized, weight redistribution from groundwater alone had changed the tilt of Earth’s axis, causing the North Pole to move three more feet! They plan to use this new understanding to look for other connections between Earth’s water and gravity, and how droughts might also have altered Earth’s axial tilt.

Unicorns of the Sea

Like the legendary unicorn it resembles, the narwhal is an enigma. It lives only in Arctic waters, in a frozen seascape far from civilization, making it hard to study and therefore not well known to science. For instance, we don’t know what it does with its most distinctive feature. We do know that its 6- to 10-foot tusk is actually a single tooth, which grows through its lip and continues growing throughout the narwhal’s life. And that narwhals living farther north have thicker tusks than their southerly cousins. Some scientists have thought the tusk is used by males to show dominance, or to joust with other males. But some females have tusks too. Some theorize it’s a sensory organ allowing the whale to test temperature or salt levels in water. But if so, all narwhals should have one. Narwhals don’t use their tusk for spearing or scaring prey, though their prey species are now changing. We’re not sure why. Narwhals are known to be some of the deepest-diving cetaceans, going down more than a mile. But we don’t know what they do down there. Scientists saw a group of narwhals swimming erratically off the coast of Greenland, so they put a tracker on one male and followed it for three months. But its path was so seemingly random they had to use chaos theory to try to make sense of it. Perhaps the narwhal is just unknowable today, making the unicorn of the sea all the more intriguing.

Extreme Antarctica

Antarctica is a continent of extremes and superlatives. It’s the coldest continent, with Earth’s lowest recorded temperature of 130 below zero Fahrenheit. It’s the driest continent, with just 6 inches of precipitation a year. It’s the windiest continent, with gusts up to 200 miles an hour. Surprisingly, it’s the highest continent, with an average altitude of 8,200 feet, largely due to ice sheets up to 15,000 feet thick. It also has an extreme range of geologic age, with rocks nearly 4 billion years old on one end, and young volcanoes forming on the other. Hundreds of millions of years ago, Antarctica sat within the supercontinent of Pangea. When Pangea began to break up, more than 80 million years ago, Antarctica ended up alone at the southern end of the Earth. When the climate cooled 34 million years ago, Antarctica began to ice over. Over millions more years, as glaciers advanced and retreated on Earth dozens of times, ice built steadily on Antarctica, such that 98% of its land is now covered in ice, hiding what lies below. As a result, we know less about Antarctica’s geology than that of the Moon or Mars. But with advanced satellite sensing, scientists are now able to look below the ice, to map a continent of massive bluffs, erosion zones and ancient river drainages that still shape the flow of ice today.

Frogsicles

Many animals hibernate, but only the wood frog can freeze solid in winter—its heart and brain stopping completely—then thaw and come back to life in the spring. Its process to do this is fascinating … and could one day save human lives. The danger with freezing is ice. If water freezes within a cell, sharp ice crystals will puncture or rupture cell walls and kill the cell, which normally kills the animal. So, as temperatures drop, the wood frog’s liver converts glycogen to glucose, a sugar syrup, and floods the frog’s system. Meanwhile, nucleating proteins push water from the frog’s cells. Glucose replaces the water, which keeps the cells from freezing. Outside the cells, the water does freeze, which stops the frog’s metabolism, including its brain and heart, and preserves its tissues. In spring, the frog thaws, its brain and organs come back to life, and breeding season begins. Several other animals use chemical antifreezes to avoid ice crystals forming within their cells. And now researchers are trying to do the same with human organs harvested for transplant. Organs currently remain viable for only a few hours outside the body. But if glucose could replace the water within them, perhaps they too could be frozen solid and later reanimated, greatly prolonging transplant time. A potentially life-saving technique for humans, learned from the humble wood frog.