Gravity

You might reasonably expect a satellite to orbit the earth in a trajectory smooth as a ball bearing in its race. Reasonable but wrong. It’s more like an Old Ford on a country road, bouncing and rattling over gravitational potholes.

The gravitational topography of the planet is less like a cue ball, more like a golf ball with all its bumps and dimples. You’ll remember your high school physics lessons on gravitation. Large objects exert gravitational force at a distance. The more dense the object, the greater the force. Massive mountain chains like the Rockies, Himalaya, and Andes create positive gravitational anomalies—areas of increased gravitational force. Negative anomalies are associated with declivities like the Mariana Trench, a rift in the sea floor of the Pacific Ocean 6.8 miles deep.

As a satellite approaches the Andes it’s pulled subtly closer to the earth. When it approaches the Mariana Islands and the adjacent sea floor trench, it bobs slightly higher. The effect is local and canceled when the satellite reenters the normal gravitational field. It’s rather like bouncing down the washboard surface of a dirt road.

The gravitational force exerted by the Andes isn’t limited to circling satellites. The roots of the Andes Mountains are washed by the Pacific Ocean. The Pacific itself is pulled toward the mountains like a massive standing wave.

It’s not only massive piles of rock that creates gravitational anomalies. The Greenland ice sheet is almost 1,500 miles long and 680 miles wide. It covers roughly 80% of the surface of Greenland—660,235 square miles. It’s typically over a mile thick and almost 2 miles at its deepest—683,751 cubic miles of ice.  And it’s the second largest body of ice on the planet.

That much ice warps the surrounding ocean, pulling it like taffy. The impact of melting Greenland ice on sea level has been recognized for some time but the effect of its reduced gravitational field has only recently been acknowledged. It further complicates a complex picture.

If all the Greenland ice sheet were to melt it’s estimated the global sea level might rise as much 23 feet but it would have little impact on sea level in the Arctic ocean. That’s counter-intuitive. The reason? Rise in sea level expected from melting ice would be countered by the fall in seal level resulting from reduced gravity. Northern Europe might be spared. New York would not.

The gravitational influence of the Greenland ice is limited to the Arctic Ocean. Melting of the northern ice would contribute to the volume of the oceans globally, increasing sea level worldwide, but that rise in the Arctic would be offset by the declining sea level resulting from reduced gravity. Areas beyond the northern ice’s gravitational influence such as the Eastern Seaboard of the US would suffer the unmitigated rise in sea level.

Western Europe isn’t without risk. If the Antarctic ice sheet melts, the sea level will likely fall in the Southern Ocean but rise dramatically in the North Atlantic.

And, of course, the change in the gravitational field would affect the Earth’s rotational momentum…

Vultures

Late in September the sun rises over the Glacier Peak Wilderness and strikes the Strait of Juan de Fuca like a temple bell. The morning resonates with light. The evening mist lifts from the water and the vultures crowding Beachey Head and Rocky Point meet the rising sun with wings outstretched, waiting for their blood to regain the warmth surrendered to the night.

They roost singly on fence posts or shoulder one another on split rails. They roost in dead trees, on the roofs of ruined barns, on water towers or barren rock at the southernmost end of Vancouver Island. They are waiting for the day to warm enough to cross the open water of the Strait and continue south.

In late September turkey vultures begin to mass at the southern edge of island like a river current against a log jam. If delayed by the weather they may cross 400, 500, or 600 a day when the log jam breaks. The Strait of Juan de Fuca is only 12 miles wide between Beachey Head and Salt Creek, between Canada and the United States, but those are 12 miles of cold water and sinking air. Vultures have evolved to soar and glide rather than beat their wings against gravity. They are not strong swimmers.

A turkey vulture may glide at 45 miles per hour,  requiring only 20 minutes to cross 12 miles, but they lose 2 feet of altitude for every second of glide. That’s an elegant glide path but it doesn’t annul gravity. Their flight must begin with at least 2,400 feet of altitude in order to make the far shore or regain altitude in flight. The cost of failure is death.

As the sun rises the air heats unevenly over the land, over freshly tilled fields, roads and towns. Warm air rises, cold air sinks. Convection cells form as localized heating increases. The vultures begin rising from their roosts, rising with the heated air, banking steeply to remain in the core of the thermals, grazing the cell walls where cooler air descends. From a distance they appear like debris carried aloft in a tornado. The behavior is called kettling, perhaps because it resembles roiling steam rising from an iron pot. Their flight is a thing of exceptional grace.

Turkey vultures are much maligned. Some of their behaviors are wonderfully practical but hardly endearing to humans. Obviously they eat carrion, sufficient reason for most people to disdain them, but the fact that they kill nothing is usually overlooked. Farmers sometimes shoot them for fear of infecting their pigs without realizing that no bacteria can survive the caustic hell of a vulture’s gut. As well most people don’t know that vultures coat their legs with their own guano or vomit in self-defense.

They gather at fish kills, on the spawning ground of salmon, on fields of harvested hay where small animals have been mangled by machinery. They forage in the cleared path of high tension power lines, on farmland, rangeland, in pastures and estuaries and tide flats, along roadsides and sewage lagoons, marshes and landfills and slaughter houses. They eat beaver and black bear, ground squirrels, muskrats, coyotes, deer, domestic cats, cows, goats, rabbits and sheep, harbor seal placenta and harbor seals themselves, voles, sea lions, opossum, porcupine, skunks and marmots, garter snakes, gopher snakes, even rattle snakes, wigeons and geese, chickens and chicken droppings, double-breasted cormorants, glaucous-winged gulls, great blue herons, swans and scooters and turkeys. There’s almost nothing dead they won’t eat but they are most drawn by the smell of the freshly dead.

Ethyl mercaptan is a chemical compound with a distinctive smell that humans can recognize in concentrations as low as one in 2.8 billion parts of air. The Guinness Book of World Records listed it as the “smelliest substance” on earth. It’s added to odorless butane and propane to alert people to a hazardous gas leak. It’s also generated in the first stages of organic decay.

Few birds have a sense of smell. Turkey vultures are an exception, adept at smelling ethyl mercaptan from miles away. They can locate carrion concealed beneath the forest canopy or a shallow grave by smell alone, providing competitive advantage over eagles, ravens and crows. Union Oil engineers supposedly abused their keen sense of smell by adding mercaptan to their pipelines, using the circling vultures like short-haired pointers to locate leaks.

Unlike raptors who kill their prey, turkey vultures are more willing to share. For a vulture, all food is a windfall. They hunt alone but are attentive to the flight patterns of others. When a vulture abandons its pattern of listless circling and becomes more purposeful in its flight, others take notice. The behavior of the others is in turn noticed. Within a short time newly discovered carrion can attract vultures from beyond the horizon. A venue of vultures (the proper name for a group of vultures on the ground) are surprisingly well behaved. They don’t haggle over scraps like gulls.

A turkey vulture’s beak is adapted for tearing flesh; it’s not strong enough to rend the tough hide of a large animal like a deer, a cow, or a bear. In the northwest they can sometimes be seen on fence posts, white legs and red heads in line abreast, patiently waiting their turn while ravens or eagles breech the hide.

Their red heads are featherless. Baldness is an advantage when thrusting your head inside the bloating belly of a goat three days dead. Feathers would trap bacteria and require constant preening. Long intestines, an industrial immune system, stomach acid that would peel the chrome off a trailer hitch, and a low pH also defend them against infection. Their naked legs are stained with guano for a similar reason and another benefit: cooling.

It’s called urohidrosis, an almost unpronounceable name for a behavior shared by storks and new world vultures. They use their feces and urine for evaporative cooling, lifting one leg at a time to precisely apply a coating of guano that dries like whitewash. The feces of a turkey vulture is sufficiently caustic to kill most bacteria acquired while walking on corpses.

Vultures aren’t aggressive but neither are they well suited for defense. Weak and clumsy on the ground and slow to take flight, they are easy prey. Their only defense is projectile vomiting. Within a six foot range they are supposedly quite accurate. It’s hard to imagine the emotional impact of a vulture’s stomach contents smack in the face. If the viability of the species is any indication, it’s an effective deterrent.

There are more reasons to admire than disdain vultures if we can disassociate them from mythology and our own mortality. They are an elegant solution to the problem of recycling resources and limiting contagion. The Cherokee called them the peace eagle because they killed nothing themselves. Perhaps they would make a better national symbol that than the thieving bald eagle. Long live the eaters of the dead!