A rocket time machine? Forget it. Only in the movies. Furthermore, even if we could join Marty and the professor in Back to the Future, we would face serious trouble at 100 feet below the surface of the ocean. For one thing, the water pressure, which gives us an uncomfortable pop when we dive 10 feet to the bottom of a swimming pool, would cause serious damage to our bodies at 100 feet. How would we breathe? Our bodies have evolved for hundreds of millions of years. Gone are the gills of our fish ancestors.
So, should we give up the idea of trying to find out what the Earth was like long ago? No way. We can become scientists, because as scientists, anything is possible.
First, we study scuba diving, about breathing from an air tank under water, about coping with the crushing pressure of 100 feet below the waves. Second, we find a real time machine-not one that looks like a rocket car, but one that lives on the ocean floor, quietly, slowly, carefully recording the Earth's history. Third, we learn the language of that strange record. Fourth, we read the record.
Dr. Jerry Wellington has completed numbers one to four. First, he is a scientist and a scuba diver. Second, he has found a living time machine off Key Largo in Florida. The time machine is a coral reef, comprising billions of tiny animals. The record of the Earth's climate lies buried in the skeletons of these animals.
It takes all of Dr. Wellington's skill as a scientist to read the record and then to speculate on what the record can tell him about the future of the Earth's climate. It also takes all of Dr. Wellington's skill as a scuba diver to survive on the ocean floor for more than a week without surfacing to the warm, friendly sun and the fresh air that gives him life.
During part of his underwater excursion, Dr. Wellington must live on air from a metal tank strapped to his back. Even when looking at the coral through his dive mask, or stopping a moment to admire a pretty fish-even then he must never forget the dangers of deep sea diving. Will he run out of air? Will his body absorb too much nitrogen? Is he sure of the route back to the habitat, his temporary home on the ocean floor? Has he told the support crew of his excursion site? Does the support crew know the exact planned time of his excursion for the day? A mistake in planning, a careless minute of inattention to his diving gauges could turn the beautiful, inviting ocean into a cruel monster ready to swallow him up.
Dr. Wellington lives for several days in a small house named AQUARIUS, on the bottom of the ocean. AQUARIUS, both a house and laboratory, sits on sand, 48 feet down, far from the turmoil of storms and waves. Of course, to call AQUARIUS a house is like calling Shaquille O'Neill a tall man. The proper name is a habitat, or pressurized cylindrical chamber with compartments, used as undersea living quarters for aquanauts.
The habitat has five viewports and only one door to the outside. Lucky for Jerry Wellington, the roof doesn't leak. The door to the outside connects to a hatch and a porch. No rocking chairs on this porch, however. No lounging here, no sunbathing. The aquanauts, the scientists, get in and out of their dive equipment on the habitat's porch. For that reason, the porch is called a wet porch. From the wet porch through a tightly sealed door, you come to the entry lock, site of the habitat's laboratory.
From the entry lock through a tightly sealed door, you come to the main lock, with six bunks, a shower, a toilet, microwave oven, refrigerator, dining table, and counter space. The habitat even has computers and air conditioning!
Scuba divers from the mobile support base-which is moored on the surface, directly above the habitat- deliver food and other supplies to the scientists below. What about water? Do they deliver water?
Almost 200 years ago, Samuel Taylor Coleridge wrote a poem about an "ancient Mariner" lost alone on a boat in the middle of the ocean. All that water around him, yet the ancient Mariner was dying of thirst:
The scientists of AQUARIUS face the same dilemma. Water surrounds them, yet they dare not drink it. But what if they could remove the salt? Then they have a glass of safe refreshment. This is done with a process called reverse osmosis. If you put a semi-permeable membrane, a type of filter, in the middle of a tank and then fill one side of the tank with sea water and the other with freshwater, the salt from the sea water will pass through the filter, turning the freshwater salty. The salt will keep slipping through the membrane until both sides of the tank have an equal amount of salt. That's osmosis. No work involved. It occurs all by itself.
The trick is to reverse the process-to force the water on the sea water side to slip through the membrane while leaving the salt behind. That's reverse osmosis, and it requires special equipment and energy. It does not occur all by itself. The habitat has electrical energy to accomplish the job. Every time Dr. Wellington returns to the habitat tired and thirsty from his excursion to the reef, he probably drinks a glass of freshwater and silently thanks the technicians and scientists who came up with the process of reverse osmosis. Too bad the ancient Mariner didn't know about it.
Compact, cylindrical AQUARIUS measures 43 feet long, 13 feet wide and 16.5 feet high. That's a tight fit for six scientists. Why do the scientists, Dr. Wellington among them, go to all the trouble of living for more than a week in the habitat? Why not simply start their dives from a surface boat, gather their information and specimens at the reef, swim back up to the boat, and conduct their experiments either on the boat or back at their laboratories on land? What is the advantage of working from the habitat instead of working from a surface boat?
Dressed in an unusual business suit, Dr. Wellington leaves for work each day at a nearby reef. No tie and jacket for him. No shiny leather shoes or neatly pressed pants. If he wants to work with the fish, he must adapt to their environment. He can't grow scales and gills, but he can wear equipment that functions like scales and gills. His daily business attire-his "scales"-consists of a soft, plastic foam suit that fits tightly over his entire body. The suit keeps him warm and protects him against sharp, rough objects on the ocean floor.
His "gills" consist of scuba:
The primary function of the buoyancy control device, or BCD, is to help the divers maintain a certain depth. The BCD has various hoses and straps attached, but basically it is an expandable bladder that can be inflated with air from the tank or by the diver blowing in one of its hoses. Deflate the BCD, and you sink. Inflate the BCD and your rise. The air tank attaches to straps on the BCD.
Getting dressed for a scuba dive takes longer than getting dressed for the high school prom. With all this equipment, Dr. Wellington can breathe easily underwater on his own. He doesn't need a crew on the surface pumping air through a hose and down to him as he works under water. Breathing under water on your own-that's where the word scuba comes from. Scuba is an acronym for Self-Contained Underwater Breathing Apparatus. "Self-contained" sounds as if a scuba diver swims alone, relying on no one for help. However, one of the rules of scuba diving is "never go alone." Even recreational divers can run into trouble down there with equipment failure, upset stomach, lost direction, dizziness. A dive partner can help you escape trouble. A dive partner often means the difference between life and death.
For a working diver who spends hours or days underwater, one partner is insufficient. Dr. Wellington, a working diver, needs a full support crew to monitor his movements, to provide expert care in an emergency, to ensure timely repairs and upkeep of the habitat, and to supply him with air tanks, scientific equipment, food, toothpaste, soap, clean clothes and most of the other items he uses every day back home, at real home on land.
Well fed, well rested, Dr. Wellington sets out from the habitat in the morning in his scuba, starting at 48 feet of depth and sometimes descending to as low as 100 feet. At 100 feet, the water pressure is 59.25 pounds per square inch (psi), more than 4 times greater than the 14.7 psi he's used to on the surface.
At 100 feet of ocean depth, Dr. Wellington has 59.25 pounds pressing on every square inch of his entire body. Assuming his chest is average size of 16 inches from shoulder to shoulder and 16 inches from collar bone to belly button, Dr. Wellington arrives at his coral reef office with 15,168 pounds pressing on his chest. About equal to four bull elephants. And that's just his chest. What is the weight pressing on his entire body ? How many elephants? An entire herd.
How can technology, such as scuba, help Dr. Wellington study coral to better understand the Earth's climate? He's a scientist, so he refuses to give up his research just because of a herd of elephants standing on his chest. He puts on scuba to breathe, see, and swim underwater, and he carefully controls that herd of elephants, that intense pressure. What is this pressure? What does it mean?
We live on the surface of the Earth's land. However, another way of looking at it is this: We live on the bottom of the Earth's air ocean. In many ways, this ocean of air behaves like the ocean of water, with waves and currents and storms and heat layers and pressure. Air has weight, just like water. If you put your hands on your bathroom scale and press, the scale will measure that pressure as weight. Scientists have measured the weight, or pressure, of the Earth's air at 14.7 pounds per square inch at sea level (where the land meets the sea).
A square column with four inches on each side and rising about 18 miles to the sky would contain 14.7 pounds of air. At 18 miles up, the air pressure falls to 0.18 pounds per square inch.
The air pressure is less on a mountain than at sea level. The air pressure is more in a coal mine or on a subway than at sea level. As you go up, there is less air above you. Less air means less weight, which means less pressure. It works the opposite way when you go down. Think of the times you've ridden an elevator up to the 12th floor. Your ears "pop" because of the sudden drop in air pressure. Think of the times you've ridden an elevator down to the first floor. Your ears "pop" then, too, because of the sudden rise in the air pressure. The same applies to water pressure: Pressure increases as you descend and decreases as you ascend.
With 59.25 psi (pounds per square inch) at 100 feet of ocean depth, Dr. Wellington should arrive for work at the coral reef squished and squashed into a pancake. Why doesn't he? Because he has balanced, or equalized, the pressure on the inside of his body with pressure on the outside of his body. A herd of elephants pushing in; a herd of elephants pushing out. Balance. Equalization. As he slowly descends, he holds his nose and blows gently to increase the pressure on the inside of his ears, saving the delicate eardrums from damage. If he fails to increase the pressure on the inside of his eardrums, he will suffer what divers call a "squeeze."
A squeeze is a pressure imbalance. When pressure is not equal on both sides of a hollow spot, it can be painful. It can be damaging. The body has many hollow spots. The major hollow spots are the lungs, the sinuses, and the ears, the space behind the eardrum. Sinuses are air sacs in the skull, around the nose and eyes. The air in the tank strapped to Dr. Wellington's back is pressurized to about 3,000 psi. A regulator attached to a hose, which is attached to an air tank, automatically releases air at the same pressure as the water around the diver. The pressure around the diver is called the ambient pressure.
Every 33 feet of depth in the ocean increases the pressure by 14.7 psi. Ambient pressure at sea level is 14.7 psi, which is called an atmosphere (atm). Ambient pressure at 33 feet below the ocean surface is 29.4 psi, or 2 atm.
At 66 feet, the ambient pressure is 44.1 psi (3 x 14.7 = 44.1). At 66 feet, the regulator releases air from the tank at 44.1 psi. The diver keeps the air pressure inside the body equal to the ambient pressure. Divers, in other words, "equalize" during the descent and the ascent. As the divers breathe out through their nose, the mask also equalizes, and remains balanced with the same psi on the outside and inside. Equalize going down and equalize going up-or suffer a squeeze.
The regulator is part of the diver's mouthpiece and usually is colored a bright orange. Divers breathe through a soft plastic mouthpiece attached to the regulator. With the brightly colored regular in his or her mouth, the diver often looks like someone sucking on a big piece of fruit. But at least Dr. Wellington doesn't look like a pancake. The trouble is, his body surely resembles a bottle of soda because it is ready to pop and fizz if the pressure around him decreases too quickly.
Soda is carbon dioxide, a common gas, and sugar dissolved in water under high air pressure. The soda is held under pressure so that the carbon dioxide pumped in at the bottling company doesn't bubble away. Open the can, release the pressure, hear a pop, watch the carbon dioxide bubble out of the soda. Our bodies, like soda, are composed mostly of water. Take away our bones, and we resemble a water balloon. The air in a diver's tank usually is the same air that we breathe at the surface: 80 percent nitrogen, 18 percent oxygen, the rest a mix of rare gases.
During ocean excursions, the nitrogen inhaled from the air tank dissolves in Dr. Wellington's water- balloon body. The high pressure at the reef forces nitrogen to dissolve in his body, just as carbon dioxide dissolves in soda. The nitrogen would cause him extreme pain if he suddenly swam to the top, ascending quickly from the coral reef at 100 feet to the surface of the ocean. The pressure on his body during the quick ascent would drop from 59.25 psi at the bottom to 14.7 psi at the top.
It would be like popping open a soda bottle: The nitrogen would bubble and fizz out of his body tissues. The bubbles of carbon dioxide from a soda bottle escape into the surrounding air. No problem. The bubbles of nitrogen can't immediately get out of a diver's body. Unable to escape into the surrounding water, the bubbles begin to fill small cavities in the diver's body. Bubbles collect in the sinuses around the eyes and nose, and in elbow and knee joints. Big pain. Bigger danger.
Divers can die from this bubbling nitrogen. The ailment is called decompression sickness. Divers curl up in agony from the pain of decompression sickness. That's why the ailment also is called "the bends." A herd of elephants pushing in, a herd of elephants pushing out . . . Deep sea diving compresses the body. To safely ascend, divers must slowly release the compression. Slow release of the pressure prevents the bends, because the nitrogen then does not bubble out of the tissues but escapes harmlessly through the lungs as the diver exhales. That's part of the lungs' normal job. They usher oxygen into the body and escort waste products out.
Our bodies use oxygen, so divers face little danger of too much oxygen building up in their tissues. Nitrogen is another story. Nitrogen molecules in the air normally stay by themselves, sort of like shy boys at a school dance. The body certainly uses nitrogen, but not when it's all alone. The body uses nitrogen only after nitrogen links with other molecules to form large, complex molecules, such as proteins.
Why doesn't nitrogen build up in our bodies when we breathe the gas on land? Because the pressure on land, 14.7 psi, isn't strong enough to force the nitrogen to dissolve in our water-balloon bodies.
But wait a minute-Dr. Wellington is a diver and a scientist. He has no intention of diving 100 feet to coral reefs if it means suffering the bends. To guard against the bends, Dr. Wellington must carefully monitor the depth and time of each dive. His gauges register depth of the dive and air left in his tank. Before leaving the habitat, he plans exactly where he's going, the depth of the coral he will examine during the dive, and exactly how long he will work outside the habitat. Nitrogen builds up in the body according to time and depth.
Several charts are available to help in planning safe dives. The charts provide information on how long divers can stay at various depths without risking decompression sickness. The charts are called dive tables. During his visit with the fish, Dr. Wellington's body has absorbed a potentially dangerous amount of nitrogen gas. It's one of the problems he must face if he wants to study the coral in its natural habitat. Too bad that what's natural for the coral is most unnatural for the human. Of course, fish also are subject to the laws of physics. They must equalize as they dart up and down in search of food. But they're born with the right equipment to handle the job.
After Dr. Wellington has lived in the habitat for several hours, his body becomes saturated with nitrogen. Saturated, like a bowl of cereal that can't absorb more sugar. Saturated, like a washcloth that can't absorb more water. The habitat serves as a laboratory and home, but only temporarily. After about 10 days, it's time to go home for real.
With all that dissolved nitrogen in his body, Dr. Wellington better not suddenly get homesick and swim to the surface and ask the support crew to sail for land. Not with his body like a soda bottle. He must spend 16 hours in the main lock of the habitat as the pressure is slowly lowered, forcing the nitrogen safely from his body. The habitat releases the compression on Dr. Wellington's body. So, quite logically, the habitat, when used this way, becomes a decompression chamber.
The chamber starts with air pressure inside at 36 psi, the same as the ambient pressure, the pressure inside and outside the habitat at 48 feet of ocean depth. The math: [48/33][14.7] + 14.7 = 36. For slightly more than 16 hours, the habitat gradually drops its pressure to force the dissolved nitrogen in Dr. Wellington's body out of solution. Because the change in pressure is very gradual, the lungs expel the nitrogen gas safely, without bubbles building up in bone joints and sinuses. The decompression process takes 16 hours when the body is saturated,
When Dr. Wellington emerges from the decompression chamber, his body still contains more dissolved nitrogen than under normal conditions at the surface. But the remaining dissolved nitrogen is within safe limits and will not bubble out as he leaves the habitat and ascends to the Launch Retrieval Transport Vessel.
You start with 1 atm at the surface of the water, and add 1 atm for every 33 feet you descend. At 33 feet of ocean depth, you have 2 atm, or 29.4 psi (2 x 14.7 = 29.4). Remember, you already start with 1 atm at sea level before descending.
1 atm = 14.7 psi (pressure at sea level)
2 atm = 29.4 psi (pressure at 33 feet of ocean depth)
33 feet of ocean depth = 2 atm (1 atm for the weight of the air, plus 1 atm for the weight of the water)
66 feet of ocean depth = 3 atm (1 atm for the air, 2 atm for the water)
99 feet of ocean depth = 4 atm (1 for the air, 3 for the water)
4 atm = 58.8 psi (4 x 14.7)
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Gene Carl Feldman
(gene@seawifs.gsfc.nasa.gov)
(301) 286-9428
Todd Carlo Viola, JASON Foundation for Education (todd@jason.org)
Revised: 17 Oct 1995