Shedding Light on the Ocean Floor
Miles below the ocean's surface, there exists another world: a dramatic landscape of vast mountain ranges, eerie spires of black rock, and dark, yawning canyons. Inhabiting this spooky setting is a most unlikely resident--light.
Cindy Van Dover is one of the scientists who detected this light, a faint glow that exists in a world of midnight black. Van Dover is a researcher with the University of Alaska Fairbanks' Institute of Marine Science.
Much of the nocturnal topography of the sea floor has been shaped by molten rock forced upward in areas where plates in Earth's crust spread apart, Van Dover said. Heaps of lava quickly cooled by sea water have formed incredible, yet unseen, mountain ranges.
"They're the most dominant geographic feature of the planet, other than the continents," Van Dover said.
Scattered amidst these mountains are tubes that spew the exhaust of the reaction between near-freezing sea water and lava. These chimneys, known as hydrothermal vents, exhale a mixture of sea water and minerals at a temperature of about 660 degrees Fahrenheit. Some of the vents are called "black smokers" for the color of their plumes.
Black smokers exist in an even blacker world. Since sea water absorbs sunlight, the ocean darkens with depth, Van Dover said. Below a few hundred feet, the only flashes of light come from fish and other tiny creatures that occasionally glow to attract mates or find prey. Scientists explore this inky darkness using submarines and shipboard-operated diving vehicles equipped with cameras and spotlights.
By illuminating the sea floor, researchers have found a quirky group of animals that live close to the vents. Giant clams, mussels, and snake-like tube worms thrive in the warm, sulfurous waters near the vents, which Van Dover calls "undersea oases." Near a vent deep under the Atlantic Ocean, researchers in 1985 found a species of shrimp that lacked the eyestalks and corneas of the shrimp we eat. Although it was assumed these deep-sea shrimp were blind, Van Dover and her colleagues found a set of visual organs on the backs of the shrimp.
What could shrimp possibly be looking at in that deep, dark world? The question led researchers to aim their cameras at the vents. They turned off the diving vehicle's spotlight, and held the camera shutter open for 10 seconds. The resultant photograph showed a crown of very faint light at the mouth of a hydrothermal vent.
Though most of the light comes from thermal radiation (the vents glow just as hot metals glow red), heat is not the only light source. Van Dover and other scientists have several theories on what else could be producing light energy in that world of darkness. One idea is that light is sometimes a byproduct of the formation of metal crystals, a process that happens at the vents as minerals from the lava react with sea water. When small bubbles collapse, they also emit light.
What's the significance of this dim night-light on the ocean floor? Van Dover hopes to find if deep sea organisms, such as tiny bacteria, can use the light for energy in the same way plants convert the sun's light to stem, stalk and fruit. Tube worms and other sea floor creatures aren't independent of the sun because they use oxygen that originated in solar-based processes.
Van Dover and her colleagues might find a creature that doesn't need sunlight to produce energy. It's hard to imagine a better earthly setting for such an alien than the mysterious world at the bottom of the sea.
