Friday, April 26, 2013

Deep-sea Souvenirs

by Hilary Close, University of Hawaii 

Evan Howard carefully attaches a bag of cups
to the rosette frame. (Hillary Close, U.H.)
In keeping with a time-honored tradition among ocean-going scientists, the DeepDOM team has been spending some of our spare time with the humble Styrofoam cup. Some lucky first-graders from Belmont, Mass., and scores of friends and family members are eagerly awaiting the results of our efforts. Why?

The same properties that make Styrofoam a perfect material for insulating your hot cocoa also make it the perfect souvenir of deep-sea expeditions: It is composed mostly of air. As our deep-sea instruments descend into the ocean, the weight of the overlying water presses down (and all around) harder and harder. When Styrofoam cups go along for the ride, this intense pressure squeezes out the air bubbles and compresses the foam material to a fraction of its original size.

To collect water from near the seafloor, we send our CTD rosette down to 5,500 meters (18,045 feet, or 3.4 miles) below the sea surface (see video in Evan’s post). When we attach a mesh bag containing our Styrofoam souvenirs very carefully to the rosette frame, they accompany it on its long journey to the seafloor. The pressure at this depth is approximately 550 times the pressure we feel up at the surface: 550 bar or 8000 PSI (pounds per square inch). That would be equivalent to several large elephants standing on the palm of your hand!

Method for measuring the volume of "unshrunken" and shrunken
cups by water displacement. (Hillary Close, U.H.)
We tried a little experiment on board to measure just how much air was pushed out of the cups. We measured the volume of an "unshrunken" cup and a shrunken cup using the principle of displacement. (Remember Archimedes? Eureka!) It proved to be very difficult to measure water volumes on a rocking ship. However, our answers converged after several trials: our shrunken cups lost about 90 percent of their original volume (all air)! If we assume that the mass of the cup remained about the same, that means that the shrunken cup is ten times denser than the unshrunken cup (density = mass/volume).

The cups also have artistic merit: the scientists on board and some very lucky first-graders carefully decorated these cups before we sent them to the depths. See more of our handiwork in the More Photos page above.

Advanced calculation: Eureka! Try reading more about Archimedes’ principle: it is actually a bit more complex that simply calculating volume from displacement. Given that it is very difficult to use a scale to measure mass on board a rocking ship, how might we use Archimedes’ principle to confirm the mass of each cup? Hint: the shrunken and unshrunken cups have different volumes, but they also float differently, or have different buoyancy.

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