HAVE SCIENTISTS DETERMINED THE DEPTH OF THE MARIANAS TRENCH YET?
Asked by Phil
Armstrong’s class at Rice Middle School
in Plano, TX
Answered by Nathan Becker, Dept of Geology &
Geophysics, University of Hawaii
The depth of the
Marianas trench has actually been known for some time. The trench is of course a long continuous
feature running from Yap to the Bonin Islands, but I'll assume you mean the
Challenger Deep, which is the deepest point along this trench. I will also apologize in advance for not
having exact dates in front of me, but it goes something like this:
Echosounders have been used as oceanographic tools since the '60s. These
work by sending a sound (or ping) out into the water and then listening for the
echo. If you know how fast sound travels in sea water, then it's simply a
matter of taking the time, T, it takes for the sound to go down to the seafloor
and echo back and multiplying it by the sound velocity in seawater, V, which is
about 1500 m/s. You also have to divide by 2 since what you're measuring is the
total time of the trip of the ping to the seafloor and then back to the ship.
Therefore to get distance, D, use:
D = 1/2 x V X T
or
D = 750 x T
And in this way the
Challenger Deep was first determined to be the deepest point in the ocean back
around 1960 or so. The name "Challenger" in fact comes from the name
of the ship that identified this point. It's about 11 km deep, or just under
seven miles, or about 36000 ft. When you fly in an airliner at cruising
altitude you are as high above the ground as we are above the seafloor when we
take the ship across the trench.
But the story doesn't end there. I
think it was around 1962 or so that a submersible, the "Trieste",
descended to the bottom of the trench with its inventor aboard, Jacques
Piccard, and a US Navy lieutenant (sorry, forgot his name). Another way to
determine depth is by measuring the water pressure which is the same as the
weight of water above you, and all submersibles carry a pressure sensor.
But there is a problem with both
techniques. The velocity of sound in water isn't constant at 1500 m/s, since
sound velocity is dependent on density, salinity, and temperature of the water,
things that all change from place to place and with depth. Pressure is also
dependent on seawater density. The solution? You need an accurate measure of
seawater velocity. This can be done by dropping a probe over the side of the
ship that measures sound velocity as it descends and sends this data back to
the ship by an attached wire. It doesn't go all the way to the bottom but since
most of the change in seawater sound velocity is in the upper 100 meters (the
thermocline) it doesn't have to. Modern
sonar surveys use such a probe prior to conducting surveys to get accurate
sound velocity and thus accurate depths.
I hope this answers your
question. Let me know if you have any
more.
Aloha,
Department of Geology
and Geophysics, University of Hawaii
Note: Nathan is a
doctoral student at UH and is currently working on a project of his
own out here. He
also is on the crew running the HMR1, so we thought he could
best answer Phil's
question. Thanks Nathan!