An elevator measures a half-mile column of water as tides and currents change
|The vertical profiler measures water conditions as it travels up and down the half-mile-long ALOHA mooring cable. Additional instruments can be mounted on the subsurface float at the top of the mooring. Poster detail: Bruce Howe, University of Washington. For full poster: 600 kb JPG, 18 Mb PDF).|
The ALOHA mooring consists of a buoy that floats 165 m (about 500 feet) below the surface, anchored to the seafloor at 891 m (2,923 feet) depth. An instrument called a vertical profiler makes measurements every second to capture a detailed motion picture of water conditions and currents at all depths around the MARS site.
The vertical profiler samples the entire length of the cable - nearly a half-mile - by crawling up and down it continuously. Complete round trips take it about 1.5 hours. (It pauses for 4 hours every fourth day to recharge batteries.) As the profiler crawls, sensors measure water temperature, salinity, depth, oxygen levels, and backscatter and fluorescence - two measures that can tell us how much microscopic life is in the water.
The float at the top of the mooring keeps the cable straight and serves as an attachment point for other instruments. Some instruments on the float can be focused upward to measure conditions such as current speeds near the surface. Others can be swapped out by an ROV as necessary, allowing the ALOHA mooring to serve as a secondary test node on the MARS system.
The mooring cable is a 21-mm (nearly an inch) thick braid with a 24,000-pound breaking strength. Copper wires and fiberoptic lines buried in the cable power the instruments at the top and convey their data back home.
The ALOHA mooring lets scientists see changes in the entire water column every couple of hours. That's often enough to watch as tides and other transient currents change the makeup of Monterey Bay. One key ingredient is the constant power supply from MARS. Without it, the profiler could not charge its batteries and would have to make many fewer trips per day up the cable.
Full-time communication via the MARS node helps scientists fine-tune their sampling. Sometimes, rare pulses of water sweep through Monterey Bay, briefly bringing unusual water conditions and sea life. When scientists see this happening, they can instruct the profiler to concentrate on the appropriate depths - and sample the area more thoroughly.
Yet another reason to install ALOHA on MARS is the cabled observatory's function as a testbed. Like MARS itself, the ALOHA mooring - from battery chargers for the vertical profiler to the instrument ports on the subsurface buoy - involve new developments. Scientists won't know how everything works until they put the mooring in the water. By installing on the MARS node, they'll have minute-to-minute notice if anything goes wrong.