MYSTERIOUS spirals in the glitter of sunlight on the sea may reveal intricate mixing beneath the waves, say oceanographers in California. Thirty years ago, Apollo astronauts noticed spirals 10 to 25 kilometres across in the pattern of sunlight reflecting off the ocean.Magazine The patterns have been extensively photographed from the space shuttle, but no one knew why they formed. Walter Munk and Larry Armi of the Scripps Institute of Oceanography in La Jolla, California, and their colleagues studied 400 photographs of the spirals in the archives of the Johnson Space Center in Houston, Texas. They conclude that subsurface currents are the cause. Wind conditions sometimes create linear "slicks" on the ocean that appear smoother than their surroundings. These regions contain increased concentrations of surfactants produced by fish and plankton. This raises the surface tension, and damps down the tiny waves that usually give the sea surface its rough texture and sparkle. The researchers suggest that currents below the surface are twisting these slicks into the spirals that are visible from space. When two bodies of water slip past each other, variations in the difference in speeds across the boundary can give birth to eddies. "Subsurface processes are winding these (spirals) up," Armi says. Studying the spirals may provide a better understanding of mixing in the oceans, though this will require more detailed observations, Armi says. To make these, the researchers plan to use an aircraft to follow the evolution of a spiral over time.