“Milky seas” observed from sea and space

For centuries, sailors have told tales of nights when, for a few surreal hours or more, their ships would come across seas that glow the color of milk. The eerie phenomenon appeared “like a plain covered with snow,” reported the captain of a U.S. clipper ship that sailed through one such “milky sea” off the coast of Java, Indonesia, July 27, 1854.

Unlike the short-lived glow of plankton that is commonly seen in the disturbed wake of a ship, milky seas can extend for tens or even hundreds of miles. They are also rare, reported just a couple of times per year, which makes them difficult for scientists to study and sample. In the past few years, a team led by Steven Miller, atmospheric scientist at Colorado State University in Fort Collins, turned to satellite images to try to identify possible cases of milky seas, and found increasing success. But they hadn’t been able to corroborate any of these potential detections with eyewitness accounts — until now.

A paper published July 11 in Proceedings of the National Academy of Sciences recounts the experiences of the crew of Ganesha, a 52-foot (16 meters) yacht that encountered a milky sea as it sailed south of Java. One of the crew, Naomi McKinnon, contacted Miller after seeing news coverage of his satellite detection of the same event.

The crew also managed to capture images with a GoPro and Samsung smartphone — perhaps the first ever eyewitness photos of milky seas. “These photos give visual testimony to the written accounts of mariners across the centuries,” writes Miller.

A chance encounter

Milky seas have long been part of nautical lore. Herman Melville included an account of one in Moby-Dick, and the Nautilus encounters one in Jules Verne’s classic 20,000 Leagues Under the Sea.

But while other sailors’ legends have become well-established in the scientific canon, like giant squid and rogue waves, “milky seas have eluded scientific inquiry,” writes Miller. Only once has a research vessel encountered a milky sea: in 1985, by chance, in the Arabian Sea. Examination of their samples of the glowing water suggested that milky seas are caused by fields of trillions of bioluminescent bacteria, communicating with each other and reaching a sort of quorum to glow in unison. Many questions about this process remain unanswered.

Around the beginning of this century, researchers began scouring satellite data to try to study milky seas. Unfortunately, identifying them in images is harder than it might sound. Their pale glow is up to 1,000 times fainter than moonlight, which can reflect off the water. Milky seas can also be easily confused with features like clouds, airglow (the soft fluorescent glow of air molecules in the upper atmosphere), and even waves of air heaving through the atmosphere. In 2005, a team of researchers led by Miller reported one milky sea in satellite data from 1995, but the data quality was too low to learn much from it. The luminous patch of ocean was identifiable only because they knew where to look, guided by a report from a British merchant vessel.

But recently, low-light satellite imaging has improved greatly thanks to a new generation of sensitive detectors launched on a pair of NOAA satellites in 2011 and 2017. Last year, Miller and his colleagues reported a dozen possible milky sea detections over eight years of data, including one in 2019 off the coast of Java that was the size of Iceland and persisted for over a month. Still, the team had no eyewitness reports to confirm any of its new candidates.

Then, McKinnon of the Ganesha contacted them.