The “milky seas” of the Earth observed from space and the sea


For centuries sailors have told tales of nights when, for a surreal few hours or more, their ships cruised seas that glistened the color of milk. The strange phenomenon appeared “like a snow-covered plain”, reported the captain of an American clipper that sailed through such a “milky sea” off Java, Indonesia on July 27, 1854.

Unlike the fleeting glow of plankton commonly seen in the disturbed wake of a ship, milky seas can stretch for tens or even hundreds of kilometers. They are also rare, reported only a few times a year, making them difficult for scientists to study and sample. In recent years, a team led by atmospheric scientist Steven Miller at Colorado State University in Fort Collins has turned to satellite images to try to identify possible cases of milky seas, and has met with increasing success. But they hadn’t been able to corroborate any of these potential detections with eyewitness accounts – until now.

An article published on July 11 in Proceedings of the National Academy of Sciences recounts the experiences of the crew of Ganesha, a 52-foot (16-meter) yacht that encountered milky seas while sailing south of Java. One of the crew, Naomi McKinnon, contacted Miller after seeing media coverage of her satellite detection of the same event.

The crew also managed to capture footage with a GoPro and Samsung smartphone – possibly the first eyewitness photos of milky seas. “These photos give visual testimony to the written stories of seafarers through the centuries,” Miller writes.

A chance encounter

Milky seas have long been part of nautical traditions. Herman Melville included a count of one in Moby-Dickand the Nautilus meets one in Jules Verne’s classic 20000 Leagues Under the Sea.

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

Around the turn of this century, researchers began combing through satellite data in an attempt to study the Milky Seas. Unfortunately, identifying them in the pictures is harder than it looks. Their pale glow is up to 1,000 times fainter than moonlight, which can reflect off water. Milky seas can also be easily confused with features such as clouds, airglow (the soft fluorescent glow of air molecules in the upper atmosphere), and even waves of air passing through the atmosphere. In 2005, a team of researchers led by Miller reported a milky sea in satellite data from 1995, but the quality of the data was too low to learn much. The bright patch of ocean was only identifiable because they knew where to look, guided by a report from a British merchant ship.

But recently, low-light satellite imagery has improved dramatically 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 Java that was the size of Iceland and lasted over a month. Still, the team had no eyewitness reports to confirm any of its new candidates.

Next, McKinnon of Ganesha contacted them.


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