What Causes Swaths of the Ocean to Glow a Magnificent Milky Green?
The sky was moonless and overcast, leaving no stars to steer by. Alone at the helm in the middle of the Arabian Sea, somewhere between Oman and India, I could see nothing in the ink-black night save for our ship’s dimly lit compass rolling on its gimbal mount as we heaved and swayed through three-meter seas. But half an hour into my shift, the sails above me began to glow, as if the moon had risen. But there was no moon, nor any stars or other ships. The light, it seemed, was coming from below and growing in intensity. Soon the entire ocean was glow-stick green, but muted, as if the light were shining through a sea of milk.
... During the voyage, I’d grown accustomed to the usual “sea sparkle” caused by dinoflagellates that ignite when the water is agitated, causing ribbons of light to twist off the Mir’s bow. But this was not that. This was the whole of the ocean, as far as I could see, glowing a uniform, opaque green. Despite the compass still wheeling in its mount, the light in the water created an optical illusion, making the sea appear perfectly calm, as if we were gliding through phosphorescent skies rather than roiling seas. ...
I woke the rest of the crew, and for over four hours we remained engulfed in this sea of green light, wonderstruck, with no idea what it was we were witnessing. Finally, a razor-sharp line appeared ahead of us where the lambent sea ended and blackness began. ... It wasn’t until we arrived at port 10 days later that we would learn the name for the eerie phenomenon that had surrounded us: a milky sea. ...
For centuries, sailors have been describing milky seas, rare occurrences where enormous expanses of the ocean light up uniformly at night, at times stretching for tens of thousands of square kilometers, or more. ...
A milky sea even made an appearance in Moby-Dick, where Melville describes a mariner sailing through a “shrouded phantom of the whitened waters” that were as “horrible to him as a real ghost.” ...
Every observation of a milky sea throughout history has been a chance encounter ... , and only once did a vessel with any scientific research capabilities happen upon one, when the USS Wilkes steamed through a milky sea for three consecutive nights off the island of Socotra, Yemen, in 1985. Onboard the Wilkes was the late marine biologist David Lapota, who was working for the navy at the time studying bioluminescence. Lapota and his team of researchers sampled the water and discovered a profusion of the bioluminescent bacterium Vibrio harveyi—a common, well-dispersed species known to luminesce—attached to bits of algae, leading them to hypothesize that legions of this bacterium and potentially other bioluminescent bacterial species as well, are the cause of milky seas. This research, conducted nearly 40 years ago, remains the only time a milky sea was ever studied in the field. ...
Steve Miller, director of the Cooperative Institute for Research in the Atmosphere, has been part of a small group of scientists leading the effort to demystify milky seas for nearly 20 years ...
Miller and his colleagues considered whether it might be possible to observe any type of marine bioluminescence from space. It was assumed that any small-scale bioluminescence, such as sea sparkle, produces far too weak a light signal to be seen from so far away. ... An atmospheric scientist by training, Miller wondered if he could use historical satellite data to locate one of these events. It didn’t take long to find what he was looking for ...
Since that initial discovery, a new generation of satellite technology has greatly improved Miller’s hunt for milky seas. Two satellites operated by the National Oceanic and Atmospheric Administration—the Suomi National Polar-orbiting Partnership and the Joint Polar Satellite System—were launched in 2011 and 2017, respectively. These modern satellites ... are equipped with specialized day/night band instruments that, at their extreme low end of sensitivity, can pick up something as dim as bioluminescence from space. ... Miller and his team have successfully identified a dozen milky seas via satellite imagery, the most significant of which was a 2019 event off the coast of Java spanning over 100,000 square kilometers—roughly the size of Iceland—which glowed continuously for at least 45 nights. ...