The “milky sea” phenomenon, sometimes known as “mareel”, is a very rare form of bioluminescence where luminous bacteria communicating with each other produce a continuous glow that illuminates the water like softly glowing snow. While the startling sight has not been fully resolved, the accidental encounter of a boat with the mareel in Indonesia gave scientists a unique opportunity to study this event and the tools we used to describe it.
A milky sea event was detected by satellites south of Java, Indonesia, between July and September 2019, but without surface observations to confirm the images, it was difficult for scientists to confirm the finding. Fortunately, a private yacht was navigating the spot at that time of year with a few eyewitnesses, a bucket, and a digital camera.
Ganesha, a private yacht captained by Johan Lemmens with a crew of six, was transiting between the Indonesian city of Lombok and the Cocos (Keeling) Islands when an unusual event occurred on 2 August.
Its records confirmed satellite observations made in 2019, pointing to an incredibly rare event that was predicted to occur globally only once or twice a year. Photos and interviews of the rare encounter were analyzed by scientists, and their findings were published in the journal Proceedings of the National Academy of Sciences.
Amazed by this shimmering event, the crew decided to pull a bucket of water for a closer look. Collecting the sample did not spoil the glow, and interestingly, mixing it caused the water to darken in contrast to the “normal” bioluminescence response.
Captain Lemmens believed that the glow settled about 10 feet below the water’s surface, contradicting the previously proposed surface slick hypothesis, which stated that the milky sea phenomenon could be the result of shiny bacteria sitting in a thin film. The deeper presence of bioluminescence in the water was supported by the fact that the movement of the yacht did not spoil the glow.
While many questions remain unanswered about the formation, structure, and inhabitants of the milky seas, Ganesha’s observations are crucial as they confirm that satellite imagery can be used to identify mareels. In the future, this could allow marine scientists to step in at the opportune moment and get a clearer idea of what’s going on.
“With new confidence in our lookouts in space, an expedition to the milky sea may now be possible,” the study authors write.
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