On some moonless nights, huge swaths of the northwest Indian Ocean and the seas around Indonesia begin to glow. This event has been attended by hundreds of sailors, but only one research vessel has ever, by sheer coincidence, encountered this bioluminescent phenomenon known as milky seas. Thanks to that vessel, samples showed that the light source was a bacteria called v. harveyi, who had colonized a microalgae called Phaocystis. But that was in 1988, and researchers have yet to be in the right place and time to see one of these events again.
Both the bacteria and the algae are common in those waters, so it’s not clear what causes these rare events. To help understand why milky seas form, researchers have gotten much better at spotting these parts of bioluminescence from the air. Using satellites, Stephen Miller, a professor of atmospheric science, has been collecting both images and eyewitness accounts of milky seas for nearly 20 years. Thanks to improvements in imaging capabilities over the decades, Miller last year published a compilation of likely milky seas spanning the time frame from 2012 to 2021, including one occurrence south of Java, Indonesia, in the summer of 2019.
But these satellite observations lacked surface confirmation — that is, until the yacht’s crew Ganesha approached Miller with their first-hand account of what they had experienced during their voyage through the seas around Java in August, recently published in PNAS. Their eyewitness confirmation — along with the first photos of a milky sea — show that these satellites are indeed a powerful tool for spotting these events.
The eyes of the sky
While milky seas can be huge — more than 100,000 square miles in the case of the 2019 sighting — the intensity of this bioluminescence is still relatively dim. By comparison, the more familiar sea sparkle of marine plankton (dinoflagellates) is 10 times stronger — and even that can be hard to spot.
To capture milky seas by satellite, researchers like Miller and his collaborators had to wait for the Day/Light Band to be installed on the National Oceanic and Atmospheric Administration’s (NOAA) next-generation environmental satellites. This low-light camera is sensitive enough to capture light 10,000 times dimmer than reflected moonlight and about 1 billion times dimmer than reflected sunlight. Day/Light Bands are installed on two satellites: the Suomi National Polar orbiting Partnership (launched in 2011) and the Joint Polar Satellite System series (launched in 2017).
These satellites enabled Miller to search 10 years of satellite data, in which he found 12 suspected milk seas between 2012 and 2021. This data showed that the events could last for weeks and that they often coincided with regional monsoons and algae blooms due to the upwelling of nutrient-rich water.
“While milky seas are a spectacular visual phenomenon with an interesting historical backstory related to maritime folklore, I think in modern times we are also very interested in understanding how and why this vast expression of our biosphere, associated with primary production (the base of the marine food chain), is happening,” Miller wrote in an email to Ars Technica. “I would like to translate this into a better awareness of the atmosphere/ocean/biosphere linkage in Earth’s climate system so that we can begin to understand how fundamental components of our planet’s ecosystem can respond in a changing climate.”
But all of Miller’s sightings came from over 500 miles in the sky—until he learned of the… Ganesha crew.
