On certain moonless nights, huge swaths of the northwest Indian Ocean and the seas around Indonesia begin to glow. This event has been witnessed by hundreds of sailors, but only one research vessel has ever, by pure chance, encountered this bioluminescent phenomenon, known as the Milky Seas. Thanks to this vessel, samples showed that the light source was a bacterium called V. harveyi, which had colonized a microalga called Phaocystis. But that was 1988, and researchers have yet to be in the right place and at the right time to catch one of these events again.
Bacteria and algae are common to these waters, so it is unclear what triggers these rare events. To help understand why the milky seas form, researchers have gotten much better at spotting these bands of bioluminescence from the sky. With the help of satellites, atmospheric science professor Stephen Miller has been collecting both images and eyewitness accounts of the Milky Seas for nearly 20 years. Thanks to improvements in imaging capabilities over the past few decades, Miller last year published a compilation of likely milky seas between 2012 and 2021, including one occurrence south of Java, Indonesia in the summer of 2019.
But these satellite observations lacked confirmation on the surface, that is, until the crew of the yacht Ganesha contacted Miller with their first-hand account of what they had experienced on their voyage across the seas around Java in August, which was recently published in PNAS. Their confirmation by eyewitnesses – as well as the first photographs of a milky sea – show that these satellites are indeed a powerful tool for spotting these events.
eyes of heaven
Although the Milky Seas can be massive – over 100,000 square kilometers in the case of the 2019 sighting – the intensity of this bioluminescence is still relatively low. In comparison, the most well-known marine burst of marine plankton (dinoflagellates) is 10 times stronger, and even that can be hard to spot.
To capture the milky seas via satellite, researchers like Miller and his collaborators had to wait for the installation of the Day/Light Band on the latest generation of environmental satellites from the National Oceanic and Atmospheric Administration (NOAA). This low-light imager is sensitive enough to capture light 10,000 times fainter than reflected moonlight and approximately 1 billion times fainter than reflected sunlight. Day/light bands have been installed on two satellites: the Suomi National Polar-orbiting Partnership (launched in 2011) and the Joint Polar Satellite System series (launched in 2017).
Using these satellites, Miller was able to sift through 10 years of satellite data, in which he found 12 suspected milky seas between 2012 and 2021. These data showed that the events could last up to weeks and often coincided with regional monsoons and algae. blooms resulting from the upwelling of nutrient-rich waters.
“While milky seas are spectacular visual phenomena with an interesting historical backstory tied to maritime folklore, I think that in modern times we are also very interested in understanding how and why this massive expression of our biosphere, associated with primary production (the very basis of the marine food chain), occurs,” Miller wrote in an email to Ars Technica. “I would like to translate this into a greater awareness of the atmosphere/ocean/biosphere coupling in Earth’s climate system, so that we can begin to understand how the fundamental components of our planet’s ecosystem may react in a changing climate.”
But all of Miller’s sightings came from over 500 miles in the sky, until he heard Ganesha crew.