Mysterious Milky Seas Are Visible From Space

Some of the tiniest beings on the planet gather together to create glowing assemblages that are so bright and so large that they can be seen from space

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Sailors and other seafarers have long been known as the source of both news of the outside world as well as fantastical tales filled with wonder and mystery about remote places. Even scientists shared amazing stories. For example, on his five year journey aboard the HMS Beagle in the 1830s, Charles Darwin unexpectedly sailed though an amazing natural phenomenon known as milky seas just off the southern tip of South America. Impressed, he wrote:

While sailing a little south of the Plata on one very dark night, the sea presented a wonderful and most beautiful spectacle. There was a fresh breeze, and every part of the surface, which during the day is seen as foam, now glowed with a pale light. . . . As far as the eye reached, the crest of every wave was bright, and the sky above the horizon, from the reflected glare of these livid flames, was not so utterly obscure as over the vault of the heavens.” (The Voyage of the Beagle: Chapter VIII: Banda Oriental And Patagonia.)

Other mariners also reported colossal phosphorescent seas that stretch as far as the eye could see, covering more than tens of thousands of square kilometers of the ocean’s surface. Milky seas were mentioned in Jules Verne’s 20,000 Leagues Under the Sea (1869) and in Herman Melville’s Moby-Dick (1851), which elevated this phenomenon further in public’s imagination.

About the same time that these two novels were published, W. E. Kingman, captain of the American clipper Shooting Star, sailed through a milky sea in 1854 and wrote:

The whole appearance of the ocean was like a plain covered with snow. There was scarce a cloud in the heavens, yet the sky … appeared as black as if a storm was raging. The scene was one of awful grandeur, the sea having turned to phosphorus, and the heavens being hung in blackness, and the stars going out, seemed to indicate that all nature was preparing for that last grand conflagration which we are taught to believe is to annihilate this material world.

Although reported more than 235 times between 1915 and 1993, milky seas are incredibly rare events, appearing roughly thrice per year (ref) — or sometimes not at all — and only in remote parts of the planet’s oceans, particularly in the northwestern Indian Ocean near the Horn of Africa and in the waters off Indonesia.

But what creates this ethereal glow? Only once has a research vessel sailed through milky seas, when a US naval research vessel, the USS Wilkes, sailed across a three-day milky sea event in the Arabian Sea, near Socotra in 1985 (ref). The curious scientists on that vessel collected a water sample. They discovered that an ‘impossibly’ huge aggregation of bioluminescent bacteria, Aliivibrio (Vibrio) harveyi, had colonized a bloom of the brown/green algae, Phaeocystis, floating near the sea’s surface, and appeared to be creating the eerie blue glow. (Aliivibrio (Vibrio) harveyi is a common, cosmopolitan luminescent species.) The presence of this bacteria led scientists to hypothesize that this bacterium, and maybe other bioluminescent bacterial species as well, are the cause of milky seas.

Bioluminescence — the emission of light by a living organism — is a common phenomenon, especially in the oceans. There are a variety of bioluminescent organisms in the sea, including fishes, tunicates, crustaceans, mollusks, and jellyfish. Dinoflagellates are a microscopic group of single-celled bioluminescent organisms that usually are referred to as marine plankton or algae or even as “sea sparkle”, even though they are also found in freshwater habitats. Dinoflagellates are ubiquitous throughout the world’s oceans, and thus, are familiar to most people because they are found near shorelines where they produce a bright blue luminescent flash that is visible at night when they are disturbed by, say, the splash of a kayak oar, the spray of a sailboat’s bow wave, or by waves crashing ashore. It is thought that dinoflagellates flash to scare away predators. But milky seas are typically very calm, so dinoflagellates, which flash when disturbed, are an unlikely source of milky sea bioluminescence. Further, scientists on the USS Wilkes ruled out dinoflagellates as the source of the milky seas bioluminescence.

Bioluminescent bacteria are much smaller than dinoflagellates and they operate entirely differently. They are either free-living in sea water or they live symbiotically in the guts of animals, or in marine sediments, or on the surfaces of decomposing fish. (As a microbiology undergrad, I cultured bioluminescent bacteria from the gills of a fish.) They are sometimes found in freshwater habitats, too. But only after their population becomes extremely dense — approximately 100 million bacterial cells per cubic centimeter of water — do they glow (ref). This glow is triggered by quorum sensing, which is a form of cell-to-cell communication where bacteria release a chemical signal into their environment. When the bacterial population density is high, this chemical signal alters gene expression in individual bacterial cells thereby triggering bioluminescence. Unlike sparkling dinoflagellates, bioluminescent bacteria produce a constant phosphorescence.

Light production is a metabolically expensive activity, so this raises a lot of questions: why do marine bacteria aggregate into massive bioluminescent clouds? Is this a mechanism to attract fish to eat them and thereby aid their dispersal into new habitats? Scientists know very little about when, how and why milky seas form, what their effects are, how the bacteria achieve the astonishing population densities sufficient to glow, how long these glowing aggregations last and why these events mainly show up in the Indian Ocean. It also doesn’t help that milky seas have so far only been discovered by accident.

To better understand the milky sea phenomenon, it is essential to predict when and where they will appear, so a research vessel can be there to conduct rigorous site sampling when the event is actually in progress. Because these glowing milky seas are remarkably huge and bright, is it possible to see them from space? This seemingly simple question has been the foundation of the professional life of Steven Miller, a professor of Atmospheric Science at Colorado State University and director of the Cooperative Institute for Research in the Atmosphere (CIRA), an agency that partners with the National Oceanic and Atmospheric Administration (NOAA). Ever since 2005, Professor Miller has specialized in using state-of-the-art satellites to study Earth with the ultimate goal of learning how to predict the occurrence of milky sea events, with the hope that research vessels could be sent out to them so scientists can better understand this phenomenon.

To this end, Professor Miller and a small team of collaborators compiled hundreds of historic reports of milky sea events observed between 1796 and 2010, many of which are recorded in logs of merchant ships. They found two areas where milky seas are most common: the northwest Indian Ocean, where 70% of all these events have been reported, and the waters surrounding Java, where 17% of sightings have occurred (Figure 1).

Whilst searching these reports, Professor Miller discovered a likely milky sea sighting logged by the S.S. Lima, as it crossed an area near Socotra on the night of 25 January 1995. After enlisting the assistance of Chris Elvidge of the National Geophysical Data Center in Boulder, Colorado, to retrieve archival data corresponding to the precise date and location reported by the S.S. Lima from the U.S. Defense Meteorological Satellite Program, Professor Miller and his collaborators saw a faint glowing smudge in that area.

“I was hooked”, Professor Miller wrote.

Professor Miller also got lucky because soon after this, newly developed equipment known as the Day/Night Band (DMB) was launched on two satellites on 2011 and on 2017, jointly operated by NASA and NOAA, along with the Suomi National Polar-orbiting Partnership and the Joint Polar Satellite System. These DNB sensors are so sensitive that they can detect light that is a billion times fainter than sunlight, making it possible to distinguish milky seas from the rest of the ocean without relying on eyewitness reports to track them down. For a decade, the US Navy has been using these highly sensitive satellite sensors to monitor the movement of ships at night by looking at their bioluminescent tracks.

But this new technology required on-the-sea confirmations of satellite imagery, so Professor Miller searched the DNB image archives collected between 2012 and 2021 from three locations where milky seas are often reported, according to his previous findings. He used sophisticated techniques to carefully analyze the satellite data to rule out other potential sources of light emission (such as moonlight reflecting off the ocean’s surface, a glowing upper atmosphere, or luminous clouds).

Professor Miller found 12 potential milky sea events, including one especially large event that covered than 100,000 square kilometers (62 square miles) south of the Indonesian island of Java, that lasted at least 45 nights during July–September 2019 (ref).

“I was browsing the imagery looking for clouds masquerading as milky seas when I stumbled upon an astounding event south of the island of Java”, Professor Miller writes in The Conversation. “I was looking at an enormous swirl of glowing ocean that spanned over 40,000 square miles (100,000 square km) — roughly the size of Kentucky”, Professor Miller continued. “As for the estimated number of bacteria involved? Approximately 100 billion trillion cells – nearly the total estimated number of stars in the observable universe!”

“The imagery from the new sensors provided a level of detail and clarity that I hadn’t imagined possible”, Professor Miller writes. “I watched in amazement as the glow slowly drifted and morphed with the ocean currents.”

Considering the extraordinary duration and colossal size of the 2019 Java milky seas event, it seemed to be an especially good test case to definitely establish the capability of the DNB satellites to detect milky seas, but could Professor Miller locate any eye-witnesses who were there at the time and who could describe what they saw and experienced?

A chance crossing of the 2019 Java milky sea by a 16m (52.5ft) private yacht, the Ganesha, which didn’t even cross the brightest part (Figure 2), Professor Miller and his collaborators finally got the eye-witness corroboration that connected the satellite data to surface observations. The crew of the Ganesha also shared their collection of raw photography of the glowing waters (Figure 3).

The Ganesha, captained by Johan Lemmens with a crew of six, was embarked on a circumnavigation of the globe. It was midway between Lombok, Indonesia, and the Cocos (Keeling) Islands in the east Indian Ocean. Their experience began on 2 August at ∼2100 local time and lasted until dawn the following morning, for a total traverse time of roughly 8 hours. An entry from Ganesha’s ship log at 2200 local time, reads:

When waking up at 2200 the sea was white. There is no moon, the sea is apparently full of ? plankton ? but the bow wave is black! It gives the impression of sailing on snow!

No one on the Ganesha knew what this phenomenon was until one crew member discovered media coverage of the satellite detections. Later, interviews with the crew added more detail to the ship’s log.

Now that Professor Miller and his collaborators have on-the-sea confirmation of the DNB satellite observations, they can begin to use this information to send research ships to study milky seas, so they can answer some of the thousands of questions that scientists have about them, such as: what water conditions — pH, sea surface temperature, currents, etc — are necessary for milky seas to develop? why do they mostly show up in the springtime? what is their significance? how deep does the bioluminescence go in the water column? Is it a surface slick of bacteria, as some scientists propose, or is it some small depth below the surface of the water? is the bioluminescent layer meters thick, or more? Considering scientists think it takes upward of 100 million bacteria per cubic centimeter of water to begin glowing, the answer to this question could change the estimated number of bacteria involved in a milky sea by billions of trillions, or possibly even trillions of trillions.

“Milky seas are simply marvelous expressions of our biosphere whose significance in nature we have not yet fathomed,” Professor Miller said in a statement. “Their very being spins an unlikely and compelling tale that ties the surface to the skies, the microscopic to the global scales, and the human experience and technology across the ages; from merchant ships of the 18th century to spaceships of the modern day.”


Steven D. Miller (2022). Boat encounter with the 2019 Java bioluminescent milky sea: Views from on-deck confirm satellite detection, Proceedings of the National Academy of Sciences 119(29):e2207612119 | doi:10.1073/pnas.2207612119


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