Hunting Milky Seas by Satellite

Hunting Milky Seas by Satellite

Like a sea captain tracking a white whale, Steve Miller has been chasing “milky seas” for decades. He has been looking for examples of a rare form of marine bioluminescence, and the arrival of new night-light sensing satellite instruments has allowed him to detect several of these rare events. It also has given scientists a better chance to sample future events.

Milky seas are a rare form of bioluminescence that mariners have described as looking like a snow field spread across the ocean. The steady white glow can stretch for vast distances, and it is not disturbed by ship wakes. Sailors have sporadically encountered this phenomenon since at least the 1600s, and Jules Verne dropped a reference to it into Twenty Thousand Leagues Under the Sea.

“A cool thing about milky seas is that they are so elusive, usually out on the high seas and away from major shipping lanes,” Miller noted. “As a result, they have remained mostly a part of maritime folklore.”

Though there has been just one direct sampling of the phenomenon, scientists believe it occurs when populations of luminous (light-making) bacteria such as Vibrio harveyi explode in connection with colonies of certain algae and phytoplankton. Unlike typical bioluminescence—where phytoplankton emit light when they are stimulated, flashing briefly like fireflies—the bacteria in milky seas can stay lit for days to weeks. However, very little is known about the conditions in which they thrive.

In the early 2000s, while working for the U.S. Naval Research Laboratory, Miller and colleagues began discussing the unique light signals that they might be able to detect with the Visible Infrared Imaging Radiometer Suite (VIIRS) that was being developed for the next generation of NOAA and NASA satellites. In particular, they were thinking about whether VIIRS would be able to detect any previously undetectable phenomena from space, such as bioluminescence in the ocean.

Miller then happened upon a ship captain’s report of a strange case of glowing seas off of Somalia in 1995. That story of the S.S. Lima led Miller to look at nighttime data from the Operational Linescan System of the U.S. Defense Meteorological Satellite Program. The signal was faint and the data were very noisy, but he found that what the Lima captain reported from the sea surface was actually visible from space. Miller and colleagues published those findings in 2005 and then waited patiently for the 2011 launch of the Suomi NPP satellite, the first to carry the new VIIRS instrument.

VIIRS was developed with a “day-night band” (DNB), a special sensor designed to detect light in a range of wavelengths from green to near-infrared. The DNB is sensitive to light levels up to 10 million times fainter than daylight, enabling scientists to distinguish signals such as airglow, auroras, city lights, and reflected moonlight. When he joined the Cooperative Institute for Research in the Atmosphere at Colorado State University in 2007, Miller continued to build a team to calibrate and explore the new features of the DNB. He believed it could help him find the elusive milky seas.

On one track, Miller built upon an established list of milky sea sightings compiled by marine biologist Peter Herring. Miller compiled more than 200 mentions of glowing seas found in historical documents and ship reports. He found one unlikely report from the captain of the C.S.S. Alabama in 1864 off the coast of Somalia that bore uncanny similarity to the 1995 Lima event. Mapping those reports from the past two centuries, Miller and colleagues found that the majority came from the northwest Indian Ocean and Arabian Sea, as well as the waters near Indonesia and the Maritime Continent.

On another track, Miller faced many challenges in determining whether the faint, ephemeral signal of milky seas could be detected by VIIRS. The day-night band is sensitive enough to detect many forms of nighttime light on and over the ocean—including lights from boats and gas flares from drilling platforms—and even in the sky—including airglow and atmospheric gravity waves. Clouds and snow also reflect light at night, muddying the DNB signals. Then there is the Moon: For half of every month, moonlight is the dominant signal reflecting off the ocean surface, making it hard to see much else.

All of these signals tend to be brighter and more ubiquitous than milky seas, so all they had to be ruled out before Miller could say whether light was coming from the ocean itself. He also noted that the DNB response to light emissions is a bit “red-shifted” away from the presumed blue/green light emissions of most forms of marine bioluminescence.

In new research published in July 2021, Miller and eight colleagues demonstrated that VIIRS could indeed detect the ghostly luminescence. Reviewing VIIRS data from 2012-2021, they found 12 instances of milky seas across the Indian Ocean and far Western Pacific. The signals from each event were invisible during the day—and so not attributable to some other reflective substance in the ocean—and persistent across several consecutive nights, drifting with the surface currents.

The largest event is shown at the top of this page. The VIIRS instrument on the NOAA-NASA Suomi NPP satellite acquired the image of Java and surrounding seas on August 4, 2019. At its largest extent, the milky sea event spanned 100,000 square kilometers, about the size of Iceland. It began at the end of July and was still visible in early September, spanning two lunar cycles. The images below show the same event alongside measurements of chlorophyll made by NASA’s Aqua satellite.

Note that the highest concentrations of chlorophyll (the green, light-harnessing pigment in phytoplankton) are adjacent to, but not matching, the brightest areas of the milky sea. Miller and colleagues suggest that while the algae are harnessing sunlight and nutrients to make food, the luminous bacteria may be consuming dead or stressed algae on the fringes of the bloom. They may also be using their light to attract fish, as the bacteria can also live within the guts of fish. There may even be a symbiotic relationship between the bacteria and the algae yet to be discovered.

To date, the only in situ study of milky seas occurred in 1985—a chance encounter by a scientific research vessel near Socotra in the Arabian Sea. Miller would like to change that. Since the Suomi NPP and NOAA-20 satellites are both equipped with VIIRS day-night bands and make daily observations, it is possible that scientists could detect a milky sea event from space and then send a research ship out to sample the waters.

“The reports over the years have been more or less consistent, but there remains a great deal of uncertainty in terms of what circumstances conspire to form one, as well as the exact composition, relevant ecology, and structure,” Miller said. “And where do they fit into nature? What they can tell us about life in the ocean? Bacteria are a very simple form of life and bioluminescence is thought to have been an essential function of some of the first life forms. What might milky seas teach us about searching for other, similar forms of basic life in the universe?”

“There is still a lot to learn,” he added. “We hope that the day-night band will help guide us toward that knowledge.”

NASA Earth Observatory images by Joshua Stevens, using VIIRS day-night band data from the Suomi National Polar-orbiting Partnership, MODIS data from NASA's Ocean Color Web, and data courtesy of Miller, S. D., et al. (2021). Story by Michael Carlowicz.

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