Hunting milky seas by satellite


Like a ship captain stalking a white whale, Steve Miller has pursued “milky seas” for decades. He looked for examples of a rare form of bioluminescence, and the arrival of new satellite night light detection instruments has enabled it to detect many of these rare events. It also gave scientists a better chance to sample future events.

Milky seas are a rare form of bioluminescence that sailors have described as resembling a snowfield strewn across the ocean. The constant white glow can spread over great distances and is not disturbed by the wake of ships. Sailors have encountered this phenomenon sporadically since at least the 1600s, and Jules Verne dropped a reference in Twenty thousand leagues under sea.

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

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

In the early 2000s, while working for the US Naval Research Laboratory, Miller and his colleagues began discussing the unique light signals they could detect with the Visible infrared imaging radiometer suite (VIIRS) which was under development for the next generation of NOAA and NASA satellites. In particular, they wondered if VIIRS would be able to detect phenomena previously undetectable from space, such as bioluminescence in the ocean.

Miller then came across a ship captain’s report of a strange case of a glowing sea off Somalia in 1995. This story of the SS Lima led Miller to examine nightly data from the operational linear scan system of the US Defense Weather Satellite Program. The signal was weak and the data very noisy, but he found that what Lima captain reported from the surface of the sea was in fact visible from space. Miller and his colleagues published these findings in 2005, then patiently waited for the launch in 2011 of the Suomi nuclear power plant satellite, the first to embed 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. DNB is sensitive to light levels up to 10 million times lower than daylight, allowing scientists to distinguish signals such as air glow, aurora, city lights and the 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 new features of DNB. He believed it might help him find the elusive Milky Seas.

On one track, Miller relied on an established list of milky sea sightings compiled by a marine biologist Pierre Herring. Miller has compiled over 200 records of glowing seas found in historical documents and ship reports. He found an unlikely report from the CSS captain Alabama in 1864 off the coast of Somalia which had strange similarities to the 1995 Lima Event. By mapping these reports from the past two centuries, Miller and his colleagues found that the majority came from the northwest Indian Ocean and Arabian Sea, as well as from waters near Indonesia and the Arabian Sea. Maritime Continent.

On another track, Miller had to overcome many challenges to determine if the weak and fleeting signal from the Milky Seas could be detected by VIIRS. The day-night strip is sensitive enough to detect many forms of night-time light on and above the ocean, including boat lights and flares from drilling rigs, and even in the sky, including the glow of the air and the waves of atmospheric gravity. Clouds and snow also reflect light at night, jamming DNB signals. Then there’s the Moon: for half of every month, moonlight is the dominant signal reflecting off the ocean’s surface, making it hard to see anything else.

All of these signals tend to be brighter and more ubiquitous than Milky Seas, so they all had to be excluded before Miller could tell if the light was coming from the ocean itself. He also noted that the DNB’s response to light emissions is a bit “red shifted”Away from the suspected 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 ghostly luminescence. Looking at VIIRS data from 2012 to 2021, they found 12 cases of milky seas in the Indian Ocean and the far western Pacific. The signals from each event were invisible during the day – and therefore not attributable to another reflective substance in the ocean – and persistent for several consecutive nights, drifting with surface currents.

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

Note that the highest concentrations of chlorophyll (the green pigment that scavenges for light in phytoplankton) are adjacent, but not consistent with, the brighter areas of the Milky Sea. Miller and colleagues suggest that while algae harness sunlight and nutrients to make food, light bacteria can consume dead or stressed algae on the margins of flowering. They can also use their light to attract fish, as bacteria can also live in the bowels of fish. There may even be a symbiotic relationship between bacteria and algae yet to be discovered.

To date, the only in situ to study 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 perform daily observations, it is possible that scientists could detect a milky sea event from space and then send a ship from research to sample the waters.

“Reports over the years have been more or less consistent, but there remains a lot of uncertainty as to the circumstances that combine to form one, as well as the exact makeup, relevant ecology and structure,” Miller said. “And where do they fit in with nature? What can they tell us about life in the ocean? Bacteria are a very simple life form, and bioluminescence is considered an essential function of some of the earliest life forms. What could the Milky Seas teach us about finding other similar basic life forms in the universe? “

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

Images from NASA’s Earth Observatory by Joshua Stevens, using data from the VIIRS day-night band of the National Suomi Partnership in Polar Orbit, the MODIS data of NASA Ocean Color Web, and the data is courtesy of Miller, SD, et al. (2021). Michael Carlowicz story.

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