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Last Updated: Tuesday, 27 September 2005, 09:10 GMT 10:10 UK

‘Milky seas’ detected from space
By Alison Ross
BBC News

Images were taken from more than 800km up

Mariners over the centuries have reported surreal, nocturnal displays of glowing sea surfaces stretching outwards to the horizon.

Little is known about these “milky seas” other than that they are probably caused by luminous bacteria.

But the first satellite detection of this strange phenomenon in the Indian Ocean may now aid future research.

The observation is described by a US team in the journal Proceedings of the National Academy of Sciences.

The glowing sea covered an area of 15,400 sq km – about the size of the traditional English county of Yorkshire – and was observed over three consecutive nights, with the first night corroborated by a ship-based account.

The scientists analysed data from satellites fitted with special sensors.

These are normally used for detecting moonlight reflections off clouds about one million times fainter than the Sun – but milky seas emit light even fainter than this.

The team searched ship reports for suitable events to match with the archived satellite data.

A reported sighting in the north-western Indian Ocean on 25 January 1995 by a British merchant vessel, the SS Lima, met the criteria.

Early warning

Despite the weak signal, the researchers were able to see a distinct feature in the satellite imagery.

“I overlaid the points of the ship they reported when they were in it, and when they left it – and they matched up,” Dr Steve Miller, from the Naval Research Laboratory in California, told the BBC News website.

The area continued to glow for three nights, and its movements correlated with known sea surface currents.

Some bacterial species have evolved the ability to convert chemical energy to light energy
The next generation of satellite sensors will be even more sensitive, enabling scientists to send out research vessels to investigate these milky seas as they occur.

“Maybe we’ll be able to detect these more often, more reliably, and in locations we don’t anticipate right now,” said Dr Miller.

Milky seas are distinct from the brief flashes of bioluminescence seen at ships’ wakes, or breaking waves, which are caused by microscopic algae called dinoflagellates.

Instead, the constant light emitted over a wide area probably comes from the luminous bacteria Vibrio harveyi, living in association with microalgal blooms.

The team was able to estimate of the number of bacteria that the observed area would have contained – an abnormally “giant” population.

“To put it into context, it’s about 200 times more than the number of background, free-living bacteria that are spread over the continental shelf waters of all the oceans,” said Dr Miller.

There have been 235 documented sightings of milky seas since 1915 – mainly concentrated in the north-western Indian Ocean and near Java, Indonesia.