NASA has unveiled a portrait of Earth sparkling at night, rivalling its iconic 'blue marble' photo and providing valuable insights into weather, climate and population and economic distribution.
One of the most iconic photographs ever taken from space is the ‘blue marble’ image of the sunlit Earth, shot 40 years ago today by Apollo 17 astronauts en route to humanity's last landing on the Moon.
Now, NASA and the U.S. National Oceanic and Atmospheric Administration (NOAA) have a ‘black marble’ image to go with it — showing the globe as it appears from space not by day, but by night.
The black marble, unveiled earlier this week in San Francisco, California, at a meeting of the American Geophysical Union, isn't a single photo. Rather, it's a composite of images taken from a new weather satellite called Suomi NPP, launched on 28 October 2011.
‘Night lights’ images are themselves nothing new. Ever since 1972, scientists have been looking at city lights by night via cameras on military weather satellites in the U.S. Defence Meteorological Satellite Program (DMSP), says Christopher Elvidge, leader of the Earth Observation Group in NOAA's National Geophysical Data Centre in Boulder Colorado.
But the older satellite's cameras were … old. The new ones have 14-bit digital capacity instead of 6-bit — and much sharper resolution: 740 metres, as opposed to 5 kilometres for the old satellite.
The camera works by repeatedly scanning each scene at multiple exposures, then integrating the images, pixel by pixel. "It's like having three simultaneous low-light cameras operating at once," says Steve Miller, a researcher at NOAA’s Colorado State University Cooperative Institute for Research in the Atmosphere. "We pick the best depending on where we're looking in the scene."
The resulting images don't just show cities as bright blobs. They resolve them into Christmas-tree spangles of light — sharp enough that in the aftermath of America's Hurricane Sandy, it was possible to see individual blacked-out districts of New York City.
Like DMSP before it, Suomi NPP was designed as a weather satellite. The ‘night-lights’ observing capacity is simply a side effect. "It's there to detect moonlit clouds," Elvidge said, "but we make as much mileage out of these instruments as we can."
But don't think of the ‘black marble’ images as nothing more than a bauble. For years, researchers have used DMSP night-lights images as global proxies for population distribution, economic activity, and urbanisation. They've also been used, Elvidge said, to help pick the darkest sites for astronomical observatories as well as to provide data for ecological studies on the effect of artificial light on birds and insects. They've even been used in health studies, assisting epidemiologists trying to determine if there is a link between bright, night-time lights, and breast cancer and cardiac disease. (The hypothesis is that bright light, especially blue-tinted light, disrupts natural sleep cycles, thereby producing stress that might be linked to chronic disease.)
Other uses involve tracking fishing boats in the Sea of Japan and the Yellow Sea, where commercial fishermen use bright lights to draw fish to their nets, and to measure natural gas flaring in dozens of countries.
But the new instrument allows even more uses. "Think of all the other things we can see at night," said Miller. "Fires, lightning flashes, aurora. And then there's the Moon. It reflects off clouds, snow, volcanic ash."
The new moonlight images even show cloud shadows, he added. "You're almost seeing a three-dimensional view."
That's more than just cool. To begin with it's useful for weather forecasters. "For all the reasons that we need to see Earth during the day, we also need to see Earth at night," Miller said.
In fact, night is when many types of clouds begin to form. And with the greater sensitivity and resolution of the new camera, it's possible not just to see large cloudbanks, but to spot tendrils of fog creeping into coastal valleys.
Nor is weather the only thing the cameras can see by moonlight. They can also track sea-ice formation in the Arctic, where the sun won't rise again until springtime. "In Alaska and the South Pole the low-light imagery is particularly useful," Miller said.
It's also possible to see volcanic eruptions by the way they light up clouds from beneath. "We can monitor volcanic activity in all weather conditions," Miller said.
Even on moonless nights, he added, a phenomenon called ‘night glow’, caused by chemical reactions in the upper atmosphere, produces a dim veil of light.
"The night is nowhere as dark as we might think," Miller said.
On Earth, night glow is visible mostly as a glowing band near the horizon. But to the new satellite's super-sensitive instruments it is yet another source of illumination.
"We discovered by accident that we can detect this on completely moonless nights," Miller said. "So not only do we have low-light imaging capability when the Moon is up, but we can see clouds reflecting that night glow."
