Earth at night

Glints of light from cities, fires, gas flares, even unregistered fishing boats speckle the dark like fireflies in a new series of satellite images released at the American Geophysical Union’s fall meeting. The Suomi National Polar-orbiting Partnership satellite, a joint project between NASA and the National Oceanic and Atmospheric Administration, launched about a year ago and has circled Earth 5000 times. Among its instruments is the Visible Infrared Imaging Radiometer Suite (VIIRS), which has dramatically improved spatial resolution compared with its predecessor (the Defense Meteorological Satellite Program, which earlier produced images of Earth at night). VIIRS includes a sensor (called a day/night band, or DNB) that can see in low-light conditions, allowing meteorologists to study moonlit clouds. But the images suggest that scientists will want to take advantage of the DNB’s images in multiple ways: not just to study clouds, but also to assess disasters such as power outages (such as before and after Superstorm Sandy last month), to study gas flares and estimate volumes of CO2 emissions, or to keep an eye on illegal unreported fishing (the boats emit light to draw in their stocks). On moonless nights—or during the dark winter months at the poles—the instrument can study Earth’s features in the dim light of the aurora, which is particularly useful when the temperature difference between atmospheric, land, and water features is not strong enough for infrared imaging. Most surprising, though, was that the DNB can even see in the dim light of Earth’s “airglow,” chemical reactions in the upper atmosphere that produce a faint radiance an order of magnitude brighter than starlight, said atmospheric scientist Steve Miller, deputy director of the Cooperative Institute for Research in the Atmosphere in Fort Collins, Colorado, at a press conference accompanying the unveiling. The DNB “is truly a paradigm shift,” he said. “This is not your father’s low-light sensor.”

Maori stones hold magnetic clues


Scientists are studying the Earth’s magnetic field using the stones that line Maori steam ovens.

The cooking process generates so much heat that the magnetic minerals in these stones will realign themselves with the current field direction.

An archaeological search is under way in New Zealand to find sites containing old ovens, or hangi as they are known.

Abandoned stones at these locations could shed light on Earth’s magnetic behaviour going back hundreds of years.

“We have very good palaeomagnetic data from across the world recording field strength and direction – especially in the Northern Hemisphere,” said Gillian Turner from Victoria University, Wellington, New Zealand.

“The southwest Pacific is the gap, and in order to complete global models, we’re rather desperate for good, high-resolved data from our part of the world,” she told BBC News.

Dr Turner was speaking here at the American Geophysical Union (AGU) Fall meeting, the world’s largest annual gathering of Earth scientists.

The NZ researcher is working on a project to retrieve information about changes in the Earth’s magnetic field stretching back over the past 10,000 years.

For data on the last few centuries, she would ordinarily have turned to pottery.

When these objects are fired, the minerals in their clay are heated above the Curie temperature and are demagnetised.

Then, as the pots cool down, those minerals become magnetised again in the direction of the prevalent field. And the strength of the magnetisation is directly related to the strength of that field.

Unfortunately for Dr Turner, the first settlers on New Zealand 700-800 years ago – the Maori – did not use pottery. However, the researcher has hit upon a fascinating alternative.

She is now exploiting the Maori cooking tradition of the steam oven.

These were pits in the ground into which were placed very hot stones, covered with baskets of food and layers of fern fronds soaked in water.

The whole construction was then topped with soil and left to cook for several hours.

Dr Turner and colleagues experimented with a modern-day hangi to see if the stones at the base of the pit could achieve the necessary Curie temperatures to reset their magnetisation – to prove they could be used as an alternative data source for their study.

“The Maori legend is that the stones achieve white hot heat,” she explained.

“Well, red hot is about 700 degrees and so white hot would be a good deal more than that. But by putting some thermocouples in the stones we were able to show they got as high as 1,100C, which of itself is quite surprising. At that temperature, rock-forming minerals start to become plastic if not melt.”

By placing a compass on top of the cooled hangi stones Dr Turner’s team was able to establish that a re-magnetisation had indeed taken place.

It turns out that hangi stones were carefully chosen, and one of the most popular types was an andesite boulder found in Central North Island.

“The Maori prefer these volcanic boulders because they don’t crack and shatter in the fire, and from our point of view they’re the best because magnetically they behave better – they’re formed with a high concentration of magnetite,” the Wellington scientist said. “But there are some sedimentary rocks which we can use also.”

Dr Turner’s team is now scouring New Zealand for archaeological digs that have uncovered hangi ovens. It is crucial that a date is recovered with the stones. This can be provided by a radiocarbon analysis of the charcoal left from the firewood used to light the oven.

Hangi stones are only likely to take Dr Turner back to the 1200s. For magnetic data deeper in time, she needs to go to other sources.

“We’re also studying volcanic rocks because they’re erupted above the Curie temperature. And the other source of information is lake sediments. Long-core sediments can give us a continuous record at specific places.”

Thanks to Kebmodee for bringing this to the attention of the It’s Interesting community.