Using satellite images, scientists have now given Antarctica its second annual physical examination, and they have found a giant crack in a glacier that may produce a major iceberg in less than 18 months (bottom right image).

The researchers from the U.S. Geological Survey (USGS) and NASA discovered the 25-kilometer-long crackspanning more than two thirds of the glacierusing NASA's Landsat 7 satellite, which passes over the continent 16 times a day on its nearly pole-to-pole orbit. The satellite takes about 300 pictures of the continent during the Antarctic summer (November through February). On the last picture of the site, taken 10 months ago, there was no crack in the ice shelf (top right image).

To determine how quickly the crack was growing, the scientists needed additional data, which they obtained by combining their findings with earlier observations from NASA's Terra satellite, the Canadian Space Agency's RADARSAT and the European Space Agency's Radar Imager. "Most of this crack formed very rapidly, in less than five weeks," says Robert Blindschadler, a glaciologist at NASA's Goddard Space Flight Center. "Right now it is growing much more slowly, at about 13 meters a day."