Icy heat: satellites look at heat flow through Antarctica's crust.

using satellite observations of our planet's magnetic field in the region overlying Antarctica, scientists can now estimate the amount of heat flowing upward through Earth's surface under kilometers-thick ice. Zones of high heat flow often match sites of volcanic activity or areas where glaciers flow much faster than normal, the scientists say.

Most of Earth's magnetic field derives from the motion of hot, iron-rich material in the planet's core. By measuring the overall magnetic field only at certain very long wavelengths, the portion of the field due to the rocks of Earth's crust stands out from the core's contribution, says Cathrine Fox Maule, a geophysicist at the Center for Planetary Science in Copenhagen. Because surface materials hold their magnetism only if their temperatures stay below a value characteristic of each mineral, the strength of the magnetic field above Earth's crust can provide insight into the amount of heat flowing out from the planet's interior.

The magnetic field data analyzed by Fox Maule and her colleagues were gathered by a Danish satellite that orbits at an altitude of about 750 km and a German craft that swoops over Antarctica at a height of 400 km. The scientists used those data to constract a model of Earth's magnetic field at an altitude of 300 km. To isolate the component of that field contributed by magnetic-crust minerals beneath Antarctica's thick shroud of ice, the researchers subtracted the portions caused by Earth's core. Where there is the least magnetic contribution, there is the greatest heat flux.

The average heat flux through Antarctic crust is 65 milliwatts per square meter (mW/[m.sup.2]), according to the researchers' calculation. That's the same value that other scientists have measured more directly for other continents, says Fox Maule.

Some Antarctic areas of elevated heat flux, which measure as high as 185 mW/[m.sup.2], correspond to locales surrounding currently active volcanoes. Other zones of high heat flux lie beneath regions where scientists have described subglacial lakes, including Lake Vostok, which lies in a seismically active region beneath 4 km of ice (SN: 3/3/01, p. 139). Still other hot spots sit beneath regions where fast-moving ice streams abound.

Fox Maule and her colleagues describe their findings in an upcoming Science.

The new research is a "solid contribution" to research, says geophysicist Donald D. Blankenship of the University of Texas at Austin. An assessment of the heat flow through Earth's crust, he notes, is "one of the hardest measurements to make on any continent, never mind one that's covered with ice."