快猫短视频

Holiday special

The seasonal movement of water between north and south polar regions regularly stretches and compresses the Earth, say US researchers.

Geophysicists analysing data from the Global Positioning System were initially puzzled when a small annual Earth movement remained after they accounted for known phenomena such as tides and plate tectonics.

A careful investigation showed the effect was 鈥渉idden in the theory鈥� describing the behavior of an elastic body like the Earth, says Geoffrey Blewett of the University of Nevada in Reno.

Each winter, snow and soil moisture accumulates in cold polar regions and the atmosphere becomes denser. The elastic body theory predicts that this extra weight should slightly compress the hemisphere experiencing winter. In the north, the maximum compression should occur in February or March, when the most water has accumulated.

The polar region should then dry out during spring and summer, while moisture settles out at the opposite pole. The result is an annual oscillation of Earth鈥檚 shape. And this is exactly the pattern shown in the GPS data.

Hemispheric shift

Sites near the north pole sank up to three millimeters in February and March, while regions near the equator moved about 1.5 millimeters north. Then the cycle reversed, with moisture accumulating near the south pole causing it to sink about three millimeters during the southern hemisphere鈥檚 winter in August and September.

鈥淭he northern hemisphere and the southern hemisphere compress,鈥� Blewett told 快猫短视频. The water that 鈥減ushes down鈥� the poles oscillates between the hemispheres.

Once they understood the oscillation pattern, the geophysicists were able to estimate that a load of about 10¹⁶ kilograms shifts between the hemispheres each year.

Journal reference: Science (vol 294, p 2342)