for National Geographic News
This is heavy.
Something massive is moving on or within the Earth and causing the planet's gravity field to get wider around the equator and flatter at the poles, according to a pair of scientists studying the field with sensitive satellite instruments.
The scientists are uncertain as to the reasons for this phenomenon, which was just the opposite for several decades prior to 1997, but think the answer possibly lies within long-term variation in the oceans.
"Starting after 1997, the world that was getting rounder started getting more oblate [flattened at the poles]," said Christopher Cox, a research scientist at the Space Geodesy Branch of NASA's Goddard Space Flight Center in Greenbelt, Maryland.
At first, the scientists had any number of explanations for this observed phenomenon—from changes in the atmosphere to the amount of water in the oceans to ocean tidal effects. But the Earth kept getting fatter at the equator and flatter at the poles.
"It has finally gotten so big that we can't explain it with any known mechanism," said Cox, who co-authored a paper in the August 2 issue of the journal Science on this change in shape of the Earth's gravity field.
The change in shape since 1997 is very subtle: an increase in equatorial radius of about one 25th of an inch (one millimeter) per year, according to measurements.
"What is interesting is that it tells us that some mass redistribution occurred inside the Earth system, probably the climate system," said Anny Cazenave, an Earth scientist at the National Center for Space Studies in Toulouse, France. "This is an interesting constraint to climate models."
Post-Glacial Rebound
Prior to 1997 the Earth was getting rounder as it recovered from thousands of years of being squished at the poles by the weight of Ice Age glaciers, an effect scientists refer to as post-glacial rebound (PGR).
As the ice sheets melted, the land that was beneath the ice started rising. As the ground rebounded in this fashion, the gravity field changed. The effect is likened to putting a finger on a rubber ball and watching it slowly bounce back.
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