Gravity feels reliable — stable and consistent enough to count on. But reality is far stranger than our intuition.
In truth, the strength of gravity varies over the Earth's surface. And it is weakest beneath the frozen continent of Antarctica after accounting for Earth's rotation
A new study reveals how achingly slow rock movements deep under the Earth's surface over tens of millions of years led to today's Antarctic gravity hole. The study highlights that the timing of changes in the Antarctic gravity low overlaps with major changes in Antarctica's climate, and future research could reveal how the shifting gravity might have encouraged the growth of the frozen continent's climate-defining ice sheets.
"If we can better understand how Earth's interior shapes gravity and sea levels, we gain insight into factors that may matter for the growth and stability of large ice sheets," said Alessandro Forte , Ph.D., a professor of geophysics at the University of Florida and co-author of the new study recreating the Antarctic gravity hole's past.
Caused by different densities of rock far beneath the Earth's surface, these variations in gravity are small in absolute terms. But they can have particularly large effects on the oceans. Where gravity is weaker, the ocean surface can sit slightly lower relative to Earth's center because water flows away toward areas of stronger gravity. Due to its gravity hole, the sea-surface height around Antarctica is measurably lower than it would otherwise be.
In the study, published recently in Scientific Reports , Forte and Petar Glišović, Ph.D., of the Paris Institute of Earth Physics, mapped the Antarctic gravity hole and revealed how it developed over millions of years. They relied on an Earth-spanning scientific project that combined global earthquake recordings with physics-based modeling to reconstruct the three-dimensional structure inside Earth.
"Imagine doing a CT scan of the whole Earth, but we don't have X-rays like we do in a medical office. We have earthquakes. Earthquake waves provide the 'light' that illuminates the interior of the planet," Forte said.
Accounting for all the rocks their earthquake waves could illuminate within Earth and physics-based modeling to predict the gravity pattern, Forte and Glišović reconstructed the gravitational map of the entire planet. The reconstructed map closely matched the gold-standard gravitational data captured by satellites, supporting the realism of their underlying models.
Then came the hard part: turning the clock backward to see how Antarctica's gravity hole developed over eons. With sophisticated computer models, they used physics-based reconstructions to rewind the flow of rocks in the interior and track changes back 70 million years, back to the time of the dinosaurs.
Those past snapshots revealed that the gravity hole started off weaker. Then, between about 50 and 30 million years ago, the gravity hole started to gain strength. The timing overlaps with major changes in Antarctica's climate system, including the onset of widespread glaciation.
Going forward, Forte hopes to test for a causal connection between this strengthening gravity hole and the ice sheets, using new modeling that links gravity, sea level and continental elevation changes.
The aim is to address one big question: "How does our climate connect to what's going on inside our planet?" Forte said.
