Antarctica sits above Earth's weakest gravitational pull - and a 70-million-year geological history explains why

Gravity is not the same everywhere - and underneath Antarctica, it is weakest of all

The ground beneath your feet pulls you down with a force that feels constant and reliable. In reality, it varies. Earth is not a perfect sphere, and the density of the rock making up its interior is not uniform. Regions with denser material beneath them have slightly stronger gravitational pull; regions with less dense material below have slightly weaker pull. These differences are small - measurable in parts per million - but their effects accumulate over planetary scales.

After accounting for Earth's rotation, which creates its own variation in effective gravity across latitudes, the single weakest gravitational region on Earth's surface sits beneath Antarctica. The Antarctic gravity low, as scientists call it, is not just a curiosity. It affects the ocean surface - where gravity is weaker, water redistributes toward regions of stronger pull, making the sea surface sit slightly lower. It may have influenced how and when Antarctica's ice sheets grew. And until recently, no one had a detailed explanation for how it formed over geological time.

Alessandro Forte of the University of Florida and Petar Glisovic of the Paris Institute of Earth Physics have published that explanation in Scientific Reports, using a combination of earthquake-based tomography and physics-based modeling to reconstruct the development of the Antarctic gravity low over the past 70 million years.

Using earthquakes as X-rays

"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 explained.

Seismic waves from earthquakes travel at speeds that depend on the temperature, composition, and density of the material they pass through. By recording how these waves arrive at seismometers distributed across Earth's surface - measuring travel times, amplitude changes, and wave distortion - scientists can construct three-dimensional maps of Earth's interior, much as X-rays build cross-sectional images of the human body. This technique, called seismic tomography, is the primary tool available for understanding Earth's deep structure.

Forte and Glisovic combined a global seismic tomography model with physics-based predictions of how the three-dimensional density structure implied by that model would produce a gravitational field at Earth's surface. The reconstructed gravitational map closely matched the gold-standard satellite gravitational data from space-based measurements, supporting the accuracy of their underlying model of Earth's interior structure.

Running the clock backward 70 million years

The harder part of the analysis was reversing direction in time. Rather than predicting a gravity map from a known interior structure, the team needed to reconstruct what Earth's interior looked like in the past - and how the slow, viscous flow of rock through the mantle changed that structure over tens of millions of years.

Mantle rock is solid by most measures but flows over geological timescales under the influence of heat and pressure. Hot, buoyant rock rises slowly; cool, dense rock sinks. This circulation drives the movement of tectonic plates at the surface and continuously reshapes the density structure of the interior. The same density changes that drive plate tectonics also cause gravitational variations to shift over millions of years.

Using sophisticated models of mantle convection physics - the equations governing how viscous material flows under temperature gradients and gravity - the researchers ran their model backward through time, reconstructing the past states of Earth's interior at intervals back to 70 million years ago, when dinosaurs still roamed the planet.

Those past snapshots reveal how the Antarctic gravity low evolved - when it was weaker or stronger than today, how its geographic footprint changed, and what mantle dynamics drove those changes. The team found that the timing of changes in the gravity low overlaps with known major shifts in Antarctic climate over the past tens of millions of years.

Ice sheets, sea level, and the implications

"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 Forte.

The connection between gravity and ice sheet formation is not intuitively obvious but follows from the same physics that makes the ocean surface lower around Antarctica today. Where gravity is weaker, the ocean sits lower - meaning the ice-ocean interface, which is crucial to ice sheet dynamics, is positioned differently than it would be with normal gravity. Changes in the strength and extent of the gravity low over geological time would have shifted where ice sheets could grow, how quickly they expanded during cold periods, and how stable they were against warming.

Whether changes in the Antarctic gravity low actually influenced the growth of the continent's ice sheets - which began forming around 34 million years ago and expanded dramatically during subsequent cooling episodes - is a question the current study raises but does not resolve. The overlap in timing between gravity changes and climate transitions is suggestive, but establishing a causal role for gravity variation in ice sheet dynamics would require integrating the geophysical modeling with climate models in ways the current work does not attempt.

The study adds Antarctica's deep gravitational history to the list of factors that researchers studying the continent's past and future climate need to consider. Ice sheet models used to project future sea level change incorporate multiple geophysical factors; how much the gravitational history matters for those projections is a question the current work helps to frame.