Scientists reveal that the giant gravitational anomaly under Antarctica is getting stronger

Although the Earth is roughly spherical, its gravitational field does not adhere to the same geometric shape. In renderings, it looks a lot like a potato, with bumps and holes.

One of the strongest of these depressions – where the gravitational field is weakest – is located under Antarctica. Now, new models of how the so-called Antarctic Geoid Low evolves over time show that it is getting stronger, driven by the long, slow movement of rocks deep beneath the Earth’s surface, like a giant shifting while it slumbers.

“If we can better understand how Earth’s interior shapes gravity and sea levels, we will gain insight into factors that may be important for the growth and stability of large ice sheets,” says geophysicist Alessandro Forti of the University of Florida.

The Earth’s heliosphere – the bumpy potato shape of the gravitational field – is unequal because gravity is related to mass, and the distribution of mass within the planet is uneven, due to different rock compositions of different densities.

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It’s not a huge difference that you’ll notice on the surface. Maps tend to exaggerate it so we can see what’s going on; If you weigh yourself at a low geode and a high geode, the difference will be just a few grams.

However, the geode represents a window into processes deep within the Earth that we cannot directly observe.

Forte and his colleague, geophysicist Petar Glisovic of the Paris Institute of Geophysics in France, created a detailed map of the Antarctic Geoid Low using another window into the Earth’s interior: earthquakes. Seismic waves generated by earthquakes travel across the planet, changing their speed and direction when they encounter materials of different compositions and densities.

“Imagine having a CT scan of the entire Earth, but not having X-rays like we do in a medical office,” Forte explains. “We have earthquakes. The seismic waves provide the ‘light’ that illuminates the interior of the planet.”

Using seismic data, the researchers built a 3D model of Earth’s mantle density and extrapolated it into a new map of the entire planetary geoid. They compared this map to gold standard gravity data collected by satellites and found that it matched exactly.

This was the easy part. The next step was to try to turn back the clock to assess how geodes evolved since the early Cenozoic, 70 million years ago.

Forte and Glisovich fed their map into a physics-based model of Earth’s mantle convection, recreating Earth’s internal geological activity to see how geodes evolved over this time frame.

Then, from the starting point, they let the model move forward to see if it could reproduce the geomembrane we see today.

They also checked whether their model reproduced real changes in the Earth’s rotation axis known as the True Polar Wander. It has reached the current geostationary and polar roving floor, indicating that it also provides an accurate representation of the evolution of the geostationary.

The results show that the Antarctic geoid’s decline is not a new development; The gravity depression has been around Antarctica for at least 70 million years. But it didn’t stay still. About 50 million years ago, its position and strength began to change dramatically — timing that matches the sharp bend in polar wandering.

According to the model, the anomaly formed when tectonic plates beneath Antarctica sank and sank deep into the mantle, changing the planet’s gravitational field at the surface. At the same time, a wide region of hot, buoyant material rose upward, becoming more influential over the past 40 million years, and strengthening the geoid depression.

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Interestingly, this may be related to the Antarctic glaciation, which began in earnest about 34 million years ago. It’s just a speculative connection, but that’s the interesting thing about the landform: it shapes the sea level. So, as the geoid shifted downward around Antarctica, the local sea surface declined with it – potentially affecting the growth of the ice sheet.

Clearly, this is a hypothesis that requires further testing. However, the work shows that different geodynamic processes, from mantle convection to geomorphology to the movement of the poles, can all be interconnected and influence each other.

The gravity hole beneath Antarctica may be subtle, but it serves as a reminder that even the slowest processes deep within the Earth can leave a lasting impression on the world above.

The research has been published in Scientific Reports.

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