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Amanda Maxwell

Jun 2nd 2021

You Weigh Less in Quebec: A Weird Case of Low Gravity

Did you know there are areas on Earth missing gravity? Or that you weigh less in Quebec?

It’s true: There are parts of Canada — mostly around Hudson’s Bay — where you weigh less. This is because gravity is quite literally missing there compared with the rest of the continent.

Although the low gravity there won’t really make much of difference to what you see on your bathroom scale, it is measurable from space. In a joint mission between NASA and the German Aerospace Center, the team flew a pair of closely coupled satellites that picked up subtle changes in the Earth’s pull. From this, researchers not only mapped out variations in gravity over the globe but also gained useful insight into climate change.

Amazing GRACE

The Gravity Recovery and Climate Experiment mission (GRACE) launched in 2002 and flew until 2017 as part of the Earth Observation Systems and global climate change studies. The paired satellites flew 500 km (around 310 miles) above the Earth’s surface and kept 220 km (almost 137 miles) apart. New Scientist describes how each was equipped with a microwave ranging system that could measure differences in distance as small as a micron (0.000039 inch).

When one of the satellites flew over an area with low gravity, it experienced less of a downward pull, and this difference could be picked up by instruments on board the other satellite. From the data gathered, scientists mapped variations in gravity all over the Earth, identifying the area around Canada’s Hudson’s Bay, among others, as missing the regular pull.

How Does Gravity Go Missing?

Gravity isn’t usually something we spend a lot of time thinking about — unless, of course, you’re a theoretical physicist. This invisible force that keeps us from floating off the planet into outer space has yet to be explained by quantum physics. But there may or may not be a particle, the graviton, involved. Quanta posits that, in the same way that photons convey light energy, the graviton might be the quantum particle that transmits the mysterious pull of gravity.

Gravity is a function of mass, in that objects with more mass exert more gravity. Bigger planets have a greater gravity. For example, Mars is much smaller than our planet, and its gravity is only 37.6% of Earth’s, according to Phys.org. When gravity acts on mass, it gives an object weight. This is why an object that weighs a kilogram on Earth only weighs 376 grams on Mars.

When mass is reduced, gravity goes missing.

What’s up With Mass on Earth?

So, if reduced mass causes lower gravity, what’s happening around Hudson’s Bay?

The GRACE mission picked up changes and fluctuations in gravity across the globe. The precise measuring instruments on board also recorded fluctuations in sea level, loss of Antarctic ice mass and changes in the Earth’s crust following seismic activity. In picking up differences in the gravity topography, researchers also found that areas around Hudson’s Bay were indented. It was as if the land had been flattened by a great weight and squished out to the sides.

On the Rebound

Going back in the climate record, the scientists proposed that the indentation is due to the Laurentide Ice Sheet. This ice layer, which is over three kilometers thick in some areas, covered the land as it slowly melted over 10,000 years. The weight of the ice pushed the Earth’s surface out to the sides, meaning that the slowly rebounding land has less mass than adjacent areas and, thus, lower gravity.

However, that’s not the whole story for Canada’s missing gravity. GRACE data calculated that only 25% to 45% is due to rebound — the remainder is due to activity in the mantle layer 60 to 124 miles (100 to 200 km) beneath the Earth’s surface. Convection currents generated by heat from the Earth’s core warm up the magma or molten rock. These affect mass in local areas by making continental plates move around.

The kind of information the GRACE mission gathered is also useful to scientists studying past climate change to predict future planet impact. They also calculate that at half an inch (12 mm) per year, it will take another 5,000 years for the 650-foot bounce back to fill in the dent and “restore” gravity.

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