Every week, the readers of our space newsletter, The Airlock, send in their questions for space reporter Neel V. Patel to answer. This week: how Earth's gravity changes according to the seasons and climate.
Someone told me recently that the gravitational pull that the Earth exerts on orbiting satellites changes in accordance with the seasonal and climatological changes in the masses of the bodies of water they pass over. I’m skeptical by nature, and my source is no expert in space technology, so I’m asking you: could that possibly be true? —Shyam
It might be hard to believe that a force like gravity can be subject to whims of the changing seasons, or from shifts in land and water on the ground. But it’s true: Earth’s gravity is indeed changed by both of these factors.
Earth’s gravitational field (the 3D representation of the planet’s gravitational influence) is not a uniform sphere. It’s full of bumps and bulges and dips and depressions, caused by mountain ranges and deep-sea trenches and other geological features. That means there are certain areas of the planet where the gravitational field is more powerful than others. This goes for every single object in the universe. That’s part of the reason why attempting to land on other planets, moons, or asteroids is so hard––gravity can vary from location to location (especially for smaller objects).
You can see how Earth’s gravity changes in these images here, created from data collected by the Gravity Recovery and Climate Experiment (GRACE) mission conducted by NASA and the German Aerospace Center. Deep-ocean floors and basins will correspond with dips in the gravitational field, since seawater is less dense than rock. Denser locations like mountain ranges and mid-ocean ridges will exert a larger gravitational attraction than places composed of less dense materials, like water.
Changes in climate mean water gets shifted around week to week, month to month, and season to season. Certain regions and bodies of water will dry up and become more shallow, or experience more rain and start to swell. These fluctuations in mass have a detectable effect in the gravitational field at these locations.
When a satellite flies over a region of significantly higher density (like a mountain range), it will accelerate a bit while approaching it, because of the increase in gravitational force, and decelerate as it flies away. These changes are incredibly small but still measurable with the right tools. (The GRACE mission actually exploited this phenomenon to map out the gravitational field of the planet.)
The study of Earth’s gravitational field actually provides scientists with another lens with which to study the effects of climate change. As Arctic warming, for example, adds mass to oceans and seas, the corresponding change in Earth’s gravity can be seen and recorded. The GRACE data (collected between 2002 and 2017) showed that 60% of the Antarctica's and Greenland’s total mass loss was an effect of Arctic warming trends, while the other 40% was due to increased ice flow into the ocean. The same data suggested that in the mission’s 15-year span, Greenland lost about 260 billion tons of ice per year, while Antarctica about 140 billion tons per year.