Gravity is one of the most fundamental forces of not just our planet, but the universe as a whole. Specifically, it’s the “pull” any object with mass exerts on every other object with mass. The strength of this force is proportional to the mass of an object, so while we don’t feel the gravitational pull of our pet hamster, we certainly feel the effects of Earth’s gravity. This is why we experience low gravity on the Moon (which is about one-hundredth the mass of the Earth), and why the Sun, with a mass about 333,000 times that of the Earth, has a strong enough gravitational field to keep an entire solar system in formation – the Moon’s gravitational pull is much weaker, and the Sun’s pull many times stronger, than that of Earth. 

Although gravity feels pretty much the same to us no matter where on Earth we go, it’s not perfectly uniform. Since gravitational force is influenced by mass, areas of high and low mass in Earth’s topography, like mountains, valleys, and ocean trenches, cause slight irregularities in the Earth’s gravity. Even the movement of water beneath the surface of the Earth shifts mass distribution enough to ever so slightly affect the pull of gravity.

To measure these changes in Earth’s gravitational field, scientists rely on NASA’s Gravity Recovery and Climate Experiment (GRACE). GRACE consists of two satellites, orbiting one behind the other and keeping careful track of their relative positions using microwave sensors. As they pass over irregularities in Earth’s gravitational field, the lead satellite speeds up or slows down enough to change that distance, allowing us to identify these spots of slightly different gravity and thus, slightly different mass down on Earth. With these data, NASA produces monthly gravitational field maps. Besides providing extremely detailed descriptions of Earth’s topography, these let us track the movement of water over the planet, which is an enormous and ongoing cause of mass transport. Thanks to GRACE, we can keep better track of groundwater changes, sea level rise, and the dynamics of deep ocean currents.