Sunday, August 15, 2021

Is gravity the same across the Earth? And why is this important?

 The Earth is rough and has different masses at different locations, and the gravitational field accompanies these variations, which occur according to the density of rocks under the ground and other factors.  After all, the distribution of mass on the planet is heterogeneous.

 The Earth's mass distribution controls the level at which seawater will be at any given time, as the instantaneous surface of the sea adjusts according to the gravity field.  In fact, there isn't even a sea level, but an average or an instantaneous sea level.

 There is a difference between the theoretically calculated gravity and the actual value of the acceleration of gravity of a particular place on the planet.  This difference is called the “gravitational anomaly”, and it can be measured based on a model that considers the dimensions, mass and rotation of the planet.

 This anomaly is observed on Earth, as it does not have the same gravitational force everywhere.  This can be measured using a device called a gravimeter, or, better yet, using satellites.  The notion that our planet has different gravitational points depending on location has been around for more than a century, but only recently have scientists been able to create an accurate model with the help of equipment positioned in Earth's orbit.

 According to this map, a person's weight would be 1% lower in India.  Below is the geoid created by GOCE, this time showing that the Earth would have a slightly potato-like shape.

 Geoid heights range from +40 meters in the Andes regions (Bolivia and Peru) to -20 meters in the northern and northeastern regions of Brazil.  But the Andes, despite being 6,000 meters high, don't have much more mass than the Amazon, according to Blitzkow.  “If we could weigh a cylinder from the surface of a mountain in the Andes and another from the Amazon, we would see that the difference in weight is not as intense as the variation in altitude”.  In other words, it doesn't matter much whether we are in a mountain or a valley - what really interferes with the gravitational force is the mass contained in the region's subsoil.

 For us humans, these differences in gravity may be insignificant, but they have a big impact on ocean dynamics and heat circulation around the planet.  Therefore, high-precision geoids can help in climate studies, understanding the Earth's internal structure, understanding the dynamics of ice and sea level change, among other phenomena that affect life on Earth.  Understanding the planet's gravitational anomaly could even help scientists learn more about the tectonic activity that causes earthquakes and volcanoes.


 Image: ESA/HPF/DLR

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