On the surface of our planet, there is a place where people weigh slightly less than anywhere else in the world. It took a lot of time to explain this anomaly.
Gravity on Earth is quite stable: if you drop a ball to the ground in England and do the same in Australia, you will find that they fall to the ground at roughly the same speed. However, scientists use the word "roughly" for a reason: gravity on Earth is not entirely uniform, and its strength depends on the distribution of mass and where you are in relation to it, writes Focus.
Essentially, it is similar to how the gravity of the Moon is stronger on Earth than the gravity of Jupiter, even though Jupiter is about 26,000 times more massive than the Moon. The reason is that the Moon is much closer. Similarly, we experience greater gravity when we are closer to a large amount of mass on the planet.
According to scientists, the Earth bulges at the equator due to its rotation, becoming a flattened spheroid rather than a true sphere. As a result, at the North Pole, where a person is closer to the main mass of the planet, our weight is slightly greater than at the equator, where a person is farther from it.
Researchers from around the world, including participants in NASA's Gravity Recovery and Climate Experiment (GRACE) mission, have attempted to map the Earth's gravitational field as accurately as possible. They used highly sensitive satellites spaced 220 kilometers apart. The mission measured gravity by linking it to the distance between the two satellites: when gravity increased, the leading satellite accelerated, and the distance between them increased. When gravity increased between the satellites, the distance between them decreased; the opposite occurred when gravity decreased in front of the pair of satellites or between them.
Scientists used this and other methods: as a result, they managed to discover several curious "gravitational anomalies" — places where gravity differs from what is expected for that region based on available data. For example, in the Hudson Bay area in Canada, your weight will be slightly less than in other parts of the planet. It is not much — about four thousandths of a percent less than the average on the surface of the Earth. However, it took a lot of research for scientists to finally explain the reason.
Researchers note that the main reason was known before: the region likely lacks some mass compared to other parts of the planet. Now scientists have also determined that this seems to be caused by several factors, including events from the last ice age, commonly referred to as the last glacial period.
It was during this time that the Laurentide Ice Sheet, which covered much of Canada, advanced across the country, pushing dense rocks and compressing them beneath itself. When the ice cover melted, the missing mass formed in this area, which is now slowly recovering.
However, according to new analysis of GRACE satellite data, this accounts for only about 25-45% of the gravitational anomaly observed in Hudson Bay. The remaining anomaly is thought to be caused by activity deep beneath the surface of the planet. For example, the rise and fall of magma create convection currents that can pull down continental plates, reducing mass in that area and making gravity there slightly weaker.
As a result, scientists have found that the largest negative mass anomaly on the planet, where gravity is weaker than elsewhere, is located in the Puerto Rican Trench. There, gravity has been measured to be about 380 milligals less than expected.
Geophysicist Peter Molnar studied the anomaly back in 1977, and previous gravity models suggested that the thickness of the Earth's crust was fairly uniform. However, upon examining the area, the scientist realized that this was not the case, and the anomaly was likely caused by a large and dense "hanging valve" of the Atlantic lithosphere beneath the trench.
So where on the surface of our planet is the place where gravity is truly the weakest, rather than just weaker than expected? Given the data known about mass and gravity, places like the summit of Mount Everest would be a reasonable assumption, as the highest mountain in the world keeps us at a significant distance from all that mass below. But the weakest measurements of gravity on the surface are actually found on Nevado Huascarán in Peru, where the acceleration due to gravity is 9.7639 meters per second squared, due to a combination of factors.
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