What is a gravity anomaly? In simple terms, a gravity anomaly occurs when there is a difference between the predicted value of the planet’s gravity field and the actual acceleration of free fall observed on a planet’s surface. Generally, these anomalies are described as a local phenomenon.
Without a question, gravity is vital to our survival. It is due to the Sun’s gravity that Earth doesn’t wander around and orbit at a fixed path. Similarly, Earth’s gravity keeps the Moon intact, which is responsible for most of the marine life cycle and stable tilt of the Earth’s axis. But do you know that gravity isn’t the same everywhere on the Earth?
Why this happens? Is there any complex reason or a simple explanation? In this edition, we will try to understand why we observe uneven gravity on the Earth’s surface.
Relationship Between Mass and Gravity
A four-dimensional description of the universe including height, width, length, and time.
According to the current understanding of physics, any object that has mass has its own gravity. If the objects has more mass then it will have more gravitational pull and vice versa. Compared to the other planetary bodies in our solar system, Earth is the densest of all the planets, and has an estimated mass of about 5.97237×1024 Kg, or in other words 5.972 sextillion (1,000 trillion) metric tons.
This is simply because everything on this planet have mass. From water to land everything on Earth has mass, however, this mass isn’t distributed uniformly. Our Earth’s gravity comes from all of its mass. And its Earth’s mass that inserts a combined gravitational pull on your body. That’s what gives you weight.
So, if you were on the moon, which has less mass as compared to the Earth, you would weigh less than you do on this planet. Similarly, if you were on a planet with greater mass than the Earth’s then you would weigh considerably more than what you do here on the earth.
The images below show how higher concentration of mass lead to strong gravitational field in that particular area and vice versa.
Mountains have much stronger gravitational fields Image Courtesy: NASA
Ocean trenches, such as Marina Trench have the lowest concentration of mass.
Gravity Anomalies Observed on the Earth
A strange phenomenon was discovered in the 1960s when Earth’s global gravity fields were charted for the very first time. Researchers were left baffled after finding that gravity in some parts of Canada, especially near the Hudson Bay region is much lower than other parts of the world.
Another region, where gravity anomaly is observed on an extremely large scale is over the British Isles. Here both negative and positive anomalies are witnessed due to the difference in density distribution of rock found beneath the surface. There are many other regions where small-scale anomalies are observed such as the Anton Dohrn, Scottish Caledonides, and Walls Boundary Fault.
In the 21st century, gravity anomalies over the Earth can be detected with the help of satellites. One of such satellites is the NASA’s GRACE mission, which was comprised of two satellites, capable of detecting even small-scale gravity anomalies on the Earth over a period of time.
Another such mission was ESA’s GOCE or Gravity Field and Steady-State Ocean Circulation Explorer, which mapped Earth’s magnetic field until it was shut down in 2013. These satellites aimed at an accurate and detailed gathering of Earth’s gravity field with sophisticated instruments, built specifically for this task. Data from these missions also helped researchers to look deep into the Earth’s interior.
So why we observe uneven Gravity on Earth?
Gravity anomaly map from NASA’s GRACE satellite
Until now, we have gathered a basic understanding that gravity depends on mass i.e., if an object has relatively greater mass, then it would have greater gravitational force or the other way around. Now I think you got the idea why we observe these gravity variations on the Earth’s surface.
Gravity anomalies are directly related to the irregular distribution of density on the Earth. And it also helps us to understand the Earth’s interior more closely. In the case of Hudson Bay region, Canada, researchers believe that the extinction of Laurentide Ice Sheet, which covered most of Canada and the northern United States almost 2 million years ago, has left a huge depression in that region.
This loss of mass has affected the Hudson bay the most because some of the Earth has been pushed to the sides by the ice sheet, and less mass means less gravity. Scientists were able to calculate the impact of the ancient ice sheet over that region from the data of the GRACE satellites which allowed them to recreate the topological settings of Hudson Bay about a million years ago during the last ice age.
The data revealed some fascinating geographical features around the area, including two big bulges on either side of Bay indicating thicker ice sheets in that region. Today, those areas are among the most affected areas of the gravity-depleted Hudson Bay.