Imagine standing in some part of the world where you weighed less than any place else. Sounds impossible? Well, it is true in a very remote region of Canada, called Hudson Bay. This region has been a puzzle for scientists for decades, as it exhibits an unusual gravitational anomaly-an area that is weaker in its gravity pull compared with the remainder of Earth. It took over 40 years of research along with state-of-the-art technology to unravel the mystery behind this low-gravity zone.
Hudson Bay in northeastern Canada has been of especial interest to both geologists and physicists for many years. The territory is gravity deficient: gravity is measurably weaker by as much as 50 parts per million. This means that a person standing here would weigh a few ounces less than in the rest of the world. But how could this be? Gravity, after all, ought to be constant everywhere on Earth, depending primarily on the mass of the Earth.
The response was not simple and required the accumulation of years of information-gathering with the most advanced satellite technology to finally unveil this secret.
In 2002, NASA in partnership with the German Aerospace Center launched a pair of satellites referred to as GRACE (Gravity Recovery and Climate Experiment). With this innovative design, they wanted to better measure Earth's gravitational variations than ever achieved by any previous experiment.
GRACE relied on two satellites flying in formation 500 kilometers above Earth's surface, 220 kilometers apart. The two satellites exchanged microwave signals that allowed them to detect even an infinitesimal change in the two satellites' positions-down to a micron (a millionth of a meter). Those shifts translate the variation in the field of gravitation below them.
There are now known to be two in number the most significant contributing factors to the Hudson Bay gravity anomaly. There are:
The Laurentide Ice Sheet: It is at the end of the last Ice Age that the entire region of what is otherwise known today as Hudson Bay was blanketed with a big ice sheet called the Laurentide Ice Sheet. It is so thick such that it compressed the crust of Earth beneath it. Approximately 10,000 years ago, as the ice sheet melted, the Earth's crust began to slowly rebound. It is termed "post-glacial rebound." Although the crust has not regained its original shape entirely, there is a mass deficit that lowers the gravitational pull in that region.
Convection Currents in the Mantle: Beneath our Earth, hot liquid rock is flowing in convection currents, circulating within the layer of the mantle. These currents redistribute mass inside our Earth, so that it alters the gravity that we experience at the surface. Hudson Bay just happens to rest on top of a region of space where there are convection currents driving mass downward beneath the surface, which just adds some additional facilitation to lowering the gravity around the region.
Discovery of the Hudson Bay gravity anomaly goes beyond being a myth; instead, it is much deeper because it provides geologists with very crucial information about the processes guiding the geological alterations in the planet, such as plate tectonics, ice mass variations, and movements of Earth's interior. Data acquired from GRACE have been quite useful for scientists analyzing climate change, the rise in sea level, and melting of glaciers.
In fact, it's this mission that shaped current understanding about the changing face of Earth. Its follow-on missions, GRACE-FO (Follow-On), launched in 2018, continue monitoring the Earth's gravity field with far greater precision.
That mystery of low gravity demonstrates just how complex our planet actually is. Even though gravity seems so constant, even its very presence can be affected by forces deep and ancient that lurk underneath the surface. For the interested in mysteries of the world, this anomaly of Hudson Bay is a powerful reminder of how much remains to be discovered about what lies beneath our feet.
Next time you stop into Canada, take a detour out to Hudson Bay-the only place on this fine earth where gravity actually feels a tad lighter!
Reference Links:
Satellites Solve Mystery of Low Gravity Over Canada
GRACE and GRACE-FO - Wikipedia
How Missing Gravity Works - HowStuffWorks
Future missions to Earth's gravitational anomalies will no doubt draw on lessons learned from the Hudson Bay example.
