How can parts of Canada be "missing" gravity?
by Jacob Silverman
Translation by Autumnson Blog
For more than 40 years, scientists have tried to figure out what's causing large parts of Canada, particularly the Hudson Bay region, to be "missing" gravity. In other words, gravity in the Hudson Bay area and surrounding regions is lower than it is in other parts of the world, a phenomenon first identified in the 1960s when the Earth's global gravity fields were being charted.
已超過40年，科學家們試圖找出什麼導致大部分的加拿大地區，特別是哈德遜灣地區，會重力“失踪”。換句話說，在哈德遜灣地區及周邊地區的重力，是低於世界其它地區的，這現象在20世紀60年代首先證明，當地球的全球重力場被繪製時。
Two theories have been proposed to account for this anomaly. But before we go over them, it's important to first consider what creates gravity. At a basic level, gravity is proportional to mass. So when the mass of an area is somehow made smaller, gravity is made smaller. Gravity can vary on different parts of the Earth. Although we usually think of it as a ball, the Earth actually bulges at the Equator and gets flatter at the poles due to its rotation. The Earth's mass is not spread out proportionally, and it can shift position over time. So scientists proposed two theories to explain how the mass of the Hudson Bay area had decreased and contributed to the area's lower gravity.
兩種理論已被提出來敘述這種異常，但在我們研究它們之前，首先重要的是要考慮什麼創造重力。在基本的層面上，引力對質量成正比。所以當一個地區的質量是莫名其妙地變得更小，重力亦變得更小。重力可以在地球的不同部份改變，雖然我們通常認為它是一個球，地球實際上在赤道凸起，由於其旋轉並在兩極變得更平。地球的質量並不是按比例伸展，並隨著時間轉移位置。因此科學家提出兩種理論來解釋，哈德遜灣地區的質量已如何下降及引致該地區的低重力。
One theory centers on a process known as convection occurring in the Earth's mantle. The mantle is a layer of molten rock called magma and exists between 60 and 124 miles (100 to 200 km) below the surface of the Earth . Magma is extremely hot and constantly whirling and shifting, rising and falling, to create convection currents. Convection drags the Earth's continental plates down, which decreases the mass in that area and decreases the gravity.
一個理論集中在一過程被稱為發生在地球地幔的對流，地幔是一層熔化的岩石稱為岩漿層及存在於地球表面以下60至124英里（100到200公里）之間。 岩漿層是十分熱和不斷地旋轉和移位、上升和下降，來創建對流。對流拖動地球的大陸板塊下降，這降低在這地區的質量和減少重力。
A new theory to account for the Hudson Bay area's missing gravity concerns the Laurentide Ice Sheet, which covered much of present-day Canada and the northern United States. This ice sheet was almost 2 miles (3.2 km) thick in most sections, and in two areas of Hudson Bay, it was 2.3 miles (3.7 km) thick. It was also very heavy and weighed down the Earth. Over a period of 10,000 years, the Laurentide Ice Sheet melted, finally disappearing 10,000 years ago. It left a deep indentation in the Earth.
一種關注到Laurentide冰蓋的新理論來解釋哈德遜灣地區的失踪重力，它涵蓋現今很大部份的加拿大和美國北部。此冰蓋差不多有2英里（3.2公里）厚在大部分路段，及在哈德遜灣的兩個地區是2.3英里（3.7公里）厚，它亦是非常重和對地球有影响。在超過 10,000年的期間Laurentide冰蓋融化了，及一萬年前最終消失，它遺下一層深深的壓痕在地球上。
To get a better idea of what happened, think about what happens when you lightly press your finger into the surface of a cake or a piece of really springy bread. Some of it moves to the sides and there's an indentation. But when you remove your finger, it bounces back to normal. A similar thing happened with the Laurentide Ice Sheet, the theory proposes -- except the Earth isn't so much "bouncing" back as it is rebounding very slowly (less than half an inch per year). In the meantime, the area around Hudson Bay has less mass because some of the Earth has been pushed to the sides by the ice sheet. Less mass means less gravity.
為了得到一個更佳的想法發生什麼事，想想當你輕輕地按你的手指到一個蛋糕或一塊真正有彈性的麵包的表面會發生什麼，它有些部份向邊位移動及有一部份縮進。但是當你移開你的手指，它反彈回到正常。 一些類似的事情發生在Laurentide冰蓋，理論提出 -- 除了地球不大“反彈”因為它是非常緩慢地回升（每年不到半英寸）。在此期間，哈德遜灣周圍的地區有較少的質量，因為地球有些部份已被推冰蓋推向邊位，而較少質量意味著較少的重力。
So which theory is correct? It turns out that both of them are. Convection and the ice sheet's rebound effect are both causing some of the decrease in gravity around Hudson Bay. First, we'll consider the ice sheet theory.
To calculate the impact of the Laurentide Ice Sheet, scientists at the Harvard-Smithsonian Center for Astrophysics used data gathered by the Gravity Recovery and Climate Experiment (GRACE) satellites between April 2002 and April 2006. The GRACE satellites are highly sophisticated machines, orbiting about 310 miles (500 km) above the Earth and 137 miles (220 km) apart. The satellites can measure distances down to a micron, so they can detect minor gravitational variations. When the lead satellite flies over the Hudson Bay area, the decrease in gravity causes the satellite to move slightly away from the Earth and from its sister satellite. This shift in distance is detected by the satellites and used to calculate the change in gravity. Any shifts detected can also be used to create maps of gravitational fields.
The GRACE data allowed scientists to create topographical maps approximating what Hudson Bay looked like during the last ice age, when it was covered by the Laurentide Ice Sheet. These maps revealed some interesting features about the area, including two bulging areas on the western and eastern sides of Hudson Bay where the ice was much thicker than the rest of the sheet. Gravity is now lower there than in other parts of the gravity-depleted bay.
Another important finding came from the GRACE data: It turns out that the ice sheet theory only accounts for 25 percent to 45 percent of the gravitational variation around Hudson Bay and the surrounding area. Subtracting the "rebound effect" from the area's gravitational signal, scientists have determined that the remaining 55 percent to 75 percent of gravitational variation is likely due to convection.
The Hudson Bay area is going to have less gravity for a long time. It's estimated that the Earth has to rebound more than 650 feet to get back to its original position, which should take about 5,000 years. But the rebound effect is still visible. Although sea levels are rising around the world, the sea level along Hudson Bay's coast is dropping as the land continues to recover from the weight of the Laurentide Ice Sheet.
While the mystery surrounding Canada's gravitational anomalies has been put to rest, the study has wider implications. Scientists involved in the Harvard-Smithsonian Center study were amazed that they were able to see how the Earth looked 20,000 years ago. And by isolating the influence of the ice sheet's rebound effect, researchers better understand how convection affects gravity and how continents change over time. Finally, the GRACE satellites have provided scientists with data on many ice sheets and glaciers. By examining climate change that took place thousands of years ago, scientists may gain a better understanding of how global warming and rising sea levels are affecting our planet today and what impact they will have on our future.
For more information about Canada's missing gravity and for a link to a map of the Laurentide Ice Sheet, please check out the links below:
How does gravity work?
How Special Relativity Works
How Weightlessness Works
Laurentide Ice Sheet
GRACE Gravity Data Constrain Ancient Ice Geometries and Continental Dynamics over Laurentia
Gravity Anomaly Solved
Sources
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Munro, Margaret. "Ancient glaciers still reshaping Canada, satellites show." CanWest News Service. May 13, 2007. http://www.canada.com/montrealgazette/news/insight/story.html? id=3d733281-d348-40e5-85a5-e7f7e67acd11
Robertson, Eugene. "The Interior of the Earth." U.S. Geological Survey. June 26, 2001. http://pubs.usgs.gov/gip/interior/
Than, Ker. "Scientists to study gash on Atlantic seafloor." LiveScience. MSNBC.com. March 1, 2007. http://www.msnbc.msn.com/id/17407745/
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Young, Kelly. "Satellites solve mystery of low gravity over Canada." NewScientist.com. May 10, 2007. http://space.newscientist.com/article/dn11826-satellites- solve-mystery-of-low-gravity-over-canada.html
"Gravity Anomaly Solved." Science Friday. May 14, 2007. http://www.sciencefriday.com/news/051407/news0514071.html
http://science.howstuffworks.com/environmental/earth/geophysics/missing-gravity.htm
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