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magazine / jf12
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January/February 2012 issue |
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À LA CARTE
The laws of attraction
Mapping the pull of gravity reveals a Mr. Potato Head planet
By Elizabeth Shilts
From space, the human eye sees Earth as a perfect sphere, but for
scientists, the lumpy globe shown at right is closer to the truth.
Produced in the spring of 2011 by the European Space Agency and the
German Aerospace Center, using data from the GOCE satellite, this
geoid is the best image yet of the effects of gravity on the planet.
Imagine Earth with its oceans at rest. Unaffected by currents,
weather and tides, water would be free to redistribute evenly around
the globe. The geoid is this resting surface, shaped only by gravity,
and is a crucial reference for measuring ocean circulation, sea-level
change and ice dynamics.
The fact that the geoid appears like a lumpy potato — and that the
gravitational field is weak or strong — is due to the varying distribution
of mass, in the form of mountains or valleys or mantle, that is more or
less dense. Less mass means weaker gravity and a lower sea level.
In regions such as central Canada, where the land mass has been
compressed by glaciers, gravitational pull is relatively weak. In areas
such as Oceania, around Australia, where there is volcanic activity,
dense magma sits close to a soft, less dense crust. This increases
gravitational pull.
The latest gravity map gives scientists a more accurate sense of
the Earth’s topography. Using such data, they can plot precise elevations
with global positioning systems and better measure gravity
changes, which is helping advance our knowledge of the processes
that cause earthquakes.
In 2013, Natural Resources Canada (NRCan) plans to update its
vertical reference system, replacing the one established in 1928 with a
geoid-based reference.
“Using these kinds of space-based techniques allows us to get elevation
data anywhere in Canada with similar accuracy,” says Denis Hains,
director of NRCan’s Geodetic Survey Division. “This is particularly
important in the North, where there are multiple reference systems.”
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