Earth’s gravity pictured in ‘HD’

By Jonathan Amos
Science correspondent, BBC News, Bergen

It is one of the most exquisite views we have ever had of the Earth.


This colourful new map traces the subtle but all pervasive influence the pull of gravity has across the globe.

Known as a geoid, it essentially defines where the level surface is on our planet; it tells us which way is “up” and which way is “down”. It is drawn from delicate measurements made by Europe’s Goce satellite, which flies so low it comes perilously close to falling out of the sky.

Scientists say the data gathered by the spacecraft will have numerous applications. One key beneficiary will be climate studies because the geoid can help researchers understand better how the great mass of ocean water is moving heat around the world.

The new map was presented here in Norway’s second city at a special Earth observation (EO) symposium dedicated to the data being acquired by Goce and other European Space Agency (Esa) missions.

Imaginary ball

Launched in 2009, the sleek satellite flies pole to pole at an altitude of just 254.9km – the lowest orbit of any research satellite in operation today.

The spacecraft carries three pairs of precision-built platinum blocks inside its gradiometer instrument that sense accelerations which are as small as 1 part in 10,000,000,000,000 of the gravity experienced on Earth.

This has allowed it to map the almost imperceptible differences in the pull exerted by the mass of the planet from one place to the next – from the great mountain ranges to the deepest ocean trenches.

Two months of observations have now been fashioned into what scientists call the geoid.

…Put a ball on this hypothetical surface and it will not roll – even though it appears to have “slopes”. These slopes can be seen in the colours which mark how the global level diverges from the generalised (an ellipsoid) shape of the Earth.

In the North Atlantic, around Iceland, the level sits about 80m above the surface of the ellipsoid; in the Indian Ocean it sits about 100m below.

MAPPING THE DIFFERENT EFFECTS OF GRAVITY

  • 1. Earth is a slightly flattened sphere – it is ellipsoidal in shape
  • 2. Goce senses tiny variations in the pull of gravity over Earth
  • 3. The data is used to construct an idealised surface, or geoid
  • 4. It traces gravity of equal ‘potential’; balls won’t roll on its ‘slopes’
  • 5. It is the shape the oceans would take without winds and currents
  • 6. So, comparing sea level and geoid data reveals ocean behaviour
  • 7. Gravity changes can betray magma movements under volcanoes
  • 8. A precise geoid underpins a universal height system for the world
  • 9. Gravity data can also reveal how much mass is lost by ice sheets
  • The geoid is of paramount interest to oceanographers because it is the shape the world’s seas would adopt if there were no tides, no winds and no currents.

    If researchers then subtract the geoid from the actual observed behaviour of the oceans, the scale of these other influences becomes apparent.

    This is information critical to climate modellers who try to represent the way the oceans manage the transfer of energy around the planet.