Wonders of the Universe

THE GEOID

Data collected by the GOCE satellite between November and December 2009 is here used to create a map of the tiny variations in Earth’s gravity field across the globe. These maps provide invaluable information for oceanographers, hydrologists and geologists in order to create accurate climate models for our planet.

This picture of Earth’s gravitational field was taken by a European Space Agency satellite, GOCE, which was launched in March 2009. GOCE is equipped with three ultra-sensitive accelerometers, arranged so that they respond to very tiny changes in the strength of Earth’s gravitational field as the satellite orbits. Skimming the edge of Earth’s atmosphere at an altitude of 250 kilometres (155 miles), GOCE spent two months gathering the data to create this extraordinary image. It’s the first time the strength of gravity across the globe has been mapped this accurately. The blue patches indicate areas that have a weak gravitational field, the green are average and the red are places where it is stronger. The reason for these fluctuations is the density of the rocks below Earth’s surface and the presence of features such as mountains or ocean trenches. More technically, the picture is presented as an equipotential surface, which means that if Earth were entirely covered in a single ocean of water, this picture would correspond to the water height at every point.

Looking at this map, it is clear that Iceland has a higher gravitational field strength than that of England. These changes are imperceptible to us, but it means that I would weigh slightly less standing at the same altitude in Manchester than I would in Reykjavik. This map was not made to show the trivial distinctions in a traveller’s weight, of course; the unparalleled level of detail will enable a deeper understanding of how our planet works, because this data is a high-precision geological tool. One particular benefit will be for oceanographers; because the map defines the baseline water surface in the absence of tides, winds and currents, it is critical to understanding the factors that determine the movement of water across the oceans of our planet. This is a very important part of understanding and predicting the way energy is transferred around our planet, which is in turn an important factor in generating accurate climate models.

The geoid therefore reveals a vast amount of detailed information about the structure of our planet, just from measuring the strength of its gravity. As far as the actual height of the ocean surface is concerned, however, the most influential factor of all is not shown: the Moon

The geoid helps us to understand unseen structures on our planet, such as here in Iceland where magma is welling upwards from Earth’s mantle, affecting the gravitational field there. In this image, taken in May 2010 from a NASA satellite, the Icelandic volcano Eyjafjallajökull can be seen erupting.