Satellite gravity data used to create 3D map of the Earth
The British Antarctic Survey is participating in the first spherical mapping of the planet – and studying the lithosphere could reveal the future pace of climate change
The 3D Earth project, a new initiative funded by the European Space Agency (ESA) that aims to produce a fully integrated 3D model for our planet, has published its initial findings following studies conducted at the Cambridge-based British Antarctic Survey (BAS) and Germany’s Kiel University.
The report, published in this week’s Scientific Reports journal, describes the attempt to unveil key geological features of the Earth’s lithosphere – the rigid outer layer that includes the crust and the upper mantle. The least understood part of this tectonic puzzle is in the Antarctic, which is where the role of BAS in the analysis is crucial.
Lead author Prof Jörg Ebbing, from Kiel University, says that progress towards a 3D model of the Earth has been accelerated by the use of satellite gravity data.
“Our new satellite gravity gradient images improve our knowledge of Earth’s deep structure,” says Prof Ebbing. “The satellite gravity data can be combined with seismological data to produce more consistent images of the crust and upper mantle in 3D. This is crucial to understanding how plate tectonics and deep mantle dynamics interact.”
Fausto Ferraccioli, science leader of the geology and geophysics team of BAS, and co-author of the study, says: “Satellite gravity is revolutionising our ability to study the lithosphere of the entire earth, including its least understood continent, Antarctica.”
Speaking exclusively to the Cambridge Independent, the Cambridge-based geophysicist explains that mapping the Earth in 3D is now possible because the new satellite gravity data from the European Space Agency (ESA) gathered from its Living Planet Programme Gravity field and steady-state Ocean Circulation Explorer (GOCE) mission can be combined with the existing seismological data. The ESA satellite was launched in 2009 and sent back data until it crashed in 2013.
“The satellite was measuring not just vertical gravity but also horizontal components,” says Dr Ferraccioli, “so you get a better impression of not just what lies directly beneath but also what lies to the sides, so you can get a really good 3D picture – but it is very complicated.
“There are six different gradients so we’ve found a compromise, which describes the curvature of the field but which is easier to look at – it’s a synthesis of the six components.
“Using these simpler gravity images we get tantalising new glimpses into the building blocks of our planet, including ancient parts of the lithosphere known as cratons.”
The authors noted that, despite their similar seismic characteristics, there are contrasts in the gravity signatures over cratons, indicating key differences in their deep structure and composition. These old features are significant as they hold records of Earth’s early history.
“GOCE also shows us fundamental similarities and unexpected differences between the Antarctic lithosphere and other continents, to which it was joined until 160 million years ago,” adds Dr Ferraccioli. “The structure of the lithosphere is the cradle on which the Antarctic ice sheets flow. There’s massive ice sheets on top of the continent and its characteristics are very important, for instance how thick the lithosphere – the rigid part – is.
“Under that is the asthenosphere, a much more plastic, gooey, sphere underneath – and that part is very hot. How thick the crust and lithosphere is influences geothermal heat flux beneath the ice sheets, and also radiogenic heat in the rocks is playing a part.”
As the ice sheets melt, the Earth underneath pushes up, but by how much is yet to be fully understood – but it matters, especially when it comes to understanding how the ice sheets will change in future with climate change.
“What we can do with gravity data is look at the thickness of the crust and of the lithosphere, that’s important for the geothermal heat flux, which in turn influences how the ice sheet flows. But also, if you have a warm bottom layer of the lithosphere, everything is much more gooey – softer – so when the ice retreats, underneath the Earth itself bounces back up quicker. How much depends on how rigid the Earth part is.”
This post-glacial rebound – the rise of land masses after the lifting of the huge weight of ice sheets during the last glacial period – is technically known as glacial isostatic adjustment (GIA).
“So as we glean more knowledge of the lithosphere in the Antarctic we’ll be able to get more of a handle also on geothermal heat flux and GIA,” concludes Dr Ferraccioli.
The project runs until the summer 2020, and a preliminary 3D model of the Earth including Antarctica will be showcased to the broader science community at the 3D Earth Science Meeting 2019 in Dublin next March.