Planet Earth Online - Physical Attractions in the Oceans

26 September 2010

Interview with

Helen Snaith, National Oceanography Centre, Southampton

A satellite designed to measure the Earth's gravitational field with unprecedented accuracy may sound like something out of a James Bond film, but it is in fact a reality.  The European GOCE spacecraft or "Gravity field and steady state Ocean Circulation Explorer" has been doing just that and has recently sent back its first results.  Richard Hollingham finds out more...

Richard -   The launch of the dart-shaped GOCE satellite from the Russian Plesetsk Cosmodrome was in March 2009.

Helen -   It is definitely the Formula 1 of satellites, if you like.  It's designed to slip through the atmosphere as easily as possible.

Richard -   Eighteen months later, in her office at the National Oceanography Centre in Southampton, Helen Snaith is studying the first data from this sleek satellite.

Helen -   Its primary mission is to measure the Earth's gravity field at very small spatial scales.  It's to see the very, very small changes in the gravity as you go around the Earth, and ultimately, to be able to use that information to get to ocean circulation.

Earth gravity modelRichard -   We'll talk about that in a second, but let's look at the first results.  You've got some of these early results through, up on your screen here.  It's an image of the Earth, but it's a lumpy, bumpy, multi-coloured, almost like a rock-like Earth.

Helen -   This is an image that's been generated to show just how lumpy the Earth actually is.  If you looked at the Earth, it's not completely smooth.  We all know it's not completely smooth.  We have mountains.  We have oceans.  We have valleys.  But they have an effect on the gravity field as well.  So the gravity field isn't the same all the way around the Earth.  When you do maths, let's say in your school, you get taught that acceleration due to gravity is 9.8 metres per second squared.  Unfortunately, that's not strictly true.  There are very small changes as you go around the Earth.  If you go over the Himalayas, there's more mass, there's more mountains, there's more gravity.  So things are pulled towards them.  If you go over deep trenches in the ocean, there's less mass, there's less gravity, and things aren't pulled towards them as much.  It's these small changes as you go around the Earth that we're trying to measure with GOCE and which you can see exaggerated an awful lot on this image.

Richard -   Okay, so you have a map of the Earth's gravity field which is what the satellite is producing.  You want to use that to work out ocean currents.  How do you make that leap?

Helen -   Because the gravity field isn't the same everywhere, the water in the oceans isn't being pulled down to the same level everywhere.  So, if the water was completely still and there were no ocean currents, it would be a bumpy surface.  But if you put a ball down on that surface, it wouldn't roll anywhere.  It would stay where it is, which is a slightly strange concept to be able to get your head around, but this is where we're coming from.  What we're trying to calculate now is the difference between that nice smooth steady state - that if there were no currents, this is the shape of the sea surface - and what we actually measure - the difference between those two is what's being caused by the ocean currents.

Richard -   So the only way you can work out really the impact of the ocean current or the height of the water as a result of these ocean currents is by removing the gravity from that.

Helen -   Exactly.  That's precisely what we want to do.  We can use an altimeter - satellite altimeter - to measure the precise height of the sea surface at any given time when the altimeter flies over.  What we don't know is how much of that height is being caused by the gravity field or that change in height.  With GOCE, we can start to get a handle on that.  That's what we're really interested in.

Richard -   Okay, you're interested in it.  Why is it important to learn or work out where the ocean currents are and what effect they're having?

Helen -   The ocean currents are a really important part of the heat transport system of the ocean.  What we're trying to do is to monitor those currents and see how consistent they are, how strong they are, whether there's changes in those currents.  To be able to do that, we need to be able to monitor them globally.  Satellites give us one of the few options we have of being able to look at the currents everywhere in a consistent way.

Add a comment