Science Interviews

Prospecting the Gravity Field

Chris - Also this week, the European Space Agency has launched the Gravity field and steady-state Ocean Circulation Explorer, or GOCE for short.  They say it’s going to bring about a whole new level of understanding of one of the Earth's most fundamental forces of nature: the gravity field. Dr Chris Hughes, from the Proudman Oceanographic Lab, is down here on Earth with us and is planning to use the data from GOCE to better understand the world’s oceans. Hello Chris. Welcome to the Naked Scientists. Tell us, what is GOCE? How does it work?

Chris H - If you imagine having six metal cubes in a box in orbit round the Earth. Because they’re all in slightly different places, in slightly different bits of the gravitational field they move differently. Ideally what you’d want to do is track them and measure the relative motion. That  tells you how the gravity field is changed from one place to another. You can’t do that because they rattle around and bounce off the walls so instead you hold them still and measure the force that you need to hold them still.

Chris - What sort of orbit is this satellite in? Is it going over the poles so the Earth is effectively turning under it and this means effectively over the course of a one month period it scans the entire of the Earth’s surface?

Chris H - That’s right. It’s not quite over the poles, it’s about 6 degrees off but it covers almost all the Earth, yes.

Chris - Why is this useful to you as an oceanographer? What can we learn by studying the Earth’s gravitational field?

Chris H - We want to know what the ocean currents are doing. We can learn an awful lot about those from sea levels, whether the sea surface is sloping or not. It’s rather like isobars on a weather map depending on which way the wind is blowing. The sea level will tell you which way the currents are going. If you want to know whether the sea’s sloped you need to know which way is up. We know it pretty well, obviously, but we don’t know it well enough. We’re talking about very small gradients: 1 in 1,000,000 is the gradient that matters. So you need to know very precisely what the gravitational field is in order to define the direction of up.

Chris - Given that the Earth is a sphere why don’t we just see gravity being uniform everywhere across the Earth’s surface?

Chris H - Because the Earth isn’t quite a sphere. Every mountain, every bump, every different mass around the Earth has its own bit of gravitational attraction towards it. If you actually look at the shape of the sea surface what you see is a whole set of wrinkles and bumps and it really looks like a map of the sea floor. Every mountain on the sea floor is pulling the water towards it. What you think of a nice smooth, round sphere is actually quite wrinkly and bumpy when you look on the very small scale.

Chris - Understanding these currents, how will this inform us about what’s going on in the oceans?

Chris H - The ocean is almost half of the climate system. It carries heat around from the equator to the poles in just the way the atmosphere does. It keeps part of the Earth warm, cools other parts and is very important for things like fisheries but also climate in Europe in particular. It’s very difficult to measure. There’s so much going on in the oceans it tends all to happen on smaller scales than it does in the atmosphere. There are fewer measurements; it’s harder to see into. We can’t measure much in the ocean from satellites. These measurements of currents are really going to give us a huge amount of new information about the basic patterns of flow in the ocean that allows us to understand how the heat gets transported around.

Chris - How long is the acquisition of the data going to take? How long before you can come back on this programme and tell us – this is what we’ve found?

Chris H - It’s going to be at least a year. It’s going to take six months or so before the whole system is celebrated and has got down to operational measurements. There’s a lot of work where the data has been collected; turning that into useful information so that we can calculate the sea level relative to the gravitational field. There may be some early results somewhere around Christmas time but it’s going to be many years  down the line before we’ve got perfect observations that we can get out of it.

Chris - That was Chris Hughes, from the Proudman Oceanographic lab. 

March 2009