PEERING beneath the surface of Mars and other planets to reveal buried geological features could get easier, thanks to a nifty new silicon gadget.
The device, called a gravity gradiometer, has been designed to measure how much the force of gravity changes from place to place, enabling it to map a planet鈥檚 gravitational field.
The idea is simple. Take two masses, each hanging from a spring. If one mass is slightly closer to a planet鈥檚 surface, it will feel stronger gravity and pull more on its spring than the other mass. Compare the pulls on the two springs, and you can work out the gravity gradient over that part of the planet. A gravity gradiometer aboard the European Space Agency鈥檚 is currently probing Earth鈥檚 gravity field, but it has a mass of hundreds of kilograms. Being so heavy, it would be prohibitively expensive to send such a device on a deep-space mission.
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So and his colleagues at the University of Twente in Enschede, the Netherlands, designed a device weighing just 1 kilogram. It uses a single wafer of silicon, which can be chiselled using methods developed for microchip fabrication.
In the design, the two test-masses are only a few centimetres apart 鈥� compared with half-a-metre in GOCE 鈥� so any difference in the gravitational force felt by the masses would be minute. To detect such small differences, the masses would be held on ultradelicate springs and the position of each mass measured to within about 1 picometre by a comb-like device whose capacitance varies as the mass moves up or down.
The team calculates that such a device, placed in a spacecraft in orbit around a planet, could sense changes in the gravity field due to geological features of about 200 kilometres across or bigger (Cryogenics, ). It could detect how deep mountains plunge into the mantle below, say, or look for subterranean oceans such as the sea suspected to lie beneath the south pole of Saturn鈥檚 moon Enceladus (pictured).
鈥淚t could pick up changes in gravity due to geological features of 200 kilometres across or bigger鈥�
鈥淲e are aiming at building a demo set-up to use on Earth in a few months,鈥� says team member Reinder Cuperus.
Bruce Bills of NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California, says that the design has potential. 鈥淚f a gradiometer of this sensitivity could be built and flown, with a mass of 1 kilogram, it would be very important for planetary gravity mapping,鈥� he says.
