Page 2/3 - Posted March 9, 2012
Scientists aren’t relying solely on space-based measurements of gravity. POLENET researchers are collaborating with French and Danish scientists who use ground-based instruments to measure gravitational changes over relatively small areas compared to the larger footprints covered by the satellites.
“It’s partly ground-truth for GRACE. It’s also an independent way of measuring uplift and mantle flow that’s related to rebound,” Wilson said.
The absolute gravity measurements are also important to separating the signals between the long-term post-glacial effect and the short-term “elastic” response of the earth from modern ice loss, according to Yves Rogister, an assistant professor at the University of Strasbourg in France.
Photo Credit: Peter Rejcek
French scientist Jean-Daniel Bernard works on the FG5 absolute gravimeter in a shelter at Scott Base.
“The GPS and gravity measurements are complementary,” he said.
Rogister and colleague Jean-Daniel Bernard brought down what’s called an FG5 absolute gravimeter, which looks a bit like a camera tripod on steroids, to McMurdo Station during the 2011-12 season to repeat a series of measurements they made two years ago.
Think about Italian scientist Galileo Galilei’s famous gravity experiment of dropping two balls of differing weights from the Tower of Pisa to get an idea of how the gravimeter works.
A vacuum is created in an upper chamber in which an object repeatedly free falls over a period of time. The acceleration of the object, measured using a laser interferometer and an atomic clock, is related to the pull of gravity, or the mass, of the earth below the site. By comparing this year’s measurements against those made two years ago, the scientists should be able to detect a subtle change in mass.
“We have the same value as two years ago, which means that gravity isn’t changing here much,” Rogister said of the preliminary data around Ross Island.
A little lighter
Unfortunately, the FG5’s extreme sensitivity makes it less than an ideal field instrument. The French scientists are limited to doing the experiment in established shelters at research stations like McMurdo, New Zealand’s nearby Scott Base and Italy’s Mario Zucchelli Station at Terra Nova Bay to the north.
A more portable version of the instrument brought to the Ice from Denmark isn’t so limited, according to Larry Hothem, a scientist from the U.S. Geological Survey who oversees the absolute gravity component of the West Antarctic-POLENET (ANET) study.
“It’s not as accurate, but it’s not as sensitive,” he said.
Jens Emil Nielsen , a scientist with the Danish National Space Center at the Technical University of Denmark , hopes the portable A10 gravimeter will prove to be a valuable tool. It takes maybe 20 minutes to set up inside a tent and can make its measurements in less than an hour versus the 24 hours required by the FG5.
Nielsen made a number of trips aboard either a helicopter or Twin Otter with the A10 to ANET sites accessed from McMurdo Station to test how the instrument would handle field conditions. If deemed successful, it will be flown to ANET sites in West Antarctica next year. Ideally, each deployment should take about three hours.
“It’s supposed to be a hit and run,” Nielsen said.
The abbreviated measurements allow more “noise” from tides and other disturbances in the A10 data, which means there’s more work for the scientists using the portable gravimeter on the back end of the study.