July 19, 2012 | Volume 42, No. 29

Gravity research returns to fairgrounds

In June 1987, researchers with Scripps Institute of Oceanography set up an absolute gravity meter in the 4-H room in the fairgrounds’ McPherson Barn, calibrating equipment for an experiment in Greenland aimed at finding a mysterious force suspected of acting in opposition to gravity.

In May, researchers for the University of Alaska’s Geophysical Institute and two universities in Japan set up a gravity meter on the very same spot at the fairgrounds. They’re using the Scripps group’s information, but their quarry isn’t nearly as exotic.

“We’re not really concerned with the laws of gravity or that level of theoretical inquiry,” said UAF research technician Max Kaufman. “We’re more concerned with understanding the physical properties of the Earth’s crust and (glacial) uplift. Southeast Alaska is a fantastic place to study how the crust of the Earth responds to loading and unloading forces.”

Launched in 2006, the International Geodetic Project is using information from the gravity meter as well as GPS measurements to track changes in uplift of the Earth’s surface, including how much of the “bounce” is due to current changes in icefields in the region.

Past and present ice loss, as well as plate tectonics, play a role in the rising of the local land mass, Kaufman said.

While academic in nature, the work of the international group has some real-world implications, including providing better information to towns like Haines about the rate of glacial uplift here for planning purposes, and gathering data that could be useful in climate change research.

“People want to know what volume of ice is in the mountains and whether that volume is increasing or decreasing over time. In general, if you’re interested in how the glacial land mass as a large region is changing, this could be a very elegant way to measure that,” Kaufman said.

The fairgrounds’ 4-H room is one of six sites surrounding the icefield in Glacier Bay where gravity measurements are being taken. The others are located at the Blanchard maintenance station near the Yukon Territory border, in Juneau at the University of Alaska-Southeast campus and near the Mendenhall Glacier, and in Glacier Bay at Bartlett Cove and Russell Island.

In three days in May, the group took 10,000 measurements in the 4-H room, or 100 sets of 100 “drops” of a reflective prism that falls in a vacuum tube. “By determining the position of the object as it falls in time, we calculate the acceleration due to gravity for each individual drop,” Kaufman said.

An absolute gravity meter measures gravity at a single spot at one point in time. Absolute gravity measurements taken by Scripps in 1987 – including some taken at the Blanchard station – are important for revealing changes in the past 25 years, Kaufman said.

Most people know from high school physics class that the acceleration due to gravity is 9.81 meters per second squared, but the acceleration changes slightly with elevation and latitude, as well as with factors such as tides, snowpack and ice accumulation, Kaufman said.

“It’s basically the same all over the Earth if you only look at those first three numbers (9.81). But acceleration due to gravity changes around the Earth if you look at the more distant decimal figures,” Kaufman said, noting that such changes are on an “infinitesimal” scale.

“Southeast Alaska has some of the largest snow accumulations in the world and some of the heaviest snowpacks. Because of that, there’s a seasonal change in the acceleration due to gravity,” Kaufman said. The rising and falling of tides has a pronounced enough effect on the researcher’s gravity meter that measurements are taken over the course of several days to cancel out the effect, he said.

Gravity changes with the seasons, reflecting the effect of snowmelt, he said.

The current study takes GPS measurements year-round from stations near Million Dollar Falls, on the airstrip at the Windy Craggy mine site, at Eldred Rock and two sites on the west arm of Glacier Bay.

GPS equipment is good for determining the rate of glacial rebound. Using the gravity meter allows scientists to differentiate between historic ice losses and more recent ones, he said. “There’s a continued, present-day unloading of the Earth’s surface due to glacier thinning and retreat…What we’re looking for is changes in the force of gravity over time at these six different places around Southeast.”

For the record, the force of gravity is lessening in Haines faster than it is at other places because the area is lifting up and away from the center of the Earth, he said, emphasizing the increments aren’t to parts per billion. “You’re not going to float away.”