Satellite capture first-ever gravity map of tides under antarcticice (Forwarded)

Research Communications
Ohio State University
Columbus, Ohio

Contact:
C.K. Shum, (614) 292-7118

Written by:
Pam Frost Gorder, (614) 292-9475

12/4/05

[These findings will be presented on Wednesday, Dec. 7, 2005, at 1:40 p.m.
PT (4:40 p.m. ET) at the American Geophysical Union meeting in San
Francisco, (Session G33B-0042, MCC Level 1).]

SATELLITES CAPTURE FIRST-EVER GRAVITY MAP OF TIDES UNDER ANTARCTIC ICE

SAN FRANCISCO -- Ohio State University scientists have used minute
fluctuations in gravity to produce the best map yet of ocean tides that
flow beneath two large Antarctic ice shelves.

They did it using the twin satellites of the Gravity Recovery and Climate
Experiment (GRACE), a joint project of NASA and the German Aerospace
Center.

Large tides flow along the ocean floor beneath the Larsen and
Filchner-Ronne Ice Shelves. Though scientists have long known of these
tides, they have not yet been modeled accurately, said C.K. Shum,
professor of civil and environmental engineering and geodetic science at
Ohio State. Yet the tides play a major role in scientists' efforts to
measure how much the ice sheets are melting or freezing, and how the
melting ice will affect global sea levels.

While the tides cause only minute fluctuations in Earth's overall gravity,
they are actually composed of massive amounts of water, he explained.

The ice is a mile thick in parts, and the tides are so large that they can
lift the shelves -- with a combined area bigger than the state of
California -- as high as 15 feet.

Scientists believe that these unseen tides can carve into the ice from
underneath and eventually cause pieces to break off, as part of the Larsen
Ice Shelf broke off in 1995.

The tides also make the job of measuring changes in the ice more
difficult. Large portions of these two ice shelves float on the water, so
the rise and fall of the ice with the tides prevents scientists from
making precise measurements of ice thickness.

The GRACE satellites offer a good way to track the tides, Shum said. Ocean
currents slightly nudge the force of gravity higher or lower in some
places around the world every day. GRACE can detect those changes.

Shin-Chan Han, a research scientist in the School of Earth Sciences at
Ohio State, presented the study Wednesday in a poster session at the
meeting of the American Geophysical Union in San Francisco.

Other research groups have tried to measure these tides terrestrially with
sensors called tide gauges. But doing so in Antarctica means first
drilling through the ice to plant the sensors on the ocean floor, and then
retrieving the sensors later to download the data. Because of the
equipment expense and the harsh conditions on the frozen continent,
scientists have been able to plant only a handful of these sensors there.

"There were measurements of these tides before, but they were confined to
very few spots," Shum said. "To get really accurate measurements -- and
make really accurate models of how the tides are interacting with the ice
-- you'd need to put tide gauges or other equipment all over the ocean
bottom underneath an ice shelf, and that's not practical."

The twin GRACE satellites have circled the globe in tandem since 2002,
effectively drawing a picture of the Earth's gravity field at least once a
month. On-board instruments measure very precisely any minute tugs the
Earth exerts on the satellites while they're in orbit.

To get a handle on the extent of the Antarctic tides, Shum and his team
used GRACE to measure the change in local gravity as water flowed beneath
the two ice shelves between August 2002 and June 2004.

Shum and his team, including Han and Koji Matsumoto of the National
Astronomical Observatory of Japan, used the gravity variations to
calculate expected tide height beneath a number of key points on the two
ice shelves.

The researchers compared their data to two Antarctic tide models created
by other groups. The two models -- which were based on sparse data
collected from tide gauges on the continent -- agreed with the GRACE data
to within 20 percent.

"We have reason to believe that GRACE data is more accurate because the
other models are based on substantially less data," Shum said. "So we
think that people who incorporate our GRACE data into their own data are
going to get better results. We also hope to help glaciologists measure
changes in the ice flow much more accurately."

Not knowing precisely where the tides are -- and how big they are --
creates two kinds of errors in scientists' measurements, he said.

As an ice sheet rises with the tide, part of the grounded portion that
normally rests on the ocean floor raises up. Researchers call the point of
contact between the ice and the rock the grounding line, and the rise of
the ice effectively moves the grounding line inland. That means that while
the tide is high, more of the ice bottom is contacting the water than when
the tide is low.

Scientists need to know where the grounding line is in order to determine
what portion of the ice shelf is being affected by direct contact with the
water. Based on the data from GRACE, Shum suspects that previous estimates
of where the grounding line is located on these two ice shelves could be
off by hundreds of meters (thousands of feet) in different locations.

Meanwhile, the rise and fall of the ice can make the surface appear to be
higher or lower than it really is when satellites measure the ice sheets'
topography. Depending on the time of day they record their elevation
measurements, scientists who use that data to calculate the mass of an ice
sheet can get totally different answers.

While people have studied the tides at Earth's middle latitudes for
centuries, tides at the poles have presented scientists with serious
obstacles, Shum said. Where harsh conditions prevent detailed study, the
terrain of the ocean bottom is not well known. The interaction between the
water and polar ice sheets isn't fully understood, either.

NASA and the National Science Foundation funded this work. Shum and Han
are continuing the project, and will focus on tides beneath Arctic ice
shelves next.

Editor's notes:

During the AGU meeting, Shum and Han can both be reached at the Holiday
Inn Golden Gateway in San Francisco at (415) 441-4000, or through Pam
Frost Gorder.

Images to accompany the story are available at

http://researchnews.osu.edu/archive/icetidepix.htm

and from Pam Frost Gorder. During the meeting, she can be reached at the
Holiday Inn Golden Gateway or in the AGU Press Room (2024 MCC) at (415)
348-4440.