Icy Fingers in the Weddell Sea

Icy Fingers in the Weddell Sea

Ice can be stunningly beautiful and also quite varied in its appearance. The most obvious differences are between the two main ice types: land ice and sea ice. But even sea ice can vary dramatically from one place to another.

On November 14, 2017, John Sonntag (mission scientist for Operation IceBridge) took this photograph of ice in the Weddell Sea. The geometric shapes are due to a phenomenon known as “finger rafting,” which occurs when two floes of thin ice collide. As a result of a collision, blocks of ice slide above and below each other in a pattern that resembles a zipper or interlocking fingers. Brine expelled from the ice forms a solution that acts as a lubricant.

For finger rafting to occur, it’s critical that the ice be thin—calculations suggest no more than 20 centimeters (8 inches). Any thicker and the ice loses its flexibility. Without flexibility, thicker ice floes that collide can result in a big pile up known as “ridging.”

“Normally you see finger rafting in bits and pieces, a few blocks here and there,” said Nathan Kurtz, IceBridge project scientist. “But this is so extensive. I’ve never seen so much rafting occurring across such a wide area.” That is not necessarily significant; it is simply a good example of the phenomenon.

The photograph above was shot on November 14, 2017, from the window of a NASA P-3 research airplane during a flight for Operation IceBridge. The second photograph (below) was captured during the same flight by the Digital Mapping System (DMS). The instrument is essentially a downward-looking camera installed on the belly of the aircraft.

The purpose of the flight had nothing to do with finger rafting; that was just a lucky sighting. Instead, the team was flying to measure the thickness of sea ice. The same flight line has been used in nearly every one of the nine years that IceBridge has been flying over Antarctica. Sea ice is important to monitor because it is an air conditioner for the planet, reflecting sunlight and keeping air temperatures cooler.

Kurtz noted that sea ice can be both an indicator of climate change and also a cause. Sea ice responds to rising temperatures by melting, which in turn can raise temperatures even more. To date, scientists have observed some regional declines in sea ice around Antarctica, but the net effect around the continent appears to be neutral. Sea ice in the Arctic, however, is telling a different story. Ice atop the Arctic Ocean has retreated significantly over the course of the satellite record.

NASA photographs courtesy of NASA/John Sonntag (top) and IceBridge Digital Mapping System (bottom). Story by Kathryn Hansen.