Operation ICE BRIDGE: Using Instrumented Aircraft to Fill the Observational Gap Between ICESat-1 and ICESat-2

The ICESat-1 satellite, which provided laser altimeter measurements of ice sheet elevations and sea ice freeboard, failed in 2009. Because its replacement, ICESat-2, is not scheduled for launch until about 2015, NASA began in 2009 a five year series of instrumented aircraft polar experiments under the name ICE BRIDGE, that are designed to fill the observational gap between the two satellites. The observations began in spring 2009 with a series of flights over the Arctic Ocean and the Greenland ice sheet that in many cases followed ICESat lines. The aircraft instruments include two scanning laser altimeters, a snow thickness radar that measures snow depth on sea ice, an ice penetrating radar that determines bedrock depth through ice as thick as 4 km, and a gravimeter. In fall 2009, the observations continued with flights over the Antarctic Peninsula, West Antarctica and the Antarctic sea ice. The second Arctic experiment will begin in March 2010. Use of the gravimeter, laser altimeters, and ice penetrating radar over the ice sheets allow for airborne mapping of the bedrock topography beneath the ice sheets and glaciers, as well as the size and shapes of the cavities beneath ice shelves and ice tongues. Examples include the pre-ICE BRIDGE discovery of a Grand Canyon scale trench beneath the Jacobshavn glacier outlet, and a sinuous channel under the Antarctic Pine Island Ice Tongue. ICE BRIDGE will have strong interaction with the ice sheet numerical modeling community, with a goal to provide Greenland bottom topography at the resolution required by the modelers. For the sea ice cover, the aircraft observations permit simultaneous measurements of the sea ice freeboard and of the snow thickness. These two measurements permit determination of sea ice thickness along the aircraft lines, which in the Arctic are designed to cover regions of first and multi-year ice. For the future, plans are being made to carry out some ICE BRIDGE observations from the un-manned Global Hawk aircraft. In summary, the five year ICE BRIDGE program continues the ICESat observations over the rapidly changing features of the ice sheets and sea ice, and through use of systematic gravity and ice-penetrating radar measurements, will provide the necessary bedrock and bottom topography measurements required by the numerical ice sheet models.