Scientists on the Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar (SIR-C/X-SAR) mission, carried aboard the space shuttle Endeavour in April and October 1994, presented findings today on the workings of Earth's climate system -- its past, present and future -- using new data from some of the hottest and coldest regions of the world.
"The radar data are providing clues to the workings of Earth's climate system that no other remote sensing system can provide," said Dr. Jeffrey J. Plaut, experiment scientist for the SIR-C/X-SAR project at NASA's Jet Propulsion Laboratory in Pasadena, Calif. The team presented their findings at the spring meeting of the American Geophysical Union in Baltimore, Md.
During the October 1994 flight of the space shuttle, observations were taken of the area near the Kufra Oasis in southeast Libya, a region of the Sahara Desert in North Africa. The SIR-C data revealed a system of old, now inactive stream valleys, called "paleodrainage" systems, which during periods of wetter climate, carried running water northward across the Sahara Desert. That region is now hyper-arid, receiving only a few millimeters of rainfall per year, and the valleys are now dry "wadis," or channels, mostly buried by windblown sand.
Prior to the spaceborne imaging radar missions, the west branch of this paleodrainage system, known as the Wadi Kufra, was recognized and much of its course had been outlined by German researchers who are now serving as co-investigators on the SIRC/X-SAR science team. The broader east branch of Wadi Kufra, however, was unknown until SIR-C revealed its dimensions: at least 5 kilometers (3 miles) wide and nearly 100 kilometers (62 miles) long. The two branches of Wadi Kufra converge at the Kufra Oasis. The farms at Kufra depend on irrigation water from the Nubian aquifer system.
"This paleodrainage pattern revealed to us by the SIR-C data suggests that the location of productive wells at the confluence of the old river valleys is no accident," said team member Dr. John F. McCauley, a researcher holding joint appointments at the U.S.Geological Survey and Northern Arizona University in Flagstaff, Ariz. "Quite likely, the water supply for the Kufra Oasis has been recharged by episodic runoff and by groundwater that moved northward in the alluvial fill of the old stream channels."
Rainfall was more abundant in this region during parts of the late Quaternary -- the second period of the Cenozoic geologic era which includes the last 2 million to 3 million years -- when "stone age" Paleolithic and later Neolithic people left their tools along the riverbanks. The SIR-C image, which clearly shows river channels cut into the surrounding bedrock, provides geoarchaeologists with a "road map" to locate artifacts and to better interpret the history of early people and climatic conditions in this region.
The recent SIR-C/X-SAR missions also obtained important new information about the Patagonian ice fields of Chile and Argentina that will enable future monitoring of this important region of the Southern Hemisphere. The ice fields are poorly known because of their remoteness and inhospitable climate. The area has only been traversed during a few very challenging mountaineering expeditions. Much of the region is nearly always covered by clouds, so that remote sensing by reflected visible or infrared radiation is often not possible.
"The SIR-C/X-SAR radar system penetrates the cloud cover to produce a unique new view of this enigmatic landscape of snow and ice," said Dr. Bryan Isacks, a geologist at Cornell University and a SIR-C/X-SAR team member.
Images of huge areas of the ice fields and outlet glaciers were obtained over successive days during each of the two radar imaging missions. They reveal glacier movements and the effects of changes in weather on the characteristics of the snow and ice. Comparisons between the two missions provided new views of two different weather conditions during fall and spring in the Southern Hemisphere.
JPL's imaging radar system is capable of discriminating different characteristics of snow and ice by using multiple radar wavelengths and being able to examine the directional characteristics -- called polarization -- of the radar echoes. The effects of the passage of a storm, for instance, can be seen clearly on successive daily images. The Patagonian ice fields are subjected to frequent storm systems, which carry enormous quantities of moisture to the mountain belt. These new SIR-C/X-SAR images have provided scientists with a first assessment of the distribution of snowfall from storms as they move across the entire ice field.
During the last few days of the April 1994 mission, a special series of radar images were taken of the west side of the northern Patagonian ice field to measure its detailed topography and the movement of the San Rafael Glacier, using a new method called "radar interferometry." These measurements confirmed previous ground observations at the glacier terminus, or glacier end, that astoundingly high rates of movement -- upward of 15 meters to 20 meters per day (49 feet to 66 feet) -- rank San Rafael as one of the fastest moving glaciers in the world. The San Rafael Glacier maintains high velocities due to complex interactions between high snowfall, topographic control and the exit of the glacier directly into an arm of the Pacific Ocean.
Limited studies have suggested that the ends of many of the Patagonian glaciers have receded during this century, but how this recession is related to changing conditions on the ice fields and in the atmosphere is still not known. These new measurements will set the stage for continued radar monitoring of the area, which in conjunction with satellite sensing of changing weather conditions, will reveal how the ice fields are changing in response to changes in global and regional climate.
The Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar project is a joint mission of the United States, German and Italian space agencies. NASA's Jet Propulsion Laboratory built the radar system and manages the SIR-C portion of the mission for NASA's Office of Mission to Planet Earth.
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