Findings from two flights of a spaceborne imaging radar aboard the space shuttle have given scientists insights into flooding in the American midwest in 1993, the course of the Nile River, and collisions between ancient supercontinents eons ago.
The findings are being presented this week at the annual meeting of the Geological Society of America in New Orleans. The space radar data were taken by the Spaceborne Imaging Radar C/XBand Synthetic Aperture Radar (SIR-C/X- SAR) during two flights of the space shuttle Endeavour in April and October 1994. The U.S. portion of the project was built and managed by NASA's Jet Propulsion Laboratory.
NASA launched its first Earth-observing synthetic aperture radar on Seasat in 1978. Two later versions of the instrument flew on the space shuttle in 1981 and 1984, each an improvement on its predecessor. X-SAR is a follow-on to the Microwave Remote Sensing Experiment, a German payload that was flown on the first shuttle Spacelab mission in 1983.
One of the most astonishing results of the 1981 mission was the discovery of ancient river beds under the sands of the Sahara Desert in North Africa. The 1984 mission enabled explorers to find the lost city of Ubar in Oman. The Magellan mission to Venus was equipped with an imaging radar that provided the first comprehensive look at the surface of that cloud-shrouded planet.
Following are highlights of the 1994 findings:
RADAR LOOKS BACK IN TIME AT ANCIENT EARTH
Hundreds of millions of years before the first humans were born, and even millions of years before the dinosaurs reigned, the Earth was dominated by one giant landmass that formed when all of the present-day continents crashed together. Now, scientists using spaceborne radar to look beneath the sands of Africa's Sahara Desert have discovered where these ancient continents collided more than 650 million years ago.
"The formation of this 'supercontinent' resulted in a massive ice age that covered the land with glaciers and set the stage for the evolution of the first complex animals. Finding the location of this collision zone is fundamental to understanding how this ancient supercontinent was formed," said Dr. Robert Stern, a member of the radar team from the University of Texas at Dallas. "This discovery also helps unravel the mystery of what controls the course of the Nile, a question that has perplexed geologists for more than a century."
"These data reveal geologic structures buried beneath the thin skin of desert sands in a manner that is reminiscent of an x-ray's ability to study the inside of a human body," Stern said. "If you're standing on the surface there is little to be seen. The geologic structures we are seeing are obscured by a few inches to a few feet of sand."
The recent discovery beneath the sands of the Sahara has helped scientists look back in time to the era of so- called supercontinents, including an exotic landscape scientists call Greater Gondwana. This landmass formed when fragments of east Gondwana (present-day Australia, Antarctica and India) crashed into west Gondwana (present- day Africa and the continents of North and South America).
"The collision zone between east and west Gondwana is buried beneath the sands of the Sahara Desert and cannot be detected using conventional field work or other types of remote sensing imagery," Stern said. "We knew from the results of the first Shuttle imaging radar experiment in 1981 that spaceborne radar could reveal amazing things beneath the Sahara. A team from the U.S. Geological Survey had discovered ancient rivers buried elsewhere in the Sahara. We thought that we could use the capability of the SIR-C/X-SAR system to penetrate ultra-dry sand and reveal the faults and folds of this collision zone, in a region of northern Sudan called the Keraf Suture. Now that we know where it is, we can move on to studying how and when the collision occurred."
BENEATH THE SANDS, CLUES ON THE COURSE OF THE NILE
The radar data also have revealed the faults and fractures in the rocks that control the course of the Nile River in the northern Sudan.
"In this region, the Nile makes a big lazy 'S', first north, then southwest, then north again," Stern said. "We can see on the radar images the structures that control the northward stretches of the Nile. We're trying to use the radar images to explain why the Nile turns southwest instead of continuing north to the Mediterranean. Understanding what controls the course of the Nile is a critical part of understanding the history of the river that is essential to millions of people in Egypt, Sudan and Ethiopia."
In addition to helping answer questions related to the collision zone and the course of the Nile, Stern said the SIRC/X-SAR radar data would be invaluable to developing nations in the area and private companies in their searches for oil, gold and water beneath the Sahara Desert.
RADAR DATA REVEAL POTENTIAL FOR FUTURE FLOODING
Scientists investigating the damage caused in the midwestern United States by the so-called "Great Flood of 1993" have developed a new technique -- using these spaceborne and imaging radar systems -- to understand the potential for future flooding and how that might impact neighboring communities.
A team of scientists used data from the spaceborne radar, along with data from two airborne radar systems, to map the Lisbon Bottoms and Jameson Island flood plains of the Missouri River in central Missouri. The flood plain was ravaged by the severe floods of 1993 and, more recently, by floods occurring this year.
The team, led by Dr. Raymond E. Arvidson, chair of Earth and Planetary Sciences at Washington University in St. Louis, analyzed SIR-C/X-SAR data and airborne radar data obtained during two flights of a NASA DC-8 aircraft in the summers of 1994 and 1995.
From this database, the team estimated that the 1993 flood added at least 5 million metric tons of sand to the flood plains study area and eroded about 3 million metric tons of soil.
"These all-weather radar systems are very sensitive to the presence of vegetation and can also be used to acquire very detailed topographical data -- the shape of the flood plain, the presence or absence of levees, the presence or absence of vegetation," Arvidson said. "These parameters are very important for damage inventory. Radar is a natural for flood-monitoring and damage assessment. It is a new way to assess flood damage because monitoring can be done during the flood and damage assessment can be done using post-flood data."
The radar systems are capable of distinguishing between soil moisture and standing bodies of water. Unlike other remote sensing systems, the radar can penetrate a forest canopy and bounce off the water, sending data back to the aircraft or spacecraft. A study of the flooded area over time allowed Arvidson and his group to look at the natural recovery of vegetation as the flood plain returned to normal.
Arvidson and his team hope the information gained from their study will provide new ways for environmentalists to manage wetlands in flood-prone areas, such as the Missouri River flood plain they are currently analyzing.
"The long-term objective is to use this area as a demonstration site for using radar to map wetlands characteristics," Arvidson said. "We're trying to use the information to predict the extent to which these wetlands alleviate flooding downstream, where there are thousands of acres of rich farm land. The management of wetlands requires periodic detailed mapping, and radar systems such as these provide the needed coverage and quantitative information."
SIR-C/X-SAR is a joint mission of the United States, German and Italian space agencies. JPL built and manages the SIR-C portion of the mission and also manages the airborne radar missions for NASA's Office of Mission to Planet Earth, Washington, DC.
News Media Contact818-354-5011