In the old Persian tale, "The Three Princes of Serendip," three young princes make several unexpected discoveries while searching for totally different things. From this story, 18th-century English author Horace Walpole coined the word "serendipity," adding that an intrinsic element of serendipity is the sagacity to recognize the significance of the unexpected.
This was certainly true of the scientists at the United States Geological Survey (USGS) who recently made a serendipitous discovery in the Sahara Desert that they are calling "astonishing." It resulted from the combination of a shortened space mission, a remarkable radar sensor, some good luck - and scientists alert to the sigificance of the unexpected.
Oddly enough, this modern story of discovery began with yet another Persian tale, this one dating back to the Persian occupation of Egypt about 500 B. C, when legends began to circulate about a "waterless river" hidden under the impassive face of the desert. These legends persisted and much later, in the 19th century, prompted a European expedition to Egypt's hostile Western Desert to search for this waterless river.
Not surprisingly, that expedition failed to find any rivers, with or without water. The Sahara's Selima Sand Sheet, after all, is an unlikely place to look for water; straddling the Egyptian-Sudanese border, it gets rain no more than two or three times in a century. This hyper-arid core of the Great Sahara Desert rates 200 on a geological "aridity index," meaning it would need 200 times the present rainfall to even have runoff. In contrast, the driest spot in the United States - near Death Valley, California - rates no more than seven on the same index.
The Sahara, however, has not always been dry. About 50 million years ago the sea flooded most of it and later large mammals roamed its lush savannahs, swamps and grasslands, indicating, scientists say, that rivers once flowed through the region. Evidence of wadis, or dry valleys, carved in western Egypt's Gilf Kebir Plateau support this hypothesis, as do the positions of small "conical hills" marking ancient divides on the surrounding planes. And three scientists who have spent considerable time there - USGS geologists Carol Breed, John McCauley and Maurice Grolier - have concluded that the wadis of the Gilf Kebir are upstream remnants of a large ancient watershed.
Though scientists have been aware of the Gilf Kebir Plateau for some time - it was discovered by Egyptian Prince Kemal al-Din in 1926 - geologists who later explored the region could come to no definitive conclusions because the wadis disappear without a trace beneath the sand at the foot of the plateau.
In November 1981, however, the Space Shuttle Columbia was launched on its second flight, carrying equipment for several scientific experiments. Included was the Shuttle Imaging Radar system (SIR-A), which scientists hoped would prove useful as a geological exploration tool. Scientists with SIR-A, in fact, had carefully targeted those regions of the Earth's surface that might be of significant geological or economic interest. Then, the first phase in this example of serendipity, a faulty fuel cell forced the scientists to shorten the 154-hour flight to 54 hours. That meant that instead of recording the targeted regions, astronauts had to settle for radar images of whatever land surfaces happened to come into view during the abbreviated mission.
After the flight, Charles Elachi of the Jet Propulsion Lab in Pasadena, California, SIR-A's principal investigator, met with other scientists - among them Gerald G. Schaber, chief of the USGS Astro-geology Branch in Flagstaff, Arizona - to examine the radar images and distribute them for preliminary analysis. Instantly noticing that the SIR-A had picked up the hyper-arid regions of the Sahara that Breed, McCauley and Grolier had explored, Schaber turned the images over to them.
Looking back now, Breed remembers exclaiming, when she took the first look at the images, "Hey! Where's the sand sheet?" because, she said, it looked as if the sand cover had been peeled off the surface of the desert to disclose an astounding geological sight: long-vanished networks of rivers, with valley floors that measured many kilometers across, and might extend for thousands of kilometers in length.
And that was but the first surprise. Despite their earlier speculation, none of the geologists who had worked in the Gilf Kebir region had come even close to guessing the enormous proportions of the ancient watershed revealed by SIR-A. Nor had they expected such dramatic results from radar imaging technology. Though radar penetration to depths of five meters (16 feet) was theoretically possible, few thought it likely, because on natural terrain near-surface moisture usually limits radar penetration to a few inches. "Nobody," said Schaber, "dreamed that there could be places on Earth dry enough for radar to penetrate the surface to these depths."
In the hyper-arid core of the Sahara, however, virtual total aridity - plus the presence of loose, porous sand – allowed the radar to perform to its limits: the radar signals bouncing back from underlying dense rock, gravel and alluvium, and outlining the river valleys with remarkable clarity. Thanks to a happy - if fortuitous - pairing of the instrument's capabilities with an ideal subject, McCauley says, "we were able to look beneath the face of the desert and use radar as a time machine."
The era to which they returned is the mid-Tertiary period - some 17 to 50 million years ago - when this part of North Africa enjoyed a moist, subtropical climate, and when rainfall on the Gilf Kebir Plateau and other highlands fed a network of large rivers flowing across grassy plains. Since then, the region has fluctuated between dry and less dry, culminating in the present extreme aridity.
In geological terms, man entered the scene rather recently - 200,000 years ago. They were probably hunters preying on zebra, rhinoceros, elephant and hippopotamus - judging by stone tools that have been discovered - but because no skeletal remains have been found, scientists can only speculate that these people were akin to Homo erectus, one of man's early ancestors. Remains of Homo erectus have been unearthed elsewhere in Africa and date to this period.
Then as now, the climate of the area determined who and what could live there - and for how long; as philosopher-historian Will Durant wrote, "Civilization exists by geological consent, subject to change without notice." These early ancestors of man apparently died out, or were driven away, when the climate changed and were succeeded later presumably by Neanderthal Man, some 30-to 50,000 years ago. Hunters and gatherers, whose stone tools can now be found lying on the desert floor, these people too eventually surrendered to the desert.
The last and most successful occupation of the region began less than 10,000 years ago when Neolithic Man entered the scene. Nomads, who herded cattle and hunted ostrich, giraffe, elephant and other animals - as vivid desert rock paintings and carvings reveal (See Aramco World, January-February 1983) - these people enjoyed a climate that nourished lush vegetation as far south as The Sudan. It was the Sahara's finest hour, but it was cut short some 5,000 years ago when the surface water dried up and man, together with virtually all other forms of life, was driven out. Since then, the desert has ruled supreme, not only concealing the clues to its past, but sometimes even altering them.
"The desert has been so modified by wind erosion," Breed says, "that many tools are no longer on the surface the people walked on." Many scientists who have worked there report artifacts from several periods of human occupation lying on the same surface. Thus, no piece of evidence can be taken at face value. For example, until the discovery of the "radar rivers," geologists had attributed the extraordinary flatness of the desert surface to wind erosion and sand deposition. But now they believe that the vanished rivers were probably responsible, carving out plains filled in by sand only much, much later.
Another case of serendipity: less than a month before the USGS scientists first reported the rivers of sand, a small expedition led by University of Arizona geologist Vance Haynes had crossed the same swath of Egypt and The Sudan swept by SIR-A. In pits they dug in the sand sheet, this expedition unearthed river sediments - some containing fossil snail shells - in localities later recognized as ancient river valleys. These deposits are now thought to be the youngest legacy of river systems that many millions of years ago may have been as extensive as the Nile.
But despite the clarity of the SIR-A images, they pose more questions than they answer. For example, McCauley says that the "trends of the ancient rivers are to the south and west, the opposite of known present-day ground water movement." Questions, therefore, arise as to whether these rivers were ever connected to the Nile and, if they were, where and when? Although he has no evidence yet, McCauley says that "it is possible they all flowed to one large basin of interior drainage as large as the Caspian Sea is now." Perhaps more intriguing was whether these rivers drained areas from which they might have carried gold, tin, and other minerals.
While such questions raised exciting possibilities, it was still no more than educated speculation - and would be - until the physical presence of these river channels was verified by digging pits. In September 1982, therefore, Dr. Bahay Issawi, director of the Egyptian Geological Survey and Mining Authority (EGSMA), led an expedition to southern Egypt to locate and examine the "radar rivers." With him went several geologists from EGSMA; Schaber, McCauley and Grolier of the USGS; Elachi and Ron Blom of the Jet Propulsion Lab; archeologist Bill McHugh of Pittsburgh's GAI Consultants; and a number of Egyptian fieldworkers.
Locating the river beds beneath the featureless Selima Sand Sheet was something of a scientific achievement in itself. In a area larger than Belgium, with virtually no landmarks, getting from one point to another is not only difficult, but also potentially dangerous, since the traditional method of navigation across the desert is by "dead-reckoning," using a compass and odometer. According to Breed, a veteran of five Sahara expeditions, the most common error travelers make is to mistake tracks in the desert for your own. "There are very few of us who have not gotten lost this way?"
At the site, field workers began to dig pits into the sand sheet where the SIR-A radar images had indicated underlying valleys, and soon began to turn up their evidence. Their labors exposed river sands and rounded stream gravels - some containing Stone Age tools that had been tumbled by running water. In other areas, buried bedrock surfaces, bright on the radar pictures, were uncovered by the diggers a meter or so (three feet) beneath the sand - "confirming without doubt," says Schaber, "that we were achieving radar penetration to that depth. We also verified penetration of several meters through small sand dunes, which are essentially invisible to the radar."
This first expedition having succeeded, another was organized and carried out by many of the same scientists in March 1983, sponsored jointly by EGSMA and the U. S. Agency for International Development (US AID) in Egypt. They once again set out in four-wheel-drive vehicles from EGSMA's field headquarters at the Kharga Oasis on the Western Deserfs eastern edge. But this time, instead of navigating by simple dead-reckoning, they navigated by means of an electronic device that calculated the position of their vehicles relative to signals from six orbiting U.S. Navy "Transit" satellites.
Since this was the first use of the satellite transit system for navigation in the desert, Breed recalled, no one knew how it would work when, on the second day out, they got lost. "Our magnetic compass headings and elapsed mileage indicated that we should be approaching camp a few miles from the Egyptian border," she said. "But the lights of camp were nowhere to be seen, and the Jeeps were very low on fuel. So rather than continue by dead-reckoning we decided to stay put and wait for a satellite fix. Then, as darkness fell, we heard the welcome 'beep-beep' of our satellite navigator as it displayed our actual position - 30 degrees off course to the east, some 20 miles away from camp. Thirty minutes later, after following the satellite navigator's directions, we reached camp safely.
The purpose of this second expedition was to define, sample and map individual channels within the 10- to 15-kilometer-wide (6.2- to 9.3-mile-wide) river valleys located on the previous expedition. With the help of the new navigation device, the scientists were able to locate many of these channels, in which they again found Stone Age tools and - to their surprise and excitement - sand that was still wet.
Breed says they were at first surprised to find damp sand because "the area is well away from the oases," and then excited because the moisture suggests the presence of "shallow water aquifers here and there that might be capable of supplying wells." In fact, surprise and excitement characterize quite well the last two years at the USGS Astrogeology Branch and EGSMA. As McCauley points out, most large river systems of the world have been explored rather thoroughly. So naturally, he says, "it's intriguing to find one that was hitherto unknown. We have a whole new set of geological phenomena to examine."
The discovery has potential applications in several disciplines. Geologists are interested not only because they can "see the past," but also because the "radar rivers" may help locate sources of groundwater or mineral deposits. Archeologists are excited at the prospect of having a "roadmap" to use in locating ancient human occupation sites. And the U.S. National Aeronautics and Space Adminstration is so encouraged by the findings in the Sahara that they not only are planning to extend the radar imaging to more of the Earth's desert regions on an August 1984 Shuttle flight, but also are considering applying similar technology to Mars, in hopes of unlocking the secrets of how the surface of that desert planet has evolved.
In the fifth century B.C., the Greek historian Herodotus wrote that "Egypt is the Nile and the Nile is Egypt." Now, almost 2,500 years later, another river's legacy has risen to claim her share of the Nile's ancient honors.
Sybil Thurman is an editor with the Tennessee Valley Authority (TVA) and a free-lance writer.