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Libyan Sahara meteorite expedition |
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by Richard Pelisson
In the 19th century, when explorers first began to cross the Sahara, no journey was considered more hazardous. Today, the GPS and the precision of satellite maps make it possible to venture off the beaten track, right into the heart of the biggest desert on the face of the Earth. The Sahara alone covers 9,000,000 sq km (3,500,000 sq mi), extending 4800 km (3000 mi) east to west and 1900 km (1200 mi) north to south across North Africa. Daytime temperatures have been recorded at over 57° C (135° F) in the shade. The surface of the desert ranges from sand dunes (erg), covering about 15%, to stone plateaus (hammada) and gravel surfaces (reg), covering about 70%, to several deeply dissected mountain massifs. The desert has held a special attraction for my brother and me for many years now. Mauritania, Algeria, the Hoggar Massif - all are destinations filled with memories. Today, a car ferry takes us towards Libya, a country where, three years ago, a tiny meteorite discovered on a dune awakened in us the curiosity which was soon to become a passionate obsession.
Hammada al Hamra Nalut, a fortified village on the cliff, Derdj... The road south often dwindles to a long straight line between two filling stations. In front of us, the horizon is dark grey. Little by little, the Sun is disappearing, and the temperature has risen by ten degrees in just a few kilometers. It's the Ghibli - a hot, very dry, sand-laden wind, which will accompany us until evening. It is our first day in the desert, and already the dust is everywhere - it has got into our cases, our belongings, and even the engines' air filters. This part of the Sahara is a huge, calcareous expanse, the site of ancient marine deposits recalling the oceans which covered it several tens of millions of years ago in the Cretaceous Period. It is chiefly a stony landscape, with only a few grassy zones surviving in depressions and oueds: dry river-beds or wadis. The southern region ends with a cliff overlooking the dunes, which spread out as far as the eye can see: the Awbary erg, an immense sea of undulating sand which seems never to end. The ever-changing colors at sunset have a magical effect; this is a desert landscape which provokes not only fascination, but also awe. The terrain seems more favorable today, with little or no vegetation and fewer boulders. I am working my way along the side of a gentle butte when I notice a black spot 50 meters to my left. In the time it takes to get there, I dream up several possibilities: it is a shadow, a piece of dark limestone or a nodule of limonite, perhaps even a tin can which has been patinated by the desert. But the closer I get, the clearer it becomes: it is a meteorite all right, lying there on the ground as if it wanted me to find it. In a terrain carved by gentle valleys, the range of my radio is not good enough to communicate the good news to my brother, who will be prospecting several kilometers further south, searching for new zones. We usually split up when hunting, but come together once or twice a day at a time and place determined each morning. When I get there, I'm alone. There is no sign of my brother, the horizon is empty. My imagination is working overtime when his 4WD finally appears, heading full speed in my direction, and accompanied by a cloud of dust.
A meteorite is run over Today is a day to be remembered. I may have found the first meteorite of our expedition, but the ten or so fragments which Roland is busy reassembling for me weigh at least 30 kg. It is a chondrite, discovered right in the middle of the track leading south. My brother's excitement still hasn't worn off, and he tells me how surprised he was to come face to face with this incredible discovery. When he first saw the large black rock right in front of the 4WD, he thought it was basalt. How could it be anything else, given that the track had already been worked over by other prospectors? He decided there would not be any point in slowing down to have a closer look, and started to lose interest. But at the last moment, he had noticed an abnormal fracturing which made him decide to carry out a routine inspection of the sort we do dozens of times a day, whenever we see a rock which looks promising. This time, there was no fusion crust, but it did show a slight magnetic attraction. Disbelief was replaced with uncertainty. One of the fragments detached, allowing him to file down the inner part a little, and revealing a speck of metal. Now the digging could begin in earnest - too bad for the track! There are times when civilization makes its presence felt at inopportune moments. I'm driving eastwards along a track so rocky that progress is difficult. I have to focus my attention just ahead of the wheels of the 4WD, avoiding the big, sharp blocks which might damage the tires or the mechanics. The terrain has got progressively worse. I could always do a U-turn, but I keep thinking there will be a smoother track soon, perhaps even a large meteorite hiding in the middle of this field of rocks. A long, dark ribbon at ground-level blocks out the horizon - it is a pipe. Or, more precisely, a pipeline - Libya is an oil-producing country. Out here in the desert, it hasn't occurred to them that someone might actually want to cross the line, and the pipe remains unburied.
It has taken us one week to cover 1500 km on the Hammada al Hamra and to collect 20 meteorites originating from at least six different falls, including an HED and a type 3 chondrite. There is every reason to expect many more discoveries. The distribution of meteorites currently stands at one for every 200 sq km for the entire western region. However, the most favorable prospecting zone, which is very clear and subject to erosion, is situated to the south-east, and shows a distribution close to one for every 20 sq km. Here, on a plateau which covers 60,000 sq km (23,000 sq mi) and has seen prospecting for almost 15 years, the ground is best observed with the naked eye. Even so, picking out an abnormal or very black rock amongst thousands of grey, beige or brown ones is no easy task.
Dar al Gani Small in size at 80 x 50 km (50 x 30 mi), Dar al Gani is the most important Saharan strewnfield, with nearly a thousand itemized meteorites, Lunar and Martian rocks, various achondrites, etc. At least 150 different falls are represented. When you approach Dar al Gani from the west, the first thing to strike you is its whiteness, as if you were looking out over mountain-tops covered in snow: a mirage in the desert. First comes a succession of terraces which then open on to a smooth, rolling expanse of white, without rocks or vegetation. Meteorites have been falling here for thousands of years, and it goes without saying that strewnfields like this one are of scientific interest. Unlike Antarctica, where ice shifts concentrate meteorites and wind scatters the fragments, things here stay in the same place from one millennium to the next. I often think of Dar al Gani as a photographic plate recording all falls over a significant time-scale of 20,000 years or more. The terrain is gentle and preserving, so that thousands of years worth of data are at present accessible.
Trajectory of the CO3 The next morning, we are forced to change our plans. The rain of the day before has completely altered the ground of the entire eastern region, all the way to the volcanic, basalt massif of the djebel Haruj. We are left with the western part of the plateau, which has been spared from the rain, but not from meteorite hunters. The evidence is there in the form of tire-tracks, some fresh, perhaps only a month old, and others obviously dating back a year or more. Every parcel of land must have been searched at least three times. Under these circumstances, it is futile to hope for a meteorite any bigger than a nut. A number of prospectors would have already been attracted to the area by the discovery of a carbonaceous chondrite in the same south-western part of Dar al Gani four years ago (DaG 005), followed by 34 other CO3 individuals within a radius of just a few kilometers. Despite all this, we decide to risk it. In the zone prospected by the teams before us, the ground is so clear and light-colored that a very black carbonaceous meteorite, even a small one, would be seen 100 m away. This type of pale, stoneless ground is every prospector's dream - provided he's the first on the scene. Needless to say, we are not. It is midday by the time we head towards the north-west. Our goal is simple: to find something which would confirm our hypothesis. In a few minutes, we reach the far end of the zone where the discoveries of past years were made. The nearest oasis is 100 km (60 mi) away as the crow flies, and there are indeed a pair of crows circling over-head, as well as some small black and white wheatears (a type of bird with a conspicuous white rump) searching for signs of human beings. It might as well be said that when exploring a new zone which bears no signs of having ever being touched, you always proceed with caution. Each of our vehicles carries more than 600 kg of water, gasoline and other materials, a minimum requirement for self-sufficiency in the desert. Descending rocky slopes and driving over soft ground demands a good amount of experience and skill if you do not want to end the day with an overheated motor, or a shovel in your hand. Ahead of me, Roland's 4WD is sending up a huge cloud of dust several meters in height. A bad sign! I'll have to tow him out. He does indeed call me by radio, but not to ask for help. When he realized he was trapped in the fech fech, he accelerated deeper into it, and actually managed to reach a small raised terrace which helped him to get out. His call is to tell me that he has come face to face with a carbonaceous chondrite weighing 4 kg.
Some days we take our time preparing and cooking our evening meal, but tonight we are the ones simmering - with understandable excitement! The stars are bright by the time we finally settle into our sleeping bags, full of hope for the next day. We have decided that one of us will continue northwest for at least 5 or 6 km to look for the missing mass, while the other will widen the search around the camp.
Discovery "I'm afraid we have a problem, what we feared most has come true!" calls Roland over the radio. I drive in his direction, knowing full well what to expect. But even so, I am more than a little surprised. There on the ground is a 95 kg meteorite, broken into a thousand pieces. It is a striking image: a pile as black as ink on a background of white. Before leaving Dar al Gani, we have a rendezvous with a friend we met two years ago. If you ever have the opportunity to drop in, bring him a few biscuits or seeds. He lives at the edge of the plateau, and he's a friendly jerboa. This little rodent will come to pay you a visit when night has fallen. Here in the desert, what could be more natural than sharing your meal? His cheeks bulging, he will head off to feed his little family.
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By the time I make it back to the bivouac, the shadows are lengthening disproportionately and the Sun is level with the horizon. The essentials for camping are already in place: a board serving as a table, two spare wheels as chairs, and most importantly, sparkling mineral water, our pre-dinner drink. The bottle is wrapped in a damp cloth: simple evaporation will lower the water's temperature, providing us with a cool, refreshing drink in half an hour. If three liters of water per person per day is enough at this time of year, in summer the daily requirement could easily exceed five liters.
You might be tempted to think that prospecting such a smooth, obstacle-free terrain is easy. But then you would be forgetting that the desert always has a surprise up its sleeve... a storm, for instance. Wind, flashes of lightning, driving rain... In just a few minutes, the landscape is completely transformed. Puddles the size of small lakes form around us. Usually firm, the ground drinks up the rain and creates traps where the 4WD sticks like glue. You really need to concentrate to zigzag between these sebkhas (salt water lakes). It is flat as far as the eye can see, and a third of the ground is soon covered in water, making it impassable. I glance at the map, studying the contour lines and looking for a raised area. When I have time, I enter the coordinates in the GPS and head in the appropriate direction. The ground does indeed become progressively less flooded, the puddles less frequent. Five kilometers further east, Roland is mopping up in vain. At least the rain has refreshed the air. And tonight, we will be camping on the shores of a lake!
The plateau ends just in front of us, the small cliff which marks its edge overlooking a lightly undulating landscape with an old oued bed, alluvial deposits and fech fech, a very fine powder caused by erosion of the clay-limestone terrain: a real trap for 4WDs! If a meteorite has fallen here, by now it will be buried under the alluvium, never to be found.
It is there on the butte: a large black stone lying on white gravel. It takes me a while to realize the importance of the discovery. Not only has our hypothesis proven to be correct, but we are once again on favorable ground. We can see more similar little buttes and terraces of limestone in front of us. By the time evening falls and we are forced to stop our searching, we have 15 kg of CO3, as well as a one-kilo chondrite clearly originating from a separate fall.
We look in all the nooks and crannies of the desert, and our perseverance is rewarded. Today, we collected more than 36 kg, the nicest individuals being an extremely attractive complete piece of 8,300 g, and a 15,500 g piece which we were able to rebuild in its original form, the majority of fragments being scattered over a few square meters following an impact on rock. These 15 kg, situated on the trajectory which we had calculated ourselves most certainly represent the final mass of the fall, and so also the culmination of our research. It is time to let the engines rest while we take stock for the first time: our efforts of the last two days have been rewarded far beyond any expectation.
