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Secrets of the Wabar craters: a team of geologists explores this enigmatic meteorite-impact site in southern Saudi Arabia.

In their explorations of the red planet, Mars Pathfinder and Mars Global Surveyor have encountered a well-known class of surface features: impact craters. Although common on the Moon and Mars, craters are rare on Earth - so far only about 156 have been confirmed worldwide. Astronomers and geologists now realize that the ancient, inactive surfaces of these neighboring worlds have preserved their long history of cosmic bombardment and cratering. In contrast, the Earth's crust is very dynamic. Tectonic plate movement, wind and water erosion, and sedimentation have conspired to obliterate most evidence of impacts, especially smaller and older ones.

These crust-changing processes have had much less effect on Mars's desertlike surface, however, and so the red planet has retained a great many impact features. Several possible analogues for Martian craters exist here on Earth. One of the youngest and best-preserved is in the Rub' al-Khali, the Empty Quarter of Saudi Arabia, which is one of the most inaccessible and formidable deserts on our planet.

The Ancient City of Wabar

In 1932 the British explorer Harry St. John Philby became only the second westerner to cross the Empty Quarter. His arduous trip took more than two months, with many of his camels dying during the journey. Philby's guides had told him of an ancient city destroyed by God because of the wickedness and impiety of its king. (The city, called 'Ubar in the Qu'ran, has since been found in northern Oman along the ancient Frankincense Trail.)

When Philby finally reached the site, which he dubbed Wabar, he was disappointed: he didn't see any ruins or evidence of human handiwork, only two shallow depressions in the buff-colored sand surrounded by lots of dark debris that he initially mistook for lava. His guides brought him handfuls of black, glassy beads - the pearls adorning the women of the destroyed city, they said. (The guides were certain they could sell them for profit in Makkah when they returned.) They also brought to Philby a chunk of rusted iron, suggesting that this was the evidence of human handiwork he was searching for.

As Philby examined the rabbit-size object, it slowly dawned on him that the craters were not of volcanic origin. With growing excitement he began to realize that he was holding the evidence of a visitor from beyond Earth's atmosphere. Laboratory examination later in Britain confirmed his suspicion: the Wabar craters were, in fact, the result of a meteorite impact.

The Expedition

Because the Earth has so few recognizable impact sites, each one demands careful scientific inspection. So in March 1995 we undertook an exhausting, 10-day expedition deep into the Empty Quarter to map and analyze the Wabar impact-crater complex. Our transportation consisted of Hummer vehicles, civilian versions of the military Humvees. It would normally take at least three days for a caravan of four-wheel-drive vehicles to reach the site, because they bog down constantly in the fine, talc-like sand. But the Hummer's variable-inflation tires can roar across dune tops at 40 kilometers per hour.

We completed the 750-km drive from Riyadh to Wabar in just under 17 hours, leaving in the early morning and camping at night in dune fields that stretch uninterrupted for hundreds of kilometers in all directions. The ride itself is endlessly fascinating, as the dunes constantly change appearance across the Empty Quarter. At Wabar they look like huge ocean waves rolling randomly in all directions.

Wabar's craters and their associated debris fields are situated at latitude 21 [degrees] 30.2[minutes] north and longitude 50 [degrees] 28.4[minutes] east. The cluster is oriented northwest-southeast and measures roughly 1,000 by 500 meters. Although numerous visitors have examined the site since Philby's first report 63 years ago, no detailed investigation had been made prior to our expedition. We spent five days conducting systematic geological and geophysical surveys of the craters and their surroundings.

At least three craters were exposed at the time of our visit (see the map on the facing page). We dubbed them the "11-meter," "Philby A" (64 meters across), and "Philby B" (116 meters). Contrary to previous reports, these impact features have been formed entirely in the loose sands of the active Rub' al-Khali. No outcrops were exposed near the craters, nor were any fragments of bedrock found in the strewnfields.

The crater walls are lined with breccia, fragments of shock-compressed sand that had been metamorphosed into a firm aggregate we called "instant rock." The exposed rims of the two largest craters are mantled with fragments of black, slaggy glass called impactite (consisting of about 10 percent iron-nickel from the shock-melted meteorite and 90 percent local sand) and chunks of instant rock that look like bleached sandstone.

At the 11-meter crater, the ejecta rim has been entirely eroded away and only the fused sand lining the lower crater walls remains visible. The fact that the craters have survived the shifting sands of the Rub' al-Khali is due solely to impactite glass and instant rock that mantle the crater rims and walls and resist scouring by the wind. The 11-meter crater was discovered by one of the authors (Wynn) during a visit in December 1994. Nobody had reported this feature before; apparently it was exposed only recently by prevailing winds.

When first seen in 1932, Philby A's rim was still exposed entirely. Sixty-three years later only its southeastern section remains visible, as the northwestern half is now covered by a prominent seif (sword-shaped) dune. Beyond the rim lies a fallout deposit of glass bombs, chunks of instant rocks, and oxidized meteorite fragments scattered across a meter-thick ejecta blanket.

We excavated a trench in the southern rim to study the layering history and obtain samples for dating the precrater sand deposit. Normally it doesn't rain for years at a time in the Rub' al-Khali, but by an amazing coincidence it rained the night before our arrival. The damp sand held the walls of the trench long enough for us to obtain a cross-section. The structure of this crater in the sand resembles that of Meteor Crater near Winslow, Arizona, with the topmost layers clearly deformed and folded back onto themselves (see page 49).

Reaching Philby B, the largest crater, we found only the rim crest and some of its breccia-lined wall remaining. The crater was well exposed in 1932 and may have been about 12 meters deep at that time. The depth was just 2 meters when Wynn visited the crater in 1994, but wind scouring increased this to about 3 1/2 meters by the following year. A fallout deposit of glassy black slag and instant rock capped the southern exposed rim, but the badly eroded eastern rim had been reduced to glassy rubble.

In the most distant, northwestern patches of the complex we spotted a smattering of glass spheres and teardrops a few millimeters in diameter - the "pearls" that Philby's guides had excitedly gathered. Most of these were likely entrained in a very hot, turbulent explosion plume that carried them downwind (roughly northwest). In effect, it rained molten glass for more than a half kilometer from the impact. Black iron meteorite fragments and crusty balls of oxidized metal are also dispersed in the area and are most abundant near the craters.

Secrets of the Camel's Hump

Analysis of the small meteorite samples we collected indicates that the Wabar impacting body was a Type IIIa medium octahedrite made of up to 94 percent iron, 3.5 to 7 percent nickel, and roughly 0.22 percent cobalt, with traces of iridium. The rest was primarily copper. As the object plunged through the Earth's upper atmosphere at hypersonic speeds it broke into several large pieces, producing the cluster of craters.

Studies of impactite samples in the 1960s gives an age of about 6,400 years for the craters, but we think that this is probably an order of magnitude too high. It seems unlikely that the craters could have survived for several thousand years in such an active desert environment. Based on circumstantial evidence, we believe the impact took place sometime between 100 and 600 years ago.

A huge fireball was reported to have passed over Riyadh in 1863 and headed southeast toward Wabar. Whether or not this meteor was associated with the impact, such an event would have been witnessed by Bedouin tribes, and the "fire from heaven" connected with the story of the destruction of the ancient city of 'Ubar apparently led to the name.

The impact, if an observer survived it, would have been spectacular indeed. Based on the crater sizes we calculate the incoming object to have been at least 4 meters wide, weighing more than 300 tons. The impact released the kinetic-energy equivalent of at least 1,000 tons of exploding TNT. The resulting flash and debris cloud would have been seen from as far away as 500 kilometers. Most of the iron-nickel body vaporized upon impact; the fraction that remained was shattered into small pieces and a substantial part was melted and mixed with shock-melted sand.

A two-ton, cone-shaped iron fragment, nicknamed the Camel's Hump, was reported to Philby, but he never found it. Moving dunes exposed it in 1965, and it was recovered by ARAMCO, Saudi Arabia's national oil company, about 200 meters southwest of Philby B.

The Wabar site is unique indeed - no wonder it's the stuff of local legend. But what we've learned from those rocks sitting in the sand should be a great boon to geologists who will soon pore over high-resolution views from Mars Global Surveyor. It wouldn't be surprising at all for "Wabar-type" craters to be found all over the red planet's desolate surface.

The authors are grateful to Sheikh Walid Y. Zahid, who supported and encouraged the expeditions to the Wabar site, as well as to Bill Chasteen, Wafa' Zawawi, Tino Fenech, Kevin Bullock, Robin De'ath, Zahid T. Zahid, Steve Butler, Taha Jarfrey, and Tim Cooley who accompanied us, worked alongside us, and survived the heat, sand, and scorpions and camel spiders that thrive in the deep desert.

JEFFREY WYNN is a research geophysicist at the U.S. Geological Survey (USGS) in Reston, Virginia. EUGENE SHOEMAKER, formerly with the USGS and Lowell Observatory in Flagstaff, Arizona, lost his life in an auto accident last July.
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Author:Wynn, Jeffrey C.; Shoemaker, Eugene M.
Publication:Sky & Telescope
Date:Nov 1, 1997
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