Wabar craters

The Wabar craters are meteorite craters found by accident by an explorer searching for the legendary city of Ubar.

The vast desert wasteland of southern Saudi Arabia known as the "Empty Quarter", or "Rub' al Khali" in Arabic, is one of the most desolate places on Earth. In 1932, a British explorer, Harry St. John "Abdullah" Philby, father of Communist spy Kim Philby, was hunting for a city named "Ubar", that the Qur'an claimed had been destroyed by God for defying the Prophet Hud.

Philby mistranslated the name of the city as "Wabar", which in a way was fortunate because he found something else that deserved a different name. After a month's journey through the wastes that was so harsh that even some of the camels died, Philby found a patch of ground about a half a square kilometer in size, littered with chunks of white sandstone, black glass, and chunks of iron-nickel meteorite. There were two large circular depressions partially filled with sand.

He brought back one of the chunks of iron. Analysis showed it to be about 90% iron and 5% nickel, with the rest consisting of various elements, including copper, cobalt, and 0.006% - an unusually high concentration - of iridium. This so-called sidereal element implied that the "Wabar" site was a meteorite impact area.

Later research located the town of Ubar elsewhere, but Philby's Wabar site remained intriguing. In 1994, the Zahid Tractor Corporation, a Saudi dealer of the "Hummer" off-road vehicle, decided to stage a publicity stunt of the vehicle by driving several of them across the Empty Quarter in the dead of summer. Few ever went deep into the Empty Quarter even in the winter and came back alive.

A US Geological Survey scientist, Jeffrey C. Wynn, was invited to come along. Zahid sponsored a total of three trips into the Empty Quarter in 1994 and 1995, and Wynn went on all of them. Even with modern technology, the trip was a difficult one. Not only were conditions harsh, reaching 61 degrees C (142 F) in May 1994, but the Wabar site was tricky to find, as it sits in the middle of an enormous dune field that has no fixed landmarks.

The Wabar site covers about 500 by 1,000 meters, and the most recent mapping shows three prominent, roughly circular craters. Two were reported by Philby, and measure 116 and 64 meters wide. The third was discovered by Wynn on the 2nd Zahid expedition and is 11 meters wide. They are all underlain by a hemispherical rim of "insta-Rock", so called because it was created from local sand by the impact shock wave, and all three are nearly full of sand.

The surface of the area partly consisted of "Insta-Rock" or "impactite", a bleached-white, coarsely-laminar sandstone-look-alike, and was littered with black glass slag and pellets. The impactite featured a form of shocked quartz known as "coesite", and is thus clearly the product of an impact event. The impact did not penetrate to bedrock but was confined to local sand, making it particularly valuable as a research site.

The presence of iron fragments at the site also pointed to a meteorite impact, as there are no iron deposits in the region. The iron was in the form of buried fist-sized cracked balls and smooth, sand-blasted fragments found on the surface. The largest fragment was recovered in a 1965 visit to Wabar and weighs 2.2 tonnes. It is known as the "Camel's Hump" and is on display at the King Saud University in Riyadh.

The sand was turned into black glass near the craters, and pellets of the glass are scattered all over the area, decreasing in size with distance from the craters due to wind-sorting. The glass is about 90% local sand and 10% meteoritic iron and nickel.

The layout of the impact area suggests that the body fell at a shallow angle, and was moving at typical meteorite entry speeds of 40,000 to 60,000 km/h. Its total mass was more than 3,500 tonnes. The shallow angle presented the body with more air resistance than it would have encountered at a steeper angle, and it broke up in the air into at least four pieces before impact. The biggest piece struck with an explosion roughly equivalent to the atom bomb that leveled Hiroshima.

Fision-track analysis of glass fragments by Storzer suggested the Wabar impact took place thousands of years ago, but the fact that the craters have filled up considerably since Philby visited them suggests their origin is much more recent. Thermoluminescence dating by Prescott, Robinson, E. shoemaker, C. Shoemaker, and Wynn (JGR, 2004) suggest the impact site is no more than 260 years old. Arab reports of a fireball passing over Riyadh, variously reported as occurring in 1863 or 1891, indicate the impact may have occurred very recently. Fragments scattered from the path of this fireball match samples found at the Wabar site.