A scar on the Jurassic world
Beneath the sands of the Kalahari Desert lies a massive secret from the age of dinosaurs. The Morokweng impact structure, completely invisible on the surface, was created 146.06 million years ago when an asteroid between 5 and 10 kilometers wide slammed into Earth. This event occurred during the Tithonian stage of the Late Jurassic period. The impact formed a crater whose size is still debated by scientists, with estimates for its diameter ranging from 70 to over 240 kilometers.
The structure's existence was first inferred in 1994, not by sight, but through geophysical surveys. Prospectors conducting magnetic and gravimetric surveys noticed a large, circular anomaly. The impact had fractured the rock deep below the surface and left behind a sheet of melted rock, altering the local magnetic field and slightly weakening the pull of gravity in the area. Subsequent drilling confirmed the presence of shock-metamorphosed rocks and an impact melt sheet, definitive evidence of a massive impact.
The impact melt sheet, a layer of rock liquefied by the intense heat of the collision, is substantial, measuring approximately 30 kilometers wide and over 870 meters thick in some areas. Isotopic dating of zircon crystals from this melt rock established the crater's age at the boundary between the Jurassic and Cretaceous periods.
A surviving piece of the impactor
Scientific drilling into the Morokweng structure yielded a remarkable discovery in 2006. At a depth of 770 meters below the surface, researchers recovered a 25-centimeter fragment of the original asteroid itself, embedded within the solidified impact melt. Finding such a large, unaltered piece of an impactor is extremely rare. The immense energy of large impacts is generally thought to vaporize the colliding object almost completely.
Analysis of the fragment, along with smaller pebble-sized pieces, revealed it to be a specific type of stony meteorite known as an LL6 ordinary chondrite. Its composition, however, is unusual. The meteorite contains iron-rich silicates and iron-nickel sulfides but lacks certain minerals like troilite and metal that are expected in this type of chondrite. This unique chemistry suggests the Morokweng asteroid might have originated from a part of the LL chondrite parent body not represented in meteorite collections today, or perhaps from a different asteroid population altogether.
The impact melt sheet also contains high levels of elements like nickel and chromium, containing 5-6% meteoritic contamination. This high percentage is matched by only one other known impact crater on Earth. The survival of the large asteroid fragment at Morokweng challenges existing models of how large projectiles interact with Earth's atmosphere and crust during an impact event.