Congo: With the help of satellite imagery, an Italian scientist team announced the discovery of Wembo-Nyama crater, a gigantic impact crater, in Democratic Republic (DR) of Congo. They have recently presented their findings at the latest annual Lunar and Planetary Science Conference, held in Houston, Texas. The new crater, which measures between 36 and 46 kilometres in diameter, is claimed by some scientists to be one of the largest craters discovered in the past ten years or so. The ring shape is clearly visible in the satellite image by TerraMetrics Inc reproduced on this page.
The Unia River flows around the ring structure, underlining its round shape. The central part of the Wembo-Nyama feature is irregular and about 550m in elevation. This is about 50-60m higher than the depression where the river flows. Although this might sound counter-intuitive, experts say that impact craters can sometimes lift up dense rocks. The surrounding rocks may subsequently erode, leaving a dome.
Italian researchers who have had a chance to look at the feature proposed a number of causes for its creation, but eventually they themselves came to the conclusions that nothing but a space impact – most likely an asteroid or a comet – could have formed this hole.
Only about 25 terrestrial impact craters are of comparable size or larger, according to the web-based Earth Impact Database. Giovanni Monegato, from the University of Padova, said “The feature was revealed only after trees were cleared from the area over the last decade.” The putative crater lacks a well-defined outer ridge, though the University of Padova team said that this could be explained by deep weathering and erosion in the tropical climate. They add that the drainage pattern in the ring is very similar to those found in large impact craters in humid environments.
Monegato said the team would now have to travel to the region to carry out field studies. Researchers would examine rocks from the site for tell-tale signs associated with space impacts. These might include shocked quartz – a form of the mineral which occurs where rocks have been hit suddenly by a massive force. It is found only at nuclear explosion sites and at asteroid impact sites.
Finding such evidence will be crucial to confirm an impact origin over other processes which might explain the structure. The researchers have considered whether volcanism or salt diapirism (a process where evaporate minerals intrude vertically into surrounding rock, forming dome-like structures on the surface) could be responsible for the annulus.
But Monegato and his colleagues say the known geology of the region – along with other features of the structure (for example, no known salt diapirs reach such a great size) – appear to rule out such explanations. “I am quite optimistic about an impact crater origin for this ring,” Mr Monegato told BBC News. “If it is an impact structure, the scientists estimate it could have been punched into the crust by a space rock measuring about 2km across. Further studies will be required to accurately determine an age for the ring, but it appears to post-date the Jurassic Period,” continued Monegato.
Some of the largest craters are:
– Vredefort: S Africa, 300km-wide, 2 billion years old
– Sudbury: Canada, 250km-wide, 1.8 billion years old
– Chicxulub: Mexico, 170km-wide, 65 million years old (buried under Mexico’s Yucatan Peninsula)
– Popigai: Russia, 100km-wide, 35.7 million years old
– Manicouagan: Canada, 100km-wide, 214 million years old