By Paul Rincon

Science reporter, BBC News, The Woodlands, Texas

A survey in the area is now required to stand up some of the ideas

Deforestation has revealed what could be a giant impact crater in Central Africa, scientists say. The 36-46km-wide feature, identified in DR Congo, may be one of the largest such structures discovered in the last decade. Italian researchers considered other origins for the ring, but say these are unlikely. They presented their findings at the recent Lunar and Planetary Science Conference in Texas, US. The ring shape is clearly visible in the satellite image by TerraMetrics Inc reproduced on this page. 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 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. Confirmation needed The putative crater lacks a well-defined outer ridge, though the University of Padova team says 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. LARGEST IMPACT CRATERS The Chicxulub crater is buried under Mexico's Yucatan Peninsula 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 Popigai: Russia, 100km-wide, 35.7 million years old Manicouagan: Canada, 100km-wide, 214 million years old Mr 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 evaporite minerals intrude vertically into surrounding rock, forming dome-like structures on the surface) could be responsible for the annulus. But Mr 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. Paul.Rincon-INTERNET@bbc.co.uk



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