Some 66m years ago Earth was hit by a space rock reckoned to have been 10km across. The resulting chaos caused the extinction of dinosaurs and many other species, opening the way for the age of mammals－and ultimately humans. It also left a big hole in what is now southern Mexico. That hole is one of only three known of similar dimensions (the other two are Vredefort in South Africa and Sudbury in Canada). And this is odd. For, during the billions of years that Earth has had a solid crust, many more than three big asteroids might have been expected to have hit it. 

That thought led Brandon Johnson of the MIT and Timothy Bowling of Purdue University in Indiana, to wonder how many other craters have vanished, either by erosion or by being swallowed into Earth’s interior as its crust moves around, and therefore whether it is likely that some have survived and been overlooked. They have just published their analysis in Geology. 

Earth’s crust formed more than 4 billion years ago, but the oldest surviving blocks of it large enough to harbor craters date back only 3.5 billion years. What is known of the sizes and orbits of modern asteroids suggests that, if things have not changed over the aeons, about 14 big asteroids (defined as having a diameter of more than 7.4km, which would cause a crater at least 85km in diameter) should hit Earth every billion years. That means 49, give or take seven, over the past 3.5 billion years. Such impacts may have been more common in the past, when more big asteroids were around. Allowing for this, Earth would have been hit by 113, give or take 11, of them. Either way, a lot of craters are missing.

    The deal struck between Iran and six world powers over its nuclear program should see sanctions against the country beginning to ease by next year. Iranians are hoping the 41 of travel restrictions could prompt an influx of international tourists. Travel agencies have already seen an increase in demand. With its ancient architecture, 42 landscapes and warm people, tour companies describe Iran as one of the unexplored jewels of the Middle East. 

    But the country’s 43 since the 1979 Islamic Revolution has seen visitor numbers from the West fall. The latest 44 show only 90,000 arrivals from North America and the European Union in 2013. Former diplomat Mehrdad Khonsari said there’s a lot of work to do. “The difficulties we’ve had in Iran have definitely hurt the tourist industry in the sense that people are afraid to go. But those people that 45 and overcame these considerations and visited Iran were never sorry,” said Khonsari.