The best candidate for a binary crater on Earth today is the Lockne crater in Sweden and a smaller crater nearby called Målingen. “We dated these structures very exactly and saw that they formed at exactly the same age,” about 450 million years ago, says Jens Ormö from the Astrobiology Center in Spain, who led analysis of the craters published in 2014. One other promising candidate pair is known, the Kamensk and Gusev craters, but their location—on the border between Russia and Ukraine—makes them difficult to study in the current global climate.
On Mars, craters can remain visible for billions of years. So using high-resolution images of the surface taken by Mars orbiters, Vavilov and his colleagues examined nearly 32,000 craters larger than 4 kilometers across to hunt for crater pairs.
Their results showed that 150 pairs appeared to be the result of binary impacts, totaling 300 individual craters. These estimates come from looking for pairs of crater shapes that would be expected following a binary asteroid collision. These include tear-drop craters, where the two craters overlap; peanut craters, where they are connected at their edges; and doublet craters, where there is a gap between the two. The orientation of the two craters depends on the position of the two asteroids at the time of impact.
“We didn’t know how many there were on Mars,” says Katarina Milijkovic at Curtin University in Australia, who performed initial modeling in 2013 to show what crater shapes might be expected but was not involved in this latest work. “Somebody had to do an exhaustive study to find all of them. I think it’s great.”
As two asteroids simultaneously strike the surface, it could lead to some intriguing physics. Elliot Sefton-Nash, the deputy project scientist on ESA’s delayed ExoMars program, says the shock waves from the impacts could collide, creating a raised ridge between the two craters or some high-pressure locations. “It’d be like going the opposite way on a motorway,” he says. “You might be able to see differences in minerals that form only under very high pressure.”
In total, the number of binary craters found on Mars accounts for only about 0.5 percent of the total craters wider than 4 kilometers on the planet—far below most estimates for how many binary asteroids should be in the solar system. That could be a result of natural weathering erasing some of the impacts, or it could be that smaller crater pairs exist below 4 kilometers across. “There are more than 100 million impact craters bigger than 100 meters,” says Anthony Lagain from Curtin University in Australia, a coauthor on the study. “If you start doing smaller craters, you have to spend a lot of time to review all of them.”
Many of the crater pairs are similar in size, which contradicts the leading theory for how binary asteroids are made. It’s thought that such systems form as a result of light from the sun hitting an asteroid, which can push material off its surface and into its orbit. As the asteroid spins, this material builds up over millions of years into a small companion, evidenced by the makeup of most of the binaries observed to date.