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When an asteroid strikes a planet, it can pack a powerful punch—as the dinosaurs discovered to their detriment 66 million years ago. But what if two asteroids strike at the same time and in the same location?
A first-of-its-kind study published in the journal Icarus investigates this phenomenon on Mars. Looking at the planet, researchers have discovered hundreds of craters that likely resulted from the impacts of a binary system, where one asteroid orbits another, like the moon orbits Earth.
“They’re really difficult to find,” says Dmitrii Vavilov at the Côte d’Azur University in France, the study’s lead author. But the findings show these binary craters are there, he says.
The first discovery of a binary asteroid was made by NASA’s Galileo spacecraft as it journeyed to Jupiter in 1993. While capturing images of an asteroid called Ida on the way, mission scientists were shocked to find a second asteroid orbiting nearby. “They were so confused,” says Harrison Agrusa, an astronomer at the University of Maryland who was not involved in this new study. “People were debating if something was wrong with the camera.”
It wasn’t. Instead, Ida was the first confirmation that asteroids could orbit in pairs, and in some cases even more. Ida’s companion, later named Dactyl, was incredibly small, yet proof of their existence. “It set off a big shock wave in the community,” says Agrusa.
Based on observations of the other millions of asteroids in the solar system, scientists today estimate that about 1 in every 6 asteroids—around 16 percent—is part of a binary system. We can see these orbiting around the solar system, particularly in the asteroid belt between Mars and Jupiter, with one of the most famous pairs—Didymos and its small companion, Dimorphos—the target of a NASA and European Space Agency (ESA) asteroid defense mission later this year.
Asteroids regularly strike planets and moons, so it would be expected that binary asteroids would too. Finding binary craters can be difficult though, especially among the myriad other craters on places like our moon. On Earth it is harder still, as geological processes quickly erase evidence of impacts.
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.
It is unlikely such a process would form binary asteroids of the same size, which are what similarly sized crater pairs suggest, says Agrusa. Instead, there may be some other formation process for binary asteroids taking place in the solar system. “It points to maybe some other mechanism that we don’t fully understand yet,” he says. “For some reason, we’re not observing those.”
One of the next steps would be to age some of these craters, which could give us a look back at the evolution of binary asteroid systems throughout the solar system, something we can’t currently do with telescope observations. “Right now, we have a static picture of the solar system,” says Quanzhi Ye, an astronomer at the University of Maryland who was not involved in the study. “This work helps us know what might have happened to binary asteroids as they grow older.”
Scientists could look beyond Mars for more evidence of binary craters elsewhere, such as on Mercury or the dwarf planet Ceres. “Ceres is in the middle of the asteroid belt,” says Lagain. “It would be interesting to see if the population of binary asteroids that collided with Ceres is different from what we see on Mars.”
As to whether there’s anything to fear from a possible binary asteroid impact on Earth in the future, there isn’t too much to be worried about just yet, as no known asteroid—binary or otherwise—is on an impact course with Earth. Yet if we did have to contend with an incoming binary asteroid one day, the problem would present an unusual dilemma.
“I don’t know what we would do,” says Ormö, adding that we’d likely have to “push the big one and hope the smaller one will follow.” On Mars, we can now see in all its glory what would happen if we didn’t.
