For more than 30 years, astronomers have known that Mars is lopsided. The southern highlands, which cover about two-thirds of the planet, are about two and a half miles higher than the northern plains. But until now, whether the disparity was caused by an asteroid impact or volcanic action on the surface has been in dispute. Three papers published in June in Nature together made a convincing case that a collision left a lasting scar on the planet’s northern half.
The northern plains form an elliptical depression 6,600 miles long and 4,000 miles wide. Since most craters are round, some believed that this formation could not have resulted from an impact and must have been caused by widespread volcanism. But in June each of the Nature papers used a different technique to arrive at the same conclusion: The northern plains are the result of a gigantic collision some 4 billion years ago.
One paper [subscription required] models the gravity and topography of Mars to distinguish impact features from volcanic activity. The second [subscription required] examines vertical impacts that might produce the thinner crust of the northern plains. The third [subscription required] simulates a set of plausible initial conditions in which a single large collision at an angle between 30 and 60 degrees results in the present topography.
Francis Nimmo, the lead author of the second study and an associate professor of earth and planetary sciences at the University of California at Santa Cruz, says that in the early solar system very large impacts were common: “Earth’s moon is the result of a big impact, Mercury has one giant impact basin, and when you move to the outer solar system, Uranus’s rotation axis is tilted because of an impact. The more places you look, the more evidence you see of these big impacts.”