How Mars Got Lopsided

摘要

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.