The new studies have "upped the odds a lot" that an impact caused the dichotomy, says planetary scientist Walter Kiefer of the Lunar and Planetary Institute in Houston. The next line of evidence, says Kiefer, would be looking for chemical differences in Martian rocks indicating that crust in the dichotomy formed from material deep inside the mantle that welled up after the impact.
The kind of asteroid needed to form the Martian dichotomy would fall in between that size and those of the rocks that formed other large craters, such as the South Pole—Aitken impact basin on the moon and the Hellas Basin in Mars's southern hemisphere, both more than 1,30 miles (2,000 kilometers) wide.
Although Earth's inhabitants still face the threat of deadly asteroid strikes, researchers say that impacts of the magnitude seen in the Martian dichotomy have long since died off, leaving only scars to tell their story.
"The early solar system was a very dangerous place to be," Andrews-Hanna says, "but if we didn't have the impacts, we wouldn't have the planets as we see them today."