NASA's Viking missions first spotted this "crustal dichotomy" in the 1970s. Twenty years later the Mars Global Surveyor mission showed that the planet’s crust was up to 30 times thicker in the south than in the north. It also detected magnetic anomalies present only in the southern hemisphere.

Three papers, published in Nature on Jun. 26, shed light on the cause of this crustal dichotomy. The first two papers show the results of computer simulations of the impact in the northern hemisphere of a large asteroid. A group led by Francis Nimmo, associate professor of Earth and planetary sciences at University of California, Santa Cruz (UCSC), authored one. This showed the results of a two dimensional simulation of the collision. The second paper, from a group led by Professor Erik Asphaug, also at UCSC, showed the results from a thee-dimensional simulation. This modelled the angle of impact, but in less detail than Nimmo’s group.

Mars	Mars Global Surveyor

Together, the simulations show that the known features of the Martian crustal dichotomy can result from the impact a body, half to two-thirds the size of the Moon striking at an angle of 30 to 60 degrees.

In the 1980s, Don Wilhelms and Steven Squyres first suggested that a giant impact caused the crustal dichotomy. But the idea found little support because there was no obvious impact crater. A rival hypothesis holds the crustal dichotomy is the result of widespread volcanic activity some 3.8 billion years ago.

The third paper from a team led by Jeffrey Andrews-Hanna, of the Massachusetts Institute of Technology, "removed" features from the Martian landscape younger than four billion years old. Once done, a huge elliptical crater, 10,600 (6,572 miles) long and 8,500 kilometers (5,270 miles) wide stands out. This is the largest crater in the solar system.

"The lowlands of Mars is an enormous elliptical projection," Andrews-Hanna told in Nature. "There’s only one process we know of that causes this kind of depression," the impact of a very large body.

"This still doesn't prove that a giant impact created the dichotomy, of course," says Squyres, "we weren't there to see it happen, and all of this is inference. But it means that it's a physically reasonable idea, and that's a significant step forward."

At the time of the impact, four billion years ago, the inner solar system was going through the final stages of planet building. Craig Agnor, a co-author on the Francis Nimmo paper told BBC News, "We think the planets formed out of a disc of rocks. As the rocks collide, you get bigger rocks and so on. Eventually, you end up with four planets and a lot of rocks - of various sizes. In terms of the process of the planets sweeping up the last bits of debris, this could have been one of the last big bits of debris."

It was during this period, that another very large body slammed into the Earth, giving rise to the Moon.

Image credits: NASA

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