The mystery behind the lopsided shape of Mars lies in its Borealis basin, whose elliptical shape led scientists to believe that the theory of an ancient impact with a giant heavenly object is a plausible one. The enormous impact crater in the northern hemisphere of the Red Planet is the largest of its kind in the entire Solar System.

As satellite observations in the 1970s have shown, Mars - the fourth planet from the Sun - has a strange lopsided shape, which has intrigued scientists over the past decades.

The theory that first tried to explain the shape of Mars through an impact with a heavenly body in the northern hemisphere was launched in 1984 by Steven W. Squires and Don Wilhelms, but was rejected by the scientific community because they didn’t believe the shape of the basin matched the normal shape of a crater.

However, the latest details provided by NASA’s Mars Reconnaissance Orbiter and Mars Global Surveyor might convince some of the more skeptical scientists that the Red Planet’s shape is in fact the result of an ancient impact.

Three studies, published this week in the journal Nature, come to support the 1984 theory, although they do not completely prove the giant impact hypothesis.

The Martian surface offers a striking contrast between its northern and southern hemispheres.

The plains flattened by lava flows in the northern hemisphere form what is considered to be one of the smoothest surfaces in the entire Solar System, while the southern hemisphere offers a rough, cratered landscape, with elevations varying from 2.5 to 5 miles compared to basin floor.

Jeffrey Andrews-Hanna of the Massachusetts Institute of Technology and his colleagues studied the Martian landscape in the northern hemisphere. With the help of computer simulations, they managed to eliminate the Tharsis volcanic range and highlight the transition between thick and thin crusts.

The result was a gigantic crater, 10,600 kilometers long and 8,500 kilometers wide. “There’s only one process we know that causes this kind of depression,” Andrew-Hanna explained, reviving the impact theory.

According to the study, the object that may have hit the Red Planet was approximately 1,200 miles across, which means it was bigger than Pluto.

The results of the computer simulations come to support Wilhelm and Squires’ theory of an ancient impact, by providing the necessary calculations that mark the transition from intuitive assumptions (logical, but not supported by calculations) to physically possible theories.

All in all, Andrew-Hanna’s study doesn’t prove that the ancient impact took place. However, as Squires pointed out, “it means that it’s a physically reasonable idea” and it’s a step forward at the same time.

The impact theory has implications in more than just the Martian evolution. As we all know, approximately four billion years ago, Earth is also believed to have been hit by a heavenly body, which is how the Moon formed.

The collisions that took place at the time extend to several planets in the solar system, which show signs of possible impacts, scientists say. This makes the ancient impact theory even more plausible.