The impact of an asteroid is the origin of the difference between the two hemispheres of Mars. Both hemispheres are very different. The northern area is flat, as if it were the bottom of an ancient ocean; moreover, from a geological point of view, the surface of the planet is thin and has a very common magnetic field. The south is the opposite, mountainous, coarse bark and with a very complex magnetic field. One study has found that these characteristics of Mars can explain an impact hypothesis.
This is a research from the University of California. Data obtained by two satellites in the orbit of Mars are Reconnaissance Orbiter and Global Surveyor. With this data a computer simulation has been performed to check if the impact of an asteroid can cause the current appearance of Mars. The answer has been yes if the asteroid's shock angle to the surface of the north pole of Mars is between 30 and 60 degrees.
This hypothesis coincides with the theory of formation within the solar system. It is believed that Mercury, Venus, Earth, Moon and Mars were formed by the existence of a ring of stones around the Sun, which over time was captured by gravity in this planetary system. The process is not over yet, but today there are not many giant asteroids in that place. At the time of the impact of Mars yes.