Mars' two hemispheres are drastically different: the northern hemisphere is mostly flat, but the southern hemisphere is full of highlands and volcanoes. But why is it like that?

New research suggests that the southern hemisphere of the red planet was once hit by a moon-sized celestial object that created volcanic activity in that area, which drastically affected the surface features there.

Researchers believe that the impact happened during the early years of the solar system, when Mars was only around 4 to 15 million years old.

When the impact occurred, Mars had a thin crust with liquid just underneath its surface. Scientists at ETH Zurich simulated conditions at the time, as well as the impact that created the planet's southern hemisphere.

The researchers started with the assumption that the body that hit Mars was at least 80 percent iron and measured about 994 miles across. It crashed into the planet at about 3 miles per second. This impact caused liquid magma beneath Mars' thin crust to erupt to the surface. Most of these volcanic plumes occurred near Mars' equator, but because of the impact, the hot liquid also ran into the south pole.

Earlier theories suggested that a similar impact, or smaller impacts, occurred on Mars' northern hemisphere. However, the ETH Zurich researchers believe their model is more accurate.

"Our scenarios more closely reflect a range of observations about Mars than the theory of a northern hemisphere impact," says Giovanni Leone, an ETH Zurich geophysicist. "Our model is an almost identical depiction of the actual distribution of volcanic identity."

Leone states that prior models haven't correctly shown this. He also points out that his simulation was "nearly realistic."

Previous research shows that volcanic activity on Mars stopped around 3.5 billion years ago. At the time, Mars also lost its magnetic field.

The ETH Zurich model confirmed the date of Mars' last magnetic field, putting it at about 4.1 billion years ago. This happened when the planet's core cooled, which also cooled the mantle and crust. This also explains why volcanic activity in the southern hemisphere stopped around that same time.

Leone also states that Mars probably never had water on its surface because it's always been a hostile environment.

"Before becoming the cold and dry desert of today, this planet was characterized by intense heat and volcanic activity, which would have evaporated any possible water and made the emergence of life highly unlikely," he says.