MARS POLES
Mars has two differently shaped hemispheres: The lowlands of the northern hemisphere and the volcanic highlands (yellow to red regions) of the southern hemisphereMOLA Science Team

A massive celestial body, made largely of iron, crashed into Mars at a speed of five kilometres per second around four to 15 million years after the Red Planet was formed.

This is the explanation being provided by ETH Zurich geophysicists for the dichotomy of the two hemispheres of Mars.

When the celestial object with a radius of at least 1,600 kilometres impacted the planet, it added a lot of iron to Mars, they say, using three dimensional computer simulations of the event.

The impact triggered strong volcanic activity lasting until three billion and half years ago, after which time the Red Planet experienced neither volcanic activity nor a magnetic field.

Giovanni Leone, the lead author of the study, believes it is unlikely that the conditions on the planet were ever conducive for oceans or life.

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

The celestial impact generated so much energy that it created a magma ocean, extending across the southern hemisphere.

This was at a time when the planet's crust was very thin.

The celestial body that struck Mars must have been at least one-tenth the mass of Mars to unleash such energy needed to create this magma ocean. The molten rock eventually solidified into the mountainous highlands that today comprise the southern hemisphere of Mars.

The model gives an accurate depiction of the uneven volcanic distribution across the two hemispheres on Mars.

The volcanoes are common and widespread on the southern hemisphere, but rare and limited to only a few small regions in the northern hemisphere which has more of flat lowlands.

The model also gives a perfect representation of the size and shape of the hemispheres.

One condition, however, is that the celestial body impacting Mars consist of 80 per cent iron.

The model developed by the ETH researchers also confirms the date on which the magnetic field on Mars ceased to exist. Corresponding to around 4.1 billion years ago, it tallies with figures previously proven by other scientists.

The paper was recently published in the journal Geophysical Research Letters.