A group of researchers claims to have solved the riddle why the northern and southern hemispheres of Mars look so different - the northern hemisphere is an enormous lowland basin which might once have held a vast ocean while the southern hemisphere comprises rugged, crater-pitted highlands with altitudes up to 8,000m greater than the north.
Researchers led by Francis Nimmo at the University of California, Santa Cruz, US, believe the huge difference between the Martian northern and southern hemispheres was caused by an impact of an asteroid that hit the northern regions about four billion years ago. This rock was about the size of Earth's moon and unleashed energy equivalent to one million billion atomic bombs like the one dropped on Nagasaki in 1945.
"It happened probably right at the end of the formation of the four terrestrial planets - Mercury, Venus, Earth and Mars," said Craig Agnor, a co-author on the Francis Nimmo study.
He 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."
Shock waves from the impact would travel through the planet and disrupt the crust on the other side, causing changes in the magnetic field recorded there.
The predicted changes are consistent with observations of magnetic anomalies in the southern hemisphere, according to Dr Nimmo.