Violent volcanic activity when Mars was young caused the planet's crust to rotate around its core, right at a time when life might have been emerging, a new study suggests.
The massive geological event also happened much more recently than previously thought, changing our understanding of the development of the planet.
Volcanic activity in the Tharsis region, the Solar System's largest complex of volcanic rocks, caused a prominent bulge on the surface of the planet – the Tharsis dome.
The event also reoriented Mars' spin axis by 20 degrees and is responsible for the present-day equatorial position of the volcanic region.
But the actual axis of the planet did not change. Instead the crust turned with respect to the core "rather like turning the flesh of an apricot around its stone", the researchers at the Université Paris-Sud say.
The tilt would be the equivalent of Paris shifting to the north pole, they say.
The Tharsis dome was thought to have influenced the direction of rivers and orientation of valleys on the red planet, and that most of the Tharsis region had already formed by 3.7 billion years ago, the end of the Noachian period, which began around 4.1 billion years ago.
It is now believed the tilt may have occurred between 3.1 billion and 3.7 billion years ago.
"This study radically changes the generally accepted scenario, according to which the Tharsis dome was thought to have mainly formed before 3.7 billion years ago and to have existed before the rivers, since it controlled their flow direction," the researchers say.
According to computer simulations, the scientists now believe most rivers would have flowed from the cratered highlands of the southern hemisphere to the low plains of the northern hemisphere and the rise of the rivers could have been entirely contemporaneous with the formation of the Tharsis dome.
The team, led by Sylvain Bouley, used simulations to reconstruct the shape of Mars before the Tharsis region formed.
"We show that the observed directions of valley networks are also consistent with topographic gradients in this configuration and thus do not require the presence of the Tharsis load," the paper says.
The sceientists say the new chronology and geography "will have to be taken into account when studying early Mars to look for traces of life or for an ocean, for instance".