The bringer of deformation NASA/JPL-Caltech/Arizona State University

Young Mars had its world turned upside-down by some hot stuff. The emergence of a titanic mount of molten rock jostled the Red Planet’s early tropics out of position, and may have helped usher in the cold, dry and dead version of the planet we know today.

That massive bulge of volcanic rock is called the Tharsis region. At 5000 kilometres across and more than 10 kilometres thick, it is the largest known volcanic complex in the solar system. When huge outpourings of lava between 4.1 and 3.7 billion years ago created it, it deformed the entire planet.

It was thought that Tharsis bent the planet’s crust and dictated the direction of Martian rivers, which formed later.


But now, Sylvain Bouley of University of Paris-South and his colleagues suggest that the rivers and their valley networks formed first, and were concentrated along the equator. The formation of Tharsis tilted the planet around so much that, if it happened on Earth, Paris would sit atop the magnetic north pole – a rearrangement that would have wild, catastrophic effects on the climate and water.

“This may make it easier to explain the climate associated with the valley networks,” says Robert Craddock, a geologist at the Smithsonian Institution in Washington DC. “It’s actually pretty obvious, but no one saw it before.”

Wandering poles

While writing his dissertation 8 years ago, Bouley noticed that valley networks that formed between 4 and 3 billion years ago were arranged almost as if they were in a circle, but tilted a bit from the equator.

Bouley and his colleagues calculated where Mars’s poles would have been before Tharsis, and worked out that the circle would have followed the equator. They also looked for evidence of a polar-like climate at what would have been the poles. The “paleo” north pole is in a region with a large amount of ice, possibly corresponding to an ancient polar ice cap, and there is evidence of water at the ancient south pole as well.

They conclude that the placement of the rivers makes the most sense if they were in place before the Tharsis region formed, or if they were forming concurrently – maybe even by rainfall or snowfall happening as the enormous volcanic structure was active.

Bouley says his next goal is studying how this crustal shifting might have contributed to Mars’s loss of water and air. As a young planet, Mars may have had warm, wet, and humid tropics at its equator just like Earth does now. But at some point, it lost is thick protective atmosphere, and much of the water went with it.

“What is certain is that the true polar wander occurred when the water and atmosphere disappeared or was disappearing,” he says. “Is the polar wander the cause? I cannot tell now.”

Journal reference: Nature, DOI: 10.1038/nature17171