Now, looking under the sediment layer, Zuber and her colleagues show the Martian crust itself is smooth. The evolving view of Mars as a once active, thriving planet, shows that first impressions are deceiving and a planet's geology is more than skin-deep. On Mars Zuber says that the best analogy for the northern plains is something akin to plate tectonics. But for other parts of the planet, that idea doesn't hold. We don't see very narrow spreading ridges and subduction zones.
Subduction zones are where one plate grinds against another, pushing it downward.
One reason why we don't see these structures could be that Mars is 4 billion years past its prime. And if you're dumping a lot of heat out of the core, she said, it could have produced a greenhouse that was hot and wet enough to nurture new life. Sleep was one of the first to suggest that Mars once had tectonic plates, like the ones found here on Earth. The first views, he said, were of the southern hemisphere which were heavily pockmarked with craters. No atmosphere, no internal heat, no life.