Data from Mars Global Surveyor indicates that Mars' crust is about 80 km thick in the southern hemisphere but only about 35 km thick in the north. With no lateral plate motion, hot-spots under the crust stay in a fixed position relative to the surface. This, along with the lower surface gravity, may account for the Tharis bulge and its enormous volcanoes. There is no evidence of current volcanic activity, however. At some time in the past there was clearly some sort of fluid on the surface.
Liquid water is the obvious fluid but other possibilities exist.
E arly in its history, Mars was much more like Earth. As with Earth almost all of its carbon dioxide was used up to form carbonate rocks. But lacking the Earth's plate tectonics , Mars is unable to recycle any of this carbon dioxide back into its atmosphere and so cannot sustain a significant greenhouse effect . The surface of Mars is therefore much colder than the Earth would be at that distance from the Sun. But it is thick enough to support very strong winds and vast dust storms that on occasion engulf the entire planet for months.