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Originally Posted by aurorae
The large canyon complex originating near the Tharsus bulge has some curious features, and the origin of it is debated. There was some thought that perhaps it was a failed rift zone. Certainly it is related in some way to the bulge and to the magma plume(s) under the bulge, but exactly how is not determined.
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Perhaps Mars expanded in the past but the expansion then stopped, at least for now. I have the rough figure that Mars is half "lowlands" and half "highlands". If we equate the lowlands with Earth-like ocean basins and the highlands with continents, then Mars has not expanded to the same relative degree as the Earth has. On Earth the continents now form only 1/4 of the surface, indicating Earth has doubled in radius. The radius of Mars has increased only by a factor of 2^1/2 or 1.41. If so maybe Mars smaller size has something to do with the lesser expansion, or maybe its core.