Is the following explanation plausible? Can you spot any flaws?
Mars doesn't have a large moon to stabilise its axis of rotation. At some point early in its formation its axis might have been pointing at the Sun with one hemisphere continually heating up and evaporating all the volatiles which circulated to the other side and froze. The extra weight of the volatiles would have compressed the surface leading to a thinner crust. As more volatiles were deposited on the frozen hemisphere compression would have continually kept Mars spheroidal by pushing the elevation of the frozen side down to match the other side's elevation.
Eventually the axis drifted away from this position so that the frozen hemisphere began to receive sunlight again and melt into an ocean. Over time Mars lost its atmosphere to space at which point the ocean also began to evaporate into space so that it lost volume resulting in the hemispheric dichotomy of Mars that we see today. As Mars lost heat to space the remnant ocean froze again.