What would be the reason for not being able to maintain its atmosphere?
A change in the diameter or orbit of Mars or perhaps a large asteroid to vaporize its ocean. The evidence shows that there was a ocean and there would have to be a atmosphere to hold the ocean in liquid form, at one time. Venus is marginally smaller D=7,521 in size, to the Earth, yet has 90X atmospheric pressure. Mars D=4,222, equivelent to Earth at 30,000 feet. What is that minimum diameter size to maintain a atmosphere?