Lowest altitude viable Mars orbit

Ben Weiss wrote:
In article ,
Ben Hallert wrote:

Of possible interest, it is my understanding that because of gravity, the
Martian atmosphere extends further from the ground then Earths, so any long
term orbit would necessarily need to be higher to avoid significant drag.
400 kilometers might not be a good place to park your crew return vehicle,
for example, unless you were confident that it could be reliably reboosted
as needed.


On the flip side, how deep a hole would you have to dig on Mars so that
the atmospheric pressure at the bottom would approach 1 atmosphere? (And
how much thicker is the atmosphere at the bottom of Vallis Marineris
(say) than the "average" pressure on Mars?


As a very, very, very rough guess, the scale height of the martian
atmosphere is going to be some 10Km.
The atmospheric pressure is 1% of earth, so you need 7 scale heights, to
hit 1 bar.
70Km.