Source: NASA
"We propose ... a vacuum airship. This concept is similar to a standard balloon, whereas a balloon uses helium or hydrogen to displace air and provide lift, a vacuum airship uses a rigid structure to maintain a vacuum to displace air and provide lift. ...
... Mars appears to have an atmosphere in which the operation of a vacuum airship would not only be possible, but beneficial over a conventional balloon or dirigible.
... The Martian atmosphere has a pressure to density ratio that is very beneficial to the operation of a vacuum airship ...
...one need not worry about the vacuum airship getting stuck in a trench or being unable to traverse terrain. The vacuum airship could be used as a communication relay for other vehicular probes ...
...Since the vacuum airship does not use a lifting gas, it can perform a near infinite number of compensation maneuvers to adjust or stabilize its altitude in a temperature variant environment. ...
...Even though the vacuum airship would be aerially base, the vehicle would still be able to touch down and perform tasks on the ground ...
...the large surface area of the vacuum airship would provide plenty of area for solar cells which would allow the vehicle to gather a large amount of solar energy ..."