Bill Sheppard wrote:

Dat's me asked,

Is Mars' atmosphere dense enough to
make either/both of those designs
viable?


Yes. In fact NASA has researched using aerial vehicles much like what
you described, based on planes that fly in the Earth's upper atmosphere.
See-
http://powerweb.grc.nasa.gov/doc/marsairplane.html
oc


Mars' atmosphere is also dense enough to support liquid water near the
equator for a suprisingly lengthy period of time. Read about the
research at http://www.aoas.org/article.php?story=20030904014348849

Dale