QUESTION:
Can you see both moons while standing on Mars? 

ANSWER from Mike Malin on February 16, 1997:
Both Phobos (the inner moon) and Deimos (the outer moon) are visible from
the surface of Mars.  Although they are very small (Phobos averages about 7
miles across, while Deimos is about 4 miles across), they are also very
close to Mars (Phobos orbits only 6000 km above the surface; Deimos about
20,000 km).

It is possible to roughly calculate what they would look like, compared to
a full Moon.  Phobos is about 60 times closer to Mars than the Moon is to
Earth, and Deimos is about 20 times closer.  The Moon has 95,000 times the
area of Phobos, and 290,000 times the area of Deimos.  The rock out of
which both Phobos and Deimos are made are about 1/3 as bright as the rocks
on the Moon.  And finally, Mars is about 1.4 times farther from the sun.
So the relative brightness of Phobos and Deimos, relative to the Moon is:

Phobos = 1 Moon X (60 X 60) X 1/3 X 1/95,000 X 1/(1.4 X 1.4) = 6/1000 as 
bright

Deimos = 1 Moon X (20 X 20) X 1/3 X 1/290,000 X 1/(1.4 X 1.4) = 2/10,000 as
bright.

Phobos is about as bright as the brightest planet (Venus) as seen from the
Earth.

Deimos is about as bright as the brightest star (Sirius) as seen from the 
Earth.