QUESTION:
The positions on Mars are given by specifying the geographic (or
martianographic?) latitude and longitude. The zero of latitude can be given
by the axis of rotation like on Earth, but how do you select the zero of the
longitude? (There is no Greenwich on Mars.)

ANSWER from Bruce Jakosky on August 11, 1997:
Setting a longitude requires an arbitrary decision as to where to draw the
zero longitude line.  On Earth, the arbitrary decision was made to draw it
through Greenwich.  On Mars, people had been looking at Mars through
telescopes for years, and had created a longitude system.  When spacecraft
got there, they needed a better one, so they looked for some geological
feature that could serve as the reference location.  They selected a small
crater that had the name Airy, and defined the longitude 0 as running
through the middle; hence, it's often called Airy-0.

Interestingly, the telescopic observers had defined the direction of
longitude on Mars so that the longitude of the location at the center of the
visible disk would increase during the night.  This is the opposite
direction from the Earth!  There's a big battle going on right now to decide
whether to change to a right-handed coordinate system.  This would mean that
all of the longitudes on Mars would change!