The base mesh is ultimately derived from NASA's Mars Orbiter Laser Altimeter (MOLA) dataset; specifically the 128 pixels per degree dataset. This is available from the Mars Global Surveyor website (more specifically, from the Mission Experiment Gridded Data Records page).
Some land class information was derived from geological shapefiles provided by pubs.usgs.gov. A potential alternative/additional set of shapefiles is the USGS Dune Database: MC1, MC2-MC29 and MC30.
The base mesh tiles ('.dsf files') were generated using MeshTool from Laminar.
The Mars package redistributes some useful files that were part of X-Plane 10.36, but not later versions:
Resources/bitmaps/skycolors/sun_glow_mars.png
Resources/bitmaps/skycolors/sky_colors_mars.png
This package also contains the following images, in modified form:
Resources/default scenery/1000 world terrain/textures10/soil/red_rock_*.dds
Resources/default scenery/1000 world terrain/textures10/soil/sand_*.dds
Resources/default scenery/1000 world terrain/textures10/apt/apt_cld_dry.dds
Airports were derived from Robin Peel's version 2006.09 set, downloaded from X-Plane Airport & Navigation Data. The original versions of the airports were conceived by Bryan Butcher. All the layouts, and choice of locations, are thanks to Bryan.
Finally, a big thanks to ISDG for providing the hosting! And for making the whole process so straightforward.