If Ganymede orbited the Sun, it would be considered a planet. The reason is that Jupiter [ http://galileo.jpl.nasa.gov/jupiter/jupiter.html ]'s moon Ganymede [ http://www.seds.org/nineplanets/nineplanets/ganymede.html ] is not only the largest moon in the Solar System [ http://www.seds.org/nineplanets/nineplanets/datamax.html ], it is larger than planets Mercury [ http://www.seds.org/nineplanets/nineplanets/mercury.html ] and Pluto [ http://dosxx.colorado.edu/plutohome.html ]. The robot spacecraft Galileo [ http://www.jpl.nasa.gov/galileo/overview.html ] currently orbiting Jupiter [ http://www.seds.org/nineplanets/nineplanets/jupiter.html ] has been able to zoom by Ganymede [ http://www.solarviews.com/eng/ganymede.htm ] several times and snap many close-up pictures. Ganymede, shown above [ http://photojournal.jpl.nasa.gov/cgi-bin/PIAGenCatalogPage.pl?PIA00716 ] in its natural colors, sports a large oval dark region known as Galileo Regio [ http://www.jpl.nasa.gov/galileo/ganymede/121896.html ]. In general, the dark regions on Ganymede [ http://sse.jpl.nasa.gov/features/planets/jupiter/ganymede.html ] are heavily cratered, implying they are very old, while the light regions are younger and dominated by unusual grooves [ http://antwrp.gsfc.nasa.gov/apod/ap960711.html ]. The origin of the grooves is still under investigation [ http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1998Icar..135..317P ].