Longitude is a bit trickier.
The Prime Meridian is defined by the International Astronomical Union. For Mercury, the Prime Meridian is defined to be the crater Hun Kal which means 'twenty' in ancient Mayan. For Venus, the center of the 'Great Eye' in the atmosphere seen in ultraviolet light is taken as zero longitude. For Mars, the tip of a feature called Fastigium Aryn near Sinus Meridiani is defined as longitude Zero degrees. System I and System II which rotate at two different speeds because at the equator the atmosphere takes 9h 50m 30s to go once around, but at the poles it takes 9h 55m 40s.