"The letters I and X are not currently used as the first letter of any ICAO identifier, and the letter J is only used in a ceremonial ICAO identifier granted to Jezero Crater on the planet Mars, JZRO."
Did they just set a precedent for J as the Mars prefix?
Yep. Nothing more permanent then temporary and all that - when Mars needs another airfield area, someone's going to Google for guidance, see that J and go "well, let's be consistent..."
Hope they add it to Microsoft Flight Simulator 2020