What is the second essential pillar of the Starship architecture required for sending a significant payload to Mars?

Answer

In-Orbit Refueling

To send a significant payload to Mars, the entire spacecraft must be refueled in Earth orbit using tanker versions of Starship; this capability is non-negotiable for Mars transit.

What is the second essential pillar of the Starship architecture required for sending a significant payload to Mars?
goalSpaceXspace exploration