In what main ways would a human return to the Moon facilitate human travel to Mars?
It is often argued that the Moon could work as a stepping stone to Mars. I want to know what the merits are of that argument. The differences seem huge. Travel time is really the main problem with going to Mars, while a Moon trip is done in a week. People have to survive away from Earth during an entire Mars opposition period of about 2 years. Atmosphere and gravity differ so that the two missions do best in having separate lander/ascent vehicle designs, I would suppose. Space suites have very different challenges too with different kinds of dust and temperature variations. The day/night cycle is a tough problem on the Moon, while Mars on the other hand has less than half the insolation, requiring different energy supply and heating/cooling systems. The Apollo lunar rover would hardly hold up in the double Martian gravity. Communication to Earth from a rotating Mars 400 000 000 km away is quite different than from the front side of the Moon. The resources which could potentially be made use of are very different too, such as water ice in eternally shadowed craters at Lunar poles and CO2 in the Martian atmosphere.
If humans returned to the Moon, what could they bring from that to also go to Mars? For example, what Apollo equipment would be useful on Mars?