No one terraformed it. The Traveller pulled a Venus out of an alternate universe. The new Venus had evolved with 1G gravity a breathable atmosphere and ecosystem. "Our" Venus he probably swapped out in it's place and confused the hell out of the locals. Same with the Moon - the Traveller swapped in a Moon which somehow has 1G gravity and also still orbits in the same position - impossible in our universe. Same with Mecury - The Traveller plopped in a Mercury with a breathable atmosphere and some method for dimming the effect of the Sun. - all of which the Vex kept running and didn't see fit to mess with. Who knows? Maybe there was some alternate Jupiter that we could walk on - however that would be so different to "real" Jupiter that I think many of the hard sci fi buffs would have real trouble accepting it. Remember, this is sci fi - you can do what you want, but your explanation for it must be plausible and not include "it's magic" "because" or any of the other wishy washy flim flam excuses for exposition employed by fantasy writers and other stupid people. So far, even "Space magic" has been meant in the Arthur C Clarke sense rather than the "oooo spooky" sense.

I don't think you understand the process of terraforming do you.. We terraform earth everyday, we use machines... We don't use a -blam!-in suit lol

you would think that an intelligent race capable of interplanetary travel in seconds would of thought off heat proof suit..

I'll wait for you to go down and Terra form it for me.