1. Mars is at the outer edge of what is called the "Goldilocks zone" for our star. That is the distance from the Sun where enough energy reaches a planet to warm it to the point where water can exist as a liquid. Venus is at the near edge of it. 2. Mars has a thin atmosphere. Too thin for us to breath, but thick enough to help trap the Sun's energy and warm the planet. So, like Earth, Mars is only pepetually cold enough to freeze water at its poles. The only "space magic" the Traveller would have had to perform on Mars to "terraform it" (make it Earth-like) would have been to introduce oxygen to the atmosphere, thicken it to the point it would support life, and set the planet's core to spinning so that it would generate a magnetic field to protect life on the surface from radiation from space. Venus and Mercury are another story. The real Venus is as hot as commercial oven with atmosphere that rains sulphuric acid....and Merucy is a dead rock (like our Moon) and is as hot as a blast furnace.