I don't understand why you are trying to use logic in a videogame. All the other planets are uninhabitable for humans for various reasons; but Destiny explains that the Traveler miraculously terraformed the planets and their atmospheres. It's supposed to be the magic of the games story. I don't understand why we would be able to go to any of the other planets if we can't go to Mercury. What makes Venus, Mars, the moon, or even the Reef okay? If you want facts, Venus is the hotter than Mercury due to the greenhouse effect because there is far more CO2 in the atmosphere which traps in the heat that the planet is receiving. Even if you didn't want to trust the fact of how hot it is, the atmosphere is about 90x more dense than Earth's. And another question to ask is how our bodies move the exact same way in the game as if all the planets had the same exact gravity? Since when did our moon have an atmosphere? Either way, it wouldn't help if we tried to make sense out of how we can play on these planets or moons, so there's no point in using logic. But it sure is fun to run around and explore Venus, so I won't argue if they let us explore Mercury.