First of all, [spoiler]Its a -blam!-ing video game[/spoiler] Second of all, [spoiler]Venus is hotter than Mercury because of its atmosphere. Since Venus's atmosphere is so thick, any heat that gets through the cracks of the atmosphere never gets out. So that heat accumulates overtime and eventually the temperature increases. In fact, Venus is the hottest planet in the solar system. Mercury doesn't even have an atmosphere... it's dead.[/spoiler]