Re: Question or two
Good questions, but there's no mistery. Even weighing half a ton may not cause a significant crater if the atmosphere they're hinting at was thick enough. But more importantly, any crater would have been small (being metallic and whole, it didn't explode on impact so no major hole) and easy for the Martian weather to erode. The rock lies exposed because over long periods of time erosion would have removed the surrounding softer rock, and left the much tougher metal behind. It is also possible that flowing water may have moved the meteorite from its original impact site to where it sits today.
This is not the first meteorite that the rovers have found. There was a smaller one - also metallic - found a few years back near one of the landing sites, and quite near a heat shield that had been shed.