Woah there, Mercury has a magnetosphere ?
That tiny thing ? The smallest of all official planets has a magnetosphere while Mars and Venus don't ? I mean hey, Mercury is smaller than our own Moon (check the mass figures) !
Well blow me down. I would've thought that, if Mars was not big enough to keep a molten core and its corresponding magnetosphere like Earth, then Mercury was damned by default, but hey, this is Real Life(TM). Things don't always go like you expect them to.