Compared to the other terrestrial planets, Mercury has always been a bit of a mysterious one. It’s internal structure is very different from its planetary siblings with its core accounting for 70% of its overall mass and an unusually thin mantle composed of silicates. One theory suggests a head-on collision between a larger proto-Mercury and a smaller object while another suggests Mercury sideswiped an Earth-mass object. It may be something completely different and a new paper suggests that a grazing collision between two similarly sized bodies led to the formation of the planet we see today.